Video for Linux Two API Specification Revision 2.6.32【轉】
轉自：https://www.linuxtv.org/downloads/legacy/video4linux/API/V4L2_API/spec-single/v4l2.htmlphp
Video for Linux Two API Specification
Revision 2.6.32
Copyright © 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 Bill Dirks, Michael H. Schimek, Hans Verkuil, Martin Rubli, Andy Walls, Mauro Carvalho Chehabgit
This document is copyrighted © 1999-2009 by Bill Dirks, Michael H. Schimek, Hans Verkuil, Martin Rubli, Andy Walls and Mauro Carvalho Chehab.web
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the appendix entitled "GNU Free Documentation License".redis
Programming examples can be used and distributed without restrictions.express
| Revision History | ||
|---|---|---|
| Revision 2.6.32 | 2009-08-31 | mcc |
| Now, revisions will match the kernel version where the V4L2 API changes will be used by the Linux Kernel. Also added Remote Controller chapter. | ||
| Revision 0.29 | 2009-08-26 | ev |
| Added documentation for string controls and for FM Transmitter controls. | ||
| Revision 0.28 | 2009-08-26 | gl |
| Added V4L2_CID_BAND_STOP_FILTER documentation. | ||
| Revision 0.27 | 2009-08-15 | mcc |
| Added libv4l and Remote Controller documentation; added v4l2grab and keytable application examples. | ||
| Revision 0.26 | 2009-07-23 | hv |
| Finalized the RDS capture API. Added modulator and RDS encoder capabilities. Added support for string controls. | ||
| Revision 0.25 | 2009-01-18 | hv |
| Added pixel formats VYUY, NV16 and NV61, and changed the debug ioctls VIDIOC_DBG_G/S_REGISTER and VIDIOC_DBG_G_CHIP_IDENT. Added camera controls V4L2_CID_ZOOM_ABSOLUTE, V4L2_CID_ZOOM_RELATIVE, V4L2_CID_ZOOM_CONTINUOUS and V4L2_CID_PRIVACY. | ||
| Revision 0.24 | 2008-03-04 | mhs |
| Added pixel formats Y16 and SBGGR16, new controls and a camera controls class. Removed VIDIOC_G/S_MPEGCOMP. | ||
| Revision 0.23 | 2007-08-30 | mhs |
| Fixed a typo in VIDIOC_DBG_G/S_REGISTER. Clarified the byte order of packed pixel formats. | ||
| Revision 0.22 | 2007-08-29 | mhs |
| Added the Video Output Overlay interface, new MPEG controls, V4L2_FIELD_INTERLACED_TB and V4L2_FIELD_INTERLACED_BT, VIDIOC_DBG_G/S_REGISTER, VIDIOC_(TRY_)ENCODER_CMD, VIDIOC_G_CHIP_IDENT, VIDIOC_G_ENC_INDEX, new pixel formats. Clarifications in the cropping chapter, about RGB pixel formats, the mmap(), poll(), select(), read() and write() functions. Typographical fixes. | ||
| Revision 0.21 | 2006-12-19 | mhs |
| Fixed a link in the VIDIOC_G_EXT_CTRLS section. | ||
| Revision 0.20 | 2006-11-24 | mhs |
| Clarified the purpose of the audioset field in struct v4l2_input and v4l2_output. | ||
| Revision 0.19 | 2006-10-19 | mhs |
| Documented V4L2_PIX_FMT_RGB444. | ||
| Revision 0.18 | 2006-10-18 | mhs |
| Added the description of extended controls by Hans Verkuil. Linked V4L2_PIX_FMT_MPEG to V4L2_CID_MPEG_STREAM_TYPE. | ||
| Revision 0.17 | 2006-10-12 | mhs |
| Corrected V4L2_PIX_FMT_HM12 description. | ||
| Revision 0.16 | 2006-10-08 | mhs |
| VIDIOC_ENUM_FRAMESIZES and VIDIOC_ENUM_FRAMEINTERVALS are now part of the API. | ||
| Revision 0.15 | 2006-09-23 | mhs |
| Cleaned up the bibliography, added BT.653 and BT.1119. capture.c/start_capturing() for user pointer I/O did not initialize the buffer index. Documented the V4L MPEG and MJPEG VID_TYPEs and V4L2_PIX_FMT_SBGGR8. Updated the list of reserved pixel formats. See the history chapter for API changes. | ||
| Revision 0.14 | 2006-09-14 | mr |
| Added VIDIOC_ENUM_FRAMESIZES and VIDIOC_ENUM_FRAMEINTERVALS proposal for frame format enumeration of digital devices. | ||
| Revision 0.13 | 2006-04-07 | mhs |
| Corrected the description of struct v4l2_window clips. New V4L2_STD_ and V4L2_TUNER_MODE_LANG1_LANG2 defines. | ||
| Revision 0.12 | 2006-02-03 | mhs |
| Corrected the description of struct v4l2_captureparm and v4l2_outputparm. | ||
| Revision 0.11 | 2006-01-27 | mhs |
| Improved the description of struct v4l2_tuner. | ||
| Revision 0.10 | 2006-01-10 | mhs |
| VIDIOC_G_INPUT and VIDIOC_S_PARM clarifications. | ||
| Revision 0.9 | 2005-11-27 | mhs |
| Improved the 525 line numbering diagram. Hans Verkuil and I rewrote the sliced VBI section. He also contributed a VIDIOC_LOG_STATUS page. Fixed VIDIOC_S_STD call in the video standard selection example. Various updates. | ||
| Revision 0.8 | 2004-10-04 | mhs |
| Somehow a piece of junk slipped into the capture example, removed. | ||
| Revision 0.7 | 2004-09-19 | mhs |
| Fixed video standard selection, control enumeration, downscaling and aspect example. Added read and user pointer i/o to video capture example. | ||
| Revision 0.6 | 2004-08-01 | mhs |
| v4l2_buffer changes, added video capture example, various corrections. | ||
| Revision 0.5 | 2003-11-05 | mhs |
| Pixel format erratum. | ||
| Revision 0.4 | 2003-09-17 | mhs |
| Corrected source and Makefile to generate a PDF. SGML fixes. Added latest API changes. Closed gaps in the history chapter. | ||
| Revision 0.3 | 2003-02-05 | mhs |
| Another draft, more corrections. | ||
| Revision 0.2 | 2003-01-15 | mhs |
| Second draft, with corrections pointed out by Gerd Knorr. | ||
| Revision 0.1 | 2002-12-01 | mhs |
| First draft, based on documentation by Bill Dirks and discussions on the V4L mailing list. | ||
Table of Contentswindows
- Introduction
- 1. Common API Elements
-
- 1.1. Opening and Closing Devices
- 1.2. Querying Capabilities
- 1.3. Application Priority
- 1.4. Video Inputs and Outputs
- 1.5. Audio Inputs and Outputs
- 1.6. Tuners and Modulators
- 1.7. Video Standards
- 1.8. User Controls
- 1.9. Extended Controls
- 1.10. Data Formats
- 1.11. Image Cropping, Insertion and Scaling
- 1.12. Streaming Parameters
- 2. Image Formats
- 3. Input/Output
- 4. Interfaces
- I. Function Reference
-
- V4L2 close() — Close a V4L2 device
- V4L2 ioctl() — Program a V4L2 device
- ioctl VIDIOC_CROPCAP — Information about the video cropping and scaling abilities
- ioctl VIDIOC_DBG_G_CHIP_IDENT — Identify the chips on a TV card
- ioctl VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER — Read or write hardware registers
- ioctl VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD — Execute an encoder command
- ioctl VIDIOC_ENUMAUDIO — Enumerate audio inputs
- ioctl VIDIOC_ENUMAUDOUT — Enumerate audio outputs
- ioctl VIDIOC_ENUM_FMT — Enumerate image formats
- ioctl VIDIOC_ENUM_FRAMESIZES — Enumerate frame sizes
- ioctl VIDIOC_ENUM_FRAMEINTERVALS — Enumerate frame intervals
- ioctl VIDIOC_ENUMINPUT — Enumerate video inputs
- ioctl VIDIOC_ENUMOUTPUT — Enumerate video outputs
- ioctl VIDIOC_ENUMSTD — Enumerate supported video standards
- ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO — Query or select the current audio input and its attributes
- ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT — Query or select the current audio output
- ioctl VIDIOC_G_CROP, VIDIOC_S_CROP — Get or set the current cropping rectangle
- ioctl VIDIOC_G_CTRL, VIDIOC_S_CTRL — Get or set the value of a control
- ioctl VIDIOC_G_ENC_INDEX — Get meta data about a compressed video stream
- ioctl VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS, VIDIOC_TRY_EXT_CTRLS — Get or set the value of several controls, try control values
- ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF — Get or set frame buffer overlay parameters
- ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT — Get or set the data format, try a format
- ioctl VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY — Get or set tuner or modulator radio frequency
- ioctl VIDIOC_G_INPUT, VIDIOC_S_INPUT — Query or select the current video input
- ioctl VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP
- ioctl VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR — Get or set modulator attributes
- ioctl VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT — Query or select the current video output
- ioctl VIDIOC_G_PARM, VIDIOC_S_PARM — Get or set streaming parameters
- ioctl VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY — Query or request the access priority associated with a file descriptor
- ioctl VIDIOC_G_SLICED_VBI_CAP — Query sliced VBI capabilities
- ioctl VIDIOC_G_STD, VIDIOC_S_STD — Query or select the video standard of the current input
- ioctl VIDIOC_G_TUNER, VIDIOC_S_TUNER — Get or set tuner attributes
- ioctl VIDIOC_LOG_STATUS — Log driver status information
- ioctl VIDIOC_OVERLAY — Start or stop video overlay
- ioctl VIDIOC_QBUF, VIDIOC_DQBUF — Exchange a buffer with the driver
- ioctl VIDIOC_QUERYBUF — Query the status of a buffer
- ioctl VIDIOC_QUERYCAP — Query device capabilities
- ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU — Enumerate controls and menu control items
- ioctl VIDIOC_QUERYSTD — Sense the video standard received by the current input
- ioctl VIDIOC_REQBUFS — Initiate Memory Mapping or User Pointer I/O
- ioctl VIDIOC_S_HW_FREQ_SEEK — Perform a hardware frequency seek
- ioctl VIDIOC_STREAMON, VIDIOC_STREAMOFF — Start or stop streaming I/O
- V4L2 mmap() — Map device memory into application address space
- V4L2 munmap() — Unmap device memory
- V4L2 open() — Open a V4L2 device
- V4L2 poll() — Wait for some event on a file descriptor
- V4L2 read() — Read from a V4L2 device
- V4L2 select() — Synchronous I/O multiplexing
- V4L2 write() — Write to a V4L2 device
- 5. V4L2 Driver Programming
- 6. Libv4l Userspace Library
- 7. Remote Controllers
- 8. Changes
-
- 8.1. Differences between V4L and V4L2
- 8.2. Changes of the V4L2 API
-
- 8.2.1. Early Versions
- 8.2.2. V4L2 Version 0.16 1999-01-31
- 8.2.3. V4L2 Version 0.18 1999-03-16
- 8.2.4. V4L2 Version 0.19 1999-06-05
- 8.2.5. V4L2 Version 0.20 (1999-09-10)
- 8.2.6. V4L2 Version 0.20 incremental changes
- 8.2.7. V4L2 Version 0.20 2000-11-23
- 8.2.8. V4L2 Version 0.20 2002-07-25
- 8.2.9. V4L2 in Linux 2.5.46, 2002-10
- 8.2.10. V4L2 2003-06-19
- 8.2.11. V4L2 2003-11-05
- 8.2.12. V4L2 in Linux 2.6.6, 2004-05-09
- 8.2.13. V4L2 in Linux 2.6.8
- 8.2.14. V4L2 spec erratum 2004-08-01
- 8.2.15. V4L2 in Linux 2.6.14
- 8.2.16. V4L2 in Linux 2.6.15
- 8.2.17. V4L2 spec erratum 2005-11-27
- 8.2.18. V4L2 spec erratum 2006-01-10
- 8.2.19. V4L2 spec erratum 2006-02-03
- 8.2.20. V4L2 spec erratum 2006-02-04
- 8.2.21. V4L2 in Linux 2.6.17
- 8.2.22. V4L2 spec erratum 2006-09-23 (Draft 0.15)
- 8.2.23. V4L2 in Linux 2.6.18
- 8.2.24. V4L2 in Linux 2.6.19
- 8.2.25. V4L2 spec erratum 2006-10-12 (Draft 0.17)
- 8.2.26. V4L2 in Linux 2.6.21
- 8.2.27. V4L2 in Linux 2.6.22
- 8.2.28. V4L2 in Linux 2.6.24
- 8.2.29. V4L2 in Linux 2.6.25
- 8.2.30. V4L2 in Linux 2.6.26
- 8.2.31. V4L2 in Linux 2.6.27
- 8.2.32. V4L2 in Linux 2.6.28
- 8.2.33. V4L2 in Linux 2.6.29
- 8.2.34. V4L2 in Linux 2.6.30
- 8.2.35. V4L2 in Linux 2.6.32
- 8.3. Relation of V4L2 to other Linux multimedia APIs
- 8.4. Experimental API Elements
- 8.5. Obsolete API Elements
- A. Video For Linux Two Header File
- B. Video Capture Example
- C. Video Grabber example using libv4l
- D. GNU Free Documentation License
-
- D.1. 0. PREAMBLE
- D.2. 1. APPLICABILITY AND DEFINITIONS
- D.3. 2. VERBATIM COPYING
- D.4. 3. COPYING IN QUANTITY
- D.5. 4. MODIFICATIONS
- D.6. 5. COMBINING DOCUMENTS
- D.7. 6. COLLECTIONS OF DOCUMENTS
- D.8. 7. AGGREGATION WITH INDEPENDENT WORKS
- D.9. 8. TRANSLATION
- D.10. 9. TERMINATION
- D.11. 10. FUTURE REVISIONS OF THIS LICENSE
- D.12. Addendum
- List of Types
- References
List of Figures
List of Tables
- 1.1. Control IDs
- 1.2. MPEG Control IDs
- 1.3. CX2341x Control IDs
- 1.4. Camera Control IDs
- 1.5. FM_TX Control IDs
- 2.1. struct v4l2_pix_format
- 2.2. enum v4l2_colorspace
- 2.3. Indexed Image Format
- 2.4. Packed RGB Image Formats
- 2.5. Packed RGB Image Formats (corrected)
- 2.6. Packed YUV Image Formats
- 2.7. Compressed Image Formats
- 2.8. Reserved Image Formats
- 3.1. struct v4l2_buffer
- 3.2. enum v4l2_buf_type
- 3.3. Buffer Flags
- 3.4. enum v4l2_memory
- 3.5. struct v4l2_timecode
- 3.6. Timecode Types
- 3.7. Timecode Flags
- 3.8. enum v4l2_field
- 4.1. struct v4l2_window
- 4.2. struct v4l2_clip
- 4.3. struct v4l2_rect
- 4.4. struct v4l2_vbi_format
- 4.5. Raw VBI Format Flags
- 4.6. struct v4l2_sliced_vbi_format
- 4.7. Sliced VBI services
- 4.8. struct v4l2_sliced_vbi_data
- 4.9. struct v4l2_mpeg_vbi_fmt_ivtv
-
4.10.
Magic Constants for struct v4l2_mpeg_vbi_fmt_ivtv
magicfield - 4.11. struct v4l2_mpeg_vbi_itv0
- 4.12. struct v4l2_mpeg_vbi_ITV0
- 4.13. struct v4l2_mpeg_vbi_itv0_line
-
4.14.
Line Identifiers for struct v4l2_mpeg_vbi_itv0_line
idfield - 4.15. struct v4l2_rds_data
- 4.16. Block description
- 4.17. Block defines
- 39. struct v4l2_cropcap
- 40. struct v4l2_rect
- 41. struct v4l2_dbg_match
- 42. struct v4l2_dbg_chip_ident
- 43. Chip Match Types
- 44. Chip Identifiers
- 45. struct v4l2_dbg_match
- 46. struct v4l2_dbg_register
- 47. Chip Match Types
- 48. struct v4l2_encoder_cmd
- 49. Encoder Commands
- 50. Encoder Command Flags
- 51. struct v4l2_fmtdesc
- 52. Image Format Description Flags
- 53. struct v4l2_frmsize_discrete
- 54. struct v4l2_frmsize_stepwise
- 55. struct v4l2_frmsizeenum
- 56. enum v4l2_frmsizetypes
- 57. struct v4l2_frmival_stepwise
- 58. struct v4l2_frmivalenum
- 59. enum v4l2_frmivaltypes
- 60. struct v4l2_input
- 61. Input Types
- 62. Input Status Flags
- 63. struct v4l2_output
- 64. Output Type
- 65. struct v4l2_standard
- 66. struct v4l2_fract
- 67. typedef v4l2_std_id
- 68. Video Standards (based on [])
- 69. struct v4l2_audio
- 70. Audio Capability Flags
- 71. Audio Mode Flags
- 72. struct v4l2_audioout
- 73. struct v4l2_crop
- 74. struct v4l2_control
- 75. struct v4l2_enc_idx
- 76. struct v4l2_enc_idx_entry
- 77. Index Entry Flags
- 78. struct v4l2_ext_control
- 79. struct v4l2_ext_controls
- 80. Control classes
- 81. struct v4l2_framebuffer
- 82. Frame Buffer Capability Flags
- 83. Frame Buffer Flags
- 84. struct v4l2_format
- 85. struct v4l2_frequency
- 86. struct v4l2_jpegcompression
- 87. JPEG Markers Flags
- 88. struct v4l2_modulator
- 89. Modulator Audio Transmission Flags
- 90. struct v4l2_streamparm
- 91. struct v4l2_captureparm
- 92. struct v4l2_outputparm
- 93. Streaming Parameters Capabilites
- 94. Capture Parameters Flags
- 95. enum v4l2_priority
- 96. struct v4l2_sliced_vbi_cap
- 97. Sliced VBI services
- 98. struct v4l2_tuner
- 99. enum v4l2_tuner_type
- 100. Tuner and Modulator Capability Flags
- 101. Tuner Audio Reception Flags
- 102. Tuner Audio Modes
- 103. Tuner Audio Matrix
- 104. struct v4l2_capability
- 105. Device Capabilities Flags
- 106. struct v4l2_queryctrl
- 107. struct v4l2_querymenu
- 108. enum v4l2_ctrl_type
- 109. Control Flags
- 110. struct v4l2_requestbuffers
- 111. struct v4l2_hw_freq_seek
- 7.1. IR default keymapping
- 7.2. Notes
- 8.1. V4L Device Types, Names and Numbers
List of Examples
- 1.1. Information about the current video input
- 1.2. Switching to the first video input
- 1.3. Information about the current audio input
- 1.4. Switching to the first audio input
- 1.5. Information about the current video standard
- 1.6. Listing the video standards supported by the current input
- 1.7. Selecting a new video standard
- 1.8. Enumerating all controls
- 1.9. Changing controls
- 1.10. Resetting the cropping parameters
- 1.11. Simple downscaling
- 1.12. Selecting an output area
- 1.13. Current scaling factor and pixel aspect
- 2.1. ITU-R Rec. BT.601 color conversion
-
2.2.
V4L2_PIX_FMT_BGR244 × 4 pixel image -
2.3.
V4L2_PIX_FMT_SBGGR84 × 4 pixel image -
2.4.
V4L2_PIX_FMT_SGBRG84 × 4 pixel image -
2.5.
V4L2_PIX_FMT_SGRBG84 × 4 pixel image -
2.6.
V4L2_PIX_FMT_SBGGR164 × 4 pixel image -
2.7.
V4L2_PIX_FMT_GREY4 × 4 pixel image -
2.8.
V4L2_PIX_FMT_Y164 × 4 pixel image -
2.9.
V4L2_PIX_FMT_YUYV4 × 4 pixel image -
2.10.
V4L2_PIX_FMT_UYVY4 × 4 pixel image -
2.11.
V4L2_PIX_FMT_YVYU4 × 4 pixel image -
2.12.
V4L2_PIX_FMT_VYUY4 × 4 pixel image -
2.13.
V4L2_PIX_FMT_Y41P8 × 4 pixel image -
2.14.
V4L2_PIX_FMT_YVU4204 × 4 pixel image -
2.15.
V4L2_PIX_FMT_YVU4104 × 4 pixel image -
2.16.
V4L2_PIX_FMT_YUV422P4 × 4 pixel image -
2.17.
V4L2_PIX_FMT_YUV411P4 × 4 pixel image -
2.18.
V4L2_PIX_FMT_NV124 × 4 pixel image -
2.19.
V4L2_PIX_FMT_NV164 × 4 pixel image - 3.1. Mapping buffers
- 3.2. Initiating streaming I/O with user pointers
- 4.1. Finding a framebuffer device for OSD
Video For Linux Two is the second version of the Video For Linux API, a kernel interface for analog radio and video capture and output drivers.
Early drivers used ad-hoc interfaces. These were replaced in Linux 2.2 by Alan Cox' V4L API, based on the interface of the bttv driver. In 1999 Bill Dirks started the development of V4L2 to fix some shortcomings of V4L and to support a wider range of devices. The API was revised again in 2002 prior to its inclusion in Linux 2.5/2.6, and work continues on improvements and additions while maintaining compatibility with existing drivers and applications. In 2006/2007 efforts began on FreeBSD drivers with a V4L2 interface.
This book documents the V4L2 API. Intended audience are driver and application writers.
If you have questions or ideas regarding the API, please write to the linux-media mailing list: https://linuxtv.org/lists.php.
The latest version of this document and the DocBook SGML sources are part of the https://linuxtv.org/repo/ repository. The online version is available here: https://linuxtv.org/downloads/video4linux/API/V4L2_API.
Table of Contents
- 1.1. Opening and Closing Devices
- 1.2. Querying Capabilities
- 1.3. Application Priority
- 1.4. Video Inputs and Outputs
- 1.5. Audio Inputs and Outputs
- 1.6. Tuners and Modulators
- 1.7. Video Standards
- 1.8. User Controls
- 1.9. Extended Controls
- 1.10. Data Formats
- 1.11. Image Cropping, Insertion and Scaling
- 1.12. Streaming Parameters
Programming a V4L2 device consists of these steps:
-
Opening the device
-
Changing device properties, selecting a video and audio input, video standard, picture brightness a. o.
-
Negotiating a data format
-
Negotiating an input/output method
-
The actual input/output loop
-
Closing the device
In practice most steps are optional and can be executed out of order. It depends on the V4L2 device type, you can read about the details in Chapter 4, Interfaces. In this chapter we will discuss the basic concepts applicable to all devices.
V4L2 drivers are implemented as kernel modules, loaded manually by the system administrator or automatically when a device is first opened. The driver modules plug into the "videodev" kernel module. It provides helper functions and a common application interface specified in this document.
Each driver thus loaded registers one or more device nodes with major number 81 and a minor number between 0 and 255. Assigning minor numbers to V4L2 devices is entirely up to the system administrator, this is primarily intended to solve conflicts between devices.[1] The module options to select minor numbers are named after the device special file with a "_nr" suffix. For example "video_nr" for /dev/video video capture devices. The number is an offset to the base minor number associated with the device type. [2] When the driver supports multiple devices of the same type more than one minor number can be assigned, separated by commas:
> insmod mydriver.o video_nr=0,1 radio_nr=0,1
In /etc/modules.conf this may be written as:
alias char-major-81-0 mydriver alias char-major-81-1 mydriver alias char-major-81-64 mydriver
options mydriver video_nr=0,1 radio_nr=0,1
|
|
When an application attempts to open a device special file with major number 81 and minor number 0, 1, or 64, load "mydriver" (and the "videodev" module it depends upon). |
|
|
Register the first two video capture devices with minor number 0 and 1 (base number is 0), the first two radio device with minor number 64 and 65 (base 64). |
When no minor number is given as module option the driver supplies a default. Chapter 4, Interfaces recommends the base minor numbers to be used for the various device types. Obviously minor numbers must be unique. When the number is already in use the offending device will not be registered.
By convention system administrators create various character device special files with these major and minor numbers in the /dev directory. The names recomended for the different V4L2 device types are listed in Chapter 4, Interfaces.
The creation of character special files (with mknod) is a privileged operation and devices cannot be opened by major and minor number. That means applications cannot reliable scan for loaded or installed drivers. The user must enter a device name, or the application can try the conventional device names.
Under the device filesystem (devfs) the minor number options are ignored. V4L2 drivers (or by proxy the "videodev" module) automatically create the required device files in the /dev/v4l directory using the conventional device names above.
Devices can support several related functions. For example video capturing, video overlay and VBI capturing are related because these functions share, amongst other, the same video input and tuner frequency. V4L and earlier versions of V4L2 used the same device name and minor number for video capturing and overlay, but different ones for VBI. Experience showed this approach has several problems[3], and to make things worse the V4L videodev module used to prohibit multiple opens of a device.
As a remedy the present version of the V4L2 API relaxed the concept of device types with specific names and minor numbers. For compatibility with old applications drivers must still register different minor numbers to assign a default function to the device. But if related functions are supported by the driver they must be available under all registered minor numbers. The desired function can be selected after opening the device as described in Chapter 4, Interfaces.
Imagine a driver supporting video capturing, video overlay, raw VBI capturing, and FM radio reception. It registers three devices with minor number 0, 64 and 224 (this numbering scheme is inherited from the V4L API). Regardless if /dev/video (81, 0) or /dev/vbi (81, 224) is opened the application can select any one of the video capturing, overlay or VBI capturing functions. Without programming (e. g. reading from the device with dd or cat) /dev/video captures video images, while /dev/vbi captures raw VBI data. /dev/radio (81, 64) is invariable a radio device, unrelated to the video functions. Being unrelated does not imply the devices can be used at the same time, however. The open() function may very well return an EBUSY error code.
Besides video input or output the hardware may also support audio sampling or playback. If so, these functions are implemented as OSS or ALSA PCM devices and eventually OSS or ALSA audio mixer. The V4L2 API makes no provisions yet to find these related devices. If you have an idea please write to the linux-media mailing list: https://linuxtv.org/lists.php.
In general, V4L2 devices can be opened more than once. When this is supported by the driver, users can for example start a "panel" application to change controls like brightness or audio volume, while another application captures video and audio. In other words, panel applications are comparable to an OSS or ALSA audio mixer application. When a device supports multiple functions like capturing and overlay simultaneously, multiple opens allow concurrent use of the device by forked processes or specialized applications.
Multiple opens are optional, although drivers should permit at least concurrent accesses without data exchange, i. e. panel applications. This implies open() can return an EBUSY error code when the device is already in use, as well as ioctl() functions initiating data exchange (namely the VIDIOC_S_FMT ioctl), and the read() and write() functions.
Mere opening a V4L2 device does not grant exclusive access.[4] Initiating data exchange however assigns the right to read or write the requested type of data, and to change related properties, to this file descriptor. Applications can request additional access privileges using the priority mechanism described in Section 1.3, 「Application Priority」.
V4L2 drivers should not support multiple applications reading or writing the same data stream on a device by copying buffers, time multiplexing or similar means. This is better handled by a proxy application in user space. When the driver supports stream sharing anyway it must be implemented transparently. The V4L2 API does not specify how conflicts are solved.
Because V4L2 covers a wide variety of devices not all aspects of the API are equally applicable to all types of devices. Furthermore devices of the same type have different capabilities and this specification permits the omission of a few complicated and less important parts of the API.
The VIDIOC_QUERYCAP ioctl is available to check if the kernel device is compatible with this specification, and to query the functions and I/O methods supported by the device. Other features can be queried by calling the respective ioctl, for example VIDIOC_ENUMINPUT to learn about the number, types and names of video connectors on the device. Although abstraction is a major objective of this API, the ioctl also allows driver specific applications to reliable identify the driver.
All V4L2 drivers must support VIDIOC_QUERYCAP. Applications should always call this ioctl after opening the device.
When multiple applications share a device it may be desirable to assign them different priorities. Contrary to the traditional "rm -rf /" school of thought a video recording application could for example block other applications from changing video controls or switching the current TV channel. Another objective is to permit low priority applications working in background, which can be preempted by user controlled applications and automatically regain control of the device at a later time.
Since these features cannot be implemented entirely in user space V4L2 defines the VIDIOC_G_PRIORITY and VIDIOC_S_PRIORITY ioctls to request and query the access priority associate with a file descriptor. Opening a device assigns a medium priority, compatible with earlier versions of V4L2 and drivers not supporting these ioctls. Applications requiring a different priority will usually call VIDIOC_S_PRIORITY after verifying the device with the VIDIOC_QUERYCAP ioctl.
Ioctls changing driver properties, such as VIDIOC_S_INPUT, return an EBUSY error code after another application obtained higher priority. An event mechanism to notify applications about asynchronous property changes has been proposed but not added yet.
Video inputs and outputs are physical connectors of a device. These can be for example RF connectors (antenna/cable), CVBS a.k.a. Composite Video, S-Video or RGB connectors. Only video and VBI capture devices have inputs, output devices have outputs, at least one each. Radio devices have no video inputs or outputs.
To learn about the number and attributes of the available inputs and outputs applications can enumerate them with the VIDIOC_ENUMINPUT and VIDIOC_ENUMOUTPUT ioctl, respectively. The struct v4l2_input returned by the VIDIOC_ENUMINPUT ioctl also contains signal status information applicable when the current video input is queried.
The VIDIOC_G_INPUT and VIDIOC_G_OUTPUT ioctl return the index of the current video input or output. To select a different input or output applications call the VIDIOC_S_INPUT and VIDIOC_S_OUTPUT ioctl. Drivers must implement all the input ioctls when the device has one or more inputs, all the output ioctls when the device has one or more outputs.
Example 1.1. Information about the current video input
struct v4l2_input input; int index; if (-1 == ioctl (fd, , &index)) { perror ("VIDIOC_G_INPUT"); exit (EXIT_FAILURE); } memset (&input, 0, sizeof (input)); input.index = index; if (-1 == ioctl (fd, , &input)) { perror ("VIDIOC_ENUMINPUT"); exit (EXIT_FAILURE); } printf ("Current input: %s\n", input.name); VIDIOC_G_INPUTVIDIOC_ENUMINPUT
Audio inputs and outputs are physical connectors of a device. Video capture devices have inputs, output devices have outputs, zero or more each. Radio devices have no audio inputs or outputs. They have exactly one tuner which in fact is an audio source, but this API associates tuners with video inputs or outputs only, and radio devices have none of these.[5] A connector on a TV card to loop back the received audio signal to a sound card is not considered an audio output.
Audio and video inputs and outputs are associated. Selecting a video source also selects an audio source. This is most evident when the video and audio source is a tuner. Further audio connectors can combine with more than one video input or output. Assumed two composite video inputs and two audio inputs exist, there may be up to four valid combinations. The relation of video and audio connectors is defined in the audioset field of the respective struct v4l2_input or struct v4l2_output, where each bit represents the index number, starting at zero, of one audio input or output.
To learn about the number and attributes of the available inputs and outputs applications can enumerate them with the VIDIOC_ENUMAUDIO and VIDIOC_ENUMAUDOUT ioctl, respectively. The struct v4l2_audio returned by the VIDIOC_ENUMAUDIO ioctl also contains signal status information applicable when the current audio input is queried.
The VIDIOC_G_AUDIO and VIDIOC_G_AUDOUT ioctl report the current audio input and output, respectively. Note that, unlike VIDIOC_G_INPUT and VIDIOC_G_OUTPUT these ioctls return a structure as VIDIOC_ENUMAUDIO and VIDIOC_ENUMAUDOUT do, not just an index.
To select an audio input and change its properties applications call the VIDIOC_S_AUDIO ioctl. To select an audio output (which presently has no changeable properties) applications call the VIDIOC_S_AUDOUT ioctl.
Drivers must implement all input ioctls when the device has one or more inputs, all output ioctls when the device has one or more outputs. When the device has any audio inputs or outputs the driver must set the V4L2_CAP_AUDIO flag in the struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl.
Example 1.3. Information about the current audio input
struct v4l2_audio audio; memset (&audio, 0, sizeof (audio)); if (-1 == ioctl (fd, , &audio)) { perror ("VIDIOC_G_AUDIO"); exit (EXIT_FAILURE); } printf ("Current input: %s\n", audio.name); VIDIOC_G_AUDIO
Example 1.4. Switching to the first audio input
struct v4l2_audio audio; memset (&audio, 0, sizeof (audio)); /* clear audio.mode, audio.reserved */ audio.index = 0; if (-1 == ioctl (fd, , &audio)) { perror ("VIDIOC_S_AUDIO"); exit (EXIT_FAILURE); } VIDIOC_S_AUDIO
Video input devices can have one or more tuners demodulating a RF signal. Each tuner is associated with one or more video inputs, depending on the number of RF connectors on the tuner. The type field of the respective struct v4l2_input returned by the VIDIOC_ENUMINPUT ioctl is set to V4L2_INPUT_TYPE_TUNER and its tuner field contains the index number of the tuner.
Radio devices have exactly one tuner with index zero, no video inputs.
To query and change tuner properties applications use the VIDIOC_G_TUNER and VIDIOC_S_TUNER ioctl, respectively. The struct v4l2_tuner returned by VIDIOC_G_TUNER also contains signal status information applicable when the tuner of the current video input, or a radio tuner is queried. Note that VIDIOC_S_TUNER does not switch the current tuner, when there is more than one at all. The tuner is solely determined by the current video input. Drivers must support both ioctls and set the V4L2_CAP_TUNER flag in the struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl when the device has one or more tuners.
Video output devices can have one or more modulators, uh, modulating a video signal for radiation or connection to the antenna input of a TV set or video recorder. Each modulator is associated with one or more video outputs, depending on the number of RF connectors on the modulator. The type field of the respective struct v4l2_output returned by the VIDIOC_ENUMOUTPUT ioctl is set to V4L2_OUTPUT_TYPE_MODULATOR and its modulator field contains the index number of the modulator. This specification does not define radio output devices.
To query and change modulator properties applications use the VIDIOC_G_MODULATOR and VIDIOC_S_MODULATOR ioctl. Note that VIDIOC_S_MODULATOR does not switch the current modulator, when there is more than one at all. The modulator is solely determined by the current video output. Drivers must support both ioctls and set the V4L2_CAP_MODULATOR flag in the struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl when the device has one or more modulators.
To get and set the tuner or modulator radio frequency applications use the VIDIOC_G_FREQUENCY and VIDIOC_S_FREQUENCY ioctl which both take a pointer to a struct v4l2_frequency. These ioctls are used for TV and radio devices alike. Drivers must support both ioctls when the tuner or modulator ioctls are supported, or when the device is a radio device.
Video devices typically support one or more different video standards or variations of standards. Each video input and output may support another set of standards. This set is reported by the std field of struct v4l2_input and struct v4l2_output returned by the VIDIOC_ENUMINPUT and VIDIOC_ENUMOUTPUT ioctl, respectively.
V4L2 defines one bit for each analog video standard currently in use worldwide, and sets aside bits for driver defined standards, e. g. hybrid standards to watch NTSC video tapes on PAL TVs and vice versa. Applications can use the predefined bits to select a particular standard, although presenting the user a menu of supported standards is preferred. To enumerate and query the attributes of the supported standards applications use the VIDIOC_ENUMSTD ioctl.
Many of the defined standards are actually just variations of a few major standards. The hardware may in fact not distinguish between them, or do so internal and switch automatically. Therefore enumerated standards also contain sets of one or more standard bits.
Assume a hypothetic tuner capable of demodulating B/PAL, G/PAL and I/PAL signals. The first enumerated standard is a set of B and G/PAL, switched automatically depending on the selected radio frequency in UHF or VHF band. Enumeration gives a "PAL-B/G" or "PAL-I" choice. Similar a Composite input may collapse standards, enumerating "PAL-B/G/H/I", "NTSC-M" and "SECAM-D/K".[6]
To query and select the standard used by the current video input or output applications call the VIDIOC_G_STD and VIDIOC_S_STD ioctl, respectively. The received standard can be sensed with the VIDIOC_QUERYSTD ioctl. Note parameter of all these ioctls is a pointer to a v4l2_std_id type (a standard set), not an index into the standard enumeration.[7] Drivers must implement all video standard ioctls when the device has one or more video inputs or outputs.
Special rules apply to USB cameras where the notion of video standards makes little sense. More generally any capture device, output devices accordingly, which is
-
incapable of capturing fields or frames at the nominal rate of the video standard, or
-
where timestamps refer to the instant the field or frame was received by the driver, not the capture time, or
-
where sequence numbers refer to the frames received by the driver, not the captured frames.
Here the driver shall set the std field of struct v4l2_input and struct v4l2_output to zero, the VIDIOC_G_STD, VIDIOC_S_STD, VIDIOC_QUERYSTD and VIDIOC_ENUMSTD ioctls shall return the EINVAL error code.[8]
Example 1.5. Information about the current video standard
v4l2_std_id std_id; struct v4l2_standard standard; if (-1 == ioctl (fd, , &std_id)) { /* Note when VIDIOC_ENUMSTD always returns EINVAL this is no video device or it falls under the USB exception, and VIDIOC_G_STD returning EINVAL is no error. */ perror ("VIDIOC_G_STD"); exit (EXIT_FAILURE); } memset (&standard, 0, sizeof (standard)); standard.index = 0; while (0 == ioctl (fd, , &standard)) { if (standard.id & std_id) { printf ("Current video standard: %s\n", standard.name); exit (EXIT_SUCCESS); } standard.index++; } /* EINVAL indicates the end of the enumeration, which cannot be empty unless this device falls under the USB exception. */ if (errno == EINVAL || standard.index == 0) { perror ("VIDIOC_ENUMSTD"); exit (EXIT_FAILURE); } VIDIOC_G_STDVIDIOC_ENUMSTD
Example 1.6. Listing the video standards supported by the current input
struct v4l2_input input; struct v4l2_standard standard; memset (&input, 0, sizeof (input)); if (-1 == ioctl (fd, , &input.index)) { perror ("VIDIOC_G_INPUT"); exit (EXIT_FAILURE); } if (-1 == ioctl (fd, , &input)) { perror ("VIDIOC_ENUM_INPUT"); exit (EXIT_FAILURE); } printf ("Current input %s supports:\n", input.name); memset (&standard, 0, sizeof (standard)); standard.index = 0; while (0 == ioctl (fd, , &standard)) { if (standard.id & input.std) printf ("%s\n", standard.name); standard.index++; } /* EINVAL indicates the end of the enumeration, which cannot be empty unless this device falls under the USB exception. */ if (errno != EINVAL || standard.index == 0) { perror ("VIDIOC_ENUMSTD"); exit (EXIT_FAILURE); } VIDIOC_G_INPUTVIDIOC_ENUMINPUTVIDIOC_ENUMSTD
Example 1.7. Selecting a new video standard
struct v4l2_input input; v4l2_std_id std_id; memset (&input, 0, sizeof (input)); if (-1 == ioctl (fd, , &input.index)) { perror ("VIDIOC_G_INPUT"); exit (EXIT_FAILURE); } if (-1 == ioctl (fd, , &input)) { perror ("VIDIOC_ENUM_INPUT"); exit (EXIT_FAILURE); } if (0 == (input.std & V4L2_STD_PAL_BG)) { fprintf (stderr, "Oops. B/G PAL is not supported.\n"); exit (EXIT_FAILURE); } /* Note this is also supposed to work when only B or G/PAL is supported. */ std_id = V4L2_STD_PAL_BG; if (-1 == ioctl (fd, , &std_id)) { perror ("VIDIOC_S_STD"); exit (EXIT_FAILURE); } VIDIOC_G_INPUTVIDIOC_ENUMINPUTVIDIOC_S_STD
Devices typically have a number of user-settable controls such as brightness, saturation and so on, which would be presented to the user on a graphical user interface. But, different devices will have different controls available, and furthermore, the range of possible values, and the default value will vary from device to device. The control ioctls provide the information and a mechanism to create a nice user interface for these controls that will work correctly with any device.
All controls are accessed using an ID value. V4L2 defines several IDs for specific purposes. Drivers can also implement their own custom controls using V4L2_CID_PRIVATE_BASE and higher values. The pre-defined control IDs have the prefix V4L2_CID_, and are listed in Table 1.1, 「Control IDs」. The ID is used when querying the attributes of a control, and when getting or setting the current value.
Generally applications should present controls to the user without assumptions about their purpose. Each control comes with a name string the user is supposed to understand. When the purpose is non-intuitive the driver writer should provide a user manual, a user interface plug-in or a driver specific panel application. Predefined IDs were introduced to change a few controls programmatically, for example to mute a device during a channel switch.
Drivers may enumerate different controls after switching the current video input or output, tuner or modulator, or audio input or output. Different in the sense of other bounds, another default and current value, step size or other menu items. A control with a certain custom ID can also change name and type.[9] Control values are stored globally, they do not change when switching except to stay within the reported bounds. They also do not change e. g. when the device is opened or closed, when the tuner radio frequency is changed or generally never without application request. Since V4L2 specifies no event mechanism, panel applications intended to cooperate with other panel applications (be they built into a larger application, as a TV viewer) may need to regularly poll control values to update their user interface.[10]
Table 1.1. Control IDs
| ID | Type | Description |
|---|---|---|
V4L2_CID_BASE |
First predefined ID, equal to V4L2_CID_BRIGHTNESS. |
|
V4L2_CID_USER_BASE |
Synonym of V4L2_CID_BASE. |
|
V4L2_CID_BRIGHTNESS |
integer | Picture brightness, or more precisely, the black level. |
V4L2_CID_CONTRAST |
integer | Picture contrast or luma gain. |
V4L2_CID_SATURATION |
integer | Picture color saturation or chroma gain. |
V4L2_CID_HUE |
integer | Hue or color balance. |
V4L2_CID_AUDIO_VOLUME |
integer | Overall audio volume. Note some drivers also provide an OSS or ALSA mixer interface. |
V4L2_CID_AUDIO_BALANCE |
integer | Audio stereo balance. Minimum corresponds to all the way left, maximum to right. |
V4L2_CID_AUDIO_BASS |
integer | Audio bass adjustment. |
V4L2_CID_AUDIO_TREBLE |
integer | Audio treble adjustment. |
V4L2_CID_AUDIO_MUTE |
boolean | Mute audio, i. e. set the volume to zero, however without affecting V4L2_CID_AUDIO_VOLUME. Like ALSA drivers, V4L2 drivers must mute at load time to avoid excessive noise. Actually the entire device should be reset to a low power consumption state. |
V4L2_CID_AUDIO_LOUDNESS |
boolean | Loudness mode (bass boost). |
V4L2_CID_BLACK_LEVEL |
integer | Another name for brightness (not a synonym of V4L2_CID_BRIGHTNESS). This control is deprecated and should not be used in new drivers and applications. |
V4L2_CID_AUTO_WHITE_BALANCE |
boolean | Automatic white balance (cameras). |
V4L2_CID_DO_WHITE_BALANCE |
button | This is an action control. When set (the value is ignored), the device will do a white balance and then hold the current setting. Contrast this with the boolean V4L2_CID_AUTO_WHITE_BALANCE, which, when activated, keeps adjusting the white balance. |
V4L2_CID_RED_BALANCE |
integer | Red chroma balance. |
V4L2_CID_BLUE_BALANCE |
integer | Blue chroma balance. |
V4L2_CID_GAMMA |
integer | Gamma adjust. |
V4L2_CID_WHITENESS |
integer | Whiteness for grey-scale devices. This is a synonym for V4L2_CID_GAMMA. This control is deprecated and should not be used in new drivers and applications. |
V4L2_CID_EXPOSURE |
integer | Exposure (cameras). [Unit?] |
V4L2_CID_AUTOGAIN |
boolean | Automatic gain/exposure control. |
V4L2_CID_GAIN |
integer | Gain control. |
V4L2_CID_HFLIP |
boolean | Mirror the picture horizontally. |
V4L2_CID_VFLIP |
boolean | Mirror the picture vertically. |
V4L2_CID_HCENTER_DEPRECATED (formerly V4L2_CID_HCENTER) |
integer | Horizontal image centering. This control is deprecated. New drivers and applications should use the Camera class controls V4L2_CID_PAN_ABSOLUTE, V4L2_CID_PAN_RELATIVE and V4L2_CID_PAN_RESET instead. |
V4L2_CID_VCENTER_DEPRECATED (formerly V4L2_CID_VCENTER) |
integer | Vertical image centering. Centering is intended to physically adjust cameras. For image cropping see Section 1.11, 「Image Cropping, Insertion and Scaling」, for clipping Section 4.2, 「Video Overlay Interface」. This control is deprecated. New drivers and applications should use the Camera class controls V4L2_CID_TILT_ABSOLUTE, V4L2_CID_TILT_RELATIVE and V4L2_CID_TILT_RESET instead. |
V4L2_CID_POWER_LINE_FREQUENCY |
enum | Enables a power line frequency filter to avoid flicker. Possible values for enum v4l2_power_line_frequency are: V4L2_CID_POWER_LINE_FREQUENCY_DISABLED (0), V4L2_CID_POWER_LINE_FREQUENCY_50HZ (1) and V4L2_CID_POWER_LINE_FREQUENCY_60HZ (2). |
V4L2_CID_HUE_AUTO |
boolean | Enables automatic hue control by the device. The effect of setting V4L2_CID_HUE while automatic hue control is enabled is undefined, drivers should ignore such request. |
V4L2_CID_WHITE_BALANCE_TEMPERATURE |
integer | This control specifies the white balance settings as a color temperature in Kelvin. A driver should have a minimum of 2800 (incandescent) to 6500 (daylight). For more information about color temperature see Wikipedia. |
V4L2_CID_SHARPNESS |
integer | Adjusts the sharpness filters in a camera. The minimum value disables the filters, higher values give a sharper picture. |
V4L2_CID_BACKLIGHT_COMPENSATION |
integer | Adjusts the backlight compensation in a camera. The minimum value disables backlight compensation. |
V4L2_CID_CHROMA_AGC |
boolean | Chroma automatic gain control. |
V4L2_CID_COLOR_KILLER |
boolean | Enable the color killer (i. e. force a black & white image in case of a weak video signal). |
V4L2_CID_COLORFX |
enum | Selects a color effect. Possible values for enum v4l2_colorfx are: V4L2_COLORFX_NONE (0), V4L2_COLORFX_BW (1) and V4L2_COLORFX_SEPIA (2). |
V4L2_CID_LASTP1 |
End of the predefined control IDs (currently V4L2_CID_COLORFX + 1). |
|
V4L2_CID_PRIVATE_BASE |
ID of the first custom (driver specific) control. Applications depending on particular custom controls should check the driver name and version, see Section 1.2, 「Querying Capabilities」. |
Applications can enumerate the available controls with the VIDIOC_QUERYCTRL and VIDIOC_QUERYMENU ioctls, get and set a control value with the VIDIOC_G_CTRL and VIDIOC_S_CTRL ioctls. Drivers must implement VIDIOC_QUERYCTRL, VIDIOC_G_CTRL and VIDIOC_S_CTRL when the device has one or more controls, VIDIOC_QUERYMENU when it has one or more menu type controls.
Example 1.8. Enumerating all controls
struct v4l2_queryctrl queryctrl; struct v4l2_querymenu querymenu; static void enumerate_menu (void) { printf (" Menu items:\n"); memset (&querymenu, 0, sizeof (querymenu)); querymenu.id = queryctrl.id; for (querymenu.index = queryctrl.minimum; querymenu.index <= queryctrl.maximum; querymenu.index++) { if (0 == ioctl (fd, , &querymenu)) { printf (" %s\n", querymenu.name); } else { perror ("VIDIOC_QUERYMENU"); exit (EXIT_FAILURE); } } } memset (&queryctrl, 0, sizeof (queryctrl)); for (queryctrl.id = V4L2_CID_BASE; queryctrl.id < V4L2_CID_LASTP1; queryctrl.id++) { if (0 == ioctl (fd, , &queryctrl)) { if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) continue; printf ("Control %s\n", queryctrl.name); if (queryctrl.type == V4L2_CTRL_TYPE_MENU) enumerate_menu (); } else { if (errno == EINVAL) continue; perror ("VIDIOC_QUERYCTRL"); exit (EXIT_FAILURE); } } for (queryctrl.id = V4L2_CID_PRIVATE_BASE;; queryctrl.id++) { if (0 == ioctl (fd, , &queryctrl)) { if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) continue; printf ("Control %s\n", queryctrl.name); if (queryctrl.type == V4L2_CTRL_TYPE_MENU) enumerate_menu (); } else { if (errno == EINVAL) break; perror ("VIDIOC_QUERYCTRL"); exit (EXIT_FAILURE); } } VIDIOC_QUERYMENUVIDIOC_QUERYCTRLVIDIOC_QUERYCTRL
Example 1.9. Changing controls
struct v4l2_queryctrl queryctrl; struct v4l2_control control; memset (&queryctrl, 0, sizeof (queryctrl)); queryctrl.id = V4L2_CID_BRIGHTNESS; if (-1 == ioctl (fd, , &queryctrl)) { if (errno != EINVAL) { perror ("VIDIOC_QUERYCTRL"); exit (EXIT_FAILURE); } else { printf ("V4L2_CID_BRIGHTNESS is not supported\n"); } } else if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) { printf ("V4L2_CID_BRIGHTNESS is not supported\n"); } else { memset (&control, 0, sizeof (control)); control.id = V4L2_CID_BRIGHTNESS; control.value = queryctrl.default_value; if (-1 == ioctl (fd, , &control)) { perror ("VIDIOC_S_CTRL"); exit (EXIT_FAILURE); } } memset (&control, 0, sizeof (control)); control.id = V4L2_CID_CONTRAST; if (0 == ioctl (fd, , &control)) { control.value += 1; /* The driver may clamp the value or return ERANGE, ignored here */ if (-1 == ioctl (fd, , &control) && errno != ERANGE) { perror ("VIDIOC_S_CTRL"); exit (EXIT_FAILURE); } /* Ignore if V4L2_CID_CONTRAST is unsupported */ } else if (errno != EINVAL) { perror ("VIDIOC_G_CTRL"); exit (EXIT_FAILURE); } control.id = V4L2_CID_AUDIO_MUTE; control.value = TRUE; /* silence */ /* Errors ignored */ ioctl (fd, VIDIOC_S_CTRL, &control); VIDIOC_QUERYCTRLVIDIOC_S_CTRLVIDIOC_G_CTRLVIDIOC_S_CTRL
The control mechanism as originally designed was meant to be used for user settings (brightness, saturation, etc). However, it turned out to be a very useful model for implementing more complicated driver APIs where each driver implements only a subset of a larger API.
The MPEG encoding API was the driving force behind designing and implementing this extended control mechanism: the MPEG standard is quite large and the currently supported hardware MPEG encoders each only implement a subset of this standard. Further more, many parameters relating to how the video is encoded into an MPEG stream are specific to the MPEG encoding chip since the MPEG standard only defines the format of the resulting MPEG stream, not how the video is actually encoded into that format.
Unfortunately, the original control API lacked some features needed for these new uses and so it was extended into the (not terribly originally named) extended control API.
Even though the MPEG encoding API was the first effort to use the Extended Control API, nowadays there are also other classes of Extended Controls, such as Camera Controls and FM Transmitter Controls. The Extended Controls API as well as all Extended Controls classes are described in the following text.
Three new ioctls are available: VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS and VIDIOC_TRY_EXT_CTRLS. These ioctls act on arrays of controls (as opposed to the VIDIOC_G_CTRL and VIDIOC_S_CTRL ioctls that act on a single control). This is needed since it is often required to atomically change several controls at once.
Each of the new ioctls expects a pointer to a struct v4l2_ext_controls. This structure contains a pointer to the control array, a count of the number of controls in that array and a control class. Control classes are used to group similar controls into a single class. For example, control class V4L2_CTRL_CLASS_USER contains all user controls (i. e. all controls that can also be set using the old VIDIOC_S_CTRL ioctl). Control class V4L2_CTRL_CLASS_MPEG contains all controls relating to MPEG encoding, etc.
All controls in the control array must belong to the specified control class. An error is returned if this is not the case.
It is also possible to use an empty control array (count == 0) to check whether the specified control class is supported.
The control array is a struct v4l2_ext_control array. The v4l2_ext_control structure is very similar to struct v4l2_control, except for the fact that it also allows for 64-bit values and pointers to be passed.
It is important to realize that due to the flexibility of controls it is necessary to check whether the control you want to set actually is supported in the driver and what the valid range of values is. So use the VIDIOC_QUERYCTRL and VIDIOC_QUERYMENU ioctls to check this. Also note that it is possible that some of the menu indices in a control of type V4L2_CTRL_TYPE_MENU may not be supported (VIDIOC_QUERYMENU will return an error). A good example is the list of supported MPEG audio bitrates. Some drivers only support one or two bitrates, others support a wider range.
The recommended way to enumerate over the extended controls is by using VIDIOC_QUERYCTRL in combination with the V4L2_CTRL_FLAG_NEXT_CTRL flag:
struct v4l2_queryctrl qctrl; qctrl.id = V4L2_CTRL_FLAG_NEXT_CTRL; while (0 == ioctl (fd, , &qctrl)) { /* ... */ qctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL; } VIDIOC_QUERYCTRL
The initial control ID is set to 0 ORed with the V4L2_CTRL_FLAG_NEXT_CTRL flag. The VIDIOC_QUERYCTRL ioctl will return the first control with a higher ID than the specified one. When no such controls are found an error is returned.
If you want to get all controls within a specific control class, then you can set the initial qctrl.id value to the control class and add an extra check to break out of the loop when a control of another control class is found:
qctrl.id = V4L2_CTRL_CLASS_MPEG | V4L2_CTRL_FLAG_NEXT_CTRL; while (0 == ioctl (fd, , &qctrl)) { if (V4L2_CTRL_ID2CLASS (qctrl.id) != V4L2_CTRL_CLASS_MPEG) break; /* ... */ qctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL; } VIDIOC_QUERYCTRL
The 32-bit qctrl.id value is subdivided into three bit ranges: the top 4 bits are reserved for flags (e. g. V4L2_CTRL_FLAG_NEXT_CTRL) and are not actually part of the ID. The remaining 28 bits form the control ID, of which the most significant 12 bits define the control class and the least significant 16 bits identify the control within the control class. It is guaranteed that these last 16 bits are always non-zero for controls. The range of 0x1000 and up are reserved for driver-specific controls. The macro V4L2_CTRL_ID2CLASS(id) returns the control class ID based on a control ID.
If the driver does not support extended controls, then VIDIOC_QUERYCTRL will fail when used in combination with V4L2_CTRL_FLAG_NEXT_CTRL. In that case the old method of enumerating control should be used (see 1.8). But if it is supported, then it is guaranteed to enumerate over all controls, including driver-private controls.
It is possible to create control panels for a graphical user interface where the user can select the various controls. Basically you will have to iterate over all controls using the method described above. Each control class starts with a control of type V4L2_CTRL_TYPE_CTRL_CLASS. VIDIOC_QUERYCTRL will return the name of this control class which can be used as the title of a tab page within a control panel.
The flags field of struct v4l2_queryctrl also contains hints on the behavior of the control. See the VIDIOC_QUERYCTRL documentation for more details.
Below all controls within the MPEG control class are described. First the generic controls, then controls specific for certain hardware.
Table 1.2. MPEG Control IDs
| ID | Type | ||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Description | |||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_CLASS |
class | ||||||||||||||||||||||||||||||||||||||||
The MPEG class descriptor. Calling VIDIOC_QUERYCTRL for this control will return a description of this control class. This description can be used as the caption of a Tab page in a GUI, for example. |
|||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_STREAM_TYPE |
enum v4l2_mpeg_stream_type | ||||||||||||||||||||||||||||||||||||||||
| The MPEG-1, -2 or -4 output stream type. One cannot assume anything here. Each hardware MPEG encoder tends to support different subsets of the available MPEG stream types. The currently defined stream types are: | |||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_STREAM_PID_PMT |
integer | ||||||||||||||||||||||||||||||||||||||||
| Program Map Table Packet ID for the MPEG transport stream (default 16) | |||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_STREAM_PID_AUDIO |
integer | ||||||||||||||||||||||||||||||||||||||||
| Audio Packet ID for the MPEG transport stream (default 256) | |||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_STREAM_PID_VIDEO |
integer | ||||||||||||||||||||||||||||||||||||||||
| Video Packet ID for the MPEG transport stream (default 260) | |||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_STREAM_PID_PCR |
integer | ||||||||||||||||||||||||||||||||||||||||
| Packet ID for the MPEG transport stream carrying PCR fields (default 259) | |||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_STREAM_PES_ID_AUDIO |
integer | ||||||||||||||||||||||||||||||||||||||||
| Audio ID for MPEG PES | |||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_STREAM_PES_ID_VIDEO |
integer | ||||||||||||||||||||||||||||||||||||||||
| Video ID for MPEG PES | |||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_STREAM_VBI_FMT |
enum v4l2_mpeg_stream_vbi_fmt | ||||||||||||||||||||||||||||||||||||||||
| Some cards can embed VBI data (e. g. Closed Caption, Teletext) into the MPEG stream. This control selects whether VBI data should be embedded, and if so, what embedding method should be used. The list of possible VBI formats depends on the driver. The currently defined VBI format types are: | |||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_SAMPLING_FREQ |
enum v4l2_mpeg_audio_sampling_freq | ||||||||||||||||||||||||||||||||||||||||
| MPEG Audio sampling frequency. Possible values are: | |||||||||||||||||||||||||||||||||||||||||
|
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V4L2_CID_MPEG_AUDIO_ENCODING |
enum v4l2_mpeg_audio_encoding | ||||||||||||||||||||||||||||||||||||||||
| MPEG Audio encoding. Possible values are: | |||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_L1_BITRATE |
enum v4l2_mpeg_audio_l1_bitrate | ||||||||||||||||||||||||||||||||||||||||
| MPEG-1/2 Layer I bitrate. Possible values are: | |||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_L2_BITRATE |
enum v4l2_mpeg_audio_l2_bitrate | ||||||||||||||||||||||||||||||||||||||||
| MPEG-1/2 Layer II bitrate. Possible values are: | |||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_L3_BITRATE |
enum v4l2_mpeg_audio_l3_bitrate | ||||||||||||||||||||||||||||||||||||||||
| MPEG-1/2 Layer III bitrate. Possible values are: | |||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_AAC_BITRATE |
integer | ||||||||||||||||||||||||||||||||||||||||
| AAC bitrate in bits per second. | |||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_AC3_BITRATE |
enum v4l2_mpeg_audio_ac3_bitrate | ||||||||||||||||||||||||||||||||||||||||
| AC-3 bitrate. Possible values are: | |||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_MODE |
enum v4l2_mpeg_audio_mode | ||||||||||||||||||||||||||||||||||||||||
| MPEG Audio mode. Possible values are: | |||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_MODE_EXTENSION |
enum v4l2_mpeg_audio_mode_extension | ||||||||||||||||||||||||||||||||||||||||
| Joint Stereo audio mode extension. In Layer I and II they indicate which subbands are in intensity stereo. All other subbands are coded in stereo. Layer III is not (yet) supported. Possible values are: | |||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_EMPHASIS |
enum v4l2_mpeg_audio_emphasis | ||||||||||||||||||||||||||||||||||||||||
| Audio Emphasis. Possible values are: | |||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_CRC |
enum v4l2_mpeg_audio_crc | ||||||||||||||||||||||||||||||||||||||||
| CRC method. Possible values are: | |||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_MUTE |
boolean | ||||||||||||||||||||||||||||||||||||||||
| Mutes the audio when capturing. This is not done by muting audio hardware, which can still produce a slight hiss, but in the encoder itself, guaranteeing a fixed and reproducable audio bitstream. 0 = unmuted, 1 = muted. | |||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_ENCODING |
enum v4l2_mpeg_video_encoding | ||||||||||||||||||||||||||||||||||||||||
| MPEG Video encoding method. Possible values are: | |||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_ASPECT |
enum v4l2_mpeg_video_aspect | ||||||||||||||||||||||||||||||||||||||||
| Video aspect. Possible values are: | |||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_B_FRAMES |
integer | ||||||||||||||||||||||||||||||||||||||||
| Number of B-Frames (default 2) | |||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_GOP_SIZE |
integer | ||||||||||||||||||||||||||||||||||||||||
| GOP size (default 12) | |||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_GOP_CLOSURE |
boolean | ||||||||||||||||||||||||||||||||||||||||
| GOP closure (default 1) | |||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_PULLDOWN |
boolean | ||||||||||||||||||||||||||||||||||||||||
| Enable 3:2 pulldown (default 0) | |||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_BITRATE_MODE |
enum v4l2_mpeg_video_bitrate_mode | ||||||||||||||||||||||||||||||||||||||||
| Video bitrate mode. Possible values are: | |||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_BITRATE |
integer | ||||||||||||||||||||||||||||||||||||||||
| Video bitrate in bits per second. | |||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_BITRATE_PEAK |
integer | ||||||||||||||||||||||||||||||||||||||||
| Peak video bitrate in bits per second. Must be larger or equal to the average video bitrate. It is ignored if the video bitrate mode is set to constant bitrate. | |||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_TEMPORAL_DECIMATION |
integer | ||||||||||||||||||||||||||||||||||||||||
| For every captured frame, skip this many subsequent frames (default 0). | |||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_MUTE |
boolean | ||||||||||||||||||||||||||||||||||||||||
| "Mutes" the video to a fixed color when capturing. This is useful for testing, to produce a fixed video bitstream. 0 = unmuted, 1 = muted. | |||||||||||||||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_MUTE_YUV |
integer | ||||||||||||||||||||||||||||||||||||||||
| Sets the "mute" color of the video. The supplied 32-bit integer is interpreted as follows (bit 0 = least significant bit): | |||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||
The following MPEG class controls deal with MPEG encoding settings that are specific to the Conexant CX23415 and CX23416 MPEG encoding chips.
Table 1.3. CX2341x Control IDs
The Camera class includes controls for mechanical (or equivalent digital) features of a device such as controllable lenses or sensors.
Table 1.4. Camera Control IDs
| ID | Type | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Description | |||||||||||
V4L2_CID_CAMERA_CLASS |
class | ||||||||||
The Camera class descriptor. Calling VIDIOC_QUERYCTRL for this control will return a description of this control class. |
|||||||||||
V4L2_CID_EXPOSURE_AUTO |
enum v4l2_exposure_auto_type | ||||||||||
| Enables automatic adjustments of the exposure time and/or iris aperture. The effect of manual changes of the exposure time or iris aperture while these features are enabled is undefined, drivers should ignore such requests. Possible values are: | |||||||||||
|
|||||||||||
V4L2_CID_EXPOSURE_ABSOLUTE |
integer | ||||||||||
| Determines the exposure time of the camera sensor. The exposure time is limited by the frame interval. Drivers should interpret the values as 100 µs units, where the value 1 stands for 1/10000th of a second, 10000 for 1 second and 100000 for 10 seconds. | |||||||||||
V4L2_CID_EXPOSURE_AUTO_PRIORITY |
boolean | ||||||||||
When V4L2_CID_EXPOSURE_AUTO is set to AUTO or APERTURE_PRIORITY, this control determines if the device may dynamically vary the frame rate. By default this feature is disabled (0) and the frame rate must remain constant. |
|||||||||||
V4L2_CID_PAN_RELATIVE |
integer | ||||||||||
| This control turns the camera horizontally by the specified amount. The unit is undefined. A positive value moves the camera to the right (clockwise when viewed from above), a negative value to the left. A value of zero does not cause motion. This is a write-only control. | |||||||||||
V4L2_CID_TILT_RELATIVE |
integer | ||||||||||
| This control turns the camera vertically by the specified amount. The unit is undefined. A positive value moves the camera up, a negative value down. A value of zero does not cause motion. This is a write-only control. | |||||||||||
V4L2_CID_PAN_RESET |
button | ||||||||||
| When this control is set, the camera moves horizontally to the default position. | |||||||||||
V4L2_CID_TILT_RESET |
button | ||||||||||
| When this control is set, the camera moves vertically to the default position. | |||||||||||
V4L2_CID_PAN_ABSOLUTE |
integer | ||||||||||
| This control turns the camera horizontally to the specified position. Positive values move the camera to the right (clockwise when viewed from above), negative values to the left. Drivers should interpret the values as arc seconds, with valid values between -180 * 3600 and +180 * 3600 inclusive. | |||||||||||
V4L2_CID_TILT_ABSOLUTE |
integer | ||||||||||
| This control turns the camera vertically to the specified position. Positive values move the camera up, negative values down. Drivers should interpret the values as arc seconds, with valid values between -180 * 3600 and +180 * 3600 inclusive. | |||||||||||
V4L2_CID_FOCUS_ABSOLUTE |
integer | ||||||||||
| This control sets the focal point of the camera to the specified position. The unit is undefined. Positive values set the focus closer to the camera, negative values towards infinity. | |||||||||||
V4L2_CID_FOCUS_RELATIVE |
integer | ||||||||||
| This control moves the focal point of the camera by the specified amount. The unit is undefined. Positive values move the focus closer to the camera, negative values towards infinity. This is a write-only control. | |||||||||||
V4L2_CID_FOCUS_AUTO |
boolean | ||||||||||
| Enables automatic focus adjustments. The effect of manual focus adjustments while this feature is enabled is undefined, drivers should ignore such requests. | |||||||||||
V4L2_CID_ZOOM_ABSOLUTE |
integer | ||||||||||
| Specify the objective lens focal length as an absolute value. The zoom unit is driver-specific and its value should be a positive integer. | |||||||||||
V4L2_CID_ZOOM_RELATIVE |
integer | ||||||||||
| Specify the objective lens focal length relatively to the current value. Positive values move the zoom lens group towards the telephoto direction, negative values towards the wide-angle direction. The zoom unit is driver-specific. This is a write-only control. | |||||||||||
V4L2_CID_ZOOM_CONTINUOUS |
integer | ||||||||||
| Move the objective lens group at the specified speed until it reaches physical device limits or until an explicit request to stop the movement. A positive value moves the zoom lens group towards the telephoto direction. A value of zero stops the zoom lens group movement. A negative value moves the zoom lens group towards the wide-angle direction. The zoom speed unit is driver-specific. | |||||||||||
V4L2_CID_PRIVACY |
boolean | ||||||||||
Prevent video from being acquired by the camera. When this control is set to TRUE (1), no image can be captured by the camera. Common means to enforce privacy are mechanical obturation of the sensor and firmware image processing, but the device is not restricted to these methods. Devices that implement the privacy control must support read access and may support write access. |
|||||||||||
V4L2_CID_BAND_STOP_FILTER |
integer | ||||||||||
| Switch the band-stop filter of a camera sensor on or off, or specify its strength. Such band-stop filters can be used, for example, to filter out the fluorescent light component. | |||||||||||
The FM Transmitter (FM_TX) class includes controls for common features of FM transmissions capable devices. Currently this class includes parameters for audio compression, pilot tone generation, audio deviation limiter, RDS transmission and tuning power features.
Table 1.5. FM_TX Control IDs
| ID | Type | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| Description | |||||||||
V4L2_CID_FM_TX_CLASS |
class | ||||||||
The FM_TX class descriptor. Calling VIDIOC_QUERYCTRL for this control will return a description of this control class. |
|||||||||
V4L2_CID_RDS_TX_DEVIATION |
integer | ||||||||
| Configures RDS signal frequency deviation level in Hz. The range and step are driver-specific. | |||||||||
V4L2_CID_RDS_TX_PI |
integer | ||||||||
| Sets the RDS Programme Identification field for transmission. | |||||||||
V4L2_CID_RDS_TX_PTY |
integer | ||||||||
| Sets the RDS Programme Type field for transmission. This encodes up to 31 pre-defined programme types. | |||||||||
V4L2_CID_RDS_TX_PS_NAME |
string | ||||||||
| Sets the Programme Service name (PS_NAME) for transmission. It is intended for static display on a receiver. It is the primary aid to listeners in programme service identification and selection. In Annex E of [EN 50067], the RDS specification, there is a full description of the correct character encoding for Programme Service name strings. Also from RDS specification, PS is usually a single eight character text. However, it is also possible to find receivers which can scroll strings sized as 8 x N characters. So, this control must be configured with steps of 8 characters. The result is it must always contain a string with size multiple of 8. | |||||||||
V4L2_CID_RDS_TX_RADIO_TEXT |
string | ||||||||
Sets the Radio Text info for transmission. It is a textual description of what is being broadcasted. RDS Radio Text can be applied when broadcaster wishes to transmit longer PS names, programme-related information or any other text. In these cases, RadioText should be used in addition to V4L2_CID_RDS_TX_PS_NAME. The encoding for Radio Text strings is also fully described in Annex E of [EN 50067]. The length of Radio Text strings depends on which RDS Block is being used to transmit it, either 32 (2A block) or 64 (2B block). However, it is also possible to find receivers which can scroll strings sized as 32 x N or 64 x N characters. So, this control must be configured with steps of 32 or 64 characters. The result is it must always contain a string with size multiple of 32 or 64. |
|||||||||
V4L2_CID_AUDIO_LIMITER_ENABLED |
boolean | ||||||||
| Enables or disables the audio deviation limiter feature. The limiter is useful when trying to maximize the audio volume, minimize receiver-generated distortion and prevent overmodulation. | |||||||||
V4L2_CID_AUDIO_LIMITER_RELEASE_TIME |
integer | ||||||||
| Sets the audio deviation limiter feature release time. Unit is in useconds. Step and range are driver-specific. | |||||||||
V4L2_CID_AUDIO_LIMITER_DEVIATION |
integer | ||||||||
| Configures audio frequency deviation level in Hz. The range and step are driver-specific. | |||||||||
V4L2_CID_AUDIO_COMPRESSION_ENABLED |
boolean | ||||||||
| Enables or disables the audio compression feature. This feature amplifies signals below the threshold by a fixed gain and compresses audio signals above the threshold by the ratio of Threshold/(Gain + Threshold). | |||||||||
V4L2_CID_AUDIO_COMPRESSION_GAIN |
integer | ||||||||
| Sets the gain for audio compression feature. It is a dB value. The range and step are driver-specific. | |||||||||
V4L2_CID_AUDIO_COMPRESSION_THRESHOLD |
integer | ||||||||
| Sets the threshold level for audio compression freature. It is a dB value. The range and step are driver-specific. | |||||||||
V4L2_CID_AUDIO_COMPRESSION_ATTACK_TIME |
integer | ||||||||
| Sets the attack time for audio compression feature. It is a useconds value. The range and step are driver-specific. | |||||||||
V4L2_CID_AUDIO_COMPRESSION_RELEASE_TIME |
integer | ||||||||
| Sets the release time for audio compression feature. It is a useconds value. The range and step are driver-specific. | |||||||||
V4L2_CID_PILOT_TONE_ENABLED |
boolean | ||||||||
| Enables or disables the pilot tone generation feature. | |||||||||
V4L2_CID_PILOT_TONE_DEVIATION |
integer | ||||||||
| Configures pilot tone frequency deviation level. Unit is in Hz. The range and step are driver-specific. | |||||||||
V4L2_CID_PILOT_TONE_FREQUENCY |
integer | ||||||||
| Configures pilot tone frequency value. Unit is in Hz. The range and step are driver-specific. | |||||||||
V4L2_CID_TUNE_PREEMPHASIS |
integer | ||||||||
| Configures the pre-emphasis value for broadcasting. A pre-emphasis filter is applied to the broadcast to accentuate the high audio frequencies. Depending on the region, a time constant of either 50 or 75 useconds is used. The enum v4l2_preemphasis defines possible values for pre-emphasis. Here they are: | |||||||||
|
|||||||||
V4L2_CID_TUNE_POWER_LEVEL |
integer | ||||||||
| Sets the output power level for signal transmission. Unit is in dBuV. Range and step are driver-specific. | |||||||||
V4L2_CID_TUNE_ANTENNA_CAPACITOR |
integer | ||||||||
| This selects the value of antenna tuning capacitor manually or automatically if set to zero. Unit, range and step are driver-specific. | |||||||||
For more details about RDS specification, refer to [EN 50067] document, from CENELEC.
Different devices exchange different kinds of data with applications, for example video images, raw or sliced VBI data, RDS datagrams. Even within one kind many different formats are possible, in particular an abundance of image formats. Although drivers must provide a default and the selection persists across closing and reopening a device, applications should always negotiate a data format before engaging in data exchange. Negotiation means the application asks for a particular format and the driver selects and reports the best the hardware can do to satisfy the request. Of course applications can also just query the current selection.
A single mechanism exists to negotiate all data formats using the aggregate struct v4l2_format and the VIDIOC_G_FMT and VIDIOC_S_FMT ioctls. Additionally the VIDIOC_TRY_FMT ioctl can be used to examine what the hardware could do, without actually selecting a new data format. The data formats supported by the V4L2 API are covered in the respective device section in Chapter 4, Interfaces. For a closer look at image formats see Chapter 2, Image Formats.
The VIDIOC_S_FMT ioctl is a major turning-point in the initialization sequence. Prior to this point multiple panel applications can access the same device concurrently to select the current input, change controls or modify other properties. The first VIDIOC_S_FMT assigns a logical stream (video data, VBI data etc.) exclusively to one file descriptor.
Exclusive means no other application, more precisely no other file descriptor, can grab this stream or change device properties inconsistent with the negotiated parameters. A video standard change for example, when the new standard uses a different number of scan lines, can invalidate the selected image format. Therefore only the file descriptor owning the stream can make invalidating changes. Accordingly multiple file descriptors which grabbed different logical streams prevent each other from interfering with their settings. When for example video overlay is about to start or already in progress, simultaneous video capturing may be restricted to the same cropping and image size.
When applications omit the VIDIOC_S_FMT ioctl its locking side effects are implied by the next step, the selection of an I/O method with the VIDIOC_REQBUFS ioctl or implicit with the first read() or write() call.
Generally only one logical stream can be assigned to a file descriptor, the exception being drivers permitting simultaneous video capturing and overlay using the same file descriptor for compatibility with V4L and earlier versions of V4L2. Switching the logical stream or returning into "panel mode" is possible by closing and reopening the device. Drivers may support a switch using VIDIOC_S_FMT.
All drivers exchanging data with applications must support the VIDIOC_G_FMT and VIDIOC_S_FMT ioctl. Implementation of the VIDIOC_TRY_FMT is highly recommended but optional.
Apart of the generic format negotiation functions a special ioctl to enumerate all image formats supported by video capture, overlay or output devices is available.[11]
The VIDIOC_ENUM_FMT ioctl must be supported by all drivers exchanging image data with applications.
Important
Drivers are not supposed to convert image formats in kernel space. They must enumerate only formats directly supported by the hardware. If necessary driver writers should publish an example conversion routine or library for integration into applications.
Some video capture devices can sample a subsection of the picture and shrink or enlarge it to an image of arbitrary size. We call these abilities cropping and scaling. Some video output devices can scale an image up or down and insert it at an arbitrary scan line and horizontal offset into a video signal.
Applications can use the following API to select an area in the video signal, query the default area and the hardware limits. Despite their name, the VIDIOC_CROPCAP, VIDIOC_G_CROP and VIDIOC_S_CROP ioctls apply to input as well as output devices.
Scaling requires a source and a target. On a video capture or overlay device the source is the video signal, and the cropping ioctls determine the area actually sampled. The target are images read by the application or overlaid onto the graphics screen. Their size (and position for an overlay) is negotiated with the VIDIOC_G_FMT and VIDIOC_S_FMT ioctls.
On a video output device the source are the images passed in by the application, and their size is again negotiated with the VIDIOC_G/S_FMT ioctls, or may be encoded in a compressed video stream. The target is the video signal, and the cropping ioctls determine the area where the images are inserted.
Source and target rectangles are defined even if the device does not support scaling or the VIDIOC_G/S_CROP ioctls. Their size (and position where applicable) will be fixed in this case. All capture and output device must support the VIDIOC_CROPCAP ioctl such that applications can determine if scaling takes place.
For capture devices the coordinates of the top left corner, width and height of the area which can be sampled is given by the bounds substructure of the struct v4l2_cropcap returned by the VIDIOC_CROPCAP ioctl. To support a wide range of hardware this specification does not define an origin or units. However by convention drivers should horizontally count unscaled samples relative to 0H (the leading edge of the horizontal sync pulse, see Figure 4.1, 「Line synchronization」). Vertically ITU-R line numbers of the first field (Figure 4.2, 「ITU-R 525 line numbering (M/NTSC and M/PAL)」, Figure 4.3, 「ITU-R 625 line numbering」), multiplied by two if the driver can capture both fields.
The top left corner, width and height of the source rectangle, that is the area actually sampled, is given by struct v4l2_crop using the same coordinate system as struct v4l2_cropcap. Applications can use the VIDIOC_G_CROP and VIDIOC_S_CROP ioctls to get and set this rectangle. It must lie completely within the capture boundaries and the driver may further adjust the requested size and/or position according to hardware limitations.
Each capture device has a default source rectangle, given by the defrect substructure of struct v4l2_cropcap. The center of this rectangle shall align with the center of the active picture area of the video signal, and cover what the driver writer considers the complete picture. Drivers shall reset the source rectangle to the default when the driver is first loaded, but not later.
For output devices these structures and ioctls are used accordingly, defining the target rectangle where the images will be inserted into the video signal.
Video hardware can have various cropping, insertion and scaling limitations. It may only scale up or down, support only discrete scaling factors, or have different scaling abilities in horizontal and vertical direction. Also it may not support scaling at all. At the same time the struct v4l2_crop rectangle may have to be aligned, and both the source and target rectangles may have arbitrary upper and lower size limits. In particular the maximum width and height in struct v4l2_crop may be smaller than the struct v4l2_cropcap.bounds area. Therefore, as usual, drivers are expected to adjust the requested parameters and return the actual values selected.
Applications can change the source or the target rectangle first, as they may prefer a particular image size or a certain area in the video signal. If the driver has to adjust both to satisfy hardware limitations, the last requested rectangle shall take priority, and the driver should preferably adjust the opposite one. The VIDIOC_TRY_FMT ioctl however shall not change the driver state and therefore only adjust the requested rectangle.
Suppose scaling on a video capture device is restricted to a factor 1:1 or 2:1 in either direction and the target image size must be a multiple of 16 × 16 pixels. The source cropping rectangle is set to defaults, which are also the upper limit in this example, of 640 × 400 pixels at offset 0, 0. An application requests an image size of 300 × 225 pixels, assuming video will be scaled down from the "full picture" accordingly. The driver sets the image size to the closest possible values 304 × 224, then chooses the cropping rectangle closest to the requested size, that is 608 × 224 (224 × 2:1 would exceed the limit 400). The offset 0, 0 is still valid, thus unmodified. Given the default cropping rectangle reported by VIDIOC_CROPCAP the application can easily propose another offset to center the cropping rectangle.
Now the application may insist on covering an area using a picture aspect ratio closer to the original request, so it asks for a cropping rectangle of 608 × 456 pixels. The present scaling factors limit cropping to 640 × 384, so the driver returns the cropping size 608 × 384 and adjusts the image size to closest possible 304 × 192.
Source and target rectangles shall remain unchanged across closing and reopening a device, such that piping data into or out of a device will work without special preparations. More advanced applications should ensure the parameters are suitable before starting I/O.
Example 1.10. Resetting the cropping parameters
(A video capture device is assumed; change V4L2_BUF_TYPE_VIDEO_CAPTURE for other devices.)
struct v4l2_cropcap cropcap; struct v4l2_crop crop; memset (&cropcap, 0, sizeof (cropcap)); cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if (-1 == ioctl (fd, , &cropcap)) { perror ("VIDIOC_CROPCAP"); exit (EXIT_FAILURE); } memset (&crop, 0, sizeof (crop)); crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; crop.c = cropcap.defrect; /* Ignore if cropping is not supported (EINVAL). */ if (-1 == ioctl (fd, , &crop) && errno != EINVAL) { perror ("VIDIOC_S_CROP"); exit (EXIT_FAILURE); } VIDIOC_CROPCAPVIDIOC_S_CROP
Example 1.11. Simple downscaling
(A video capture device is assumed.)
struct v4l2_cropcap cropcap; struct v4l2_format format; reset_cropping_parameters (); /* Scale down to 1/4 size of full picture. */ memset (&format, 0, sizeof (format)); /* defaults */ format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; format.fmt.pix.width = cropcap.defrect.width >> 1; format.fmt.pix.height = cropcap.defrect.height >> 1; format.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV; if (-1 == ioctl (fd, , &format)) { perror ("VIDIOC_S_FORMAT"); exit (EXIT_FAILURE); } /* We could check the actual image size now, the actual scaling factor or if the driver can scale at all. */ VIDIOC_S_FMT
Example 1.12. Selecting an output area
struct v4l2_cropcap cropcap; struct v4l2_crop crop; memset (&cropcap, 0, sizeof (cropcap)); cropcap.type = V4L2_BUF_TYPE_VIDEO_OUTPUT; if (-1 == ioctl (fd, VIDIOC_CROPCAP;, &cropcap)) { perror ("VIDIOC_CROPCAP"); exit (EXIT_FAILURE); } memset (&crop, 0, sizeof (crop)); crop.type = V4L2_BUF_TYPE_VIDEO_OUTPUT; crop.c = cropcap.defrect; /* Scale the width and height to 50 % of their original size and center the output. */ crop.c.width /= 2; crop.c.height /= 2; crop.c.left += crop.c.width / 2; crop.c.top += crop.c.height / 2; /* Ignore if cropping is not supported (EINVAL). */ if (-1 == ioctl (fd, VIDIOC_S_CROP, &crop) && errno != EINVAL) { perror ("VIDIOC_S_CROP"); exit (EXIT_FAILURE); }
Example 1.13. Current scaling factor and pixel aspect
(A video capture device is assumed.)
struct v4l2_cropcap cropcap; struct v4l2_crop crop; struct v4l2_format format; double hscale, vscale; double aspect; int dwidth, dheight; memset (&cropcap, 0, sizeof (cropcap)); cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if (-1 == ioctl (fd, , &cropcap)) { perror ("VIDIOC_CROPCAP"); exit (EXIT_FAILURE); } memset (&crop, 0, sizeof (crop)); crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if (-1 == ioctl (fd, , &crop)) { if (errno != EINVAL) { perror ("VIDIOC_G_CROP"); exit (EXIT_FAILURE); } /* Cropping not supported. */ crop.c = cropcap.defrect; } memset (&format, 0, sizeof (format)); format.fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if (-1 == ioctl (fd, , &format)) { perror ("VIDIOC_G_FMT"); exit (EXIT_FAILURE); } /* The scaling applied by the driver. */ hscale = format.fmt.pix.width / (double) crop.c.width; vscale = format.fmt.pix.height / (double) crop.c.height; aspect = cropcap.pixelaspect.numerator / (double) cropcap.pixelaspect.denominator; aspect = aspect * hscale / vscale; /* Devices following ITU-R BT.601 do not capture square pixels. For playback on a computer monitor we should scale the images to this size. */ dwidth = format.fmt.pix.width / aspect; dheight = format.fmt.pix.height; VIDIOC_CROPCAPVIDIOC_G_CROPVIDIOC_G_FMT
Streaming parameters are intended to optimize the video capture process as well as I/O. Presently applications can request a high quality capture mode with the VIDIOC_S_PARM ioctl.
The current video standard determines a nominal number of frames per second. If less than this number of frames is to be captured or output, applications can request frame skipping or duplicating on the driver side. This is especially useful when using the read() or write(), which are not augmented by timestamps or sequence counters, and to avoid unneccessary data copying.
Finally these ioctls can be used to determine the number of buffers used internally by a driver in read/write mode. For implications see the section discussing the read() function.
To get and set the streaming parameters applications call the VIDIOC_G_PARM and VIDIOC_S_PARM ioctl, respectively. They take a pointer to a struct v4l2_streamparm, which contains a union holding separate parameters for input and output devices.
These ioctls are optional, drivers need not implement them. If so, they return the EINVAL error code.
[1] Access permissions are associated with character device special files, hence we must ensure device numbers cannot change with the module load order. To this end minor numbers are no longer automatically assigned by the "videodev" module as in V4L but requested by the driver. The defaults will suffice for most people unless two drivers compete for the same minor numbers.
[2] In earlier versions of the V4L2 API the module options where named after the device special file with a "unit_" prefix, expressing the minor number itself, not an offset. Rationale for this change is unknown. Lastly the naming and semantics are just a convention among driver writers, the point to note is that minor numbers are not supposed to be hardcoded into drivers.
[3] Given a device file name one cannot reliable find related devices. For once names are arbitrary and in a system with multiple devices, where only some support VBI capturing, a /dev/video2 is not necessarily related to /dev/vbi2. The V4L VIDIOCGUNIT ioctl would require a search for a device file with a particular major and minor number.
[4] Drivers could recognize the O_EXCL open flag. Presently this is not required, so applications cannot know if it really works.
[5] Actually struct v4l2_audio ought to have a tuner field like struct v4l2_input, not only making the API more consistent but also permitting radio devices with multiple tuners.
[6] Some users are already confused by technical terms PAL, NTSC and SECAM. There is no point asking them to distinguish between B, G, D, or K when the software or hardware can do that automatically.
[7] An alternative to the current scheme is to use pointers to indices as arguments of VIDIOC_G_STD and VIDIOC_S_STD, the struct v4l2_input and struct v4l2_output std field would be a set of indices like audioset.
Indices are consistent with the rest of the API and identify the standard unambiguously. In the present scheme of things an enumerated standard is looked up by v4l2_std_id. Now the standards supported by the inputs of a device can overlap. Just assume the tuner and composite input in the example above both exist on a device. An enumeration of "PAL-B/G", "PAL-H/I" suggests a choice which does not exist. We cannot merge or omit sets, because applications would be unable to find the standards reported by VIDIOC_G_STD. That leaves separate enumerations for each input. Also selecting a standard by v4l2_std_id can be ambiguous. Advantage of this method is that applications need not identify the standard indirectly, after enumerating.
So in summary, the lookup itself is unavoidable. The difference is only whether the lookup is necessary to find an enumerated standard or to switch to a standard by v4l2_std_id.
[8] See Section 3.5, 「Buffers」 for a rationale. Probably even USB cameras follow some well known video standard. It might have been better to explicitly indicate elsewhere if a device cannot live up to normal expectations, instead of this exception.
[9] It will be more convenient for applications if drivers make use of the V4L2_CTRL_FLAG_DISABLED flag, but that was never required.
[10] Applications could call an ioctl to request events. After another process called VIDIOC_S_CTRL or another ioctl changing shared properties the select() function would indicate readability until any ioctl (querying the properties) is called.
[11] Enumerating formats an application has no a-priori knowledge of (otherwise it could explicitely ask for them and need not enumerate) seems useless, but there are applications serving as proxy between drivers and the actual video applications for which this is useful.
Table of Contents
The V4L2 API was primarily designed for devices exchanging image data with applications. The v4l2_pix_format structure defines the format and layout of an image in memory. Image formats are negotiated with the VIDIOC_S_FMT ioctl. (The explanations here focus on video capturing and output, for overlay frame buffer formats see also VIDIOC_G_FBUF.)
Table 2.1. struct v4l2_pix_format
| __u32 | width |
Image width in pixels. |
| __u32 | height |
Image height in pixels. |
Applications set these fields to request an image size, drivers return the closest possible values. In case of planar formats the width and height applies to the largest plane. To avoid ambiguities drivers must return values rounded up to a multiple of the scale factor of any smaller planes. For example when the image format is YUV 4:2:0, width and height must be multiples of two. |
||
| __u32 | pixelformat |
The pixel format or type of compression, set by the application. This is a little endian four character code. V4L2 defines standard RGB formats in Table 2.4, 「Packed RGB Image Formats」, YUV formats in Section 2.5, 「YUV Formats」, and reserved codes in Table 2.8, 「Reserved Image Formats」 |
| enum v4l2_field | field |
Video images are typically interlaced. Applications can request to capture or output only the top or bottom field, or both fields interlaced or sequentially stored in one buffer or alternating in separate buffers. Drivers return the actual field order selected. For details see Section 3.6, 「Field Order」. |
| __u32 | bytesperline |
Distance in bytes between the leftmost pixels in two adjacent lines. |
| Both applications and drivers can set this field to request padding bytes at the end of each line. Drivers however may ignore the value requested by the application, returning Video hardware may access padding bytes, therefore they must reside in accessible memory. Consider cases where padding bytes after the last line of an image cross a system page boundary. Input devices may write padding bytes, the value is undefined. Output devices ignore the contents of padding bytes. When the image format is planar the |
||
| __u32 | sizeimage |
Size in bytes of the buffer to hold a complete image, set by the driver. Usually this is bytesperline times height. When the image consists of variable length compressed data this is the maximum number of bytes required to hold an image. |
| enum v4l2_colorspace | colorspace |
This information supplements the pixelformat and must be set by the driver, see Section 2.2, 「Colorspaces」. |
| __u32 | priv |
Reserved for custom (driver defined) additional information about formats. When not used drivers and applications must set this field to zero. |
In order to exchange images between drivers and applications, it is necessary to have standard image data formats which both sides will interpret the same way. V4L2 includes several such formats, and this section is intended to be an unambiguous specification of the standard image data formats in V4L2.
V4L2 drivers are not limited to these formats, however. Driver-specific formats are possible. In that case the application may depend on a codec to convert images to one of the standard formats when needed. But the data can still be stored and retrieved in the proprietary format. For example, a device may support a proprietary compressed format. Applications can still capture and save the data in the compressed format, saving much disk space, and later use a codec to convert the images to the X Windows screen format when the video is to be displayed.
Even so, ultimately, some standard formats are needed, so the V4L2 specification would not be complete without well-defined standard formats.
The V4L2 standard formats are mainly uncompressed formats. The pixels are always arranged in memory from left to right, and from top to bottom. The first byte of data in the image buffer is always for the leftmost pixel of the topmost row. Following that is the pixel immediately to its right, and so on until the end of the top row of pixels. Following the rightmost pixel of the row there may be zero or more bytes of padding to guarantee that each row of pixel data has a certain alignment. Following the pad bytes, if any, is data for the leftmost pixel of the second row from the top, and so on. The last row has just as many pad bytes after it as the other rows.
In V4L2 each format has an identifier which looks like PIX_FMT_XXX, defined in the videodev.h header file. These identifiers represent four character codes which are also listed below, however they are not the same as those used in the Windows world.
[intro]
- Gamma Correction
-
[to do]
E'R = f(R)
E'G = f(G)
E'B = f(B)
- Construction of luminance and color-difference signals
-
[to do]
E'Y = CoeffR E'R + CoeffG E'G + CoeffB E'B
(E'R - E'Y) = E'R - CoeffR E'R - CoeffG E'G - CoeffB E'B
(E'B - E'Y) = E'B - CoeffR E'R - CoeffG E'G - CoeffB E'B
- Re-normalized color-difference signals
-
The color-difference signals are scaled back to unity range [-0.5;+0.5]:
KB = 0.5 / (1 - CoeffB)
KR = 0.5 / (1 - CoeffR)
PB = KB (E'B - E'Y) = 0.5 (CoeffR / CoeffB) E'R + 0.5 (CoeffG / CoeffB) E'G + 0.5 E'B
PR = KR (E'R - E'Y) = 0.5 E'R + 0.5 (CoeffG / CoeffR) E'G + 0.5 (CoeffB / CoeffR) E'B
- Quantization
-
[to do]
Y' = (Lum. Levels - 1) · E'Y + Lum. Offset
CB = (Chrom. Levels - 1) · PB + Chrom. Offset
CR = (Chrom. Levels - 1) · PR + Chrom. Offset
Rounding to the nearest integer and clamping to the range [0;255] finally yields the digital color components Y'CbCr stored in YUV images.
Example 2.1. ITU-R Rec. BT.601 color conversion
Forward Transformation
int ER, EG, EB; /* gamma corrected RGB input [0;255] */
int Y1, Cb, Cr; /* output [0;255] */
double r, g, b; /* temporaries */
double y1, pb, pr;
int
clamp (double x)
{
int r = x; /* round to nearest */
if (r < 0) return 0;
else if (r > 255) return 255;
else return r;
}
r = ER / 255.0;
g = EG / 255.0;
b = EB / 255.0;
y1 = 0.299 * r + 0.587 * g + 0.114 * b;
pb = -0.169 * r - 0.331 * g + 0.5 * b;
pr = 0.5 * r - 0.419 * g - 0.081 * b;
Y1 = clamp (219 * y1 + 16);
Cb = clamp (224 * pb + 128);
Cr = clamp (224 * pr + 128);
/* or shorter */
y1 = 0.299 * ER + 0.587 * EG + 0.114 * EB;
Y1 = clamp ( (219 / 255.0) * y1 + 16);
Cb = clamp (((224 / 255.0) / (2 - 2 * 0.114)) * (EB - y1) + 128);
Cr = clamp (((224 / 255.0) / (2 - 2 * 0.299)) * (ER - y1) + 128);
Inverse Transformation
int Y1, Cb, Cr; /* gamma pre-corrected input [0;255] */
int ER, EG, EB; /* output [0;255] */
double r, g, b; /* temporaries */
double y1, pb, pr;
int
clamp (double x)
{
int r = x; /* round to nearest */
if (r < 0) return 0;
else if (r > 255) return 255;
else return r;
}
y1 = (255 / 219.0) * (Y1 - 16);
pb = (255 / 224.0) * (Cb - 128);
pr = (255 / 224.0) * (Cr - 128);
r = 1.0 * y1 + 0 * pb + 1.402 * pr;
g = 1.0 * y1 - 0.344 * pb - 0.714 * pr;
b = 1.0 * y1 + 1.772 * pb + 0 * pr;
ER = clamp (r * 255); /* [ok? one should prob. limit y1,pb,pr] */
EG = clamp (g * 255);
EB = clamp (b * 255);
Table 2.2. enum v4l2_colorspace
| Identifier | Value | Description | Chromaticities[a] | White Point | Gamma Correction | Luminance E'Y | Quantization | |||
|---|---|---|---|---|---|---|---|---|---|---|
| Red | Green | Blue | Y' | Cb, Cr | ||||||
V4L2_COLORSPACE_SMPTE170M |
1 | NTSC/PAL according to [SMPTE 170M], [ITU BT.601] | x = 0.630, y = 0.340 | x = 0.310, y = 0.595 | x = 0.155, y = 0.070 | x = 0.3127, y = 0.3290, Illuminant D65 | E' = 4.5 I for I ≤0.018, 1.099 I0.45 - 0.099 for 0.018 < I | 0.299 E'R + 0.587 E'G + 0.114 E'B | 219 E'Y + 16 | 224 PB,R + 128 |
V4L2_COLORSPACE_SMPTE240M |
2 | 1125-Line (US) HDTV, see [SMPTE 240M] | x = 0.630, y = 0.340 | x = 0.310, y = 0.595 | x = 0.155, y = 0.070 | x = 0.3127, y = 0.3290, Illuminant D65 | E' = 4 I for I ≤0.0228, 1.1115 I0.45 - 0.1115 for 0.0228 < I | 0.212 E'R + 0.701 E'G + 0.087 E'B | 219 E'Y + 16 | 224 PB,R + 128 |
V4L2_COLORSPACE_REC709 |
3 | HDTV and modern devices, see [ITU BT.709] | x = 0.640, y = 0.330 | x = 0.300, y = 0.600 | x = 0.150, y = 0.060 | x = 0.3127, y = 0.3290, Illuminant D65 | E' = 4.5 I for I ≤0.018, 1.099 I0.45 - 0.099 for 0.018 < I | 0.2125 E'R + 0.7154 E'G + 0.0721 E'B | 219 E'Y + 16 | 224 PB,R + 128 |
V4L2_COLORSPACE_BT878 |
4 | Broken Bt878 extents[b], [ITU BT.601] | ? | ? | ? | ? | ? | 0.299 E'R + 0.587 E'G + 0.114 E'B | 237 E'Y + 16 | 224 PB,R + 128 (probably) |
V4L2_COLORSPACE_470_SYSTEM_M |
5 | M/NTSC[c] according to [ITU BT.470], [ITU BT.601] | x = 0.67, y = 0.33 | x = 0.21, y = 0.71 | x = 0.14, y = 0.08 | x = 0.310, y = 0.316, Illuminant C | ? | 0.299 E'R + 0.587 E'G + 0.114 E'B | 219 E'Y + 16 | 224 PB,R + 128 |
V4L2_COLORSPACE_470_SYSTEM_BG |
6 | 625-line PAL and SECAM systems according to [ITU BT.470], [ITU BT.601] | x = 0.64, y = 0.33 | x = 0.29, y = 0.60 | x = 0.15, y = 0.06 | x = 0.313, y = 0.329, Illuminant D65 | ? | 0.299 E'R + 0.587 E'G + 0.114 E'B | 219 E'Y + 16 | 224 PB,R + 128 |
V4L2_COLORSPACE_JPEG |
7 | JPEG Y'CbCr, see [JFIF], [ITU BT.601] | ? | ? | ? | ? | ? | 0.299 E'R + 0.587 E'G + 0.114 E'B | 256 E'Y + 16[d] | 256 PB,R + 128 |
V4L2_COLORSPACE_SRGB |
8 | [?] | x = 0.640, y = 0.330 | x = 0.300, y = 0.600 | x = 0.150, y = 0.060 | x = 0.3127, y = 0.3290, Illuminant D65 | E' = 4.5 I for I ≤0.018, 1.099 I0.45 - 0.099 for 0.018 < I | n/a | ||
|
[a] The coordinates of the color primaries are given in the CIE system (1931) [b] The ubiquitous Bt878 video capture chip quantizes E'Y to 238 levels, yielding a range of Y' = 16 … 253, unlike Rec. 601 Y' = 16 … 235. This is not a typo in the Bt878 documentation, it has been implemented in silicon. The chroma extents are unclear. [c] No identifier exists for M/PAL which uses the chromaticities of M/NTSC, the remaining parameters are equal to B and G/PAL. [d] Note JFIF quantizes Y'PBPR in range [0;+1] and [-0.5;+0.5] to 257 levels, however Y'CbCr signals are still clamped to [0;255]. |
||||||||||
In this format each pixel is represented by an 8 bit index into a 256 entry ARGB palette. It is intended for Video Output Overlays only. There are no ioctls to access the palette, this must be done with ioctls of the Linux framebuffer API.
Name
Packed RGB formats — Packed RGB formats
Description
These formats are designed to match the pixel formats of typical PC graphics frame buffers. They occupy 8, 16, 24 or 32 bits per pixel. These are all packed-pixel formats, meaning all the data for a pixel lie next to each other in memory.
When one of these formats is used, drivers shall report the colorspace V4L2_COLORSPACE_SRGB.
Table 2.4. Packed RGB Image Formats
| Identifier | Code | Byte 0 in memory | Byte 1 | Byte 2 | Byte 3 | ||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | |||||
V4L2_PIX_FMT_RGB332 |
'RGB1' | b1 | b0 | g2 | g1 | g0 | r2 | r1 | r0 | ||||||||||||||||||||||||||||
V4L2_PIX_FMT_RGB444 |
'R444' | g3 | g2 | g1 | g0 | b3 | b2 | b1 | b0 | a3 | a2 | a1 | a0 | r3 | r2 | r1 | r0 | ||||||||||||||||||||
V4L2_PIX_FMT_RGB555 |
'RGBO' | g2 | g1 | g0 | r4 | r3 | r2 | r1 | r0 | a | b4 | b3 | b2 | b1 | b0 | g4 | g3 | ||||||||||||||||||||
V4L2_PIX_FMT_RGB565 |
'RGBP' | g2 | g1 | g0 | r4 | r3 | r2 | r1 | r0 | b4 | b3 | b2 | b1 | b0 | g5 | g4 | g3 | ||||||||||||||||||||
V4L2_PIX_FMT_RGB555X |
'RGBQ' | a | b4 | b3 | b2 | b1 | b0 | g4 | g3 | g2 | g1 | g0 | r4 | r3 | r2 | r1 | r0 | ||||||||||||||||||||
V4L2_PIX_FMT_RGB565X |
'RGBR' | b4 | b3 | b2 | b1 | b0 | g5 | g4 | g3 | g2 | g1 | g0 | r4 | r3 | r2 | r1 | r0 | ||||||||||||||||||||
V4L2_PIX_FMT_BGR24 |
'BGR3' | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 | g7 | g6 | g5 | g4 | g3 | g2 | g1 | g0 | r7 | r6 | r5 | r4 | r3 | r2 | r1 | r0 | ||||||||||||
V4L2_PIX_FMT_RGB24 |
'RGB3' | r7 | r6 | r5 | r4 | r3 | r2 | r1 | r0 | g7 | g6 | g5 | g4 | g3 | g2 | g1 | g0 | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 | ||||||||||||
V4L2_PIX_FMT_BGR32 |
'BGR4' | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 | g7 | g6 | g5 | g4 | g3 | g2 | g1 | g0 | r7 | r6 | r5 | r4 | r3 | r2 | r1 | r0 | a7 | a6 | a5 | a4 | a3 | a2 | a1 | a0 | ||||
V4L2_PIX_FMT_RGB32 |
'RGB4' | r7 | r6 | r5 | r4 | r3 | r2 | r1 | r0 | g7 | g6 | g5 | g4 | g3 | g2 | g1 | g0 | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 | a7 | a6 | a5 | a4 | a3 | a2 | a1 | a0 | ||||
Bit 7 is the most significant bit. The value of a = alpha bits is undefined when reading from the driver, ignored when writing to the driver, except when alpha blending has been negotiated for a Video Overlay or Video Output Overlay.
Example 2.2. V4L2_PIX_FMT_BGR24 4 × 4 pixel image
Byte Order. Each cell is one byte.
| start + 0: | B00 | G00 | R00 | B01 | G01 | R01 | B02 | G02 | R02 | B03 | G03 | R03 |
| start + 12: | B10 | G10 | R10 | B11 | G11 | R11 | B12 | G12 | R12 | B13 | G13 | R13 |
| start + 24: | B20 | G20 | R20 | B21 | G21 | R21 | B22 | G22 | R22 | B23 | G23 | R23 |
| start + 36: | B30 | G30 | R30 | B31 | G31 | R31 | B32 | G32 | R32 | B33 | G33 | R33 |
Important
Drivers may interpret these formats differently.
Some RGB formats above are uncommon and were probably defined in error. Drivers may interpret them as in Table 2.5, 「Packed RGB Image Formats (corrected)」.
Table 2.5. Packed RGB Image Formats (corrected)
| Identifier | Code | Byte 0 in memory | Byte 1 | Byte 2 | Byte 3 | ||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | |||||
V4L2_PIX_FMT_RGB332 |
'RGB1' | r2 | r1 | r0 | g2 | g1 | g0 | b1 | b0 | ||||||||||||||||||||||||||||
V4L2_PIX_FMT_RGB444 |
'R444' | g3 | g2 | g1 | g0 | b3 | b2 | b1 | b0 | a3 | a2 | a1 | a0 | r3 | r2 | r1 | r0 | ||||||||||||||||||||
V4L2_PIX_FMT_RGB555 |
'RGBO' | g2 | g1 | g0 | b4 | b3 | b2 | b1 | b0 | a | r4 | r3 | r2 | r1 | r0 | g4 | g3 | ||||||||||||||||||||
V4L2_PIX_FMT_RGB565 |
'RGBP' | g2 | g1 | g0 | b4 | b3 | b2 | b1 | b0 | r4 | r3 | r2 | r1 | r0 | g5 | g4 | g3 | ||||||||||||||||||||
V4L2_PIX_FMT_RGB555X |
'RGBQ' | a | r4 | r3 | r2 | r1 | r0 | g4 | g3 | g2 | g1 | g0 | b4 | b3 | b2 | b1 | b0 | ||||||||||||||||||||
V4L2_PIX_FMT_RGB565X |
'RGBR' | r4 | r3 | r2 | r1 | r0 | g5 | g4 | g3 | g2 | g1 | g0 | b4 | b3 | b2 | b1 | b0 | ||||||||||||||||||||
V4L2_PIX_FMT_BGR24 |
'BGR3' | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 | g7 | g6 | g5 | g4 | g3 | g2 | g1 | g0 | r7 | r6 | r5 | r4 | r3 | r2 | r1 | r0 | ||||||||||||
V4L2_PIX_FMT_RGB24 |
'RGB3' | r7 | r6 | r5 | r4 | r3 | r2 | r1 | r0 | g7 | g6 | g5 | g4 | g3 | g2 | g1 | g0 | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 | ||||||||||||
V4L2_PIX_FMT_BGR32 |
'BGR4' | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 | g7 | g6 | g5 | g4 | g3 | g2 | g1 | g0 | r7 | r6 | r5 | r4 | r3 | r2 | r1 | r0 | a7 | a6 | a5 | a4 | a3 | a2 | a1 | a0 | ||||
V4L2_PIX_FMT_RGB32 |
'RGB4' | a7 | a6 | a5 | a4 | a3 | a2 | a1 | a0 | r7 | r6 | r5 | r4 | r3 | r2 | r1 | r0 | g7 | g6 | g5 | g4 | g3 | g2 | g1 | g0 | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 | ||||
A test utility to determine which RGB formats a driver actually supports is available from the LinuxTV v4l-dvb repository. See https://linuxtv.org/repo/ for access instructions.
Name
V4L2_PIX_FMT_SBGGR8 — Bayer RGB format
Description
This is commonly the native format of digital cameras, reflecting the arrangement of sensors on the CCD device. Only one red, green or blue value is given for each pixel. Missing components must be interpolated from neighbouring pixels. From left to right the first row consists of a blue and green value, the second row of a green and red value. This scheme repeats to the right and down for every two columns and rows.
Name
V4L2_PIX_FMT_SGBRG8 — Bayer RGB format
Description
This is commonly the native format of digital cameras, reflecting the arrangement of sensors on the CCD device. Only one red, green or blue value is given for each pixel. Missing components must be interpolated from neighbouring pixels. From left to right the first row consists of a green and blue value, the second row of a red and green value. This scheme repeats to the right and down for every two columns and rows.
Name
V4L2_PIX_FMT_SGRBG8 — Bayer RGB format
Description
This is commonly the native format of digital cameras, reflecting the arrangement of sensors on the CCD device. Only one red, green or blue value is given for each pixel. Missing components must be interpolated from neighbouring pixels. From left to right the first row consists of a green and blue value, the second row of a red and green value. This scheme repeats to the right and down for every two columns and rows.
Name
V4L2_PIX_FMT_SBGGR16 — Bayer RGB format
Description
This format is similar to V4L2_PIX_FMT_SBGGR8, except each pixel has a depth of 16 bits. The least significant byte is stored at lower memory addresses (little-endian). Note the actual sampling precision may be lower than 16 bits, for example 10 bits per pixel with values in range 0 to 1023.
Example 2.6. V4L2_PIX_FMT_SBGGR16 4 × 4 pixel image
Byte Order. Each cell is one byte.
| start + 0: | B00low | B00high | G01low | G01high | B02low | B02high | G03low | G03high |
| start + 8: | G10low | G10high | R11low | R11high | G12low | G12high | R13low | R13high |
| start + 16: | B20low | B20high | G21low | G21high | B22low | B22high | G23low | G23high |
| start + 24: | G30low | G30high | R31low | R31high | G32low | G32high | R33low | R33high |
YUV is the format native to TV broadcast and composite video signals. It separates the brightness information (Y) from the color information (U and V or Cb and Cr). The color information consists of red and blue color difference signals, this way the green component can be reconstructed by subtracting from the brightness component. See Section 2.2, 「Colorspaces」 for conversion examples. YUV was chosen because early television would only transmit brightness information. To add color in a way compatible with existing receivers a new signal carrier was added to transmit the color difference signals. Secondary in the YUV format the U and V components usually have lower resolution than the Y component. This is an analog video compression technique taking advantage of a property of the human visual system, being more sensitive to brightness information.
Name
Packed YUV formats — Packed YUV formats
Description
Similar to the packed RGB formats these formats store the Y, Cb and Cr component of each pixel in one 16 or 32 bit word.
Table 2.6. Packed YUV Image Formats
| Identifier | Code | Byte 0 in memory | Byte 1 | Byte 2 | Byte 3 | ||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | |||||
V4L2_PIX_FMT_YUV444 |
'Y444' | Cb3 | Cb2 | Cb1 | Cb0 | Cr3 | Cr2 | Cr1 | Cr0 | a3 | a2 | a1 | a0 | Y'3 | Y'2 | Y'1 | Y'0 | ||||||||||||||||||||
V4L2_PIX_FMT_YUV555 |
'YUVO' | Cb2 | Cb1 | Cb0 | Cr4 | Cr3 | Cr2 | Cr1 | Cr0 | a | Y'4 | Y'3 | Y'2 | Y'1 | Y'0 | Cb4 | Cb3 | ||||||||||||||||||||
V4L2_PIX_FMT_YUV565 |
'YUVP' | Cb2 | Cb1 | Cb0 | Cr4 | Cr3 | Cr2 | Cr1 | Cr0 | Y'4 | Y'3 | Y'2 | Y'1 | Y'0 | Cb5 | Cb4 | Cb3 | ||||||||||||||||||||
V4L2_PIX_FMT_YUV32 |
'YUV4' | a7 | a6 | a5 | a4 | a3 | a2 | a1 | a0 | Y'7 | Y'6 | Y'5 | Y'4 | Y'3 | Y'2 | Y'1 | Y'0 | Cb7 | Cb6 | Cb5 | Cb4 | Cb3 | Cb2 | Cb1 | Cb0 | Cr7 | Cr6 | Cr5 | Cr4 | Cr3 | Cr2 | Cr1 | Cr0 | ||||
Bit 7 is the most significant bit. The value of a = alpha bits is undefined when reading from the driver, ignored when writing to the driver, except when alpha blending has been negotiated for a Video Overlay or Video Output Overlay.
Name
V4L2_PIX_FMT_GREY — Grey-scale image
Name
V4L2_PIX_FMT_Y16 — Grey-scale image
Description
This is a grey-scale image with a depth of 16 bits per pixel. The least significant byte is stored at lower memory addresses (little-endian). Note the actual sampling precision may be lower than 16 bits, for example 10 bits per pixel with values in range 0 to 1023.
Example 2.8. V4L2_PIX_FMT_Y16 4 × 4 pixel image
Byte Order. Each cell is one byte.
| start + 0: | Y'00low | Y'00high | Y'01low | Y'01high | Y'02low | Y'02high | Y'03low | Y'03high |
| start + 8: | Y'10low | Y'10high | Y'11low | Y'11high | Y'12low | Y'12high | Y'13low | Y'13high |
| start + 16: | Y'20low | Y'20high | Y'21low | Y'21high | Y'22low | Y'22high | Y'23low | Y'23high |
| start + 24: | Y'30low | Y'30high | Y'31low | Y'31high | Y'32low | Y'32high | Y'33low | Y'33high |
Name
V4L2_PIX_FMT_YUYV — Packed format with ½ horizontal chroma resolution, also known as YUV 4:2:2
Description
In this format each four bytes is two pixels. Each four bytes is two Y's, a Cb and a Cr. Each Y goes to one of the pixels, and the Cb and Cr belong to both pixels. As you can see, the Cr and Cb components have half the horizontal resolution of the Y component. V4L2_PIX_FMT_YUYV is known in the Windows environment as YUY2.
Example 2.9. V4L2_PIX_FMT_YUYV 4 × 4 pixel image
Byte Order. Each cell is one byte.
| start + 0: | Y'00 | Cb00 | Y'01 | Cr00 | Y'02 | Cb01 | Y'03 | Cr01 |
| start + 8: | Y'10 | Cb10 | Y'11 | Cr10 | Y'12 | Cb11 | Y'13 | Cr11 |
| start + 16: | Y'20 | Cb20 | Y'21 | Cr20 | Y'22 | Cb21 | Y'23 | Cr21 |
| start + 24: | Y'30 | Cb30 | Y'31 | Cr30 | Y'32 | Cb31 | Y'33 | Cr31 |
Color Sample Location.
| 0 | 1 | 2 | 3 | ||||
| 0 | Y | C | Y | Y | C | Y | |
| 1 | Y | C | Y | Y | C | Y | |
| 2 | Y | C | Y | Y | C | Y | |
| 3 | Y | C | Y | Y | C | Y |
Name
V4L2_PIX_FMT_UYVY — Variation of V4L2_PIX_FMT_YUYV with different order of samples in memory
Description
In this format each four bytes is two pixels. Each four bytes is two Y's, a Cb and a Cr. Each Y goes to one of the pixels, and the Cb and Cr belong to both pixels. As you can see, the Cr and Cb components have half the horizontal resolution of the Y component.
Example 2.10. V4L2_PIX_FMT_UYVY 4 × 4 pixel image
Byte Order. Each cell is one byte.
| start + 0: | Cb00 | Y'00 | Cr00 | Y'01 | Cb01 | Y'02 | Cr01 | Y'03 |
| start + 8: | Cb10 | Y'10 | Cr10 | Y'11 | Cb11 | Y'12 | Cr11 | Y'13 |
| start + 16: | Cb20 | Y'20 | Cr20 | Y'21 | Cb21 | Y'22 | Cr21 | Y'23 |
| start + 24: | Cb30 | Y'30 | Cr30 | Y'31 | Cb31 | Y'32 | Cr31 | Y'33 |
Color Sample Location.
| 0 | 1 | 2 | 3 | ||||
| 0 | Y | C | Y | Y | C | Y | |
| 1 | Y | C | Y | Y | C | Y | |
| 2 | Y | C | Y | Y | C | Y | |
| 3 | Y | C | Y | Y | C | Y |
Name
V4L2_PIX_FMT_YVYU — Variation of V4L2_PIX_FMT_YUYV with different order of samples in memory
Description
In this format each four bytes is two pixels. Each four bytes is two Y's, a Cb and a Cr. Each Y goes to one of the pixels, and the Cb and Cr belong to both pixels. As you can see, the Cr and Cb components have half the horizontal resolution of the Y component.
Example 2.11. V4L2_PIX_FMT_YVYU 4 × 4 pixel image
Byte Order. Each cell is one byte.
| start + 0: | Y'00 | Cr00 | Y'01 | Cb00 | Y'02 | Cr01 | Y'03 | Cb01 |
| start + 8: | Y'10 | Cr10 | Y'11 | Cb10 | Y'12 | Cr11 | Y'13 | Cb11 |
| start + 16: | Y'20 | Cr20 | Y'21 | Cb20 | Y'22 | Cr21 | Y'23 | Cb21 |
| start + 24: | Y'30 | Cr30 | Y'31 | Cb30 | Y'32 | Cr31 | Y'33 | Cb31 |
Color Sample Location.
| 0 | 1 | 2 | 3 | ||||
| 0 | Y | C | Y | Y | C | Y | |
| 1 | Y | C | Y | Y | C | Y | |
| 2 | Y | C | Y | Y | C | Y | |
| 3 | Y | C | Y | Y | C | Y |
Name
V4L2_PIX_FMT_VYUY — Variation of V4L2_PIX_FMT_YUYV with different order of samples in memory
Description
In this format each four bytes is two pixels. Each four bytes is two Y's, a Cb and a Cr. Each Y goes to one of the pixels, and the Cb and Cr belong to both pixels. As you can see, the Cr and Cb components have half the horizontal resolution of the Y component.
Example 2.12. V4L2_PIX_FMT_VYUY 4 × 4 pixel image
Byte Order. Each cell is one byte.
| start + 0: | Cr00 | Y'00 | Cb00 | Y'01 | Cr01 | Y'02 | Cb01 | Y'03 |
| start + 8: | Cr10 | Y'10 | Cb10 | Y'11 | Cr11 | Y'12 | Cb11 | Y'13 |
| start + 16: | Cr20 | Y'20 | Cb20 | Y'21 | Cr21 | Y'22 | Cb21 | Y'23 |
| start + 24: | Cr30 | Y'30 | Cb30 | Y'31 | Cr31 | Y'32 | Cb31 | Y'33 |
Color Sample Location.
| 0 | 1 | 2 | 3 | ||||
| 0 | Y | C | Y | Y | C | Y | |
| 1 | Y | C | Y | Y | C | Y | |
| 2 | Y | C | Y | Y | C | Y | |
| 3 | Y | C | Y | Y | C | Y |
Name
V4L2_PIX_FMT_Y41P — Format with ¼ horizontal chroma resolution, also known as YUV 4:1:1
Description
In this format each 12 bytes is eight pixels. In the twelve bytes are two CbCr pairs and eight Y's. The first CbCr pair goes with the first four Y's, and the second CbCr pair goes with the other four Y's. The Cb and Cr components have one fourth the horizontal resolution of the Y component.
Do not confuse this format with V4L2_PIX_FMT_YUV411P. Y41P is derived from "YUV 4:1:1 packed", while YUV411P stands for "YUV 4:1:1 planar".
Example 2.13. V4L2_PIX_FMT_Y41P 8 × 4 pixel image
Byte Order. Each cell is one byte.
| start + 0: | Cb00 | Y'00 | Cr00 | Y'01 | Cb01 | Y'02 | Cr01 | Y'03 | Y'04 | Y'05 | Y'06 | Y'07 |
| start + 12: | Cb10 | Y'10 | Cr10 | Y'11 | Cb11 | Y'12 | Cr11 | Y'13 | Y'14 | Y'15 | Y'16 | Y'17 |
| start + 24: | Cb20 | Y'20 | Cr20 | Y'21 | Cb21 | Y'22 | Cr21 | Y'23 | Y'24 | Y'25 | Y'26 | Y'27 |
| start + 36: | Cb30 | Y'30 | Cr30 | Y'31 | Cb31 | Y'32 | Cr31 | Y'33 | Y'34 | Y'35 | Y'36 | Y'37 |
Color Sample Location.
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | ||||||||
| 0 | Y | Y | C | Y | Y | Y | Y | C | Y | Y | |||||
| 1 | Y | Y | C | Y | Y | Y | Y | C | Y | Y | |||||
| 2 | Y | Y | C | Y | Y | Y | Y | C | Y | Y | |||||
| 3 | Y | Y | C | Y | Y | Y | Y | C | Y | Y |
Name
V4L2_PIX_FMT_YVU420, V4L2_PIX_FMT_YUV420 — Planar formats with ½ horizontal and vertical chroma resolution, also known as YUV 4:2:0
Description
These are planar formats, as opposed to a packed format. The three components are separated into three sub- images or planes. The Y plane is first. The Y plane has one byte per pixel. For V4L2_PIX_FMT_YVU420, the Cr plane immediately follows the Y plane in memory. The Cr plane is half the width and half the height of the Y plane (and of the image). Each Cr belongs to four pixels, a two-by-two square of the image. For example, Cr0 belongs to Y'00, Y'01, Y'10, and Y'11. Following the Cr plane is the Cb plane, just like the Cr plane. V4L2_PIX_FMT_YUV420 is the same except the Cb plane comes first, then the Cr plane.
If the Y plane has pad bytes after each row, then the Cr and Cb planes have half as many pad bytes after their rows. In other words, two Cx rows (including padding) is exactly as long as one Y row (including padding).
Example 2.14. V4L2_PIX_FMT_YVU420 4 × 4 pixel image
Byte Order. Each cell is one byte.
| start + 0: | Y'00 | Y'01 | Y'02 | Y'03 |
| start + 4: | Y'10 | Y'11 | Y'12 | Y'13 |
| start + 8: | Y'20 | Y'21 | Y'22 | Y'23 |
| start + 12: | Y'30 | Y'31 | Y'32 | Y'33 |
| start + 16: | Cr00 | Cr01 | ||
| start + 18: | Cr10 | Cr11 | ||
| start + 20: | Cb00 | Cb01 | ||
| start + 22: | Cb10 | Cb11 |
Color Sample Location.
| 0 | 1 | 2 | 3 | ||||
| 0 | Y | Y | Y | Y | |||
| C | C | ||||||
| 1 | Y | Y | Y | Y | |||
| 2 | Y | Y | Y | Y | |||
| C | C | ||||||
| 3 | Y | Y | Y | Y |
Name
V4L2_PIX_FMT_YVU410, V4L2_PIX_FMT_YUV410 — Planar formats with ¼ horizontal and vertical chroma resolution, also known as YUV 4:1:0
Description
These are planar formats, as opposed to a packed format. The three components are separated into three sub-images or planes. The Y plane is first. The Y plane has one byte per pixel. For V4L2_PIX_FMT_YVU410, the Cr plane immediately follows the Y plane in memory. The Cr plane is ¼ the width and ¼ the height of the Y plane (and of the image). Each Cr belongs to 16 pixels, a four-by-four square of the image. Following the Cr plane is the Cb plane, just like the Cr plane. V4L2_PIX_FMT_YUV410 is the same, except the Cb plane comes first, then the Cr plane.
If the Y plane has pad bytes after each row, then the Cr and Cb planes have ¼ as many pad bytes after their rows. In other words, four Cx rows (including padding) are exactly as long as one Y row (including padding).
Example 2.15. V4L2_PIX_FMT_YVU410 4 × 4 pixel image
Byte Order. Each cell is one byte.
| start + 0: | Y'00 | Y'01 | Y'02 | Y'03 |
| start + 4: | Y'10 | Y'11 | Y'12 | Y'13 |
| start + 8: | Y'20 | Y'21 | Y'22 | Y'23 |
| start + 12: | Y'30 | Y'31 | Y'32 | Y'33 |
| start + 16: | Cr00 | |||
| start + 17: | Cb00 |
Color Sample Location.
| 0 | 1 | 2 | 3 | ||||
| 0 | Y | Y | Y | Y | |||
| 1 | Y | Y | Y | Y | |||
| C | |||||||
| 2 | Y | Y | Y | Y | |||
| 3 | Y | Y | Y | Y |
Name
V4L2_PIX_FMT_YUV422P — Format with ½ horizontal chroma resolution, also known as YUV 4:2:2. Planar layout as opposed to V4L2_PIX_FMT_YUYV
Description
This format is not commonly used. This is a planar version of the YUYV format. The three components are separated into three sub-images or planes. The Y plane is first. The Y plane has one byte per pixel. The Cb plane immediately follows the Y plane in memory. The Cb plane is half the width of the Y plane (and of the image). Each Cb belongs to two pixels. For example, Cb0 belongs to Y'00, Y'01. Following the Cb plane is the Cr plane, just like the Cb plane.
If the Y plane has pad bytes after each row, then the Cr and Cb planes have half as many pad bytes after their rows. In other words, two Cx rows (including padding) is exactly as long as one Y row (including padding).
Example 2.16. V4L2_PIX_FMT_YUV422P 4 × 4 pixel image
Byte Order. Each cell is one byte.
| start + 0: | Y'00 | Y'01 | Y'02 | Y'03 |
| start + 4: | Y'10 | Y'11 | Y'12 | Y'13 |
| start + 8: | Y'20 | Y'21 | Y'22 | Y'23 |
| start + 12: | Y'30 | Y'31 | Y'32 | Y'33 |
| start + 16: | Cb00 | Cb01 | ||
| start + 18: | Cb10 | Cb11 | ||
| start + 20: | Cb20 | Cb21 | ||
| start + 22: | Cb30 | Cb31 | ||
| start + 24: | Cr00 | Cr01 | ||
| start + 26: | Cr10 | Cr11 | ||
| start + 28: | Cr20 | Cr21 | ||
| start + 30: | Cr30 | Cr31 |
Color Sample Location.
| 0 | 1 | 2 | 3 | ||||
| 0 | Y | C | Y | Y | C | Y | |
| 1 | Y | C | Y | Y | C | Y | |
| 2 | Y | C | Y | Y | C | Y | |
| 3 | Y | C | Y | Y | C | Y |
Name
V4L2_PIX_FMT_YUV411P — Format with ¼ horizontal chroma resolution, also known as YUV 4:1:1. Planar layout as opposed to V4L2_PIX_FMT_Y41P
Description
This format is not commonly used. This is a planar format similar to the 4:2:2 planar format except with half as many chroma. The three components are separated into three sub-images or planes. The Y plane is first. The Y plane has one byte per pixel. The Cb plane immediately follows the Y plane in memory. The Cb plane is ¼ the width of the Y plane (and of the image). Each Cb belongs to 4 pixels all on the same row. For example, Cb0 belongs to Y'00, Y'01, Y'02 and Y'03. Following the Cb plane is the Cr plane, just like the Cb plane.
If the Y plane has pad bytes after each row, then the Cr and Cb planes have ¼ as many pad bytes after their rows. In other words, four C x rows (including padding) is exactly as long as one Y row (including padding).
Example 2.17. V4L2_PIX_FMT_YUV411P 4 × 4 pixel image
Byte Order. Each cell is one byte.
| start + 0: | Y'00 | Y'01 | Y'02 | Y'03 |
| start + 4: | Y'10 | Y'11 | Y'12 | Y'13 |
| start + 8: | Y'20 | Y'21 | Y'22 | Y'23 |
| start + 12: | Y'30 | Y'31 | Y'32 | Y'33 |
| start + 16: | Cb00 | |||
| start + 17: | Cb10 | |||
| start + 18: | Cb20 | |||
| start + 19: | Cb30 | |||
| start + 20: | Cr00 | |||
| start + 21: | Cr10 | |||
| start + 22: | Cr20 | |||
| start + 23: | Cr30 |
Color Sample Location.
| 0 | 1 | 2 | 3 | ||||
| 0 | Y | Y | C | Y | Y | ||
| 1 | Y | Y | C | Y | Y | ||
| 2 | Y | Y | C | Y | Y | ||
| 3 | Y | Y | C | Y | Y |
Name
V4L2_PIX_FMT_NV12, V4L2_PIX_FMT_NV21 — Formats with ½ horizontal and vertical chroma resolution, also known as YUV 4:2:0. One luminance and one chrominance plane with alternating chroma samples as opposed to V4L2_PIX_FMT_YVU420
Description
These are two-plane versions of the YUV 4:2:0 format. The three components are separated into two sub-images or planes. The Y plane is first. The Y plane has one byte per pixel. For V4L2_PIX_FMT_NV12, a combined CbCr plane immediately follows the Y plane in memory. The CbCr plane is the same width, in bytes, as the Y plane (and of the image), but is half as tall in pixels. Each CbCr pair belongs to four pixels. For example, Cb0/Cr0 belongs to Y'00, Y'01, Y'10, Y'11. V4L2_PIX_FMT_NV21 is the same except the Cb and Cr bytes are swapped, the CrCb plane starts with a Cr byte.
If the Y plane has pad bytes after each row, then the CbCr plane has as many pad bytes after its rows.
Example 2.18. V4L2_PIX_FMT_NV12 4 × 4 pixel image
Byte Order. Each cell is one byte.
| start + 0: | Y'00 | Y'01 | Y'02 | Y'03 |
| start + 4: | Y'10 | Y'11 | Y'12 | Y'13 |
| start + 8: | Y'20 | Y'21 | Y'22 | Y'23 |
| start + 12: | Y'30 | Y'31 | Y'32 | Y'33 |
| start + 16: | Cb00 | Cr00 | Cb01 | Cr01 |
| start + 20: | Cb10 | Cr10 | Cb11 | Cr11 |
Color Sample Location.
| 0 | 1 | 2 | 3 | ||||
| 0 | Y | Y | Y | Y | |||
| C | C | ||||||
| 1 | Y | Y | Y | Y | |||
| 2 | Y | Y | Y | Y | |||
| C | C | ||||||
| 3 | Y | Y | Y | Y |
Name
V4L2_PIX_FMT_NV16, V4L2_PIX_FMT_NV61 — Formats with ½ horizontal chroma resolution, also known as YUV 4:2:2. One luminance and one chrominance plane with alternating chroma samples as opposed to V4L2_PIX_FMT_YVU420
Description
These are two-plane versions of the YUV 4:2:2 format. The three components are separated into two sub-images or planes. The Y plane is first. The Y plane has one byte per pixel. For V4L2_PIX_FMT_NV16, a combined CbCr plane immediately follows the Y plane in memory. The CbCr plane is the same width and height, in bytes, as the Y plane (and of the image). Each CbCr pair belongs to two pixels. For example, Cb0/Cr0 belongs to Y'00, Y'01. V4L2_PIX_FMT_NV61 is the same except the Cb and Cr bytes are swapped, the CrCb plane starts with a Cr byte.
If the Y plane has pad bytes after each row, then the CbCr plane has as many pad bytes after its rows.
Example 2.19. V4L2_PIX_FMT_NV16 4 × 4 pixel image
Byte Order. Each cell is one byte.
| start + 0: | Y'00 | Y'01 | Y'02 | Y'03 |
| start + 4: | Y'10 | Y'11 | Y'12 | Y'13 |
| start + 8: | Y'20 | Y'21 | Y'22 | Y'23 |
| start + 12: | Y'30 | Y'31 | Y'32 | Y'33 |
| start + 16: | Cb00 | Cr00 | Cb01 | Cr01 |
| start + 20: | Cb10 | Cr10 | Cb11 | Cr11 |
| start + 24: | Cb20 | Cr20 | Cb21 | Cr21 |
| start + 28: | Cb30 | Cr30 | Cb31 | Cr31 |
Color Sample Location.
| 0 | 1 | 2 | 3 | ||||
| 0 | Y | Y | Y | Y | |||
| C | C | ||||||
| 1 | Y | Y | Y | Y | |||
| C | C | ||||||
| 2 | Y | Y | Y | Y | |||
| C | C | ||||||
| 3 | Y | Y | Y | Y | |||
| C | C |
Table 2.7. Compressed Image Formats
| Identifier | Code | Details |
|---|---|---|
V4L2_PIX_FMT_JPEG |
'JPEG' | TBD. See also VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP. |
V4L2_PIX_FMT_MPEG |
'MPEG' | MPEG stream. The actual format is determined by extended control V4L2_CID_MPEG_STREAM_TYPE, see Table 1.2, 「MPEG Control IDs」. |
These formats are not defined by this specification, they are just listed for reference and to avoid naming conflicts. If you want to register your own format, send an e-mail to the linux-media mailing list https://linuxtv.org/lists.php for inclusion in the videodev2.h file. If you want to share your format with other developers add a link to your documentation and send a copy to the linux-media mailing list for inclusion in this section. If you think your format should be listed in a standard format section please make a proposal on the linux-media mailing list.
Table 2.8. Reserved Image Formats
| Identifier | Code | Details |
|---|---|---|
V4L2_PIX_FMT_DV |
'dvsd' | unknown |
V4L2_PIX_FMT_ET61X251 |
'E625' | Compressed format of the ET61X251 driver. |
V4L2_PIX_FMT_HI240 |
'HI24' | 8 bit RGB format used by the BTTV driver. |
V4L2_PIX_FMT_HM12 |
'HM12' | YUV 4:2:0 format used by the IVTV driver, http://www.ivtvdriver.org/ The format is documented in the kernel sources in the file |
V4L2_PIX_FMT_SPCA501 |
'S501' | YUYV per line used by the gspca driver. |
V4L2_PIX_FMT_SPCA505 |
'S505' | YYUV per line used by the gspca driver. |
V4L2_PIX_FMT_SPCA508 |
'S508' | YUVY per line used by the gspca driver. |
V4L2_PIX_FMT_SPCA561 |
'S561' | Compressed GBRG Bayer format used by the gspca driver. |
V4L2_PIX_FMT_SGRBG10 |
'DA10' | 10 bit raw Bayer, expanded to 16 bits. |
V4L2_PIX_FMT_SGRBG10DPCM8 |
'DB10' | 10 bit raw Bayer DPCM compressed to 8 bits. |
V4L2_PIX_FMT_PAC207 |
'P207' | Compressed BGGR Bayer format used by the gspca driver. |
V4L2_PIX_FMT_MR97310A |
'M310' | Compressed BGGR Bayer format used by the gspca driver. |
V4L2_PIX_FMT_OV511 |
'O511' | OV511 JPEG format used by the gspca driver. |
V4L2_PIX_FMT_OV518 |
'O518' | OV518 JPEG format used by the gspca driver. |
V4L2_PIX_FMT_PJPG |
'PJPG' | Pixart 73xx JPEG format used by the gspca driver. |
V4L2_PIX_FMT_SQ905C |
'905C' | Compressed RGGB bayer format used by the gspca driver. |
V4L2_PIX_FMT_MJPEG |
'MJPG' | Compressed format used by the Zoran driver |
V4L2_PIX_FMT_PWC1 |
'PWC1' | Compressed format of the PWC driver. |
V4L2_PIX_FMT_PWC2 |
'PWC2' | Compressed format of the PWC driver. |
V4L2_PIX_FMT_SN9C10X |
'S910' | Compressed format of the SN9C102 driver. |
V4L2_PIX_FMT_SN9C20X_I420 |
'S920' | YUV 4:2:0 format of the gspca sn9c20x driver. |
V4L2_PIX_FMT_WNVA |
'WNVA' | Used by the Winnov Videum driver, http://www.thedirks.org/winnov/ |
V4L2_PIX_FMT_YYUV |
'YYUV' | unknown |
Table of Contents
The V4L2 API defines several different methods to read from or write to a device. All drivers exchanging data with applications must support at least one of them.
The classic I/O method using the read() and write() function is automatically selected after opening a V4L2 device. When the driver does not support this method attempts to read or write will fail at any time.
Other methods must be negotiated. To select the streaming I/O method with memory mapped or user buffers applications call the VIDIOC_REQBUFS ioctl. The asynchronous I/O method is not defined yet.
Video overlay can be considered another I/O method, although the application does not directly receive the image data. It is selected by initiating video overlay with the VIDIOC_S_FMT ioctl. For more information see Section 4.2, 「Video Overlay Interface」.
Generally exactly one I/O method, including overlay, is associated with each file descriptor. The only exceptions are applications not exchanging data with a driver ("panel applications", see Section 1.1, 「Opening and Closing Devices」) and drivers permitting simultaneous video capturing and overlay using the same file descriptor, for compatibility with V4L and earlier versions of V4L2.
VIDIOC_S_FMT and VIDIOC_REQBUFS would permit this to some degree, but for simplicity drivers need not support switching the I/O method (after first switching away from read/write) other than by closing and reopening the device.
The following sections describe the various I/O methods in more detail.
Input and output devices support the read() and write() function, respectively, when the V4L2_CAP_READWRITE flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl is set.
Drivers may need the CPU to copy the data, but they may also support DMA to or from user memory, so this I/O method is not necessarily less efficient than other methods merely exchanging buffer pointers. It is considered inferior though because no meta-information like frame counters or timestamps are passed. This information is necessary to recognize frame dropping and to synchronize with other data streams. However this is also the simplest I/O method, requiring little or no setup to exchange data. It permits command line stunts like this (the vidctrl tool is fictitious):
> vidctrl /dev/video --input=0 --format=YUYV --size=352x288 > dd if=/dev/video of=myimage.422 bs=202752 count=1
To read from the device applications use the read() function, to write the write() function. Drivers must implement one I/O method if they exchange data with applications, but it need not be this.[12] When reading or writing is supported, the driver must also support the select() and poll() function.[13]
Input and output devices support this I/O method when the V4L2_CAP_STREAMING flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl is set. There are two streaming methods, to determine if the memory mapping flavor is supported applications must call the VIDIOC_REQBUFS ioctl.
Streaming is an I/O method where only pointers to buffers are exchanged between application and driver, the data itself is not copied. Memory mapping is primarily intended to map buffers in device memory into the application's address space. Device memory can be for example the video memory on a graphics card with a video capture add-on. However, being the most efficient I/O method available for a long time, many other drivers support streaming as well, allocating buffers in DMA-able main memory.
A driver can support many sets of buffers. Each set is identified by a unique buffer type value. The sets are independent and each set can hold a different type of data. To access different sets at the same time different file descriptors must be used.[14]
To allocate device buffers applications call the VIDIOC_REQBUFS ioctl with the desired number of buffers and buffer type, for example V4L2_BUF_TYPE_VIDEO_CAPTURE. This ioctl can also be used to change the number of buffers or to free the allocated memory, provided none of the buffers are still mapped.
Before applications can access the buffers they must map them into their address space with the mmap() function. The location of the buffers in device memory can be determined with the VIDIOC_QUERYBUF ioctl. The m.offset and length returned in a struct v4l2_buffer are passed as sixth and second parameter to the mmap() function. The offset and length values must not be modified. Remember the buffers are allocated in physical memory, as opposed to virtual memory which can be swapped out to disk. Applications should free the buffers as soon as possible with the munmap() function.
Example 3.1. Mapping buffers
struct v4l2_requestbuffers reqbuf; struct { void *start; size_t length; } *buffers; unsigned int i; memset (&reqbuf, 0, sizeof (reqbuf)); reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; reqbuf.memory = V4L2_MEMORY_MMAP; reqbuf.count = 20; if (-1 == ioctl (fd, , &reqbuf)) { if (errno == EINVAL) printf ("Video capturing or mmap-streaming is not supported\n"); else perror ("VIDIOC_REQBUFS"); exit (EXIT_FAILURE); } /* We want at least five buffers. */ if (reqbuf.count < 5) { /* You may need to free the buffers here. */ printf ("Not enough buffer memory\n"); exit (EXIT_FAILURE); } buffers = calloc (reqbuf.count, sizeof (*buffers)); assert (buffers != NULL); for (i = 0; i < reqbuf.count; i++) { struct v4l2_buffer buffer; memset (&buffer, 0, sizeof (buffer)); buffer.type = reqbuf.type; buffer.memory = V4L2_MEMORY_MMAP; buffer.index = i; if (-1 == ioctl (fd, , &buffer)) { perror ("VIDIOC_QUERYBUF"); exit (EXIT_FAILURE); } buffers[i].length = buffer.length; /* remember for munmap() */ buffers[i].start = mmap (NULL, buffer.length, PROT_READ | PROT_WRITE, /* recommended */ MAP_SHARED, /* recommended */ fd, buffer.m.offset); if (MAP_FAILED == buffers[i].start) { /* If you do not exit here you should unmap() and free() the buffers mapped so far. */ perror ("mmap"); exit (EXIT_FAILURE); } } /* Cleanup. */ for (i = 0; i < reqbuf.count; i++) munmap (buffers[i].start, buffers[i].length); VIDIOC_REQBUFSVIDIOC_QUERYBUF
Conceptually streaming drivers maintain two buffer queues, an incoming and an outgoing queue. They separate the synchronous capture or output operation locked to a video clock from the application which is subject to random disk or network delays and preemption by other processes, thereby reducing the probability of data loss. The queues are organized as FIFOs, buffers will be output in the order enqueued in the incoming FIFO, and were captured in the order dequeued from the outgoing FIFO.
The driver may require a minimum number of buffers enqueued at all times to function, apart of this no limit exists on the number of buffers applications can enqueue in advance, or dequeue and process. They can also enqueue in a different order than buffers have been dequeued, and the driver can fill enqueued empty buffers in any order. [15] The index number of a buffer (struct v4l2_buffer index) plays no role here, it only identifies the buffer.
Initially all mapped buffers are in dequeued state, inaccessible by the driver. For capturing applications it is customary to first enqueue all mapped buffers, then to start capturing and enter the read loop. Here the application waits until a filled buffer can be dequeued, and re-enqueues the buffer when the data is no longer needed. Output applications fill and enqueue buffers, when enough buffers are stacked up the output is started with VIDIOC_STREAMON. In the write loop, when the application runs out of free buffers, it must wait until an empty buffer can be dequeued and reused.
To enqueue and dequeue a buffer applications use the VIDIOC_QBUF and VIDIOC_DQBUF ioctl. The status of a buffer being mapped, enqueued, full or empty can be determined at any time using the VIDIOC_QUERYBUF ioctl. Two methods exist to suspend execution of the application until one or more buffers can be dequeued. By default VIDIOC_DQBUF blocks when no buffer is in the outgoing queue. When the O_NONBLOCK flag was given to the open() function, VIDIOC_DQBUF returns immediately with an EAGAIN error code when no buffer is available. The select() or poll() function are always available.
To start and stop capturing or output applications call the VIDIOC_STREAMON and VIDIOC_STREAMOFF ioctl. Note VIDIOC_STREAMOFF removes all buffers from both queues as a side effect. Since there is no notion of doing anything "now" on a multitasking system, if an application needs to synchronize with another event it should examine the struct v4l2_buffer timestamp of captured buffers, or set the field before enqueuing buffers for output.
Drivers implementing memory mapping I/O must support the VIDIOC_REQBUFS, VIDIOC_QUERYBUF, VIDIOC_QBUF, VIDIOC_DQBUF, VIDIOC_STREAMON and VIDIOC_STREAMOFF ioctl, the mmap(), munmap(), select() and poll() function.[16]
[capture example]
Input and output devices support this I/O method when the V4L2_CAP_STREAMING flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl is set. If the particular user pointer method (not only memory mapping) is supported must be determined by calling the VIDIOC_REQBUFS ioctl.
This I/O method combines advantages of the read/write and memory mapping methods. Buffers are allocated by the application itself, and can reside for example in virtual or shared memory. Only pointers to data are exchanged, these pointers and meta-information are passed in struct v4l2_buffer. The driver must be switched into user pointer I/O mode by calling the VIDIOC_REQBUFS with the desired buffer type. No buffers are allocated beforehands, consequently they are not indexed and cannot be queried like mapped buffers with the VIDIOC_QUERYBUF ioctl.
Example 3.2. Initiating streaming I/O with user pointers
struct v4l2_requestbuffers reqbuf; memset (&reqbuf, 0, sizeof (reqbuf)); reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; reqbuf.memory = V4L2_MEMORY_USERPTR; if (ioctl (fd, , &reqbuf) == -1) { if (errno == EINVAL) printf ("Video capturing or user pointer streaming is not supported\n"); else perror ("VIDIOC_REQBUFS"); exit (EXIT_FAILURE); } VIDIOC_REQBUFS
Buffer addresses and sizes are passed on the fly with the VIDIOC_QBUF ioctl. Although buffers are commonly cycled, applications can pass different addresses and sizes at each VIDIOC_QBUF call. If required by the hardware the driver swaps memory pages within physical memory to create a continuous area of memory. This happens transparently to the application in the virtual memory subsystem of the kernel. When buffer pages have been swapped out to disk they are brought back and finally locked in physical memory for DMA.[17]
Filled or displayed buffers are dequeued with the VIDIOC_DQBUF ioctl. The driver can unlock the memory pages at any time between the completion of the DMA and this ioctl. The memory is also unlocked when VIDIOC_STREAMOFF is called, VIDIOC_REQBUFS, or when the device is closed. Applications must take care not to free buffers without dequeuing. For once, the buffers remain locked until further, wasting physical memory. Second the driver will not be notified when the memory is returned to the application's free list and subsequently reused for other purposes, possibly completing the requested DMA and overwriting valuable data.
For capturing applications it is customary to enqueue a number of empty buffers, to start capturing and enter the read loop. Here the application waits until a filled buffer can be dequeued, and re-enqueues the buffer when the data is no longer needed. Output applications fill and enqueue buffers, when enough buffers are stacked up output is started. In the write loop, when the application runs out of free buffers it must wait until an empty buffer can be dequeued and reused. Two methods exist to suspend execution of the application until one or more buffers can be dequeued. By default VIDIOC_DQBUF blocks when no buffer is in the outgoing queue. When the O_NONBLOCK flag was given to the open() function, VIDIOC_DQBUF returns immediately with an EAGAIN error code when no buffer is available. The select() or poll() function are always available.
To start and stop capturing or output applications call the VIDIOC_STREAMON and VIDIOC_STREAMOFF ioctl. Note VIDIOC_STREAMOFF removes all buffers from both queues and unlocks all buffers as a side effect. Since there is no notion of doing anything "now" on a multitasking system, if an application needs to synchronize with another event it should examine the struct v4l2_buffer timestamp of captured buffers, or set the field before enqueuing buffers for output.
Drivers implementing user pointer I/O must support the VIDIOC_REQBUFS, VIDIOC_QBUF, VIDIOC_DQBUF, VIDIOC_STREAMON and VIDIOC_STREAMOFF ioctl, the select() and poll() function.[18]
A buffer contains data exchanged by application and driver using one of the Streaming I/O methods. Only pointers to buffers are exchanged, the data itself is not copied. These pointers, together with meta-information like timestamps or field parity, are stored in a struct v4l2_buffer, argument to the VIDIOC_QUERYBUF, VIDIOC_QBUF and VIDIOC_DQBUF ioctl.
Nominally timestamps refer to the first data byte transmitted. In practice however the wide range of hardware covered by the V4L2 API limits timestamp accuracy. Often an interrupt routine will sample the system clock shortly after the field or frame was stored completely in memory. So applications must expect a constant difference up to one field or frame period plus a small (few scan lines) random error. The delay and error can be much larger due to compression or transmission over an external bus when the frames are not properly stamped by the sender. This is frequently the case with USB cameras. Here timestamps refer to the instant the field or frame was received by the driver, not the capture time. These devices identify by not enumerating any video standards, see Section 1.7, 「Video Standards」.
Similar limitations apply to output timestamps. Typically the video hardware locks to a clock controlling the video timing, the horizontal and vertical synchronization pulses. At some point in the line sequence, possibly the vertical blanking, an interrupt routine samples the system clock, compares against the timestamp and programs the hardware to repeat the previous field or frame, or to display the buffer contents.
Apart of limitations of the video device and natural inaccuracies of all clocks, it should be noted system time itself is not perfectly stable. It can be affected by power saving cycles, warped to insert leap seconds, or even turned back or forth by the system administrator affecting long term measurements. [19]
Table 3.1. struct v4l2_buffer
| __u32 | index |
Number of the buffer, set by the application. This field is only used for memory mapping I/O and can range from zero to the number of buffers allocated with the VIDIOC_REQBUFS ioctl (struct v4l2_requestbuffers count) minus one. |
|
| enum v4l2_buf_type | type |
Type of the buffer, same as struct v4l2_format type or struct v4l2_requestbuffers type, set by the application. |
|
| __u32 | bytesused |
The number of bytes occupied by the data in the buffer. It depends on the negotiated data format and may change with each buffer for compressed variable size data like JPEG images. Drivers must set this field when type refers to an input stream, applications when an output stream. |
|
| __u32 | flags |
Flags set by the application or driver, see Table 3.3, 「Buffer Flags」. | |
| enum v4l2_field | field |
Indicates the field order of the image in the buffer, see Table 3.8, 「enum v4l2_field」. This field is not used when the buffer contains VBI data. Drivers must set it when type refers to an input stream, applications when an output stream. |
|
| struct timeval | timestamp |
For input streams this is the system time (as returned by the |
|
| struct v4l2_timecode | timecode |
When type is V4L2_BUF_TYPE_VIDEO_CAPTURE and the V4L2_BUF_FLAG_TIMECODE flag is set in flags, this structure contains a frame timecode. In V4L2_FIELD_ALTERNATE mode the top and bottom field contain the same timecode. Timecodes are intended to help video editing and are typically recorded on video tapes, but also embedded in compressed formats like MPEG. This field is independent of the timestamp and sequence fields. |
|
| __u32 | sequence |
Set by the driver, counting the frames in the sequence. | |
| In V4L2_FIELD_ALTERNATE mode the top and bottom field have the same sequence number. The count starts at zero and includes dropped or repeated frames. A dropped frame was received by an input device but could not be stored due to lack of free buffer space. A repeated frame was displayed again by an output device because the application did not pass new data in time. Note this may count the frames received e.g. over USB, without taking into account the frames dropped by the remote hardware due to limited compression throughput or bus bandwidth. These devices identify by not enumerating any video standards, see Section 1.7, 「Video Standards」. |
|||
| enum v4l2_memory | memory |
This field must be set by applications and/or drivers in accordance with the selected I/O method. | |
| union | m |
||
| __u32 | offset |
When memory is V4L2_MEMORY_MMAP this is the offset of the buffer from the start of the device memory. The value is returned by the driver and apart of serving as parameter to the mmap() function not useful for applications. See Section 3.2, 「Streaming I/O (Memory Mapping)」 for details. |
|
| unsigned long | userptr |
When memory is V4L2_MEMORY_USERPTR this is a pointer to the buffer (casted to unsigned long type) in virtual memory, set by the application. See Section 3.3, 「Streaming I/O (User Pointers)」 for details. |
|
| __u32 | length |
Size of the buffer (not the payload) in bytes. | |
| __u32 | input |
Some video capture drivers support rapid and synchronous video input changes, a function useful for example in video surveillance applications. For this purpose applications set the V4L2_BUF_FLAG_INPUT flag, and this field to the number of a video input as in struct v4l2_input field index. |
|
| __u32 | reserved |
A place holder for future extensions and custom (driver defined) buffer types V4L2_BUF_TYPE_PRIVATE and higher. |
|
Table 3.2. enum v4l2_buf_type
V4L2_BUF_TYPE_VIDEO_CAPTURE |
1 | Buffer of a video capture stream, see Section 4.1, 「Video Capture Interface」. |
V4L2_BUF_TYPE_VIDEO_OUTPUT |
2 | Buffer of a video output stream, see Section 4.3, 「Video Output Interface」. |
V4L2_BUF_TYPE_VIDEO_OVERLAY |
3 | Buffer for video overlay, see Section 4.2, 「Video Overlay Interface」. |
V4L2_BUF_TYPE_VBI_CAPTURE |
4 | Buffer of a raw VBI capture stream, see Section 4.7, 「Raw VBI Data Interface」. |
V4L2_BUF_TYPE_VBI_OUTPUT |
5 | Buffer of a raw VBI output stream, see Section 4.7, 「Raw VBI Data Interface」. |
V4L2_BUF_TYPE_SLICED_VBI_CAPTURE |
6 | Buffer of a sliced VBI capture stream, see Section 4.8, 「Sliced VBI Data Interface」. |
V4L2_BUF_TYPE_SLICED_VBI_OUTPUT |
7 | Buffer of a sliced VBI output stream, see Section 4.8, 「Sliced VBI Data Interface」. |
V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY |
8 | Buffer for video output overlay (OSD), see Section 4.4, 「Video Output Overlay Interface」. Status: Experimental. |
V4L2_BUF_TYPE_PRIVATE |
0x80 | This and higher values are reserved for custom (driver defined) buffer types. |
Table 3.3. Buffer Flags
V4L2_BUF_FLAG_MAPPED |
0x0001 | The buffer resides in device memory and has been mapped into the application's address space, see Section 3.2, 「Streaming I/O (Memory Mapping)」 for details. Drivers set or clear this flag when the VIDIOC_QUERYBUF, VIDIOC_QBUF or VIDIOC_DQBUF ioctl is called. Set by the driver. |
V4L2_BUF_FLAG_QUEUED |
0x0002 | Internally drivers maintain two buffer queues, an incoming and outgoing queue. When this flag is set, the buffer is currently on the incoming queue. It automatically moves to the outgoing queue after the buffer has been filled (capture devices) or displayed (output devices). Drivers set or clear this flag when the VIDIOC_QUERYBUF ioctl is called. After (successful) calling the VIDIOC_QBUF ioctl it is always set and after VIDIOC_DQBUF always cleared. |
V4L2_BUF_FLAG_DONE |
0x0004 | When this flag is set, the buffer is currently on the outgoing queue, ready to be dequeued from the driver. Drivers set or clear this flag when the VIDIOC_QUERYBUF ioctl is called. After calling the VIDIOC_QBUF or VIDIOC_DQBUF it is always cleared. Of course a buffer cannot be on both queues at the same time, the V4L2_BUF_FLAG_QUEUED and V4L2_BUF_FLAG_DONE flag are mutually exclusive. They can be both cleared however, then the buffer is in "dequeued" state, in the application domain to say so. |
V4L2_BUF_FLAG_KEYFRAME |
0x0008 | Drivers set or clear this flag when calling the VIDIOC_DQBUF ioctl. It may be set by video capture devices when the buffer contains a compressed image which is a key frame (or field), i. e. can be decompressed on its own. |
V4L2_BUF_FLAG_PFRAME |
0x0010 | Similar to V4L2_BUF_FLAG_KEYFRAME this flags predicted frames or fields which contain only differences to a previous key frame. |
V4L2_BUF_FLAG_BFRAME |
0x0020 | Similar to V4L2_BUF_FLAG_PFRAME this is a bidirectional predicted frame or field. [ooc tbd] |
V4L2_BUF_FLAG_TIMECODE |
0x0100 | The timecode field is valid. Drivers set or clear this flag when the VIDIOC_DQBUF ioctl is called. |
V4L2_BUF_FLAG_INPUT |
0x0200 | The input field is valid. Applications set or clear this flag before calling the VIDIOC_QBUF ioctl. |
Table 3.4. enum v4l2_memory
V4L2_MEMORY_MMAP |
1 | The buffer is used for memory mapping I/O. |
V4L2_MEMORY_USERPTR |
2 | The buffer is used for user pointer I/O. |
V4L2_MEMORY_OVERLAY |
3 | [to do] |
The v4l2_timecode structure is designed to hold a [SMPTE 12M] or similar timecode. (struct timeval timestamps are stored in struct v4l2_buffer field timestamp.)
Table 3.5. struct v4l2_timecode
| __u32 | type |
Frame rate the timecodes are based on, see Table 3.6, 「Timecode Types」. |
| __u32 | flags |
Timecode flags, see Table 3.7, 「Timecode Flags」. |
| __u8 | frames |
Frame count, 0 ... 23/24/29/49/59, depending on the type of timecode. |
| __u8 | seconds |
Seconds count, 0 ... 59. This is a binary, not BCD number. |
| __u8 | minutes |
Minutes count, 0 ... 59. This is a binary, not BCD number. |
| __u8 | hours |
Hours count, 0 ... 29. This is a binary, not BCD number. |
| __u8 | userbits[4] |
The "user group" bits from the timecode. |
Table 3.6. Timecode Types
V4L2_TC_TYPE_24FPS |
1 | 24 frames per second, i. e. film. |
V4L2_TC_TYPE_25FPS |
2 | 25 frames per second, i. e. PAL or SECAM video. |
V4L2_TC_TYPE_30FPS |
3 | 30 frames per second, i. e. NTSC video. |
V4L2_TC_TYPE_50FPS |
4 | |
V4L2_TC_TYPE_60FPS |
5 |
Table 3.7. Timecode Flags
V4L2_TC_FLAG_DROPFRAME |
0x0001 | Indicates "drop frame" semantics for counting frames in 29.97 fps material. When set, frame numbers 0 and 1 at the start of each minute, except minutes 0, 10, 20, 30, 40, 50 are omitted from the count. |
V4L2_TC_FLAG_COLORFRAME |
0x0002 | The "color frame" flag. |
V4L2_TC_USERBITS_field |
0x000C | Field mask for the "binary group flags". |
V4L2_TC_USERBITS_USERDEFINED |
0x0000 | Unspecified format. |
V4L2_TC_USERBITS_8BITCHARS |
0x0008 | 8-bit ISO characters. |
We have to distinguish between progressive and interlaced video. Progressive video transmits all lines of a video image sequentially. Interlaced video divides an image into two fields, containing only the odd and even lines of the image, respectively. Alternating the so called odd and even field are transmitted, and due to a small delay between fields a cathode ray TV displays the lines interleaved, yielding the original frame. This curious technique was invented because at refresh rates similar to film the image would fade out too quickly. Transmitting fields reduces the flicker without the necessity of doubling the frame rate and with it the bandwidth required for each channel.
It is important to understand a video camera does not expose one frame at a time, merely transmitting the frames separated into fields. The fields are in fact captured at two different instances in time. An object on screen may well move between one field and the next. For applications analysing motion it is of paramount importance to recognize which field of a frame is older, the temporal order.
When the driver provides or accepts images field by field rather than interleaved, it is also important applications understand how the fields combine to frames. We distinguish between top and bottom fields, the spatial order: The first line of the top field is the first line of an interlaced frame, the first line of the bottom field is the second line of that frame.
However because fields were captured one after the other, arguing whether a frame commences with the top or bottom field is pointless. Any two successive top and bottom, or bottom and top fields yield a valid frame. Only when the source was progressive to begin with, e. g. when transferring film to video, two fields may come from the same frame, creating a natural order.
Counter to intuition the top field is not necessarily the older field. Whether the older field contains the top or bottom lines is a convention determined by the video standard. Hence the distinction between temporal and spatial order of fields. The diagrams below should make this clearer.
All video capture and output devices must report the current field order. Some drivers may permit the selection of a different order, to this end applications initialize the field field of struct v4l2_pix_format before calling the VIDIOC_S_FMT ioctl. If this is not desired it should have the value V4L2_FIELD_ANY (0).
Table 3.8. enum v4l2_field
V4L2_FIELD_ANY |
0 | Applications request this field order when any one of the V4L2_FIELD_NONE, V4L2_FIELD_TOP, V4L2_FIELD_BOTTOM, or V4L2_FIELD_INTERLACED formats is acceptable. Drivers choose depending on hardware capabilities or e. g. the requested image size, and return the actual field order. struct v4l2_buffer field can never be V4L2_FIELD_ANY. |
V4L2_FIELD_NONE |
1 | Images are in progressive format, not interlaced. The driver may also indicate this order when it cannot distinguish between V4L2_FIELD_TOP and V4L2_FIELD_BOTTOM. |
V4L2_FIELD_TOP |
2 | Images consist of the top field only. |
V4L2_FIELD_BOTTOM |
3 | Images consist of the bottom field only. Applications may wish to prevent a device from capturing interlaced images because they will have "comb" or "feathering" artefacts around moving objects. |
V4L2_FIELD_INTERLACED |
4 | Images contain both fields, interleaved line by line. The temporal order of the fields (whether the top or bottom field is first transmitted) depends on the current video standard. M/NTSC transmits the bottom field first, all other standards the top field first. |
V4L2_FIELD_SEQ_TB |
5 | Images contain both fields, the top field lines are stored first in memory, immediately followed by the bottom field lines. Fields are always stored in temporal order, the older one first in memory. Image sizes refer to the frame, not fields. |
V4L2_FIELD_SEQ_BT |
6 | Images contain both fields, the bottom field lines are stored first in memory, immediately followed by the top field lines. Fields are always stored in temporal order, the older one first in memory. Image sizes refer to the frame, not fields. |
V4L2_FIELD_ALTERNATE |
7 | The two fields of a frame are passed in separate buffers, in temporal order, i. e. the older one first. To indicate the field parity (whether the current field is a top or bottom field) the driver or application, depending on data direction, must set struct v4l2_buffer field to V4L2_FIELD_TOP or V4L2_FIELD_BOTTOM. Any two successive fields pair to build a frame. If fields are successive, without any dropped fields between them (fields can drop individually), can be determined from the struct v4l2_buffer sequence field. Image sizes refer to the frame, not fields. This format cannot be selected when using the read/write I/O method. |
V4L2_FIELD_INTERLACED_TB |
8 | Images contain both fields, interleaved line by line, top field first. The top field is transmitted first. |
V4L2_FIELD_INTERLACED_BT |
9 | Images contain both fields, interleaved line by line, top field first. The bottom field is transmitted first. |
[12] It would be desirable if applications could depend on drivers supporting all I/O interfaces, but as much as the complex memory mapping I/O can be inadequate for some devices we have no reason to require this interface, which is most useful for simple applications capturing still images.
[13] At the driver level select() and poll() are the same, and select() is too important to be optional.
[14] One could use one file descriptor and set the buffer type field accordingly when calling VIDIOC_QBUF etc., but it makes the select() function ambiguous. We also like the clean approach of one file descriptor per logical stream. Video overlay for example is also a logical stream, although the CPU is not needed for continuous operation.
[15] Random enqueue order permits applications processing images out of order (such as video codecs) to return buffers earlier, reducing the probability of data loss. Random fill order allows drivers to reuse buffers on a LIFO-basis, taking advantage of caches holding scatter-gather lists and the like.
[16] At the driver level select() and poll() are the same, and select() is too important to be optional. The rest should be evident.
[17] We expect that frequently used buffers are typically not swapped out. Anyway, the process of swapping, locking or generating scatter-gather lists may be time consuming. The delay can be masked by the depth of the incoming buffer queue, and perhaps by maintaining caches assuming a buffer will be soon enqueued again. On the other hand, to optimize memory usage drivers can limit the number of buffers locked in advance and recycle the most recently used buffers first. Of course, the pages of empty buffers in the incoming queue need not be saved to disk. Output buffers must be saved on the incoming and outgoing queue because an application may share them with other processes.
[18] At the driver level select() and poll() are the same, and select() is too important to be optional. The rest should be evident.
[19] Since no other Linux multimedia API supports unadjusted time it would be foolish to introduce here. We must use a universally supported clock to synchronize different media, hence time of day.
Table of Contents
Video capture devices sample an analog video signal and store the digitized images in memory. Today nearly all devices can capture at full 25 or 30 frames/second. With this interface applications can control the capture process and move images from the driver into user space.
Conventionally V4L2 video capture devices are accessed through character device special files named /dev/video and /dev/video0 to /dev/video63 with major number 81 and minor numbers 0 to 63. /dev/video is typically a symbolic link to the preferred video device. Note the same device files are used for video output devices.
Devices supporting the video capture interface set the V4L2_CAP_VIDEO_CAPTURE flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. As secondary device functions they may also support the video overlay (V4L2_CAP_VIDEO_OVERLAY) and the raw VBI capture (V4L2_CAP_VBI_CAPTURE) interface. At least one of the read/write or streaming I/O methods must be supported. Tuners and audio inputs are optional.
Video capture devices shall support audio input, tuner, controls, cropping and scaling and streaming parameter ioctls as needed. The video input and video standard ioctls must be supported by all video capture devices.
The result of a capture operation is determined by cropping and image format parameters. The former select an area of the video picture to capture, the latter how images are stored in memory, i. e. in RGB or YUV format, the number of bits per pixel or width and height. Together they also define how images are scaled in the process.
As usual these parameters are not reset at open() time to permit Unix tool chains, programming a device and then reading from it as if it was a plain file. Well written V4L2 applications ensure they really get what they want, including cropping and scaling.
Cropping initialization at minimum requires to reset the parameters to defaults. An example is given in Section 1.11, 「Image Cropping, Insertion and Scaling」.
To query the current image format applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VIDEO_CAPTURE and call the VIDIOC_G_FMT ioctl with a pointer to this structure. Drivers fill the struct v4l2_pix_format pix member of the fmt union.
To request different parameters applications set the type field of a struct v4l2_format as above and initialize all fields of the struct v4l2_pix_format vbi member of the fmt union, or better just modify the results of VIDIOC_G_FMT, and call the VIDIOC_S_FMT ioctl with a pointer to this structure. Drivers may adjust the parameters and finally return the actual parameters as VIDIOC_G_FMT does.
Like VIDIOC_S_FMT the VIDIOC_TRY_FMT ioctl can be used to learn about hardware limitations without disabling I/O or possibly time consuming hardware preparations.
The contents of struct v4l2_pix_format are discussed in Chapter 2, Image Formats. See also the specification of the VIDIOC_G_FMT, VIDIOC_S_FMT and VIDIOC_TRY_FMT ioctls for details. Video capture devices must implement both the VIDIOC_G_FMT and VIDIOC_S_FMT ioctl, even if VIDIOC_S_FMT ignores all requests and always returns default parameters as VIDIOC_G_FMT does. VIDIOC_TRY_FMT is optional.
A video capture device may support the read() function and/or streaming (memory mapping or user pointer) I/O. See Chapter 3, Input/Output for details.
Video overlay devices have the ability to genlock (TV-)video into the (VGA-)video signal of a graphics card, or to store captured images directly in video memory of a graphics card, typically with clipping. This can be considerable more efficient than capturing images and displaying them by other means. In the old days when only nuclear power plants needed cooling towers this used to be the only way to put live video into a window.
Video overlay devices are accessed through the same character special files as video capture devices. Note the default function of a /dev/video device is video capturing. The overlay function is only available after calling the VIDIOC_S_FMT ioctl.
The driver may support simultaneous overlay and capturing using the read/write and streaming I/O methods. If so, operation at the nominal frame rate of the video standard is not guaranteed. Frames may be directed away from overlay to capture, or one field may be used for overlay and the other for capture if the capture parameters permit this.
Applications should use different file descriptors for capturing and overlay. This must be supported by all drivers capable of simultaneous capturing and overlay. Optionally these drivers may also permit capturing and overlay with a single file descriptor for compatibility with V4L and earlier versions of V4L2.[20]
Devices supporting the video overlay interface set the V4L2_CAP_VIDEO_OVERLAY flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. The overlay I/O method specified below must be supported. Tuners and audio inputs are optional.
Video overlay devices shall support audio input, tuner, controls, cropping and scaling and streaming parameter ioctls as needed. The video input and video standard ioctls must be supported by all video overlay devices.
Before overlay can commence applications must program the driver with frame buffer parameters, namely the address and size of the frame buffer and the image format, for example RGB 5:6:5. The VIDIOC_G_FBUF and VIDIOC_S_FBUF ioctls are available to get and set these parameters, respectively. The VIDIOC_S_FBUF ioctl is privileged because it allows to set up DMA into physical memory, bypassing the memory protection mechanisms of the kernel. Only the superuser can change the frame buffer address and size. Users are not supposed to run TV applications as root or with SUID bit set. A small helper application with suitable privileges should query the graphics system and program the V4L2 driver at the appropriate time.
Some devices add the video overlay to the output signal of the graphics card. In this case the frame buffer is not modified by the video device, and the frame buffer address and pixel format are not needed by the driver. The VIDIOC_S_FBUF ioctl is not privileged. An application can check for this type of device by calling the VIDIOC_G_FBUF ioctl.
A driver may support any (or none) of five clipping/blending methods:
-
Chroma-keying displays the overlaid image only where pixels in the primary graphics surface assume a certain color.
-
A bitmap can be specified where each bit corresponds to a pixel in the overlaid image. When the bit is set, the corresponding video pixel is displayed, otherwise a pixel of the graphics surface.
-
A list of clipping rectangles can be specified. In these regions no video is displayed, so the graphics surface can be seen here.
-
The framebuffer has an alpha channel that can be used to clip or blend the framebuffer with the video.
-
A global alpha value can be specified to blend the framebuffer contents with video images.
When simultaneous capturing and overlay is supported and the hardware prohibits different image and frame buffer formats, the format requested first takes precedence. The attempt to capture (VIDIOC_S_FMT) or overlay (VIDIOC_S_FBUF) may fail with an EBUSY error code or return accordingly modified parameters..
The overlaid image is determined by cropping and overlay window parameters. The former select an area of the video picture to capture, the latter how images are overlaid and clipped. Cropping initialization at minimum requires to reset the parameters to defaults. An example is given in Section 1.11, 「Image Cropping, Insertion and Scaling」.
The overlay window is described by a struct v4l2_window. It defines the size of the image, its position over the graphics surface and the clipping to be applied. To get the current parameters applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VIDEO_OVERLAY and call the VIDIOC_G_FMT ioctl. The driver fills the v4l2_window substructure named win. It is not possible to retrieve a previously programmed clipping list or bitmap.
To program the overlay window applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VIDEO_OVERLAY, initialize the win substructure and call the VIDIOC_S_FMT ioctl. The driver adjusts the parameters against hardware limits and returns the actual parameters as VIDIOC_G_FMT does. Like VIDIOC_S_FMT, the VIDIOC_TRY_FMT ioctl can be used to learn about driver capabilities without actually changing driver state. Unlike VIDIOC_S_FMT this also works after the overlay has been enabled.
The scaling factor of the overlaid image is implied by the width and height given in struct v4l2_window and the size of the cropping rectangle. For more information see Section 1.11, 「Image Cropping, Insertion and Scaling」.
When simultaneous capturing and overlay is supported and the hardware prohibits different image and window sizes, the size requested first takes precedence. The attempt to capture or overlay as well (VIDIOC_S_FMT) may fail with an EBUSY error code or return accordingly modified parameters.
Table 4.1. struct v4l2_window
| struct v4l2_rect | w |
Size and position of the window relative to the top, left corner of the frame buffer defined with VIDIOC_S_FBUF. The window can extend the frame buffer width and height, the x and y coordinates can be negative, and it can lie completely outside the frame buffer. The driver clips the window accordingly, or if that is not possible, modifies its size and/or position. |
| enum v4l2_field | field |
Applications set this field to determine which video field shall be overlaid, typically one of V4L2_FIELD_ANY (0), V4L2_FIELD_TOP, V4L2_FIELD_BOTTOM or V4L2_FIELD_INTERLACED. Drivers may have to choose a different field order and return the actual setting here. |
| __u32 | chromakey |
When chroma-keying has been negotiated with VIDIOC_S_FBUF applications set this field to the desired pixel value for the chroma key. The format is the same as the pixel format of the framebuffer (struct v4l2_framebuffer fmt.pixelformat field), with bytes in host order. E. g. for V4L2_PIX_FMT_BGR24 the value should be 0xRRGGBB on a little endian, 0xBBGGRR on a big endian host. |
| struct v4l2_clip * | clips |
When chroma-keying has not been negotiated and VIDIOC_G_FBUF indicated this capability, applications can set this field to point to an array of clipping rectangles. |
Like the window coordinates w, clipping rectangles are defined relative to the top, left corner of the frame buffer. However clipping rectangles must not extend the frame buffer width and height, and they must not overlap. If possible applications should merge adjacent rectangles. Whether this must create x-y or y-x bands, or the order of rectangles, is not defined. When clip lists are not supported the driver ignores this field. Its contents after calling VIDIOC_S_FMT are undefined. |
||
| __u32 | clipcount |
When the application set the clips field, this field must contain the number of clipping rectangles in the list. When clip lists are not supported the driver ignores this field, its contents after calling VIDIOC_S_FMT are undefined. When clip lists are supported but no clipping is desired this field must be set to zero. |
| void * | bitmap |
When chroma-keying has not been negotiated and VIDIOC_G_FBUF indicated this capability, applications can set this field to point to a clipping bit mask. |
| It must be of the same size as the window, ((__u8 *) )[ * y + x / 8] & (1 << (x & 7))bitmapw.width where When a clipping bit mask is not supported the driver ignores this field, its contents after calling Applications need not create a clip list or bit mask. When they pass both, or despite negotiating chroma-keying, the results are undefined. Regardless of the chosen method, the clipping abilities of the hardware may be limited in quantity or quality. The results when these limits are exceeded are undefined.[b] |
||
| __u8 | global_alpha |
The global alpha value used to blend the framebuffer with video images, if global alpha blending has been negotiated (V4L2_FBUF_FLAG_GLOBAL_ALPHA, see VIDIOC_S_FBUF, Table 83, 「Frame Buffer Flags」). |
| Note this field was added in Linux 2.6.23, extending the structure. However the VIDIOC_G/S/TRY_FMT ioctls, which take a pointer to a v4l2_format parent structure with padding bytes at the end, are not affected. | ||
|
[a] Should we require [b] When the image is written into frame buffer memory it will be undesirable if the driver clips out less pixels than expected, because the application and graphics system are not aware these regions need to be refreshed. The driver should clip out more pixels or not write the image at all. |
||
Table 4.2. struct v4l2_clip[21]
| struct v4l2_rect | c |
Coordinates of the clipping rectangle, relative to the top, left corner of the frame buffer. Only window pixels outside all clipping rectangles are displayed. |
| struct v4l2_clip * | next |
Pointer to the next clipping rectangle, NULL when this is the last rectangle. Drivers ignore this field, it cannot be used to pass a linked list of clipping rectangles. |
Table 4.3. struct v4l2_rect
| __s32 | left |
Horizontal offset of the top, left corner of the rectangle, in pixels. |
| __s32 | top |
Vertical offset of the top, left corner of the rectangle, in pixels. Offsets increase to the right and down. |
| __s32 | width |
Width of the rectangle, in pixels. |
| __s32 | height |
Height of the rectangle, in pixels. Width and height cannot be negative, the fields are signed for hysterical reasons. |
To start or stop the frame buffer overlay applications call the VIDIOC_OVERLAY ioctl.
Video output devices encode stills or image sequences as analog video signal. With this interface applications can control the encoding process and move images from user space to the driver.
Conventionally V4L2 video output devices are accessed through character device special files named /dev/video and /dev/video0 to /dev/video63 with major number 81 and minor numbers 0 to 63. /dev/video is typically a symbolic link to the preferred video device. Note the same device files are used for video capture devices.
Devices supporting the video output interface set the V4L2_CAP_VIDEO_OUTPUT flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. As secondary device functions they may also support the raw VBI output (V4L2_CAP_VBI_OUTPUT) interface. At least one of the read/write or streaming I/O methods must be supported. Modulators and audio outputs are optional.
Video output devices shall support audio output, modulator, controls, cropping and scaling and streaming parameter ioctls as needed. The video output and video standard ioctls must be supported by all video output devices.
The output is determined by cropping and image format parameters. The former select an area of the video picture where the image will appear, the latter how images are stored in memory, i. e. in RGB or YUV format, the number of bits per pixel or width and height. Together they also define how images are scaled in the process.
As usual these parameters are not reset at open() time to permit Unix tool chains, programming a device and then writing to it as if it was a plain file. Well written V4L2 applications ensure they really get what they want, including cropping and scaling.
Cropping initialization at minimum requires to reset the parameters to defaults. An example is given in Section 1.11, 「Image Cropping, Insertion and Scaling」.
To query the current image format applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VIDEO_OUTPUT and call the VIDIOC_G_FMT ioctl with a pointer to this structure. Drivers fill the struct v4l2_pix_format pix member of the fmt union.
To request different parameters applications set the type field of a struct v4l2_format as above and initialize all fields of the struct v4l2_pix_format vbi member of the fmt union, or better just modify the results of VIDIOC_G_FMT, and call the VIDIOC_S_FMT ioctl with a pointer to this structure. Drivers may adjust the parameters and finally return the actual parameters as VIDIOC_G_FMT does.
Like VIDIOC_S_FMT the VIDIOC_TRY_FMT ioctl can be used to learn about hardware limitations without disabling I/O or possibly time consuming hardware preparations.
The contents of struct v4l2_pix_format are discussed in Chapter 2, Image Formats. See also the specification of the VIDIOC_G_FMT, VIDIOC_S_FMT and VIDIOC_TRY_FMT ioctls for details. Video output devices must implement both the VIDIOC_G_FMT and VIDIOC_S_FMT ioctl, even if VIDIOC_S_FMT ignores all requests and always returns default parameters as VIDIOC_G_FMT does. VIDIOC_TRY_FMT is optional.
A video output device may support the write() function and/or streaming (memory mapping or user pointer) I/O. See Chapter 3, Input/Output for details.
Experimental
This is an experimental interface and may change in the future.
Some video output devices can overlay a framebuffer image onto the outgoing video signal. Applications can set up such an overlay using this interface, which borrows structures and ioctls of the Video Overlay interface.
The OSD function is accessible through the same character special file as the Video Output function. Note the default function of such a /dev/video device is video capturing or output. The OSD function is only available after calling the VIDIOC_S_FMT ioctl.
Devices supporting the Video Output Overlay interface set the V4L2_CAP_VIDEO_OUTPUT_OVERLAY flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl.
Contrary to the Video Overlay interface the framebuffer is normally implemented on the TV card and not the graphics card. On Linux it is accessible as a framebuffer device (/dev/fbN). Given a V4L2 device, applications can find the corresponding framebuffer device by calling the VIDIOC_G_FBUF ioctl. It returns, amongst other information, the physical address of the framebuffer in the base field of struct v4l2_framebuffer. The framebuffer device ioctl FBIOGET_FSCREENINFO returns the same address in the smem_start field of struct fb_fix_screeninfo. The FBIOGET_FSCREENINFO ioctl and struct fb_fix_screeninfo are defined in the linux/fb.h header file.
The width and height of the framebuffer depends on the current video standard. A V4L2 driver may reject attempts to change the video standard (or any other ioctl which would imply a framebuffer size change) with an EBUSY error code until all applications closed the framebuffer device.
Example 4.1. Finding a framebuffer device for OSD
#include <linux/fb.h> struct v4l2_framebuffer fbuf; unsigned int i; int fb_fd; if (-1 == ioctl (fd, VIDIOC_G_FBUF, &fbuf)) { perror ("VIDIOC_G_FBUF"); exit (EXIT_FAILURE); } for (i = 0; i > 30; ++i) { char dev_name[16]; struct fb_fix_screeninfo si; snprintf (dev_name, sizeof (dev_name), "/dev/fb%u", i); fb_fd = open (dev_name, O_RDWR); if (-1 == fb_fd) { switch (errno) { case ENOENT: /* no such file */ case ENXIO: /* no driver */ continue; default: perror ("open"); exit (EXIT_FAILURE); } } if (0 == ioctl (fb_fd, FBIOGET_FSCREENINFO, &si)) { if (si.smem_start == (unsigned long) fbuf.base) break; } else { /* Apparently not a framebuffer device. */ } close (fb_fd); fb_fd = -1; } /* fb_fd is the file descriptor of the framebuffer device for the video output overlay, or -1 if no device was found. */
The overlay is controlled by source and target rectangles. The source rectangle selects a subsection of the framebuffer image to be overlaid, the target rectangle an area in the outgoing video signal where the image will appear. Drivers may or may not support scaling, and arbitrary sizes and positions of these rectangles. Further drivers may support any (or none) of the clipping/blending methods defined for the Video Overlay interface.
A struct v4l2_window defines the size of the source rectangle, its position in the framebuffer and the clipping/blending method to be used for the overlay. To get the current parameters applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY and call the VIDIOC_G_FMT ioctl. The driver fills the v4l2_window substructure named win. It is not possible to retrieve a previously programmed clipping list or bitmap.
To program the source rectangle applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY, initialize the win substructure and call the VIDIOC_S_FMT ioctl. The driver adjusts the parameters against hardware limits and returns the actual parameters as VIDIOC_G_FMT does. Like VIDIOC_S_FMT, the VIDIOC_TRY_FMT ioctl can be used to learn about driver capabilities without actually changing driver state. Unlike VIDIOC_S_FMT this also works after the overlay has been enabled.
A struct v4l2_crop defines the size and position of the target rectangle. The scaling factor of the overlay is implied by the width and height given in struct v4l2_window and struct v4l2_crop. The cropping API applies to Video Output and Video Output Overlay devices in the same way as to Video Capture and Video Overlay devices, merely reversing the direction of the data flow. For more information see Section 1.11, 「Image Cropping, Insertion and Scaling」.
Suspended
This interface has been be suspended from the V4L2 API implemented in Linux 2.6 until we have more experience with codec device interfaces.
A V4L2 codec can compress, decompress, transform, or otherwise convert video data from one format into another format, in memory. Applications send data to be converted to the driver through a write() call, and receive the converted data through a read() call. For efficiency a driver may also support streaming I/O.
[to do]
Suspended
This interface has been be suspended from the V4L2 API implemented in Linux 2.6 until we have more experience with effect device interfaces.
A V4L2 video effect device can do image effects, filtering, or combine two or more images or image streams. For example video transitions or wipes. Applications send data to be processed and receive the result data either with read() and write() functions, or through the streaming I/O mechanism.
[to do]
VBI is an abbreviation of Vertical Blanking Interval, a gap in the sequence of lines of an analog video signal. During VBI no picture information is transmitted, allowing some time while the electron beam of a cathode ray tube TV returns to the top of the screen. Using an oscilloscope you will find here the vertical synchronization pulses and short data packages ASK modulated[22] onto the video signal. These are transmissions of services such as Teletext or Closed Caption.
Subject of this interface type is raw VBI data, as sampled off a video signal, or to be added to a signal for output. The data format is similar to uncompressed video images, a number of lines times a number of samples per line, we call this a VBI image.
Conventionally V4L2 VBI devices are accessed through character device special files named /dev/vbi and /dev/vbi0 to /dev/vbi31 with major number 81 and minor numbers 224 to 255. /dev/vbi is typically a symbolic link to the preferred VBI device. This convention applies to both input and output devices.
To address the problems of finding related video and VBI devices VBI capturing and output is also available as device function under /dev/video. To capture or output raw VBI data with these devices applications must call the VIDIOC_S_FMT ioctl. Accessed as /dev/vbi, raw VBI capturing or output is the default device function.
Devices supporting the raw VBI capturing or output API set the V4L2_CAP_VBI_CAPTURE or V4L2_CAP_VBI_OUTPUT flags, respectively, in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. At least one of the read/write, streaming or asynchronous I/O methods must be supported. VBI devices may or may not have a tuner or modulator.
VBI devices shall support video input or output, tuner or modulator, and controls ioctls as needed. The video standard ioctls provide information vital to program a VBI device, therefore must be supported.
Raw VBI sampling abilities can vary, in particular the sampling frequency. To properly interpret the data V4L2 specifies an ioctl to query the sampling parameters. Moreover, to allow for some flexibility applications can also suggest different parameters.
As usual these parameters are not reset at open() time to permit Unix tool chains, programming a device and then reading from it as if it was a plain file. Well written V4L2 applications should always ensure they really get what they want, requesting reasonable parameters and then checking if the actual parameters are suitable.
To query the current raw VBI capture parameters applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VBI_CAPTURE or V4L2_BUF_TYPE_VBI_OUTPUT, and call the VIDIOC_G_FMT ioctl with a pointer to this structure. Drivers fill the struct v4l2_vbi_format vbi member of the fmt union.
To request different parameters applications set the type field of a struct v4l2_format as above and initialize all fields of the struct v4l2_vbi_format vbi member of the fmt union, or better just modify the results of VIDIOC_G_FMT, and call the VIDIOC_S_FMT ioctl with a pointer to this structure. Drivers return an EINVAL error code only when the given parameters are ambiguous, otherwise they modify the parameters according to the hardware capabilites and return the actual parameters. When the driver allocates resources at this point, it may return an EBUSY error code to indicate the returned parameters are valid but the required resources are currently not available. That may happen for instance when the video and VBI areas to capture would overlap, or when the driver supports multiple opens and another process already requested VBI capturing or output. Anyway, applications must expect other resource allocation points which may return EBUSY, at the VIDIOC_STREAMON ioctl and the first read(), write() and select() call.
VBI devices must implement both the VIDIOC_G_FMT and VIDIOC_S_FMT ioctl, even if VIDIOC_S_FMT ignores all requests and always returns default parameters as VIDIOC_G_FMT does. VIDIOC_TRY_FMT is optional.
Table 4.4. struct v4l2_vbi_format
| __u32 | sampling_rate |
Samples per second, i. e. unit 1 Hz. |
| __u32 | offset |
Horizontal offset of the VBI image, relative to the leading edge of the line synchronization pulse and counted in samples: The first sample in the VBI image will be located |
| __u32 | samples_per_line |
|
| __u32 | sample_format |
Defines the sample format as in Chapter 2, Image Formats, a four-character-code.[a] Usually this is |
| __u32 | start[2] |
This is the scanning system line number associated with the first line of the VBI image, of the first and the second field respectively. See Figure 4.2, 「ITU-R 525 line numbering (M/NTSC and M/PAL)」 and Figure 4.3, 「ITU-R 625 line numbering」 for valid values. VBI input drivers can return start values 0 if the hardware cannot reliable identify scanning lines, VBI acquisition may not require this information. |
| __u32 | count[2] |
The number of lines in the first and second field image, respectively. |
| Drivers should be as flexibility as possible. For example, it may be possible to extend or move the VBI capture window down to the picture area, implementing a 'full field mode' to capture data service transmissions embedded in the picture. An application can set the first or second Both To initialize the |
||
| __u32 | flags |
See Table 4.5, 「Raw VBI Format Flags」 below. Currently only drivers set flags, applications must set this field to zero. |
| __u32 | reserved[2] |
This array is reserved for future extensions. Drivers and applications must set it to zero. |
|
[a] A few devices may be unable to sample VBI data at all but can extend the video capture window to the VBI region. |
||
Table 4.5. Raw VBI Format Flags
V4L2_VBI_UNSYNC |
0x0001 | This flag indicates hardware which does not properly distinguish between fields. Normally the VBI image stores the first field (lower scanning line numbers) first in memory. This may be a top or bottom field depending on the video standard. When this flag is set the first or second field may be stored first, however the fields are still in correct temporal order with the older field first in memory.[a] |
V4L2_VBI_INTERLACED |
0x0002 | By default the two field images will be passed sequentially; all lines of the first field followed by all lines of the second field (compare Section 3.6, 「Field Order」 V4L2_FIELD_SEQ_TB and V4L2_FIELD_SEQ_BT, whether the top or bottom field is first in memory depends on the video standard). When this flag is set, the two fields are interlaced (cf. V4L2_FIELD_INTERLACED). The first line of the first field followed by the first line of the second field, then the two second lines, and so on. Such a layout may be necessary when the hardware has been programmed to capture or output interlaced video images and is unable to separate the fields for VBI capturing at the same time. For simplicity setting this flag implies that both count values are equal and non-zero. |
|
[a] Most VBI services transmit on both fields, but some have different semantics depending on the field number. These cannot be reliable decoded or encoded when |
||
Figure 4.2. ITU-R 525 line numbering (M/NTSC and M/PAL)
(1) For the purpose of this specification field 2 starts in line 264 and not 263.5 because half line capturing is not supported.
Figure 4.3. ITU-R 625 line numbering
(1) For the purpose of this specification field 2 starts in line 314 and not 313.5 because half line capturing is not supported.
Remember the VBI image format depends on the selected video standard, therefore the application must choose a new standard or query the current standard first. Attempts to read or write data ahead of format negotiation, or after switching the video standard which may invalidate the negotiated VBI parameters, should be refused by the driver. A format change during active I/O is not permitted.
To assure synchronization with the field number and easier implementation, the smallest unit of data passed at a time is one frame, consisting of two fields of VBI images immediately following in memory.
The total size of a frame computes as follows:
([0] + [1]) * * sample size in bytescountcountsamples_per_line
The sample size is most likely always one byte, applications must check the sample_format field though, to function properly with other drivers.
A VBI device may support read/write and/or streaming (memory mapping or user pointer) I/O. The latter bears the possibility of synchronizing video and VBI data by using buffer timestamps.
Remember the VIDIOC_STREAMON ioctl and the first read(), write() and select() call can be resource allocation points returning an EBUSY error code if the required hardware resources are temporarily unavailable, for example the device is already in use by another process.
VBI stands for Vertical Blanking Interval, a gap in the sequence of lines of an analog video signal. During VBI no picture information is transmitted, allowing some time while the electron beam of a cathode ray tube TV returns to the top of the screen.
Sliced VBI devices use hardware to demodulate data transmitted in the VBI. V4L2 drivers shall not do this by software, see also the raw VBI interface. The data is passed as short packets of fixed size, covering one scan line each. The number of packets per video frame is variable.
Sliced VBI capture and output devices are accessed through the same character special files as raw VBI devices. When a driver supports both interfaces, the default function of a /dev/vbi device is raw VBI capturing or output, and the sliced VBI function is only available after calling the VIDIOC_S_FMT ioctl as defined below. Likewise a /dev/video device may support the sliced VBI API, however the default function here is video capturing or output. Different file descriptors must be used to pass raw and sliced VBI data simultaneously, if this is supported by the driver.
Devices supporting the sliced VBI capturing or output API set the V4L2_CAP_SLICED_VBI_CAPTURE or V4L2_CAP_SLICED_VBI_OUTPUT flag respectively, in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. At least one of the read/write, streaming or asynchronous I/O methods must be supported. Sliced VBI devices may have a tuner or modulator.
Sliced VBI devices shall support video input or output and tuner or modulator ioctls if they have these capabilities, and they may support control ioctls. The video standard ioctls provide information vital to program a sliced VBI device, therefore must be supported.
To find out which data services are supported by the hardware applications can call the VIDIOC_G_SLICED_VBI_CAP ioctl. All drivers implementing the sliced VBI interface must support this ioctl. The results may differ from those of the VIDIOC_S_FMT ioctl when the number of VBI lines the hardware can capture or output per frame, or the number of services it can identify on a given line are limited. For example on PAL line 16 the hardware may be able to look for a VPS or Teletext signal, but not both at the same time.
To determine the currently selected services applications set the type field of struct v4l2_format to V4L2_BUF_TYPE_SLICED_VBI_CAPTURE or V4L2_BUF_TYPE_SLICED_VBI_OUTPUT, and the VIDIOC_G_FMT ioctl fills the fmt.sliced member, a struct v4l2_sliced_vbi_format.
Applications can request different parameters by initializing or modifying the fmt.sliced member and calling the VIDIOC_S_FMT ioctl with a pointer to the v4l2_format structure.
The sliced VBI API is more complicated than the raw VBI API because the hardware must be told which VBI service to expect on each scan line. Not all services may be supported by the hardware on all lines (this is especially true for VBI output where Teletext is often unsupported and other services can only be inserted in one specific line). In many cases, however, it is sufficient to just set the service_set field to the required services and let the driver fill the service_lines array according to hardware capabilities. Only if more precise control is needed should the programmer set the service_lines array explicitly.
The VIDIOC_S_FMT ioctl modifies the parameters according to hardware capabilities. When the driver allocates resources at this point, it may return an EBUSY error code if the required resources are temporarily unavailable. Other resource allocation points which may return EBUSY can be the VIDIOC_STREAMON ioctl and the first read(), write() and select() call.
Table 4.6. struct v4l2_sliced_vbi_format
| __u32 | service_set |
If On return the driver sets this field to the union of all elements of the returned |
||
| __u16 | service_lines[2][24] |
Applications initialize this array with sets of data services the driver shall look for or insert on the respective scan line. Subject to hardware capabilities drivers return the requested set, a subset, which may be just a single service, or an empty set. When the hardware cannot handle multiple services on the same line the driver shall choose one. No assumptions can be made on which service the driver chooses. Data services are defined in Table 4.7, 「Sliced VBI services」. Array indices map to ITU-R line numbers (see also Figure 4.2, 「ITU-R 525 line numbering (M/NTSC and M/PAL)」 and Figure 4.3, 「ITU-R 625 line numbering」) as follows: |
||
| Element | 525 line systems | 625 line systems | ||
service_lines[0][1] |
1 | 1 | ||
service_lines[0][23] |
23 | 23 | ||
service_lines[1][1] |
264 | 314 | ||
service_lines[1][23] |
286 | 336 | ||
Drivers must set service_lines[0][0] and service_lines[1][0] to zero. |
||||
| __u32 | io_size |
Maximum number of bytes passed by one read() or write() call, and the buffer size in bytes for the VIDIOC_QBUF and VIDIOC_DQBUF ioctl. Drivers set this field to the size of struct v4l2_sliced_vbi_data times the number of non-zero elements in the returned service_lines array (that is the number of lines potentially carrying data). |
||
| __u32 | reserved[2] |
This array is reserved for future extensions. Applications and drivers must set it to zero. | ||
|
[a] According to ETS 300 706 lines 6-22 of the first field and lines 5-22 of the second field may carry Teletext data. |
||||
Table 4.7. Sliced VBI services
| Symbol | Value | Reference | Lines, usually | Payload |
|---|---|---|---|---|
V4L2_SLICED_TELETEXT_B (Teletext System B) |
0x0001 | [ETS 300 706], [ITU BT.653] | PAL/SECAM line 7-22, 320-335 (second field 7-22) | Last 42 of the 45 byte Teletext packet, that is without clock run-in and framing code, lsb first transmitted. |
V4L2_SLICED_VPS |
0x0400 | [ETS 300 231] | PAL line 16 | Byte number 3 to 15 according to Figure 9 of ETS 300 231, lsb first transmitted. |
V4L2_SLICED_CAPTION_525 |
0x1000 | [EIA 608-B] | NTSC line 21, 284 (second field 21) | Two bytes in transmission order, including parity bit, lsb first transmitted. |
V4L2_SLICED_WSS_625 |
0x4000 | [ITU BT.1119], [EN 300 294] | PAL/SECAM line 23 | Byte 0 1
msb lsb msb lsb
Bit 7 6 5 4 3 2 1 0 x x 13 12 11 10 9
|
V4L2_SLICED_VBI_525 |
0x1000 | Set of services applicable to 525 line systems. | ||
V4L2_SLICED_VBI_625 |
0x4401 | Set of services applicable to 625 line systems. | ||
Drivers may return an EINVAL error code when applications attempt to read or write data without prior format negotiation, after switching the video standard (which may invalidate the negotiated VBI parameters) and after switching the video input (which may change the video standard as a side effect). The VIDIOC_S_FMT ioctl may return an EBUSY error code when applications attempt to change the format while i/o is in progress (between a VIDIOC_STREAMON and VIDIOC_STREAMOFF call, and after the first read() or write() call).
A single read() or write() call must pass all data belonging to one video frame. That is an array of v4l2_sliced_vbi_data structures with one or more elements and a total size not exceeding io_size bytes. Likewise in streaming I/O mode one buffer of io_size bytes must contain data of one video frame. The id of unused v4l2_sliced_vbi_data elements must be zero.
Table 4.8. struct v4l2_sliced_vbi_data
| __u32 | id |
A flag from Table 97, 「Sliced VBI services」 identifying the type of data in this packet. Only a single bit must be set. When the id of a captured packet is zero, the packet is empty and the contents of other fields are undefined. Applications shall ignore empty packets. When the id of a packet for output is zero the contents of the data field are undefined and the driver must no longer insert data on the requested field and line. |
| __u32 | field |
The video field number this data has been captured from, or shall be inserted at. 0 for the first field, 1 for the second field. |
| __u32 | line |
The field (as opposed to frame) line number this data has been captured from, or shall be inserted at. See Figure 4.2, 「ITU-R 525 line numbering (M/NTSC and M/PAL)」 and Figure 4.3, 「ITU-R 625 line numbering」 for valid values. Sliced VBI capture devices can set the line number of all packets to 0 if the hardware cannot reliably identify scan lines. The field number must always be valid. |
| __u32 | reserved |
This field is reserved for future extensions. Applications and drivers must set it to zero. |
| __u8 | data[48] |
The packet payload. See Table 97, 「Sliced VBI services」 for the contents and number of bytes passed for each data type. The contents of padding bytes at the end of this array are undefined, drivers and applications shall ignore them. |
Packets are always passed in ascending line number order, without duplicate line numbers. The write() function and the VIDIOC_QBUF ioctl must return an EINVAL error code when applications violate this rule. They must also return an EINVAL error code when applications pass an incorrect field or line number, or a combination of field, line and id which has not been negotiated with the VIDIOC_G_FMT or VIDIOC_S_FMT ioctl. When the line numbers are unknown the driver must pass the packets in transmitted order. The driver can insert empty packets with id set to zero anywhere in the packet array.
To assure synchronization and to distinguish from frame dropping, when a captured frame does not carry any of the requested data services drivers must pass one or more empty packets. When an application fails to pass VBI data in time for output, the driver must output the last VPS and WSS packet again, and disable the output of Closed Caption and Teletext data, or output data which is ignored by Closed Caption and Teletext decoders.
A sliced VBI device may support read/write and/or streaming (memory mapping and/or user pointer) I/O. The latter bears the possibility of synchronizing video and VBI data by using buffer timestamps.
If a device can produce an MPEG output stream, it may be capable of providing negotiated sliced VBI services as data embedded in the MPEG stream. Users or applications control this sliced VBI data insertion with the V4L2_CID_MPEG_STREAM_VBI_FMT control.
If the driver does not provide the V4L2_CID_MPEG_STREAM_VBI_FMT control, or only allows that control to be set to V4L2_MPEG_STREAM_VBI_FMT_NONE, then the device cannot embed sliced VBI data in the MPEG stream.
The V4L2_CID_MPEG_STREAM_VBI_FMT control does not implicitly set the device driver to capture nor cease capturing sliced VBI data. The control only indicates to embed sliced VBI data in the MPEG stream, if an application has negotiated sliced VBI service be captured.
It may also be the case that a device can embed sliced VBI data in only certain types of MPEG streams: for example in an MPEG-2 PS but not an MPEG-2 TS. In this situation, if sliced VBI data insertion is requested, the sliced VBI data will be embedded in MPEG stream types when supported, and silently omitted from MPEG stream types where sliced VBI data insertion is not supported by the device.
The following subsections specify the format of the embedded sliced VBI data.
The V4L2_MPEG_STREAM_VBI_FMT_NONE embedded sliced VBI format shall be interpreted by drivers as a control to cease embedding sliced VBI data in MPEG streams. Neither the device nor driver shall insert "empty" embedded sliced VBI data packets in the MPEG stream when this format is set. No MPEG stream data structures are specified for this format.
The V4L2_MPEG_STREAM_VBI_FMT_IVTV embedded sliced VBI format, when supported, indicates to the driver to embed up to 36 lines of sliced VBI data per frame in an MPEG-2 Private Stream 1 PES packet encapsulated in an MPEG-2 Program Pack in the MPEG stream.
Historical context: This format specification originates from a custom, embedded, sliced VBI data format used by the ivtv driver. This format has already been informally specified in the kernel sources in the file Documentation/video4linux/cx2341x/README.vbi . The maximum size of the payload and other aspects of this format are driven by the CX23415 MPEG decoder's capabilities and limitations with respect to extracting, decoding, and displaying sliced VBI data embedded within an MPEG stream.
This format's use is not exclusive to the ivtv driver nor exclusive to CX2341x devices, as the sliced VBI data packet insertion into the MPEG stream is implemented in driver software. At least the cx18 driver provides sliced VBI data insertion into an MPEG-2 PS in this format as well.
The following definitions specify the payload of the MPEG-2 Private Stream 1 PES packets that contain sliced VBI data when V4L2_MPEG_STREAM_VBI_FMT_IVTV is set. (The MPEG-2 Private Stream 1 PES packet header and encapsulating MPEG-2 Program Pack header are not detailed here. Please refer to the MPEG-2 specifications for details on those packet headers.)
The payload of the MPEG-2 Private Stream 1 PES packets that contain sliced VBI data is specified by struct v4l2_mpeg_vbi_fmt_ivtv. The payload is variable length, depending on the actual number of lines of sliced VBI data present in a video frame. The payload may be padded at the end with unspecified fill bytes to align the end of the payload to a 4-byte boundary. The payload shall never exceed 1552 bytes (2 fields with 18 lines/field with 43 bytes of data/line and a 4 byte magic number).
Table 4.9. struct v4l2_mpeg_vbi_fmt_ivtv
| __u8 | magic[4] |
A "magic" constant from Table 4.10, 「Magic Constants for struct v4l2_mpeg_vbi_fmt_ivtv magic field」 that indicates this is a valid sliced VBI data payload and also indicates which member of the anonymous union, itv0 or ITV0, to use for the payload data. |
|
| union | (anonymous) | ||
| struct v4l2_mpeg_vbi_itv0 | itv0 |
The primary form of the sliced VBI data payload that contains anywhere from 1 to 35 lines of sliced VBI data. Line masks are provided in this form of the payload indicating which VBI lines are provided. | |
| struct v4l2_mpeg_vbi_ITV0 | ITV0 |
An alternate form of the sliced VBI data payload used when 36 lines of sliced VBI data are present. No line masks are provided in this form of the payload; all valid line mask bits are implcitly set. |
Table 4.10. Magic Constants for struct v4l2_mpeg_vbi_fmt_ivtv magic field
| Defined Symbol | Value | Description |
|---|---|---|
V4L2_MPEG_VBI_IVTV_MAGIC0 |
"itv0" | Indicates the itv0 member of the union in struct v4l2_mpeg_vbi_fmt_ivtv is valid. |
V4L2_MPEG_VBI_IVTV_MAGIC1 |
"ITV0" | Indicates the ITV0 member of the union in struct v4l2_mpeg_vbi_fmt_ivtv is valid and that 36 lines of sliced VBI data are present. |
Table 4.11. struct v4l2_mpeg_vbi_itv0
| __le32 | linemask[2] |
Bitmasks indicating the VBI service lines present. These [0] blinemask0 : line 6 first field [0] blinemask17 : line 23 first field [0] blinemask18 : line 6 second field [0] blinemask31 : line 19 second field [1] blinemask0 : line 20 second field [1] blinemask3 : line 23 second field [1] blinemask4 -b31 : unused and set to 0 |
| struct v4l2_mpeg_vbi_itv0_line | line[35] |
This is a variable length array that holds from 1 to 35 lines of sliced VBI data. The sliced VBI data lines present correspond to the bits set in the linemask array, starting from b0 of linemask[0] up through b31 of linemask[0], and from b0 of linemask[1] up through b 3 of linemask[1]. line[0] corresponds to the first bit found set in the linemask array, line[1] corresponds to the second bit found set in the linemask array, etc. If no linemask array bits are set, then line[0] may contain one line of unspecified data that should be ignored by applications. |
Table 4.12. struct v4l2_mpeg_vbi_ITV0
| struct v4l2_mpeg_vbi_itv0_line | line[36] |
A fixed length array of 36 lines of sliced VBI data. line[0] through line [17] correspond to lines 6 through 23 of the first field. line[18] through line[35] corresponds to lines 6 through 23 of the second field. |
Table 4.13. struct v4l2_mpeg_vbi_itv0_line
| __u8 | id |
A line identifier value from Table 4.14, 「Line Identifiers for struct v4l2_mpeg_vbi_itv0_line id field」 that indicates the type of sliced VBI data stored on this line. |
| __u8 | data[42] |
The sliced VBI data for the line. |
Table 4.14. Line Identifiers for struct v4l2_mpeg_vbi_itv0_line id field
| Defined Symbol | Value | Description |
|---|---|---|
V4L2_MPEG_VBI_IVTV_TELETEXT_B |
1 | Refer to Sliced VBI services for a description of the line payload. |
V4L2_MPEG_VBI_IVTV_CAPTION_525 |
4 | Refer to Sliced VBI services for a description of the line payload. |
V4L2_MPEG_VBI_IVTV_WSS_625 |
5 | Refer to Sliced VBI services for a description of the line payload. |
V4L2_MPEG_VBI_IVTV_VPS |
7 | Refer to Sliced VBI services for a description of the line payload. |
This interface aims at devices receiving and demodulating Teletext data [[ETS 300 706], [ITU BT.653]], evaluating the Teletext packages and storing formatted pages in cache memory. Such devices are usually implemented as microcontrollers with serial interface (I2C) and can be found on older TV cards, dedicated Teletext decoding cards and home-brew devices connected to the PC parallel port.
The Teletext API was designed by Martin Buck. It is defined in the kernel header file linux/videotext.h, the specification is available from http://home.pages.de/~videotext/. (Videotext is the name of the German public television Teletext service.) Conventional character device file names are /dev/vtx and /dev/vttuner, with device number 83, 0 and 83, 16 respectively. A similar interface exists for the Philips SAA5249 Teletext decoder [specification?] with character device file names /dev/tlkN, device number 102, N.
Eventually the Teletext API was integrated into the V4L API with character device file names /dev/vtx0 to /dev/vtx31, device major number 81, minor numbers 192 to 223. For reference the V4L Teletext API specification is reproduced here in full: "Teletext interfaces talk the existing VTX API." Teletext devices with major number 83 and 102 will be removed in Linux 2.6.
There are no plans to replace the Teletext API or to integrate it into V4L2. Please write to the linux-media mailing list: https://linuxtv.org/lists.php when the need arises.
This interface is intended for AM and FM (analog) radio receivers and transmitters.
Conventionally V4L2 radio devices are accessed through character device special files named /dev/radio and /dev/radio0 to /dev/radio63 with major number 81 and minor numbers 64 to 127.
Devices supporting the radio interface set the V4L2_CAP_RADIO and V4L2_CAP_TUNER or V4L2_CAP_MODULATOR flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. Other combinations of capability flags are reserved for future extensions.
Radio devices can support controls, and must support the tuner or modulator ioctls.
They do not support the video input or output, audio input or output, video standard, cropping and scaling, compression and streaming parameter, or overlay ioctls. All other ioctls and I/O methods are reserved for future extensions.
Radio devices may have a couple audio controls (as discussed in Section 1.8, 「User Controls」) such as a volume control, possibly custom controls. Further all radio devices have one tuner or modulator (these are discussed in Section 1.6, 「Tuners and Modulators」) with index number zero to select the radio frequency and to determine if a monaural or FM stereo program is received/emitted. Drivers switch automatically between AM and FM depending on the selected frequency. The VIDIOC_G_TUNER or VIDIOC_G_MODULATOR ioctl reports the supported frequency range.
The Radio Data System transmits supplementary information in binary format, for example the station name or travel information, on an inaudible audio subcarrier of a radio program. This interface is aimed at devices capable of receiving and decoding RDS information.
For more information see the core RDS standard [EN 50067] and the RBDS standard [NRSC-4].
Note that the RBDS standard as is used in the USA is almost identical to the RDS standard. Any RDS decoder can also handle RBDS. Only some of the fields have slightly different meanings. See the RBDS standard for more information.
The RBDS standard also specifies support for MMBS (Modified Mobile Search). This is a proprietary format which seems to be discontinued. The RDS interface does not support this format. Should support for MMBS (or the so-called 'E blocks' in general) be needed, then please contact the linux-media mailing list: https://linuxtv.org/lists.php.
Devices supporting the RDS capturing API set the V4L2_CAP_RDS_CAPTURE flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. Any tuner that supports RDS will set the V4L2_TUNER_CAP_RDS flag in the capability field of struct v4l2_tuner. Whether an RDS signal is present can be detected by looking at the rxsubchans field of struct v4l2_tuner: the V4L2_TUNER_SUB_RDS will be set if RDS data was detected.
Devices supporting the RDS output API set the V4L2_CAP_RDS_OUTPUT flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. Any modulator that supports RDS will set the V4L2_TUNER_CAP_RDS flag in the capability field of struct v4l2_modulator. In order to enable the RDS transmission one must set the V4L2_TUNER_SUB_RDS bit in the txsubchans field of struct v4l2_modulator.
RDS data can be read from the radio device with the read() function. The data is packed in groups of three bytes, as follows:
Table 4.15. struct v4l2_rds_data
| __u8 | lsb |
Least Significant Byte of RDS Block |
| __u8 | msb |
Most Significant Byte of RDS Block |
| __u8 | block |
Block description |
Table 4.16. Block description
| Bits 0-2 | Block (aka offset) of the received data. |
| Bits 3-5 | Deprecated. Currently identical to bits 0-2. Do not use these bits. |
| Bit 6 | Corrected bit. Indicates that an error was corrected for this data block. |
| Bit 7 | Error bit. Indicates that an uncorrectable error occurred during reception of this block. |
Table 4.17. Block defines
| V4L2_RDS_BLOCK_MSK | 7 | Mask for bits 0-2 to get the block ID. |
| V4L2_RDS_BLOCK_A | 0 | Block A. |
| V4L2_RDS_BLOCK_B | 1 | Block B. |
| V4L2_RDS_BLOCK_C | 2 | Block C. |
| V4L2_RDS_BLOCK_D | 3 | Block D. |
| V4L2_RDS_BLOCK_C_ALT | 4 | Block C'. |
| V4L2_RDS_BLOCK_INVALID | 7 | An invalid block. |
| V4L2_RDS_BLOCK_CORRECTED | 0x40 | A bit error was detected but corrected. |
| V4L2_RDS_BLOCK_ERROR | 0x80 | An incorrectable error occurred. |
[20] A common application of two file descriptors is the XFree86 Xv/V4L interface driver and a V4L2 application. While the X server controls video overlay, the application can take advantage of memory mapping and DMA.
In the opinion of the designers of this API, no driver writer taking the efforts to support simultaneous capturing and overlay will restrict this ability by requiring a single file descriptor, as in V4L and earlier versions of V4L2. Making this optional means applications depending on two file descriptors need backup routines to be compatible with all drivers, which is considerable more work than using two fds in applications which do not. Also two fd's fit the general concept of one file descriptor for each logical stream. Hence as a complexity trade-off drivers must support two file descriptors and may support single fd operation.
[21] The X Window system defines "regions" which are vectors of struct BoxRec { short x1, y1, x2, y2; } with width = x2 - x1 and height = y2 - y1, so one cannot pass X11 clip lists directly.
[22] ASK: Amplitude-Shift Keying. A high signal level represents a '1' bit, a low level a '0' bit.
Table of Contents
- V4L2 close() — Close a V4L2 device
- V4L2 ioctl() — Program a V4L2 device
- ioctl VIDIOC_CROPCAP — Information about the video cropping and scaling abilities
- ioctl VIDIOC_DBG_G_CHIP_IDENT — Identify the chips on a TV card
- ioctl VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER — Read or write hardware registers
- ioctl VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD — Execute an encoder command
- ioctl VIDIOC_ENUMAUDIO — Enumerate audio inputs
- ioctl VIDIOC_ENUMAUDOUT — Enumerate audio outputs
- ioctl VIDIOC_ENUM_FMT — Enumerate image formats
- ioctl VIDIOC_ENUM_FRAMESIZES — Enumerate frame sizes
- ioctl VIDIOC_ENUM_FRAMEINTERVALS — Enumerate frame intervals
- ioctl VIDIOC_ENUMINPUT — Enumerate video inputs
- ioctl VIDIOC_ENUMOUTPUT — Enumerate video outputs
- ioctl VIDIOC_ENUMSTD — Enumerate supported video standards
- ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO — Query or select the current audio input and its attributes
- ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT — Query or select the current audio output
- ioctl VIDIOC_G_CROP, VIDIOC_S_CROP — Get or set the current cropping rectangle
- ioctl VIDIOC_G_CTRL, VIDIOC_S_CTRL — Get or set the value of a control
- ioctl VIDIOC_G_ENC_INDEX — Get meta data about a compressed video stream
- ioctl VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS, VIDIOC_TRY_EXT_CTRLS — Get or set the value of several controls, try control values
- ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF — Get or set frame buffer overlay parameters
- ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT — Get or set the data format, try a format
- ioctl VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY — Get or set tuner or modulator radio frequency
- ioctl VIDIOC_G_INPUT, VIDIOC_S_INPUT — Query or select the current video input
- ioctl VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP
- ioctl VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR — Get or set modulator attributes
- ioctl VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT — Query or select the current video output
- ioctl VIDIOC_G_PARM, VIDIOC_S_PARM — Get or set streaming parameters
- ioctl VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY — Query or request the access priority associated with a file descriptor
- ioctl VIDIOC_G_SLICED_VBI_CAP — Query sliced VBI capabilities
- ioctl VIDIOC_G_STD, VIDIOC_S_STD — Query or select the video standard of the current input
- ioctl VIDIOC_G_TUNER, VIDIOC_S_TUNER — Get or set tuner attributes
- ioctl VIDIOC_LOG_STATUS — Log driver status information
- ioctl VIDIOC_OVERLAY — Start or stop video overlay
- ioctl VIDIOC_QBUF, VIDIOC_DQBUF — Exchange a buffer with the driver
- ioctl VIDIOC_QUERYBUF — Query the status of a buffer
- ioctl VIDIOC_QUERYCAP — Query device capabilities
- ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU — Enumerate controls and menu control items
- ioctl VIDIOC_QUERYSTD — Sense the video standard received by the current input
- ioctl VIDIOC_REQBUFS — Initiate Memory Mapping or User Pointer I/O
- ioctl VIDIOC_S_HW_FREQ_SEEK — Perform a hardware frequency seek
- ioctl VIDIOC_STREAMON, VIDIOC_STREAMOFF — Start or stop streaming I/O
- V4L2 mmap() — Map device memory into application address space
- V4L2 munmap() — Unmap device memory
- V4L2 open() — Open a V4L2 device
- V4L2 poll() — Wait for some event on a file descriptor
- V4L2 read() — Read from a V4L2 device
- V4L2 select() — Synchronous I/O multiplexing
- V4L2 write() — Write to a V4L2 device
Name
v4l2-close — Close a V4L2 device
Synopsis
#include <unistd.h>
int close( |
int | fd); |
Name
v4l2-ioctl — Program a V4L2 device
Synopsis
#include <sys/ioctl.h>
int ioctl( |
int | fd, |
| int | request, | |
| void * | argp); |
Arguments
-
fd -
File descriptor returned by
open(). -
request -
V4L2 ioctl request code as defined in the videodev.h header file, for example VIDIOC_QUERYCAP.
-
argp -
Pointer to a function parameter, usually a structure.
Description
The ioctl() function is used to program V4L2 devices. The argument fd must be an open file descriptor. An ioctl request has encoded in it whether the argument is an input, output or read/write parameter, and the size of the argument argp in bytes. Macros and defines specifying V4L2 ioctl requests are located in the videodev.h header file. Applications should use their own copy, not include the version in the kernel sources on the system they compile on. All V4L2 ioctl requests, their respective function and parameters are specified in Function Reference.
Return Value
On success the ioctl() function returns 0 and does not reset the errno variable. On failure -1 is returned, when the ioctl takes an output or read/write parameter it remains unmodified, and the errno variable is set appropriately. See below for possible error codes. Generic errors like EBADF or EFAULT are not listed in the sections discussing individual ioctl requests.
Note ioctls may return undefined error codes. Since errors may have side effects such as a driver reset applications should abort on unexpected errors.
- EBADF
-
fdis not a valid open file descriptor. - EBUSY
-
The property cannot be changed right now. Typically this error code is returned when I/O is in progress or the driver supports multiple opens and another process locked the property.
- EFAULT
-
argpreferences an inaccessible memory area. - ENOTTY
-
fdis not associated with a character special device. - EINVAL
-
The
requestor the data pointed to byargpis not valid. This is a very common error code, see the individual ioctl requests listed in Function Reference for actual causes. - ENOMEM
-
Not enough physical or virtual memory was available to complete the request.
- ERANGE
-
The application attempted to set a control with the
VIDIOC_S_CTRLioctl to a value which is out of bounds.
Name
VIDIOC_CROPCAP — Information about the video cropping and scaling abilities
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_cropcap * | argp); |
Description
Applications use this function to query the cropping limits, the pixel aspect of images and to calculate scale factors. They set the type field of a v4l2_cropcap structure to the respective buffer (stream) type and call the VIDIOC_CROPCAP ioctl with a pointer to this structure. Drivers fill the rest of the structure. The results are constant except when switching the video standard. Remember this switch can occur implicit when switching the video input or output.
Table 39. struct v4l2_cropcap
| enum v4l2_buf_type | type |
Type of the data stream, set by the application. Only these types are valid here: V4L2_BUF_TYPE_VIDEO_CAPTURE, V4L2_BUF_TYPE_VIDEO_OUTPUT, V4L2_BUF_TYPE_VIDEO_OVERLAY, and custom (driver defined) types with code V4L2_BUF_TYPE_PRIVATE and higher. |
| struct v4l2_rect | bounds |
Defines the window within capturing or output is possible, this may exclude for example the horizontal and vertical blanking areas. The cropping rectangle cannot exceed these limits. Width and height are defined in pixels, the driver writer is free to choose origin and units of the coordinate system in the analog domain. |
| struct v4l2_rect | defrect |
Default cropping rectangle, it shall cover the "whole picture". Assuming pixel aspect 1/1 this could be for example a 640 × 480 rectangle for NTSC, a 768 × 576 rectangle for PAL and SECAM centered over the active picture area. The same co-ordinate system as for bounds is used. |
| struct v4l2_fract | pixelaspect |
This is the pixel aspect (y / x) when no scaling is applied, the ratio of the actual sampling frequency and the frequency required to get square pixels. When cropping coordinates refer to square pixels, the driver sets |
Table 40. struct v4l2_rect
| __s32 | left |
Horizontal offset of the top, left corner of the rectangle, in pixels. |
| __s32 | top |
Vertical offset of the top, left corner of the rectangle, in pixels. |
| __s32 | width |
Width of the rectangle, in pixels. |
| __s32 | height |
Height of the rectangle, in pixels. Width and height cannot be negative, the fields are signed for hysterical reasons. |
Return Value
On success 0 is returned, on error -1 and the errno variable is set appropriately:
- EINVAL
-
The struct v4l2_cropcap
typeis invalid or the ioctl is not supported. This is not permitted for video capture, output and overlay devices, which must supportVIDIOC_CROPCAP.
Name
VIDIOC_DBG_G_CHIP_IDENT — Identify the chips on a TV card
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_dbg_chip_ident * | argp); |
Description
Experimental
This is an experimental interface and may change in the future.
For driver debugging purposes this ioctl allows test applications to query the driver about the chips present on the TV card. Regular applications must not use it. When you found a chip specific bug, please contact the linux-media mailing list (https://linuxtv.org/lists.php) so it can be fixed.
To query the driver applications must initialize the match.type and match.addr or match.name fields of a struct v4l2_dbg_chip_ident and call VIDIOC_DBG_G_CHIP_IDENT with a pointer to this structure. On success the driver stores information about the selected chip in the ident and revision fields. On failure the structure remains unchanged.
When match.type is V4L2_CHIP_MATCH_HOST, match.addr selects the nth non-I2C chip on the TV card. You can enumerate all chips by starting at zero and incrementing match.addr by one until VIDIOC_DBG_G_CHIP_IDENT fails with an EINVAL error code. The number zero always selects the host chip, e. g. the chip connected to the PCI or USB bus.
When match.type is V4L2_CHIP_MATCH_I2C_DRIVER, match.name contains the I2C driver name. For instance "saa7127" will match any chip supported by the saa7127 driver, regardless of its I2C bus address. When multiple chips supported by the same driver are present, the ioctl will return V4L2_IDENT_AMBIGUOUS in the ident field.
When match.type is V4L2_CHIP_MATCH_I2C_ADDR, match.addr selects a chip by its 7 bit I2C bus address.
When match.type is V4L2_CHIP_MATCH_AC97, match.addr selects the nth AC97 chip on the TV card. You can enumerate all chips by starting at zero and incrementing match.addr by one until VIDIOC_DBG_G_CHIP_IDENT fails with an EINVAL error code.
On success, the ident field will contain a chip ID from the Linux media/v4l2-chip-ident.h header file, and the revision field will contain a driver specific value, or zero if no particular revision is associated with this chip.
When the driver could not identify the selected chip, ident will contain V4L2_IDENT_UNKNOWN. When no chip matched the ioctl will succeed but the ident field will contain V4L2_IDENT_NONE. If multiple chips matched, ident will contain V4L2_IDENT_AMBIGUOUS. In all these cases the revision field remains unchanged.
This ioctl is optional, not all drivers may support it. It was introduced in Linux 2.6.21, but the API was changed to the one described here in 2.6.29.
We recommended the v4l2-dbg utility over calling this ioctl directly. It is available from the LinuxTV v4l-dvb repository; see https://linuxtv.org/repo/ for access instructions.
Table 41. struct v4l2_dbg_match
| __u32 | type |
See Table 43, 「Chip Match Types」 for a list of possible types. | |
| union | (anonymous) | ||
| __u32 | addr |
Match a chip by this number, interpreted according to the type field. |
|
| char | name[32] |
Match a chip by this name, interpreted according to the type field. |
Table 42. struct v4l2_dbg_chip_ident
| struct v4l2_dbg_match | match |
How to match the chip, see Table 41, 「struct v4l2_dbg_match」. |
| __u32 | ident |
A chip identifier as defined in the Linux media/v4l2-chip-ident.h header file, or one of the values from Table 44, 「Chip Identifiers」. |
| __u32 | revision |
A chip revision, chip and driver specific. |
Table 43. Chip Match Types
V4L2_CHIP_MATCH_HOST |
0 | Match the nth chip on the card, zero for the host chip. Does not match I2C chips. |
V4L2_CHIP_MATCH_I2C_DRIVER |
1 | Match an I2C chip by its driver name. |
V4L2_CHIP_MATCH_I2C_ADDR |
2 | Match a chip by its 7 bit I2C bus address. |
V4L2_CHIP_MATCH_AC97 |
3 | Match the nth anciliary AC97 chip. |
Name
VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER — Read or write hardware registers
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_dbg_register * | argp); |
int ioctl( |
int | fd, |
| int | request, | |
| const struct v4l2_dbg_register * | argp); |
Arguments
-
fd -
File descriptor returned by
open(). -
request -
VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER
-
argp
Description
Experimental
This is an experimental interface and may change in the future.
For driver debugging purposes these ioctls allow test applications to access hardware registers directly. Regular applications must not use them.
Since writing or even reading registers can jeopardize the system security, its stability and damage the hardware, both ioctls require superuser privileges. Additionally the Linux kernel must be compiled with the CONFIG_VIDEO_ADV_DEBUG option to enable these ioctls.
To write a register applications must initialize all fields of a struct v4l2_dbg_register and call VIDIOC_DBG_S_REGISTER with a pointer to this structure. The match.type and match.addr or match.name fields select a chip on the TV card, the reg field specifies a register number and the val field the value to be written into the register.
To read a register applications must initialize the match.type, match.chip or match.name and reg fields, and call VIDIOC_DBG_G_REGISTER with a pointer to this structure. On success the driver stores the register value in the val field. On failure the structure remains unchanged.
When match.type is V4L2_CHIP_MATCH_HOST, match.addr selects the nth non-I2C chip on the TV card. The number zero always selects the host chip, e. g. the chip connected to the PCI or USB bus. You can find out which chips are present with the VIDIOC_DBG_G_CHIP_IDENT ioctl.
When match.type is V4L2_CHIP_MATCH_I2C_DRIVER, match.name contains the I2C driver name. For instance "saa7127" will match any chip supported by the saa7127 driver, regardless of its I2C bus address. When multiple chips supported by the same driver are present, the effect of these ioctls is undefined. Again with the VIDIOC_DBG_G_CHIP_IDENT ioctl you can find out which I2C chips are present.
When match.type is V4L2_CHIP_MATCH_I2C_ADDR, match.addr selects a chip by its 7 bit I2C bus address.
When match.type is V4L2_CHIP_MATCH_AC97, match.addr selects the nth AC97 chip on the TV card.
Success not guaranteed
Due to a flaw in the Linux I2C bus driver these ioctls may return successfully without actually reading or writing a register. To catch the most likely failure we recommend a VIDIOC_DBG_G_CHIP_IDENT call confirming the presence of the selected I2C chip.
These ioctls are optional, not all drivers may support them. However when a driver supports these ioctls it must also support VIDIOC_DBG_G_CHIP_IDENT. Conversely it may support VIDIOC_DBG_G_CHIP_IDENT but not these ioctls.
VIDIOC_DBG_G_REGISTER and VIDIOC_DBG_S_REGISTER were introduced in Linux 2.6.21, but their API was changed to the one described here in kernel 2.6.29.
We recommended the v4l2-dbg utility over calling these ioctls directly. It is available from the LinuxTV v4l-dvb repository; see https://linuxtv.org/repo/ for access instructions.
Table 45. struct v4l2_dbg_match
| __u32 | type |
See Table 43, 「Chip Match Types」 for a list of possible types. | |
| union | (anonymous) | ||
| __u32 | addr |
Match a chip by this number, interpreted according to the type field. |
|
| char | name[32] |
Match a chip by this name, interpreted according to the type field. |
Table 46. struct v4l2_dbg_register
| struct v4l2_dbg_match | match |
How to match the chip, see Table 45, 「struct v4l2_dbg_match」. | |
| __u64 | reg |
A register number. | |
| __u64 | val |
The value read from, or to be written into the register. |
Table 47. Chip Match Types
V4L2_CHIP_MATCH_HOST |
0 | Match the nth chip on the card, zero for the host chip. Does not match I2C chips. |
V4L2_CHIP_MATCH_I2C_DRIVER |
1 | Match an I2C chip by its driver name. |
V4L2_CHIP_MATCH_I2C_ADDR |
2 | Match a chip by its 7 bit I2C bus address. |
V4L2_CHIP_MATCH_AC97 |
3 | Match the nth anciliary AC97 chip. |
Return Value
On success 0 is returned, on error -1 and the errno variable is set appropriately:
- EINVAL
-
The driver does not support this ioctl, or the kernel was not compiled with the
CONFIG_VIDEO_ADV_DEBUGoption, or thematch_typeis invalid, or the selected chip or register does not exist. - EPERM
-
Insufficient permissions. Root privileges are required to execute these ioctls.
Name
VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD — Execute an encoder command
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_encoder_cmd * | argp); |
Arguments
-
fd -
File descriptor returned by
open(). -
request -
VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD
-
argp
Description
Experimental
This is an experimental interface and may change in the future.
These ioctls control an audio/video (usually MPEG-) encoder. VIDIOC_ENCODER_CMD sends a command to the encoder, VIDIOC_TRY_ENCODER_CMD can be used to try a command without actually executing it.
To send a command applications must initialize all fields of a struct v4l2_encoder_cmd and call VIDIOC_ENCODER_CMD or VIDIOC_TRY_ENCODER_CMD with a pointer to this structure.
The cmd field must contain the command code. The flags field is currently only used by the STOP command and contains one bit: If the V4L2_ENC_CMD_STOP_AT_GOP_END flag is set, encoding will continue until the end of the current Group Of Pictures, otherwise it will stop immediately.
A read() call sends a START command to the encoder if it has not been started yet. After a STOP command, read() calls will read the remaining data buffered by the driver. When the buffer is empty, read() will return zero and the next read() call will restart the encoder.
A close() call sends an immediate STOP to the encoder, and all buffered data is discarded.
These ioctls are optional, not all drivers may support them. They were introduced in Linux 2.6.21.
Table 48. struct v4l2_encoder_cmd
| __u32 | cmd |
The encoder command, see Table 49, 「Encoder Commands」. |
| __u32 | flags |
Flags to go with the command, see Table 50, 「Encoder Command Flags」. If no flags are defined for this command, drivers and applications must set this field to zero. |
| __u32 | data[8] |
Reserved for future extensions. Drivers and applications must set the array to zero. |
Table 49. Encoder Commands
V4L2_ENC_CMD_START |
0 | Start the encoder. When the encoder is already running or paused, this command does nothing. No flags are defined for this command. |
V4L2_ENC_CMD_STOP |
1 | Stop the encoder. When the V4L2_ENC_CMD_STOP_AT_GOP_END flag is set, encoding will continue until the end of the current Group Of Pictures, otherwise encoding will stop immediately. When the encoder is already stopped, this command does nothing. |
V4L2_ENC_CMD_PAUSE |
2 | Pause the encoder. When the encoder has not been started yet, the driver will return an EPERM error code. When the encoder is already paused, this command does nothing. No flags are defined for this command. |
V4L2_ENC_CMD_RESUME |
3 | Resume encoding after a PAUSE command. When the encoder has not been started yet, the driver will return an EPERM error code. When the encoder is already running, this command does nothing. No flags are defined for this command. |
Name
VIDIOC_ENUMAUDIO — Enumerate audio inputs
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_audio * | argp); |
Description
To query the attributes of an audio input applications initialize the index field and zero out the reserved array of a struct v4l2_audio and call the VIDIOC_ENUMAUDIO ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all audio inputs applications shall begin at index zero, incrementing by one until the driver returns EINVAL.
See ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO(2) for a description of struct v4l2_audio.
Name
VIDIOC_ENUMAUDOUT — Enumerate audio outputs
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_audioout * | argp); |
Description
To query the attributes of an audio output applications initialize the index field and zero out the reserved array of a struct v4l2_audioout and call the VIDIOC_G_AUDOUT ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all audio outputs applications shall begin at index zero, incrementing by one until the driver returns EINVAL.
Note connectors on a TV card to loop back the received audio signal to a sound card are not audio outputs in this sense.
See ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT(2) for a description of struct v4l2_audioout.
Name
VIDIOC_ENUM_FMT — Enumerate image formats
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_fmtdesc * | argp); |
Description
To enumerate image formats applications initialize the type and index field of struct v4l2_fmtdesc and call the VIDIOC_ENUM_FMT ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code. All formats are enumerable by beginning at index zero and incrementing by one until EINVAL is returned.
Table 51. struct v4l2_fmtdesc
| __u32 | index |
Number of the format in the enumeration, set by the application. This is in no way related to the pixelformat field. |
| enum v4l2_buf_type | type |
Type of the data stream, set by the application. Only these types are valid here: V4L2_BUF_TYPE_VIDEO_CAPTURE, V4L2_BUF_TYPE_VIDEO_OUTPUT, V4L2_BUF_TYPE_VIDEO_OVERLAY, and custom (driver defined) types with code V4L2_BUF_TYPE_PRIVATE and higher. |
| __u32 | flags |
See Table 52, 「Image Format Description Flags」 |
| __u8 | description[32] |
Description of the format, a NUL-terminated ASCII string. This information is intended for the user, for example: "YUV 4:2:2". |
| __u32 | pixelformat |
The image format identifier. This is a four character code as computed by the v4l2_fourcc() macro: |
#define v4l2_fourcc(a,b,c,d) (((__u32)(a)<<0)|((__u32)(b)<<8)|((__u32)(c)<<16)|((__u32)(d)<<24)) Several image formats are already defined by this specification in Chapter 2, Image Formats. Note these codes are not the same as those used in the Windows world. |
||
| __u32 | reserved[4] |
Reserved for future extensions. Drivers must set the array to zero. |
Return Value
On success 0 is returned, on error -1 and the errno variable is set appropriately:
- EINVAL
-
The struct v4l2_fmtdesc
typeis not supported or theindexis out of bounds.
Name
VIDIOC_ENUM_FRAMESIZES — Enumerate frame sizes
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_frmsizeenum * | argp); |
Arguments
-
fd -
File descriptor returned by
open(). -
request -
VIDIOC_ENUM_FRAMESIZES
-
argp -
Pointer to a struct v4l2_frmsizeenum that contains an index and pixel format and receives a frame width and height.
Description
Experimental
This is an experimental interface and may change in the future.
This ioctl allows applications to enumerate all frame sizes (i. e. width and height in pixels) that the device supports for the given pixel format.
The supported pixel formats can be obtained by using the VIDIOC_ENUM_FMT function.
The return value and the content of the v4l2_frmsizeenum.type field depend on the type of frame sizes the device supports. Here are the semantics of the function for the different cases:
-
Discrete: The function returns success if the given index value (zero-based) is valid. The application should increase the index by one for each call until
EINVALis returned. Thev4l2_frmsizeenum.typefield is set toV4L2_FRMSIZE_TYPE_DISCRETEby the driver. Of the union only thediscretemember is valid. -
Step-wise: The function returns success if the given index value is zero and
EINVALfor any other index value. Thev4l2_frmsizeenum.typefield is set toV4L2_FRMSIZE_TYPE_STEPWISEby the driver. Of the union only thestepwisemember is valid. -
Continuous: This is a special case of the step-wise type above. The function returns success if the given index value is zero and
EINVALfor any other index value. Thev4l2_frmsizeenum.typefield is set toV4L2_FRMSIZE_TYPE_CONTINUOUSby the driver. Of the union only thestepwisemember is valid and thestep_widthandstep_heightvalues are set to 1.
When the application calls the function with index zero, it must check the type field to determine the type of frame size enumeration the device supports. Only for the V4L2_FRMSIZE_TYPE_DISCRETE type does it make sense to increase the index value to receive more frame sizes.
Note that the order in which the frame sizes are returned has no special meaning. In particular does it not say anything about potential default format sizes.
Applications can assume that the enumeration data does not change without any interaction from the application itself. This means that the enumeration data is consistent if the application does not perform any other ioctl calls while it runs the frame size enumeration.
Structs
In the structs below, IN denotes a value that has to be filled in by the application, OUT denotes values that the driver fills in. The application should zero out all members except for the IN fields.
Table 53. struct v4l2_frmsize_discrete
| __u32 | width |
Width of the frame [pixel]. |
| __u32 | height |
Height of the frame [pixel]. |
Table 54. struct v4l2_frmsize_stepwise
| __u32 | min_width |
Minimum frame width [pixel]. |
| __u32 | max_width |
Maximum frame width [pixel]. |
| __u32 | step_width |
Frame width step size [pixel]. |
| __u32 | min_height |
Minimum frame height [pixel]. |
| __u32 | max_height |
Maximum frame height [pixel]. |
| __u32 | step_height |
Frame height step size [pixel]. |
Table 55. struct v4l2_frmsizeenum
| __u32 | index |
IN: Index of the given frame size in the enumeration. | |
| __u32 | pixel_format |
IN: Pixel format for which the frame sizes are enumerated. | |
| __u32 | type |
OUT: Frame size type the device supports. | |
| union | OUT: Frame size with the given index. | ||
| struct v4l2_frmsize_discrete | discrete |
||
| struct v4l2_frmsize_stepwise | stepwise |
||
| __u32 | reserved[2] |
Reserved space for future use. |
Name
VIDIOC_ENUM_FRAMEINTERVALS — Enumerate frame intervals
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_frmivalenum * | argp); |
Arguments
-
fd -
File descriptor returned by
open(). -
request -
VIDIOC_ENUM_FRAMEINTERVALS
-
argp -
Pointer to a struct v4l2_frmivalenum structure that contains a pixel format and size and receives a frame interval.
Description
This ioctl allows applications to enumerate all frame intervals that the device supports for the given pixel format and frame size.
The supported pixel formats and frame sizes can be obtained by using the VIDIOC_ENUM_FMT and VIDIOC_ENUM_FRAMESIZES functions.
The return value and the content of the v4l2_frmivalenum.type field depend on the type of frame intervals the device supports. Here are the semantics of the function for the different cases:
-
Discrete: The function returns success if the given index value (zero-based) is valid. The application should increase the index by one for each call until
EINVALis returned. The `v4l2_frmivalenum.type` field is set to `V4L2_FRMIVAL_TYPE_DISCRETE` by the driver. Of the union only the `discrete` member is valid. -
Step-wise: The function returns success if the given index value is zero and
EINVALfor any other index value. Thev4l2_frmivalenum.typefield is set toV4L2_FRMIVAL_TYPE_STEPWISEby the driver. Of the union only thestepwisemember is valid. -
Continuous: This is a special case of the step-wise type above. The function returns success if the given index value is zero and
EINVALfor any other index value. Thev4l2_frmivalenum.typefield is set toV4L2_FRMIVAL_TYPE_CONTINUOUSby the driver. Of the union only thestepwisemember is valid and thestepvalue is set to 1.
When the application calls the function with index zero, it must check the type field to determine the type of frame interval enumeration the device supports. Only for the V4L2_FRMIVAL_TYPE_DISCRETE type does it make sense to increase the index value to receive more frame intervals.
Note that the order in which the frame intervals are returned has no special meaning. In particular does it not say anything about potential default frame intervals.
Applications can assume that the enumeration data does not change without any interaction from the application itself. This means that the enumeration data is consistent if the application does not perform any other ioctl calls while it runs the frame interval enumeration.
Notes
-
Frame intervals and frame rates: The V4L2 API uses frame intervals instead of frame rates. Given the frame interval the frame rate can be computed as follows:
frame_rate = 1 / frame_interval
Structs
In the structs below, IN denotes a value that has to be filled in by the application, OUT denotes values that the driver fills in. The application should zero out all members except for the IN fields.
Table 57. struct v4l2_frmival_stepwise
| struct v4l2_fract | min |
Minimum frame interval [s]. |
| struct v4l2_fract | max |
Maximum frame interval [s]. |
| struct v4l2_fract | step |
Frame interval step size [s]. |
Table 58. struct v4l2_frmivalenum
| __u32 | index |
IN: Index of the given frame interval in the enumeration. | |
| __u32 | pixel_format |
IN: Pixel format for which the frame intervals are enumerated. | |
| __u32 | width |
IN: Frame width for which the frame intervals are enumerated. | |
| __u32 | height |
IN: Frame height for which the frame intervals are enumerated. | |
| __u32 | type |
OUT: Frame interval type the device supports. | |
| union | OUT: Frame interval with the given index. | ||
| struct v4l2_fract | discrete |
Frame interval [s]. | |
| struct v4l2_frmival_stepwise | stepwise |
||
| __u32 | reserved[2] |
Reserved space for future use. |
Name
VIDIOC_ENUMINPUT — Enumerate video inputs
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_input * | argp); |
Description
To query the attributes of a video input applications initialize the index field of struct v4l2_input and call the VIDIOC_ENUMINPUT ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all inputs applications shall begin at index zero, incrementing by one until the driver returns EINVAL.
Table 60. struct v4l2_input
| __u32 | index |
Identifies the input, set by the application. |
| __u8 | name[32] |
Name of the video input, a NUL-terminated ASCII string, for example: "Vin (Composite 2)". This information is intended for the user, preferably the connector label on the device itself. |
| __u32 | type |
Type of the input, see Table 61, 「Input Types」. |
| __u32 | audioset |
Drivers can enumerate up to 32 video and audio inputs. This field shows which audio inputs were selectable as audio source if this was the currently selected video input. It is a bit mask. The LSB corresponds to audio input 0, the MSB to input 31. Any number of bits can be set, or none. When the driver does not enumerate audio inputs no bits must be set. Applications shall not interpret this as lack of audio support. Some drivers automatically select audio sources and do not enumerate them since there is no choice anyway. For details on audio inputs and how to select the current input see Section 1.5, 「Audio Inputs and Outputs」. |
| __u32 | tuner |
Capture devices can have zero or more tuners (RF demodulators). When the type is set to V4L2_INPUT_TYPE_TUNER this is an RF connector and this field identifies the tuner. It corresponds to struct v4l2_tuner field index. For details on tuners see Section 1.6, 「Tuners and Modulators」. |
| v4l2_std_id | std |
Every video input supports one or more different video standards. This field is a set of all supported standards. For details on video standards and how to switch see Section 1.7, 「Video Standards」. |
| __u32 | status |
This field provides status information about the input. See Table 62, 「Input Status Flags」 for flags. With the exception of the sensor orientation bits status is only valid when this is the current input. |
| __u32 | reserved[4] |
Reserved for future extensions. Drivers must set the array to zero. |
Table 61. Input Types
V4L2_INPUT_TYPE_TUNER |
1 | This input uses a tuner (RF demodulator). |
V4L2_INPUT_TYPE_CAMERA |
2 | Analog baseband input, for example CVBS / Composite Video, S-Video, RGB. |
Table 62. Input Status Flags
| General | ||
V4L2_IN_ST_NO_POWER |
0x00000001 | Attached device is off. |
V4L2_IN_ST_NO_SIGNAL |
0x00000002 | |
V4L2_IN_ST_NO_COLOR |
0x00000004 | The hardware supports color decoding, but does not detect color modulation in the signal. |
| Sensor Orientation | ||
V4L2_IN_ST_HFLIP |
0x00000010 | The input is connected to a device that produces a signal that is flipped horizontally and does not correct this before passing the signal to userspace. |
V4L2_IN_ST_VFLIP |
0x00000020 | The input is connected to a device that produces a signal that is flipped vertically and does not correct this before passing the signal to userspace. Note that a 180 degree rotation is the same as HFLIP | VFLIP |
| Analog Video | ||
V4L2_IN_ST_NO_H_LOCK |
0x00000100 | No horizontal sync lock. |
V4L2_IN_ST_COLOR_KILL |
0x00000200 | A color killer circuit automatically disables color decoding when it detects no color modulation. When this flag is set the color killer is enabled and has shut off color decoding. |
| Digital Video | ||
V4L2_IN_ST_NO_SYNC |
0x00010000 | No synchronization lock. |
V4L2_IN_ST_NO_EQU |
0x00020000 | No equalizer lock. |
V4L2_IN_ST_NO_CARRIER |
0x00040000 | Carrier recovery failed. |
| VCR and Set-Top Box | ||
V4L2_IN_ST_MACROVISION |
0x01000000 | Macrovision is an analog copy prevention system mangling the video signal to confuse video recorders. When this flag is set Macrovision has been detected. |
V4L2_IN_ST_NO_ACCESS |
0x02000000 | Conditional access denied. |
V4L2_IN_ST_VTR |
0x04000000 | VTR time constant. [?] |
Return Value
On success 0 is returned, on error -1 and the errno variable is set appropriately:
- EINVAL
-
The struct v4l2_input
indexis out of bounds.
Name
VIDIOC_ENUMOUTPUT — Enumerate video outputs
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_output * | argp); |
Description
To query the attributes of a video outputs applications initialize the index field of struct v4l2_output and call the VIDIOC_ENUMOUTPUT ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all outputs applications shall begin at index zero, incrementing by one until the driver returns EINVAL.
Table 63. struct v4l2_output
| __u32 | index |
Identifies the output, set by the application. |
| __u8 | name[32] |
Name of the video output, a NUL-terminated ASCII string, for example: "Vout". This information is intended for the user, preferably the connector label on the device itself. |
| __u32 | type |
Type of the output, see Table 64, 「Output Type」. |
| __u32 | audioset |
Drivers can enumerate up to 32 video and audio outputs. This field shows which audio outputs were selectable as the current output if this was the currently selected video output. It is a bit mask. The LSB corresponds to audio output 0, the MSB to output 31. Any number of bits can be set, or none. When the driver does not enumerate audio outputs no bits must be set. Applications shall not interpret this as lack of audio support. Drivers may automatically select audio outputs without enumerating them. For details on audio outputs and how to select the current output see Section 1.5, 「Audio Inputs and Outputs」. |
| __u32 | modulator |
Output devices can have zero or more RF modulators. When the type is V4L2_OUTPUT_TYPE_MODULATOR this is an RF connector and this field identifies the modulator. It corresponds to struct v4l2_modulator field index. For details on modulators see Section 1.6, 「Tuners and Modulators」. |
| v4l2_std_id | std |
Every video output supports one or more different video standards. This field is a set of all supported standards. For details on video standards and how to switch see Section 1.7, 「Video Standards」. |
| __u32 | reserved[4] |
Reserved for future extensions. Drivers must set the array to zero. |
Return Value
On success 0 is returned, on error -1 and the errno variable is set appropriately:
- EINVAL
-
The struct v4l2_output
indexis out of bounds.
Name
VIDIOC_ENUMSTD — Enumerate supported video standards
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_standard * | argp); |
Description
To query the attributes of a video standard, especially a custom (driver defined) one, applications initialize the index field of struct v4l2_standard and call the VIDIOC_ENUMSTD ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all standards applications shall begin at index zero, incrementing by one until the driver returns EINVAL. Drivers may enumerate a different set of standards after switching the video input or output.[23]
Table 65. struct v4l2_standard
| __u32 | index |
Number of the video standard, set by the application. |
| v4l2_std_id | id |
The bits in this field identify the standard as one of the common standards listed in Table 67, 「typedef v4l2_std_id」, or if bits 32 to 63 are set as custom standards. Multiple bits can be set if the hardware does not distinguish between these standards, however separate indices do not indicate the opposite. The id must be unique. No other enumerated v4l2_standard structure, for this input or output anyway, can contain the same set of bits. |
| __u8 | name[24] |
Name of the standard, a NUL-terminated ASCII string, for example: "PAL-B/G", "NTSC Japan". This information is intended for the user. |
| struct v4l2_fract | frameperiod |
The frame period (not field period) is numerator / denominator. For example M/NTSC has a frame period of 1001 / 30000 seconds. |
| __u32 | framelines |
Total lines per frame including blanking, e. g. 625 for B/PAL. |
| __u32 | reserved[4] |
Reserved for future extensions. Drivers must set the array to zero. |
Table 67. typedef v4l2_std_id
| __u64 | v4l2_std_id |
This type is a set, each bit representing another video standard as listed below and in Table 68, 「Video Standards (based on [])」. The 32 most significant bits are reserved for custom (driver defined) video standards. |
#define V4L2_STD_PAL_B ((v4l2_std_id)0x00000001) #define V4L2_STD_PAL_B1 ((v4l2_std_id)0x00000002) #define V4L2_STD_PAL_G ((v4l2_std_id)0x00000004) #define V4L2_STD_PAL_H ((v4l2_std_id)0x00000008) #define V4L2_STD_PAL_I ((v4l2_std_id)0x00000010) #define V4L2_STD_PAL_D ((v4l2_std_id)0x00000020) #define V4L2_STD_PAL_D1 ((v4l2_std_id)0x00000040) #define V4L2_STD_PAL_K ((v4l2_std_id)0x00000080) #define V4L2_STD_PAL_M ((v4l2_std_id)0x00000100) #define V4L2_STD_PAL_N ((v4l2_std_id)0x00000200) #define V4L2_STD_PAL_Nc ((v4l2_std_id)0x00000400) #define V4L2_STD_PAL_60 ((v4l2_std_id)0x00000800)
V4L2_STD_PAL_60 is a hybrid standard with 525 lines, 60 Hz refresh rate, and PAL color modulation with a 4.43 MHz color subcarrier. Some PAL video recorders can play back NTSC tapes in this mode for display on a 50/60 Hz agnostic PAL TV.
#define V4L2_STD_NTSC_M ((v4l2_std_id)0x00001000) #define V4L2_STD_NTSC_M_JP ((v4l2_std_id)0x00002000) #define V4L2_STD_NTSC_443 ((v4l2_std_id)0x00004000)
V4L2_STD_NTSC_443 is a hybrid standard with 525 lines, 60 Hz refresh rate, and NTSC color modulation with a 4.43 MHz color subcarrier.
#define V4L2_STD_NTSC_M_KR ((v4l2_std_id)0x00008000) #define V4L2_STD_SECAM_B ((v4l2_std_id)0x00010000) #define V4L2_STD_SECAM_D ((v4l2_std_id)0x00020000) #define V4L2_STD_SECAM_G ((v4l2_std_id)0x00040000) #define V4L2_STD_SECAM_H ((v4l2_std_id)0x00080000) #define V4L2_STD_SECAM_K ((v4l2_std_id)0x00100000) #define V4L2_STD_SECAM_K1 ((v4l2_std_id)0x00200000) #define V4L2_STD_SECAM_L ((v4l2_std_id)0x00400000) #define V4L2_STD_SECAM_LC ((v4l2_std_id)0x00800000) /* ATSC/HDTV */ #define V4L2_STD_ATSC_8_VSB ((v4l2_std_id)0x01000000) #define V4L2_STD_ATSC_16_VSB ((v4l2_std_id)0x02000000)
V4L2_STD_ATSC_8_VSB and V4L2_STD_ATSC_16_VSB are U.S. terrestrial digital TV standards. Presently the V4L2 API does not support digital TV. See also the Linux DVB API at https://linuxtv.org.
#define V4L2_STD_PAL_BG (V4L2_STD_PAL_B |\ V4L2_STD_PAL_B1 |\ V4L2_STD_PAL_G) #define V4L2_STD_B (V4L2_STD_PAL_B |\ V4L2_STD_PAL_B1 |\ V4L2_STD_SECAM_B) #define V4L2_STD_GH (V4L2_STD_PAL_G |\ V4L2_STD_PAL_H |\ V4L2_STD_SECAM_G |\ V4L2_STD_SECAM_H) #define V4L2_STD_PAL_DK (V4L2_STD_PAL_D |\ V4L2_STD_PAL_D1 |\ V4L2_STD_PAL_K) #define V4L2_STD_PAL (V4L2_STD_PAL_BG |\ V4L2_STD_PAL_DK |\ V4L2_STD_PAL_H |\ V4L2_STD_PAL_I) #define V4L2_STD_NTSC (V4L2_STD_NTSC_M |\ V4L2_STD_NTSC_M_JP |\ V4L2_STD_NTSC_M_KR) #define V4L2_STD_MN (V4L2_STD_PAL_M |\ V4L2_STD_PAL_N |\ V4L2_STD_PAL_Nc |\ V4L2_STD_NTSC) #define V4L2_STD_SECAM_DK (V4L2_STD_SECAM_D |\ V4L2_STD_SECAM_K |\ V4L2_STD_SECAM_K1) #define V4L2_STD_DK (V4L2_STD_PAL_DK |\ V4L2_STD_SECAM_DK) #define V4L2_STD_SECAM (V4L2_STD_SECAM_B |\ V4L2_STD_SECAM_G |\ V4L2_STD_SECAM_H |\ V4L2_STD_SECAM_DK |\ V4L2_STD_SECAM_L |\ V4L2_STD_SECAM_LC) #define V4L2_STD_525_60 (V4L2_STD_PAL_M |\ V4L2_STD_PAL_60 |\ V4L2_STD_NTSC |\ V4L2_STD_NTSC_443) #define V4L2_STD_625_50 (V4L2_STD_PAL |\ V4L2_STD_PAL_N |\ V4L2_STD_PAL_Nc |\ V4L2_STD_SECAM) #define V4L2_STD_UNKNOWN 0 #define V4L2_STD_ALL (V4L2_STD_525_60 |\ V4L2_STD_625_50)
Table 68. Video Standards (based on [[ITU BT.470]])
| Characteristics | M/NTSC[a] |
M/PAL | N/PAL[b] |
B, B1, G/PAL | D, D1, K/PAL | H/PAL | I/PAL | B, G/SECAM | D, K/SECAM | K1/SECAM | L/SECAM |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Frame lines | 525 | 625 | |||||||||
| Frame period (s) | 1001/30000 | 1/25 | |||||||||
| Chrominance sub-carrier frequency (Hz) | 3579545 ± 10 | 3579611.49 ± 10 | 4433618.75 ± 5 (3582056.25 ± 5) | 4433618.75 ± 5 | 4433618.75 ± 1 | fOR = 4406250 ± 2000, fOB = 4250000 ± 2000 | |||||
| Nominal radio-frequency channel bandwidth (MHz) | 6 | 6 | 6 | B: 7; B1, G: 8 | 8 | 8 | 8 | 8 | 8 | 8 | 8 |
| Sound carrier relative to vision carrier (MHz) | + 4.5 | + 4.5 | + 4.5 | + 6.5 ± 0.001 | + 5.5 | + 5.9996 ± 0.0005 | + 5.5 ± 0.001 | + 6.5 ± 0.001 | + 6.5 | + 6.5 [g] |
|
|
[a] Japan uses a standard similar to M/NTSC (V4L2_STD_NTSC_M_JP). [b] The values in brackets apply to the combination N/PAL a.k.a. NC used in Argentina (V4L2_STD_PAL_Nc). [c] In the Federal Republic of Germany, Austria, Italy, the Netherlands, Slovakia and Switzerland a system of two sound carriers is used, the frequency of the second carrier being 242.1875 kHz above the frequency of the first sound carrier. For stereophonic sound transmissions a similar system is used in Australia. [d] New Zealand uses a sound carrier displaced 5.4996 ± 0.0005 MHz from the vision carrier. [e] In Denmark, Finland, New Zealand, Sweden and Spain a system of two sound carriers is used. In Iceland, Norway and Poland the same system is being introduced. The second carrier is 5.85 MHz above the vision carrier and is DQPSK modulated with 728 kbit/s sound and data multiplex. (NICAM system) [f] In the United Kingdom, a system of two sound carriers is used. The second sound carrier is 6.552 MHz above the vision carrier and is DQPSK modulated with a 728 kbit/s sound and data multiplex able to carry two sound channels. (NICAM system) [g] In France, a digital carrier 5.85 MHz away from the vision carrier may be used in addition to the main sound carrier. It is modulated in differentially encoded QPSK with a 728 kbit/s sound and data multiplexer capable of carrying two sound channels. (NICAM system) |
|||||||||||
Return Value
On success 0 is returned, on error -1 and the errno variable is set appropriately:
- EINVAL
-
The struct v4l2_standard
indexis out of bounds.
[23] The supported standards may overlap and we need an unambiguous set to find the current standard returned by VIDIOC_G_STD.
Name
VIDIOC_G_AUDIO, VIDIOC_S_AUDIO — Query or select the current audio input and its attributes
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_audio * | argp); |
int ioctl( |
int | fd, |
| int | request, | |
| const struct v4l2_audio * | argp); |
Description
To query the current audio input applications zero out the reserved array of a struct v4l2_audio and call the VIDIOC_G_AUDIO ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the device has no audio inputs, or none which combine with the current video input.
Audio inputs have one writable property, the audio mode. To select the current audio input and change the audio mode, applications initialize the index and mode fields, and the reserved array of a v4l2_audio structure and call the VIDIOC_S_AUDIO ioctl. Drivers may switch to a different audio mode if the request cannot be satisfied. However, this is a write-only ioctl, it does not return the actual new audio mode.
Table 69. struct v4l2_audio
| __u32 | index |
Identifies the audio input, set by the driver or application. |
| __u8 | name[32] |
Name of the audio input, a NUL-terminated ASCII string, for example: "Line In". This information is intended for the user, preferably the connector label on the device itself. |
| __u32 | capability |
Audio capability flags, see Table 70, 「Audio Capability Flags」. |
| __u32 | mode |
Audio mode flags set by drivers and applications (on VIDIOC_S_AUDIO ioctl), see Table 71, 「Audio Mode Flags」. |
| __u32 | reserved[2] |
Reserved for future extensions. Drivers and applications must set the array to zero. |
Table 70. Audio Capability Flags
V4L2_AUDCAP_STEREO |
0x00001 | This is a stereo input. The flag is intended to automatically disable stereo recording etc. when the signal is always monaural. The API provides no means to detect if stereo is received, unless the audio input belongs to a tuner. |
V4L2_AUDCAP_AVL |
0x00002 | Automatic Volume Level mode is supported. |
Return Value
On success 0 is returned, on error -1 and the errno variable is set appropriately:
- EINVAL
-
No audio inputs combine with the current video input, or the number of the selected audio input is out of bounds or it does not combine, or there are no audio inputs at all and the ioctl is not supported.
- EBUSY
-
I/O is in progress, the input cannot be switched.
Name
VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT — Query or select the current audio output
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_audioout * | argp); |
int ioctl( |
int | fd, |
| int | request, | |
| const struct v4l2_audioout * | argp); |
Description
To query the current audio output applications zero out the reserved array of a struct v4l2_audioout and call the VIDIOC_G_AUDOUT ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the device has no audio inputs, or none which combine with the current video output.
Audio outputs have no writable properties. Nevertheless, to select the current audio output applications can initialize the index field and reserved array (which in the future may contain writable properties) of a v4l2_audioout structure and call the VIDIOC_S_AUDOUT ioctl. Drivers switch to the requested output or return the EINVAL error code when the index is out of bounds. This is a write-only ioctl, it does not return the current audio output attributes as VIDIOC_G_AUDOUT does.
Note connectors on a TV card to loop back the received audio signal to a sound card are not audio outputs in this sense.
Table 72. struct v4l2_audioout
| __u32 | index |
Identifies the audio output, set by the driver or application. |
| __u8 | name[32] |
Name of the audio output, a NUL-terminated ASCII string, for example: "Line Out". This information is intended for the user, preferably the connector label on the device itself. |
| __u32 | capability |
Audio capability flags, none defined yet. Drivers must set this field to zero. |
| __u32 | mode |
Audio mode, none defined yet. Drivers and applications (on VIDIOC_S_AUDOUT) must set this field to zero. |
| __u32 | reserved[2] |
Reserved for future extensions. Drivers and applications must set the array to zero. |
Return Value
On success 0 is returned, on error -1 and the errno variable is set appropriately:
- EINVAL
-
No audio outputs combine with the current video output, or the number of the selected audio output is out of bounds or it does not combine, or there are no audio outputs at all and the ioctl is not supported.
- EBUSY
-
I/O is in progress, the output cannot be switched.
Name
VIDIOC_G_CROP, VIDIOC_S_CROP — Get or set the current cropping rectangle
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_crop * | argp); |
int ioctl( |
int | fd, |
| int | request, | |
| const struct v4l2_crop * | argp); |
Description
To query the cropping rectangle size and position applications set the type field of a v4l2_crop structure to the respective buffer (stream) type and call the VIDIOC_G_CROP ioctl with a pointer to this structure. The driver fills the rest of the structure or returns the EINVAL error code if cropping is not supported.
To change the cropping rectangle applications initialize the type and struct v4l2_rect substructure named c of a v4l2_crop structure and call the VIDIOC_S_CROP ioctl with a pointer to this structure.
The driver first adjusts the requested dimensions against hardware limits, i. e. the bounds given by the capture/output window, and it rounds to the closest possible values of horizontal and vertical offset, width and height. In particular the driver must round the vertical offset of the cropping rectangle to frame lines modulo two, such that the field order cannot be confused.
Second the driver adjusts the image size (the opposite rectangle of the scaling process, source or target depending on the data direction) to the closest size possible while maintaining the current horizontal and vertical scaling factor.
Finally the driver programs the hardware with the actual cropping and image parameters. VIDIOC_S_CROP is a write-only ioctl, it does not return the actual parameters. To query them applications must call VIDIOC_G_CROP and VIDIOC_G_FMT. When the parameters are unsuitable the application may modify the cropping or image parameters and repeat the cycle until satisfactory parameters have been negotiated.
When cropping is not supported then no parameters are changed and VIDIOC_S_CROP returns the EINVAL error code.
Table 73. struct v4l2_crop
| enum v4l2_buf_type | type |
Type of the data stream, set by the application. Only these types are valid here: V4L2_BUF_TYPE_VIDEO_CAPTURE, V4L2_BUF_TYPE_VIDEO_OUTPUT, V4L2_BUF_TYPE_VIDEO_OVERLAY, and custom (driver defined) types with code V4L2_BUF_TYPE_PRIVATE and higher. |
| struct v4l2_rect | c |
Cropping rectangle. The same co-ordinate system as for struct v4l2_cropcap bounds is used. |
Name
VIDIOC_G_CTRL, VIDIOC_S_CTRL — Get or set the value of a control
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_control * | argp); |
Description
To get the current value of a control applications initialize the id field of a struct v4l2_control and call the VIDIOC_G_CTRL ioctl with a pointer to this structure. To change the value of a control applications initialize the id and value fields of a struct v4l2_control and call the VIDIOC_S_CTRL ioctl.
When the id is invalid drivers return an EINVAL error code. When the value is out of bounds drivers can choose to take the closest valid value or return an ERANGE error code, whatever seems more appropriate. However, VIDIOC_S_CTRL is a write-only ioctl, it does not return the actual new value.
These ioctls work only with user controls. For other control classes the VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS or VIDIOC_TRY_EXT_CTRLS must be used.
Return Value
On success 0 is returned, on error -1 and the errno variable is set appropriately:
- EINVAL
-
The struct v4l2_control
idis invalid. - ERANGE
-
The struct v4l2_control
valueis out of bounds. - EBUSY
-
The control is temporarily not changeable, possibly because another applications took over control of the device function this control belongs to.
Name
VIDIOC_G_ENC_INDEX — Get meta data about a compressed video stream
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_enc_idx * | argp); |
Description
Experimental
This is an experimental interface and may change in the future.
The VIDIOC_G_ENC_INDEX ioctl provides meta data about a compressed video stream the same or another application currently reads from the driver, which is useful for random access into the stream without decoding it.
To read the data applications must call VIDIOC_G_ENC_INDEX with a pointer to a struct v4l2_enc_idx. On success the driver fills the entry array, stores the number of elements written in the entries field, and initializes the entries_cap field.
Each element of the entry array contains meta data about one picture. A VIDIOC_G_ENC_INDEX call reads up to V4L2_ENC_IDX_ENTRIES entries from a driver buffer, which can hold up to entries_cap entries. This number can be lower or higher than V4L2_ENC_IDX_ENTRIES, but not zero. When the application fails to read the meta data in time the oldest entries will be lost. When the buffer is empty or no capturing/encoding is in progress, entries will be zero.
Currently this ioctl is only defined for MPEG-2 program streams and video elementary streams.
Table 75. struct v4l2_enc_idx
| __u32 | entries |
The number of entries the driver stored in the entry array. |
||
| __u32 | entries_cap |
The number of entries the driver can buffer. Must be greater than zero. | ||
| __u32 | reserved[4] |
Reserved for future extensions. Drivers must set the array to zero. | ||
| struct v4l2_enc_idx_entry | entry[V4L2_ENC_IDX_ENTRIES] |
Meta data about a compressed video stream. Each element of the array corresponds to one picture, sorted in ascending order by their offset. |
||
Table 76. struct v4l2_enc_idx_entry
| __u64 | offset |
The offset in bytes from the beginning of the compressed video stream to the beginning of this picture, that is a PES packet header as defined in [ISO 13818-1] or a picture header as defined in [ISO 13818-2]. When the encoder is stopped, the driver resets the offset to zero. |
| __u64 | pts |
The 33 bit Presentation Time Stamp of this picture as defined in [ISO 13818-1]. |
| __u32 | length |
The length of this picture in bytes. |
| __u32 | flags |
Flags containing the coding type of this picture, see Table 77, 「Index Entry Flags」. |
| __u32 | reserved[2] |
Reserved for future extensions. Drivers must set the array to zero. |
Table 77. Index Entry Flags
V4L2_ENC_IDX_FRAME_I |
0x00 | This is an Intra-coded picture. |
V4L2_ENC_IDX_FRAME_P |
0x01 | This is a Predictive-coded picture. |
V4L2_ENC_IDX_FRAME_B |
0x02 | This is a Bidirectionally predictive-coded picture. |
V4L2_ENC_IDX_FRAME_MASK |
0x0F | AND the flags field with this mask to obtain the picture coding type. |
Name
VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS, VIDIOC_TRY_EXT_CTRLS — Get or set the value of several controls, try control values
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_ext_controls * | argp); |
Arguments
-
fd -
File descriptor returned by
open(). -
request -
VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS, VIDIOC_TRY_EXT_CTRLS
-
argp
Description
These ioctls allow the caller to get or set multiple controls atomically. Control IDs are grouped into control classes (see Table 80, 「Control classes」) and all controls in the control array must belong to the same control class.
Applications must always fill in the count, ctrl_class, controls and reserved fields of struct v4l2_ext_controls, and initialize the struct v4l2_ext_control array pointed to by the controls fields.
To get the current value of a set of controls applications initialize the id, size and reserved2 fields of each struct v4l2_ext_control and call the VIDIOC_G_EXT_CTRLS ioctl. String controls controls must also set the string field.
If the size is too small to receive the control result (only relevant for pointer-type controls like strings), then the driver will set size to a valid value and return an ENOSPC error code. You should re-allocate the string memory to this new size and try again. It is possible that the same issue occurs again if the string has grown in the meantime. It is recommended to call VIDIOC_QUERYCTRL first and use maximum+1 as the new size value. It is guaranteed that that is sufficient memory.
To change the value of a set of controls applications initialize the id, size, reserved2 and value/string fields of each struct v4l2_ext_control and call the VIDIOC_S_EXT_CTRLS ioctl. The controls will only be set if all control values are valid.
To check if a set of controls have correct values applications initialize the id, size, reserved2 and value/string fields of each struct v4l2_ext_control and call the VIDIOC_TRY_EXT_CTRLS ioctl. It is up to the driver whether wrong values are automatically adjusted to a valid value or if an error is returned.
When the id or ctrl_class is invalid drivers return an EINVAL error code. When the value is out of bounds drivers can choose to take the closest valid value or return an ERANGE error code, whatever seems more appropriate. In the first case the new value is set in struct v4l2_ext_control.
The driver will only set/get these controls if all control values are correct. This prevents the situation where only some of the controls were set/get. Only low-level errors (e. g. a failed i2c command) can still cause this situation.
Table 78. struct v4l2_ext_control
| __u32 | id |
Identifies the control, set by the application. | |
| __u32 | size |
The total size in bytes of the payload of this control. This is normally 0, but for pointer controls this should be set to the size of the memory containing the payload, or that will receive the payload. If VIDIOC_G_EXT_CTRLS finds that this value is less than is required to store the payload result, then it is set to a value large enough to store the payload result and ENOSPC is returned. Note that for string controls this size field should not be confused with the length of the string. This field refers to the size of the memory that contains the string. The actual length of the string may well be much smaller. |
|
| __u32 | reserved2[1] |
Reserved for future extensions. Drivers and applications must set the array to zero. | |
| union | (anonymous) | ||
| __s32 | value |
New value or current value. | |
| __s64 | value64 |
New value or current value. | |
| char * | string |
A pointer to a string. |
Table 79. struct v4l2_ext_controls
| __u32 | ctrl_class |
The control class to which all controls belong, see Table 80, 「Control classes」. |
| __u32 | count |
The number of controls in the controls array. May also be zero. |
| __u32 | error_idx |
Set by the driver in case of an error. It is the index of the control causing the error or equal to 'count' when the error is not associated with a particular control. Undefined when the ioctl returns 0 (success). |
| __u32 | reserved[2] |
Reserved for future extensions. Drivers and applications must set the array to zero. |
| struct v4l2_ext_control * | controls |
Pointer to an array of count v4l2_ext_control structures. Ignored if count equals zero. |
Table 80. Control classes
V4L2_CTRL_CLASS_USER |
0x980000 | The class containing user controls. These controls are described in Section 1.8, 「User Controls」. All controls that can be set using the VIDIOC_S_CTRL and VIDIOC_G_CTRL ioctl belong to this class. |
V4L2_CTRL_CLASS_MPEG |
0x990000 | The class containing MPEG compression controls. These controls are described in Section 1.9.5, 「MPEG Control Reference」. |
V4L2_CTRL_CLASS_CAMERA |
0x9a0000 | The class containing camera controls. These controls are described in Section 1.9.6, 「Camera Control Reference」. |
V4L2_CTRL_CLASS_FM_TX |
0x9b0000 | The class containing FM Transmitter (FM TX) controls. These controls are described in Section 1.9.7, 「FM Transmitter Control Reference」. |
Return Value
On success 0 is returned, on error -1 and the errno variable is set appropriately:
- EINVAL
-
The struct v4l2_ext_control
idis invalid or the struct v4l2_ext_controlsctrl_classis invalid. This error code is also returned by theVIDIOC_S_EXT_CTRLSandVIDIOC_TRY_EXT_CTRLSioctls if two or more control values are in conflict. - ERANGE
-
The struct v4l2_ext_control
valueis out of bounds. - EBUSY
-
The control is temporarily not changeable, possibly because another applications took over control of the device function this control belongs to.
- ENOSPC
-
The space reserved for the control's payload is insufficient. The field
sizeis set to a value that is enough to store the payload and this error code is returned.
Name
VIDIOC_G_FBUF, VIDIOC_S_FBUF — Get or set frame buffer overlay parameters
Synopsis
int ioctl( |
int | fd, |
| int | request, | |
| struct v4l2_framebuffer * | argp); |
int ioctl( |
int | fd, |
| int | request, | |
| const struct v4l2_framebuffer * | argp); |
Description
Applications can use the VIDIOC_G_FBUF and VIDIOC_S_FBUF ioctl to get and set the framebuffer parameters for a Video Overlay or Video Output Overlay (OSD). The type of overlay is implied by the device type (capture or output device) and can be determined with the VIDIOC_QUERYCAP ioctl. One /dev/videoN device must not support both kinds of overlay.
The V4L2 API distinguishes destructive and non-destructive overlays. A destructive overlay copies captured video images into the video memory of a graphics card. A non-destructive overlay blends video images into a VGA signal or graphics into a video signal. Video Output Overlays are always non-destructive.
To get the current parameters applications call the VIDIOC_G_FBUF ioctl with a pointer to a v4l2_framebuffer structure. The driver fills all fields of the structure or returns an EINVAL error code when overlays are not supported.
To set the parameters for a Video Output Overlay, applications must initialize the flags field of a struct v4l2_framebuffer. Since the framebuffer is implemented on the TV card all other parameters are determined by the driver. When an application calls VIDIOC_S_FBUF with a pointer to this structure, the driver prepares for the overlay and returns the framebuffer parameters as VIDIOC_G_FBUF does, or it returns an error code.
To set the parameters for a non-destructive Video Overlay, applications must initialize the flags field, the fmt substructure, and call VIDIOC_S_FBUF. Again the driver prepares for the overlay and returns the framebuffer parameters as VIDIOC_G_FBUF does, or it returns an error code.
For a destructive Video Overlay applications must additionally provide a base address. Setting up a DMA to a random memory location can jeopardize the system security, its stability or even damage the hardware, therefore only the superuser can set the parameters for a destructive video overlay.
Table 81. struct v4l2_framebuffer
| __u32 | capability |
Overlay capability flags set by the driver, see Table 82, 「Frame Buffer Capability Flags」. | |
| __u32 | flags |
Overlay control flags set by application and driver, see Table 83, 「Frame Buffer Flags」 | |
| void * | base |
Physical base address of the framebuffer, that is the address of the pixel in the top left corner of the framebuffer.[a] | |
| This field is irrelevant to non-destructive Video Overlays. For destructive Video Overlays applications must provide a base address. The driver may accept only base addresses which are a multiple of two, four or eight bytes. For Video Output Overlays the driver must return a valid base address, so applications can find the corresponding Linux framebuffer device (see Section 4.4, 「Video Output Overlay Interface」). | |||
| struct v4l2_pix_format | fmt |
Layout of the frame buffer. The v4l2_pix_format structure is defined in Chapter 2, Image Formats, for clarification the fields and acceptable values are listed below: | |
| __u32 | width |
Width of the frame buffer in pixels. | |
| __u32 | height |
Height of the frame buffer in pixels. | |
| __u32 | pixelformat |
The pixel format of the framebuffer. | |
For non-destructive Video Overlays this field only defines a format for the struct v4l2_window chromakey field. |
|||
| For destructive Video Overlays applications must initialize this field. For Video Output Overlays the driver must return a valid format. | |||
Usually this is an RGB format (for example V4L2_PIX_FMT_RGB565) but YUV formats (only packed YUV formats when chroma keying is used, not including V4L2_PIX_FMT_YUYV and V4L2_PIX_FMT_UYVY) and the |
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- 2. 譯：Convolutional Two-Stream Network Fusion for Video Action Recognition
- 3. ORA-00119: invalid specification for system parameter LOCAL_LISTENER - 轉
- 4. Battery Charging Specification, Revision 1.2, December 7, 2010 中文詳解
- 5. SlowFast Networks for Video Recognition
- 6. 視頻動作識別--Convolutional Two-Stream Network Fusion for Video Action Recognition
- 7. Appearance-and-Relation Networks for Video Classification
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每一个你不满意的现在，都有一个你没有努力的曾经。
- 1. 六、USB Video Class Specification
- 2. 譯：Convolutional Two-Stream Network Fusion for Video Action Recognition
- 3. ORA-00119: invalid specification for system parameter LOCAL_LISTENER - 轉
- 4. Battery Charging Specification, Revision 1.2, December 7, 2010 中文詳解
- 5. SlowFast Networks for Video Recognition
- 6. 視頻動作識別--Convolutional Two-Stream Network Fusion for Video Action Recognition
- 7. Appearance-and-Relation Networks for Video Classification
- 8. DASH Adaptive Streaming for HTML 5 Video
- 9. XJTLU CSE103 revision note
- 10. Xilinx reVISION開搞!【reVISION】