FFmpeg數據結構AVFrame

時間 2019-11-12

標籤 ffmpeg 數據結構 avframe 简体版

原文原文鏈接

本文爲做者原創，轉載請註明出處：http://www.javashuo.com/article/p-cweaorzk-q.htmlhtml

本文基於FFmpeg 4.1版本。express

1. 數據結構定義

struct AVFrame定義於<libavutil/frame.h>數組

struct AVFrame frame;

AVFrame中存儲的是通過解碼後的原始數據。在解碼中，AVFrame是解碼器的輸出；在編碼中，AVFrame是編碼器的輸入。下圖中，「decoded frames」的數據類型就是AVFrame：安全

_______              ______________
|       |            |              |
| input |  demuxer   | encoded data |   decoder
| file  | ---------> | packets      | -----+
|_______|            |______________|      |
                                           v
                                       _________
                                      |         |
                                      | decoded |
                                      | frames  |
                                      |_________|
 ________             ______________       |
|        |           |              |      |
| output | <-------- | encoded data | <----+
| file   |   muxer   | packets      |   encoder
|________|           |______________|

AVFrame數據結構很是重要，它的成員很是多，致使數據結構定義篇幅很長。下面引用的數據結構定義中省略冗長的註釋以及大部分紅員，先整體說明AVFrame的用法，而後再將一些重要成員摘錄出來單獨進行說明：數據結構

/**
 * This structure describes decoded (raw) audio or video data.
 *
 * AVFrame must be allocated using av_frame_alloc(). Note that this only
 * allocates the AVFrame itself, the buffers for the data must be managed
 * through other means (see below).
 * AVFrame must be freed with av_frame_free().
 *
 * AVFrame is typically allocated once and then reused multiple times to hold
 * different data (e.g. a single AVFrame to hold frames received from a
 * decoder). In such a case, av_frame_unref() will free any references held by
 * the frame and reset it to its original clean state before it
 * is reused again.
 *
 * The data described by an AVFrame is usually reference counted through the
 * AVBuffer API. The underlying buffer references are stored in AVFrame.buf /
 * AVFrame.extended_buf. An AVFrame is considered to be reference counted if at
 * least one reference is set, i.e. if AVFrame.buf[0] != NULL. In such a case,
 * every single data plane must be contained in one of the buffers in
 * AVFrame.buf or AVFrame.extended_buf.
 * There may be a single buffer for all the data, or one separate buffer for
 * each plane, or anything in between.
 *
 * sizeof(AVFrame) is not a part of the public ABI, so new fields may be added
 * to the end with a minor bump.
 *
 * Fields can be accessed through AVOptions, the name string used, matches the
 * C structure field name for fields accessible through AVOptions. The AVClass
 * for AVFrame can be obtained from avcodec_get_frame_class()
 */
typedef struct AVFrame {
    uint8_t *data[AV_NUM_DATA_POINTERS];
    int linesize[AV_NUM_DATA_POINTERS];
    uint8_t **extended_data;
    int width, height;
    int nb_samples;
    int format;
    int key_frame;
    enum AVPictureType pict_type;
    AVRational sample_aspect_ratio;
    int64_t pts;
    ......
} AVFrame;

AVFrame的用法：less

AVFrame對象必須調用av_frame_alloc()在堆上分配，注意此處指的是AVFrame對象自己，AVFrame對象必須調用av_frame_free()進行銷燬。
AVFrame中包含的數據緩衝區是
AVFrame一般只需分配一次，而後能夠屢次重用，每次重用前應調用av_frame_unref()將frame復位到原始的乾淨可用的狀態。

下面將一些重要的成員摘錄出來進行說明：
dataide

/**
     * pointer to the picture/channel planes.
     * This might be different from the first allocated byte
     *
     * Some decoders access areas outside 0,0 - width,height, please
     * see avcodec_align_dimensions2(). Some filters and swscale can read
     * up to 16 bytes beyond the planes, if these filters are to be used,
     * then 16 extra bytes must be allocated.
     *
     * NOTE: Except for hwaccel formats, pointers not needed by the format
     * MUST be set to NULL.
     */
    uint8_t *data[AV_NUM_DATA_POINTERS];

存儲原始幀數據(未編碼的原始圖像或音頻格式，做爲解碼器的輸出或編碼器的輸入)。
data是一個指針數組，數組的每個元素是一個指針，指向視頻中圖像的某一plane或音頻中某一聲道的plane。
關於圖像plane的詳細說明參考「色彩空間與像素格式」，音頻plane的詳細說明參數「ffplay源碼解析6-音頻重採樣 6.1.1節」。下面簡單說明：
對於packet格式，一幅YUV圖像的Y、U、V交織存儲在一個plane中，形如YUVYUV...，data[0]指向這個plane；
一個雙聲道的音頻幀其左聲道L、右聲道R交織存儲在一個plane中，形如LRLRLR...，data[0]指向這個plane。
對於planar格式，一幅YUV圖像有Y、U、V三個plane，data[0]指向Y plane，data[1]指向U plane，data[2]指向V plane；
一個雙聲道的音頻幀有左聲道L和右聲道R兩個plane，data[0]指向L plane，data[1]指向R plane。函數

linesize佈局

/**
     * For video, size in bytes of each picture line.
     * For audio, size in bytes of each plane.
     *
     * For audio, only linesize[0] may be set. For planar audio, each channel
     * plane must be the same size.
     *
     * For video the linesizes should be multiples of the CPUs alignment
     * preference, this is 16 or 32 for modern desktop CPUs.
     * Some code requires such alignment other code can be slower without
     * correct alignment, for yet other it makes no difference.
     *
     * @note The linesize may be larger than the size of usable data -- there
     * may be extra padding present for performance reasons.
     */
    int linesize[AV_NUM_DATA_POINTERS];

對於視頻來講，linesize是每行圖像的大小(字節數)。注意有對齊要求。
對於音頻來講，linesize是每一個plane的大小(字節數)。音頻只使用linesize[0]。對於planar音頻來講，每一個plane的大小必須同樣。
linesize可能會因性能上的考慮而填充一些額外的數據，所以linesize可能比實際對應的音視頻數據尺寸要大。性能

extended_data

/**
     * pointers to the data planes/channels.
     *
     * For video, this should simply point to data[].
     *
     * For planar audio, each channel has a separate data pointer, and
     * linesize[0] contains the size of each channel buffer.
     * For packed audio, there is just one data pointer, and linesize[0]
     * contains the total size of the buffer for all channels.
     *
     * Note: Both data and extended_data should always be set in a valid frame,
     * but for planar audio with more channels that can fit in data,
     * extended_data must be used in order to access all channels.
     */
    uint8_t **extended_data;

????extended_data是幹啥的????
對於視頻來講，直接指向data[]成員。
對於音頻來講，packet格式音頻只有一個plane，一個音頻幀中各個聲道的採樣點交織存儲在此plane中；planar格式音頻每一個聲道一個plane。在多聲道planar格式音頻中，必須使用extended_data才能訪問全部聲道，什麼意思？
在有效的視頻/音頻frame中，data和extended_data兩個成員都必須設置有效值。

width, height

/**
     * @name Video dimensions
     * Video frames only. The coded dimensions (in pixels) of the video frame,
     * i.e. the size of the rectangle that contains some well-defined values.
     *
     * @note The part of the frame intended for display/presentation is further
     * restricted by the @ref cropping "Cropping rectangle".
     * @{
     */
    int width, height;

視頻幀寬和高(像素)。

nb_samples

/**
     * number of audio samples (per channel) described by this frame
     */
    int nb_samples;

音頻幀中單個聲道中包含的採樣點數。

format

/**
     * format of the frame, -1 if unknown or unset
     * Values correspond to enum AVPixelFormat for video frames,
     * enum AVSampleFormat for audio)
     */
    int format;

幀格式。若是是未知格式或未設置，則值爲-1。
對於視頻幀，此值對應於「enum AVPixelFormat」結構：

enum AVPixelFormat {
    AV_PIX_FMT_NONE = -1,
    AV_PIX_FMT_YUV420P,   ///< planar YUV 4:2:0, 12bpp, (1 Cr & Cb sample per 2x2 Y samples)
    AV_PIX_FMT_YUYV422,   ///< packed YUV 4:2:2, 16bpp, Y0 Cb Y1 Cr
    AV_PIX_FMT_RGB24,     ///< packed RGB 8:8:8, 24bpp, RGBRGB...
    AV_PIX_FMT_BGR24,     ///< packed RGB 8:8:8, 24bpp, BGRBGR...
    ......
}

對於音頻幀，此值對應於「enum AVSampleFormat」格式：

enum AVSampleFormat {
    AV_SAMPLE_FMT_NONE = -1,
    AV_SAMPLE_FMT_U8,          ///< unsigned 8 bits
    AV_SAMPLE_FMT_S16,         ///< signed 16 bits
    AV_SAMPLE_FMT_S32,         ///< signed 32 bits
    AV_SAMPLE_FMT_FLT,         ///< float
    AV_SAMPLE_FMT_DBL,         ///< double

    AV_SAMPLE_FMT_U8P,         ///< unsigned 8 bits, planar
    AV_SAMPLE_FMT_S16P,        ///< signed 16 bits, planar
    AV_SAMPLE_FMT_S32P,        ///< signed 32 bits, planar
    AV_SAMPLE_FMT_FLTP,        ///< float, planar
    AV_SAMPLE_FMT_DBLP,        ///< double, planar
    AV_SAMPLE_FMT_S64,         ///< signed 64 bits
    AV_SAMPLE_FMT_S64P,        ///< signed 64 bits, planar

    AV_SAMPLE_FMT_NB           ///< Number of sample formats. DO NOT USE if linking dynamically
};

key_frame

/**
     * 1 -> keyframe, 0-> not
     */
    int key_frame;

視頻幀是不是關鍵幀的標識，1->關鍵幀，0->非關鍵幀。

pict_type

/**
     * Picture type of the frame.
     */
    enum AVPictureType pict_type;

視頻幀類型(I、B、P等)。以下：

/**
 * @}
 * @}
 * @defgroup lavu_picture Image related
 *
 * AVPicture types, pixel formats and basic image planes manipulation.
 *
 * @{
 */

enum AVPictureType {
    AV_PICTURE_TYPE_NONE = 0, ///< Undefined
    AV_PICTURE_TYPE_I,     ///< Intra
    AV_PICTURE_TYPE_P,     ///< Predicted
    AV_PICTURE_TYPE_B,     ///< Bi-dir predicted
    AV_PICTURE_TYPE_S,     ///< S(GMC)-VOP MPEG-4
    AV_PICTURE_TYPE_SI,    ///< Switching Intra
    AV_PICTURE_TYPE_SP,    ///< Switching Predicted
    AV_PICTURE_TYPE_BI,    ///< BI type
};

sample_aspect_ratio

/**
     * Sample aspect ratio for the video frame, 0/1 if unknown/unspecified.
     */
    AVRational sample_aspect_ratio;

視頻幀的寬高比。

pts

/**
     * Presentation timestamp in time_base units (time when frame should be shown to user).
     */
    int64_t pts;

顯示時間戳。單位是time_base。

pkt_pts

#if FF_API_PKT_PTS
    /**
     * PTS copied from the AVPacket that was decoded to produce this frame.
     * @deprecated use the pts field instead
     */
    attribute_deprecated
    int64_t pkt_pts;
#endif

此frame對應的packet中的顯示時間戳。是從對應packet(解碼生成此frame)中拷貝PTS獲得此值。

pkt_dts

/**
     * DTS copied from the AVPacket that triggered returning this frame. (if frame threading isn't used)
     * This is also the Presentation time of this AVFrame calculated from
     * only AVPacket.dts values without pts values.
     */
    int64_t pkt_dts;

此frame對應的packet中的解碼時間戳。是從對應packet(解碼生成此frame)中拷貝DTS獲得此值。
若是對應的packet中只有dts而未設置pts，則此值也是此frame的pts。

coded_picture_number

/**
     * picture number in bitstream order
     */
    int coded_picture_number;

在編碼流中當前圖像的序號。

display_picture_number

/**
     * picture number in display order
     */
    int display_picture_number;

在顯示序列中當前圖像的序號。

interlaced_frame

/**
     * The content of the picture is interlaced.
     */
    int interlaced_frame;

圖像逐行/隔行模式標識。

sample_rate

/**
     * Sample rate of the audio data.
     */
    int sample_rate;

音頻採樣率。

channel_layout

/**
     * Channel layout of the audio data.
     */
    uint64_t channel_layout;

音頻聲道佈局。每bit表明一個特定的聲道，參考channel_layout.h中的定義，一目瞭然：

/**
 * @defgroup channel_masks Audio channel masks
 *
 * A channel layout is a 64-bits integer with a bit set for every channel.
 * The number of bits set must be equal to the number of channels.
 * The value 0 means that the channel layout is not known.
 * @note this data structure is not powerful enough to handle channels
 * combinations that have the same channel multiple times, such as
 * dual-mono.
 *
 * @{
 */
#define AV_CH_FRONT_LEFT             0x00000001
#define AV_CH_FRONT_RIGHT            0x00000002
#define AV_CH_FRONT_CENTER           0x00000004
#define AV_CH_LOW_FREQUENCY          0x00000008
......

/**
 * @}
 * @defgroup channel_mask_c Audio channel layouts
 * @{
 * */
#define AV_CH_LAYOUT_MONO              (AV_CH_FRONT_CENTER)
#define AV_CH_LAYOUT_STEREO            (AV_CH_FRONT_LEFT|AV_CH_FRONT_RIGHT)
#define AV_CH_LAYOUT_2POINT1           (AV_CH_LAYOUT_STEREO|AV_CH_LOW_FREQUENCY)

buf

/**
     * AVBuffer references backing the data for this frame. If all elements of
     * this array are NULL, then this frame is not reference counted. This array
     * must be filled contiguously -- if buf[i] is non-NULL then buf[j] must
     * also be non-NULL for all j < i.
     *
     * There may be at most one AVBuffer per data plane, so for video this array
     * always contains all the references. For planar audio with more than
     * AV_NUM_DATA_POINTERS channels, there may be more buffers than can fit in
     * this array. Then the extra AVBufferRef pointers are stored in the
     * extended_buf array.
     */
    AVBufferRef *buf[AV_NUM_DATA_POINTERS];

此幀的數據能夠由AVBufferRef管理，AVBufferRef提供AVBuffer引用機制。這裏涉及到緩衝區引用計數概念：
AVBuffer是FFmpeg中很經常使用的一種緩衝區，緩衝區使用引用計數(reference-counted)機制。
AVBufferRef則對AVBuffer緩衝區提供了一層封裝，最主要的是做引用計數處理，實現了一種安全機制。用戶不該直接訪問AVBuffer，應經過AVBufferRef來訪問AVBuffer，以保證安全。
FFmpeg中不少基礎的數據結構都包含了AVBufferRef成員，來間接使用AVBuffer緩衝區。
相關內容參考「FFmpeg數據結構AVBuffer」
????幀的數據緩衝區AVBuffer就是前面的data成員，用戶不該直接使用data成員，應經過buf成員間接使用data成員。那extended_data又是作什麼的呢????

若是buf[]的全部元素都爲NULL，則此幀不會被引用計數。必須連續填充buf[] - 若是buf[i]爲非NULL，則對於全部j<i，buf[j]也必須爲非NULL。
每一個plane最多能夠有一個AVBuffer，一個AVBufferRef指針指向一個AVBuffer，一個AVBuffer引用指的就是一個AVBufferRef指針。
對於視頻來講，buf[]包含全部AVBufferRef指針。對於具備多於AV_NUM_DATA_POINTERS個聲道的planar音頻來講，可能buf[]存不下全部的AVBbufferRef指針，多出的AVBufferRef指針存儲在extended_buf數組中。

extended_buf&nb_extended_buf

/**
     * For planar audio which requires more than AV_NUM_DATA_POINTERS
     * AVBufferRef pointers, this array will hold all the references which
     * cannot fit into AVFrame.buf.
     *
     * Note that this is different from AVFrame.extended_data, which always
     * contains all the pointers. This array only contains the extra pointers,
     * which cannot fit into AVFrame.buf.
     *
     * This array is always allocated using av_malloc() by whoever constructs
     * the frame. It is freed in av_frame_unref().
     */
    AVBufferRef **extended_buf;
    /**
     * Number of elements in extended_buf.
     */
    int        nb_extended_buf;

對於具備多於AV_NUM_DATA_POINTERS個聲道的planar音頻來講，可能buf[]存不下全部的AVBbufferRef指針，多出的AVBufferRef指針存儲在extended_buf數組中。
注意此處的extended_buf和AVFrame.extended_data的不一樣，AVFrame.extended_data包含全部指向各plane的指針，而extended_buf只包含AVFrame.buf中裝不下的指針。
extended_buf是構造frame時av_frame_alloc()中自動調用av_malloc()來分配空間的。調用av_frame_unref會釋放掉extended_buf。
nb_extended_buf是extended_buf中的元素數目。

best_effort_timestamp

/**
     * frame timestamp estimated using various heuristics, in stream time base
     * - encoding: unused
     * - decoding: set by libavcodec, read by user.
     */
    int64_t best_effort_timestamp;

????

pkt_pos

/**
     * reordered pos from the last AVPacket that has been input into the decoder
     * - encoding: unused
     * - decoding: Read by user.
     */
    int64_t pkt_pos;

記錄最後一個扔進解碼器的packet在輸入文件中的位置偏移量。

pkt_duration

/**
     * duration of the corresponding packet, expressed in
     * AVStream->time_base units, 0 if unknown.
     * - encoding: unused
     * - decoding: Read by user.
     */
    int64_t pkt_duration;

對應packet的時長，單位是AVStream->time_base。

channels

/**
     * number of audio channels, only used for audio.
     * - encoding: unused
     * - decoding: Read by user.
     */
    int channels;

音頻聲道數量。

pkt_size

/**
     * size of the corresponding packet containing the compressed
     * frame.
     * It is set to a negative value if unknown.
     * - encoding: unused
     * - decoding: set by libavcodec, read by user.
     */
    int pkt_size;

對應packet的大小。

crop_

/**
     * @anchor cropping
     * @name Cropping
     * Video frames only. The number of pixels to discard from the the
     * top/bottom/left/right border of the frame to obtain the sub-rectangle of
     * the frame intended for presentation.
     * @{
     */
    size_t crop_top;
    size_t crop_bottom;
    size_t crop_left;
    size_t crop_right;
    /**
     * @}
     */

用於視頻幀圖像裁切。四個值分別爲從frame的上/下/左/右邊界裁切的像素數。

2. 相關函數使用說明

2.1 av_frame_alloc()

/**
 * Allocate an AVFrame and set its fields to default values.  The resulting
 * struct must be freed using av_frame_free().
 *
 * @return An AVFrame filled with default values or NULL on failure.
 *
 * @note this only allocates the AVFrame itself, not the data buffers. Those
 * must be allocated through other means, e.g. with av_frame_get_buffer() or
 * manually.
 */
AVFrame *av_frame_alloc(void);

構造一個frame，對象各成員被設爲默認值。
此函數只分配AVFrame對象自己，而不分配AVFrame中的數據緩衝區。

2.2 av_frame_free()

/**
 * Free the frame and any dynamically allocated objects in it,
 * e.g. extended_data. If the frame is reference counted, it will be
 * unreferenced first.
 *
 * @param frame frame to be freed. The pointer will be set to NULL.
 */
void av_frame_free(AVFrame **frame);

釋放一個frame。

2.3 av_frame_ref()

/**
 * Set up a new reference to the data described by the source frame.
 *
 * Copy frame properties from src to dst and create a new reference for each
 * AVBufferRef from src.
 *
 * If src is not reference counted, new buffers are allocated and the data is
 * copied.
 *
 * @warning: dst MUST have been either unreferenced with av_frame_unref(dst),
 *           or newly allocated with av_frame_alloc() before calling this
 *           function, or undefined behavior will occur.
 *
 * @return 0 on success, a negative AVERROR on error
 */
int av_frame_ref(AVFrame *dst, const AVFrame *src);

爲src中的數據創建一個新的引用。
將src中幀的各屬性拷到dst中，而且爲src中每一個AVBufferRef建立一個新的引用。
若是src未使用引用計數，則dst中會分配新的數據緩衝區，將將src中緩衝區的數據拷貝到dst中的緩衝區。

2.4 av_frame_clone()

/**
 * Create a new frame that references the same data as src.
 *
 * This is a shortcut for av_frame_alloc()+av_frame_ref().
 *
 * @return newly created AVFrame on success, NULL on error.
 */
AVFrame *av_frame_clone(const AVFrame *src);

建立一個新的frame，新的frame和src使用同一數據緩衝區，緩衝區管理使用引用計數機制。
本函數至關於av_frame_alloc()+av_frame_ref()

2.5 av_frame_unref()

/**
 * Unreference all the buffers referenced by frame and reset the frame fields.
 */
void av_frame_unref(AVFrame *frame);

解除本frame對本frame中全部緩衝區的引用，並復位frame中各成員。

2.6 av_frame_move_ref()

/**
 * Move everything contained in src to dst and reset src.
 *
 * @warning: dst is not unreferenced, but directly overwritten without reading
 *           or deallocating its contents. Call av_frame_unref(dst) manually
 *           before calling this function to ensure that no memory is leaked.
 */
void av_frame_move_ref(AVFrame *dst, AVFrame *src);

將src中全部數據拷貝到dst中，並復位src。
爲避免內存泄漏，在調用av_frame_move_ref(dst, src)以前應先調用av_frame_unref(dst) 。

2.7 av_frame_get_buffer()

/**
 * Allocate new buffer(s) for audio or video data.
 *
 * The following fields must be set on frame before calling this function:
 * - format (pixel format for video, sample format for audio)
 * - width and height for video
 * - nb_samples and channel_layout for audio
 *
 * This function will fill AVFrame.data and AVFrame.buf arrays and, if
 * necessary, allocate and fill AVFrame.extended_data and AVFrame.extended_buf.
 * For planar formats, one buffer will be allocated for each plane.
 *
 * @warning: if frame already has been allocated, calling this function will
 *           leak memory. In addition, undefined behavior can occur in certain
 *           cases.
 *
 * @param frame frame in which to store the new buffers.
 * @param align Required buffer size alignment. If equal to 0, alignment will be
 *              chosen automatically for the current CPU. It is highly
 *              recommended to pass 0 here unless you know what you are doing.
 *
 * @return 0 on success, a negative AVERROR on error.
 */
int av_frame_get_buffer(AVFrame *frame, int align);

爲音頻或視頻數據分配新的緩衝區。
調用本函數前，幀中的以下成員必須先設置好：

format (視頻像素格式或音頻採樣格式)
width、height(視頻畫面和寬和高)
nb_samples、channel_layout(音頻單個聲道中的採樣點數目和聲道佈局)

本函數會填充AVFrame.data和AVFrame.buf數組，若是有須要，還會分配和填充AVFrame.extended_data和AVFrame.extended_buf。
對於planar格式，會爲每一個plane分配一個緩衝區。

2.8 av_frame_copy()

/**
 * Copy the frame data from src to dst.
 *
 * This function does not allocate anything, dst must be already initialized and
 * allocated with the same parameters as src.
 *
 * This function only copies the frame data (i.e. the contents of the data /
 * extended data arrays), not any other properties.
 *
 * @return >= 0 on success, a negative AVERROR on error.
 */
int av_frame_copy(AVFrame *dst, const AVFrame *src);

將src中的幀數據拷貝到dst中。
本函數並不會有任何分配緩衝區的動做，調用此函數前dst必須已經使用了和src一樣的參數完成了初始化。
本函數只拷貝幀中的數據緩衝區的內容(data/extended_data數組中的內容)，而不涉及幀中任何其餘的屬性。