FFmpeg來源簡單分析：結構會員管理系統-AVClass

所有和AVOption有關的數據都存儲在AVClass結構體中。假設一個結構體（好比AVFormatContext或者AVCodecContext）想要支持AVOption的話，它的第一個成員變量必須是一個指向AVClass結構體的指針。該AVClass中的成員變量option必須指向一個AVOption類型的靜態數組。

何爲AVOption？

AVOption是用來設置FFmpeg中變量的值的結構體。可能講到這個做用有的人會奇怪：設置系統中變量的值，直接使用等於號「=」就可以。爲何還要專門定義一個結構體呢？事實上AVOption的特色就在於它賦值時候的靈活性。AVOption可以使用字符串爲不論什麼類型的變量賦值。傳統意義上。假設變量類型爲int，則需要使用整數來賦值；假設變量爲double，則需要使用小數來賦值。假設變量類型爲char *，才需要使用字符串來賦值。

而AVOption將這些賦值「歸一化」了，統一使用字符串賦值。好比給int型變量qp設定值爲20，經過AVOption需要傳遞進去一個內容爲「20」的字符串。
此外，AVOption中變量的名稱也使用字符串來表示。結合上面提到的使用字符串賦值的特性，咱們可以發現使用AVOption以後。傳遞兩個字符串（一個是變量的名稱，一個是變量的值）就可以改變系統中變量的值。
上文提到的這樣的方法的意義在哪裏？我我的感受對於直接使用C語言進行開發的人來講。做用不是很是明顯：全然可以使用等於號「=」就可以進行各類變量的賦值。但是對於從外部系統中調用FFmpeg的人來講，做用就很是大了：從外部系統中僅僅可以傳遞字符串給內部系統。比方說對於直接調用ffmpeg.exe的人來講，他們是沒法改動FFmpeg內部各個變量的數值的，這樣的狀況下僅僅能經過輸入「名稱」和「值」這樣的字符串，經過AVOption改變FFmpeg內部變量的值。因而可知，使用AVOption可以使FFmpeg更加適應多種多樣的外部系統。

忽然想到了JavaEE開發中也有這樣的類似的機制。互聯網上僅僅可以傳輸字符串，便是沒有方法傳輸整形、浮點型這樣的的數據。而Java系統中卻包括整形、浮點型等各類數據類型。

所以開發JSP中的Servlet的時候常常需要將整數字符串手工轉化成一個整型的變量。使用最多的一個函數就是Integer.parseInt()方法。好比如下代碼可以將字符串「123」轉化成整數123。

int a=Integer.parseInt("123");

而在使用JavaEE中的Struts2進行開發的時候。就不需要進行手動轉換處理了。Struts2中包括了類似AVOption的這樣的數據類型本身主動轉換機制，可以將互聯網上收到的字符串「名稱」和「值」的組合本身主動賦值給對應名稱的變量。

由此發現了一個結論：編程語言之間真的是相通的！

現在回到AVOption。事實上除了可以對FFmpeg常用結構體AVFormatContext。AVCodecContext等進行賦值以外，還可以對它們的私有數據priv_data進行賦值。

這個字段裏一般存儲了各類編碼器特有的結構體。而這些結構體的定義在FFmpeg的SDK中是找不到的。好比使用libx264進行編碼的時候。經過AVCodecContext的priv_data字段可以對X264Context結構體中的變量進行賦值，設置preset，profile等。

使用libx265進行編碼的時候，經過AVCodecContext的priv_data字段可以對libx265Context結構體中的變量進行賦值，設置preset，tune等。

何爲AVClass？

AVClass最基本的做用就是給結構體（好比AVFormatContext等）添加AVOption功能的支持。

換句話說AVClass就是AVOption和目標結構體之間的「橋樑」。AVClass要求必須聲明爲目標結構體的第一個變量。

AVClass中有一個option數組用於存儲目標結構體的所有的AVOption。舉個樣例，AVFormatContext結構體。AVClass和AVOption之間的關係例如如下圖所看到的。

圖中AVFormatContext結構體的第一個變量爲AVClass類型的指針av_class，它在AVFormatContext結構體初始化的時候，被賦值指向了全局靜態變量av_format_context_class結構體（定義位於libavformat\options.c）。而AVClass類型的av_format_context_class結構體中的option變量指向了全局靜態數組avformat_options（定義位於libavformat\options_table.h）。

AVOption

如下開始從代碼的角度記錄AVOption。

AVOption結構體的定義例如如下所看到的。

/**
 * AVOption
 */
typedef struct AVOption {
    const char *name;

    /**
     * short English help text
     * @todo What about other languages?
     */
    const char *help;

    /**
     * The offset relative to the context structure where the option
     * value is stored. It should be 0 for named constants.
     */
    int offset;
    enum AVOptionType type;

    /**
     * the default value for scalar options
     */
    union {
        int64_t i64;
        double dbl;
        const char *str;
        /* TODO those are unused now */
        AVRational q;
    } default_val;
    double min;                 ///< minimum valid value for the option
    double max;                 ///< maximum valid value for the option

    int flags;
#define AV_OPT_FLAG_ENCODING_PARAM  1   ///< a generic parameter which can be set by the user for muxing or encoding
#define AV_OPT_FLAG_DECODING_PARAM  2   ///< a generic parameter which can be set by the user for demuxing or decoding
#if FF_API_OPT_TYPE_METADATA
#define AV_OPT_FLAG_METADATA        4   ///< some data extracted or inserted into the file like title, comment, ...
#endif
#define AV_OPT_FLAG_AUDIO_PARAM     8
#define AV_OPT_FLAG_VIDEO_PARAM     16
#define AV_OPT_FLAG_SUBTITLE_PARAM  32
/**
 * The option is inteded for exporting values to the caller.
 */
#define AV_OPT_FLAG_EXPORT          64
/**
 * The option may not be set through the AVOptions API, only read.
 * This flag only makes sense when AV_OPT_FLAG_EXPORT is also set.
 */
#define AV_OPT_FLAG_READONLY        128
#define AV_OPT_FLAG_FILTERING_PARAM (1<<16) ///< a generic parameter which can be set by the user for filtering
//FIXME think about enc-audio, ... style flags

    /**
     * The logical unit to which the option belongs. Non-constant
     * options and corresponding named constants share the same
     * unit. May be NULL.
     */
    const char *unit;
} AVOption;

如下簡單解釋一下AVOption的幾個成員變量：

name：名稱。
help：簡短的幫助。

offset：選項相對結構體首部地址的偏移量（這個很是重要）。

type：選項的類型。
default_val：選項的默認值。
min：選項的最小值。
max：選項的最大值。
flags：一些標記。
unit：該選項所屬的邏輯單元，可以爲空。

當中。default_val是一個union類型的變量。可以依據選項數據類型的不一樣。取int，double，char*。AVRational（表示分數）幾種類型。type是一個AVOptionType類型的變量。AVOptionType是一個枚舉類型，定義例如如下。

enum AVOptionType{
    AV_OPT_TYPE_FLAGS,
    AV_OPT_TYPE_INT,
    AV_OPT_TYPE_INT64,
    AV_OPT_TYPE_DOUBLE,
    AV_OPT_TYPE_FLOAT,
    AV_OPT_TYPE_STRING,
    AV_OPT_TYPE_RATIONAL,
    AV_OPT_TYPE_BINARY,  ///< offset must point to a pointer immediately followed by an int for the length
    AV_OPT_TYPE_DICT,
    AV_OPT_TYPE_CONST = 128,
    AV_OPT_TYPE_IMAGE_SIZE = MKBETAG('S','I','Z','E'), ///< offset must point to two consecutive integers
    AV_OPT_TYPE_PIXEL_FMT  = MKBETAG('P','F','M','T'),
    AV_OPT_TYPE_SAMPLE_FMT = MKBETAG('S','F','M','T'),
    AV_OPT_TYPE_VIDEO_RATE = MKBETAG('V','R','A','T'), ///< offset must point to AVRational
    AV_OPT_TYPE_DURATION   = MKBETAG('D','U','R',' '),
    AV_OPT_TYPE_COLOR      = MKBETAG('C','O','L','R'),
    AV_OPT_TYPE_CHANNEL_LAYOUT = MKBETAG('C','H','L','A'),
#if FF_API_OLD_AVOPTIONS
    FF_OPT_TYPE_FLAGS = 0,
    FF_OPT_TYPE_INT,
    FF_OPT_TYPE_INT64,
    FF_OPT_TYPE_DOUBLE,
    FF_OPT_TYPE_FLOAT,
    FF_OPT_TYPE_STRING,
    FF_OPT_TYPE_RATIONAL,
    FF_OPT_TYPE_BINARY,  ///< offset must point to a pointer immediately followed by an int for the length
    FF_OPT_TYPE_CONST=128,
#endif
};

AVClass

AVClass中存儲了AVOption類型的數組option。用於存儲選項信息。AVClass有一個特色就是它必須位於其支持的結構體的第一個位置。好比。AVFormatContext和AVCodecContext都支持AVClass，觀察它們結構體的定義可以發現他們結構體的第一個變量都是AVClass。截取一小段AVFormatContext的定義的開頭部分，例如如下所看到的。

typedef struct AVFormatContext {
    /**
     * A class for logging and @ref avoptions. Set by avformat_alloc_context().
     * Exports (de)muxer private options if they exist.
     */
    const AVClass *av_class;

    /**
     * The input container format.
     *
     * Demuxing only, set by avformat_open_input().
     */
    struct AVInputFormat *iformat;

    /**
     * The output container format.
     *
     * Muxing only, must be set by the caller before avformat_write_header().
     */
struct AVOutputFormat *oformat;
//後文略

截取一小段AVCodecContext的定義的開頭部分，例如如下所看到的。

typedef struct AVCodecContext {
    /**
     * information on struct for av_log
     * - set by avcodec_alloc_context3
     */
    const AVClass *av_class;
    int log_level_offset;

    enum AVMediaType codec_type; /* see AVMEDIA_TYPE_xxx */
    const struct AVCodec  *codec;
	//後文略

如下來看一下AVClass的定義，例如如下所看到的。

/**
 * Describe the class of an AVClass context structure. That is an
 * arbitrary struct of which the first field is a pointer to an
 * AVClass struct (e.g. AVCodecContext, AVFormatContext etc.).
 */
typedef struct AVClass {
    /**
     * The name of the class; usually it is the same name as the
     * context structure type to which the AVClass is associated.
     */
    const char* class_name;

    /**
     * A pointer to a function which returns the name of a context
     * instance ctx associated with the class.
     */
    const char* (*item_name)(void* ctx);

    /**
     * a pointer to the first option specified in the class if any or NULL
     *
     * @see av_set_default_options()
     */
    const struct AVOption *option;

    /**
     * LIBAVUTIL_VERSION with which this structure was created.
     * This is used to allow fields to be added without requiring major
     * version bumps everywhere.
     */

    int version;

    /**
     * Offset in the structure where log_level_offset is stored.
     * 0 means there is no such variable
     */
    int log_level_offset_offset;

    /**
     * Offset in the structure where a pointer to the parent context for
     * logging is stored. For example a decoder could pass its AVCodecContext
     * to eval as such a parent context, which an av_log() implementation
     * could then leverage to display the parent context.
     * The offset can be NULL.
     */
    int parent_log_context_offset;

    /**
     * Return next AVOptions-enabled child or NULL
     */
    void* (*child_next)(void *obj, void *prev);

    /**
     * Return an AVClass corresponding to the next potential
     * AVOptions-enabled child.
     *
     * The difference between child_next and this is that
     * child_next iterates over _already existing_ objects, while
     * child_class_next iterates over _all possible_ children.
     */
    const struct AVClass* (*child_class_next)(const struct AVClass *prev);

    /**
     * Category used for visualization (like color)
     * This is only set if the category is equal for all objects using this class.
     * available since version (51 << 16 | 56 << 8 | 100)
     */
    AVClassCategory category;

    /**
     * Callback to return the category.
     * available since version (51 << 16 | 59 << 8 | 100)
     */
    AVClassCategory (*get_category)(void* ctx);

    /**
     * Callback to return the supported/allowed ranges.
     * available since version (52.12)
     */
    int (*query_ranges)(struct AVOptionRanges **, void *obj, const char *key, int flags);
} AVClass;

如下簡單解釋一下AVClass的幾個已經理解的成員變量：

class_name：AVClass名稱。

item_name：函數。獲取與AVClass相關聯的結構體實例的名稱。
option：AVOption類型的數組（最重要）。
version：完畢該AVClass的時候的LIBAVUTIL_VERSION。
category：AVClass的類型，是一個類型爲AVClassCategory的枚舉型變量。

當中AVClassCategory定義例如如下。

typedef enum {
    AV_CLASS_CATEGORY_NA = 0,
    AV_CLASS_CATEGORY_INPUT,
    AV_CLASS_CATEGORY_OUTPUT,
    AV_CLASS_CATEGORY_MUXER,
    AV_CLASS_CATEGORY_DEMUXER,
    AV_CLASS_CATEGORY_ENCODER,
    AV_CLASS_CATEGORY_DECODER,
    AV_CLASS_CATEGORY_FILTER,
    AV_CLASS_CATEGORY_BITSTREAM_FILTER,
    AV_CLASS_CATEGORY_SWSCALER,
    AV_CLASS_CATEGORY_SWRESAMPLER,
    AV_CLASS_CATEGORY_DEVICE_VIDEO_OUTPUT = 40,
    AV_CLASS_CATEGORY_DEVICE_VIDEO_INPUT,
    AV_CLASS_CATEGORY_DEVICE_AUDIO_OUTPUT,
    AV_CLASS_CATEGORY_DEVICE_AUDIO_INPUT,
    AV_CLASS_CATEGORY_DEVICE_OUTPUT,
    AV_CLASS_CATEGORY_DEVICE_INPUT,
    AV_CLASS_CATEGORY_NB, ///< not part of ABI/API
}AVClassCategory;

上面解釋字段仍是比較抽象的。如下經過詳細的樣例看一下AVClass這個結構體。

咱們看幾個詳細的樣例：

AVFormatContext中的AVClass
AVCodecContext中的AVClass
AVFrame中的AVClass
各類組件（libRTMP，libx264，libx265）裏面特有的AVClass。

AVFormatContext

AVFormatContext 中的AVClass定義位於libavformat\options.c中。是一個名稱爲av_format_context_class的靜態結構體。例如如下所看到的。

static const AVClass av_format_context_class = {
    .class_name     = "AVFormatContext",
    .item_name      = format_to_name,
    .option         = avformat_options,
    .version        = LIBAVUTIL_VERSION_INT,
    .child_next     = format_child_next,
    .child_class_next = format_child_class_next,
    .category       = AV_CLASS_CATEGORY_MUXER,
    .get_category   = get_category,
};

從源碼可以看出如下幾點
（1）class_name
該AVClass名稱是「AVFormatContext」。
（2）item_name
item_name指向一個函數format_to_name()，該函數定義例如如下所看到的。

static const char* format_to_name(void* ptr)
{
    AVFormatContext* fc = (AVFormatContext*) ptr;
    if(fc->iformat) return fc->iformat->name;
    else if(fc->oformat) return fc->oformat->name;
    else return "NULL";
}

從函數的定義可以看出，假設AVFormatContext結構體中的AVInputFormat結構體不爲空，則返回AVInputFormat的name，而後嘗試返回AVOutputFormat的name，假設AVOutputFormat也爲空，則返回「NULL」。

（3）option

option字段則指向一個元素個數很是多的靜態數組avformat_options。該數組單獨定義於libavformat\options_table.h中。

當中包括了AVFormatContext支持的所有的AVOption，例如如下所看到的。

/*
 * 雷霄驊
 * leixiaohua1020@126.com
 * 中國傳媒大學/數字電視技術
 * http://blog.csdn.net/leixiaohua1020
 *
 */

#ifndef AVFORMAT_OPTIONS_TABLE_H
#define AVFORMAT_OPTIONS_TABLE_H

#include <limits.h>

#include "libavutil/opt.h"
#include "avformat.h"
#include "internal.h"

#define OFFSET(x) offsetof(AVFormatContext,x)
#define DEFAULT 0 //should be NAN but it does not work as it is not a constant in glibc as required by ANSI/ISO C
//these names are too long to be readable
#define E AV_OPT_FLAG_ENCODING_PARAM
#define D AV_OPT_FLAG_DECODING_PARAM

static const AVOption avformat_options[] = {
{"avioflags", NULL, OFFSET(avio_flags), AV_OPT_TYPE_FLAGS, {.i64 = DEFAULT }, INT_MIN, INT_MAX, D|E, "avioflags"},
{"direct", "reduce buffering", 0, AV_OPT_TYPE_CONST, {.i64 = AVIO_FLAG_DIRECT }, INT_MIN, INT_MAX, D|E, "avioflags"},
{"probesize", "set probing size", OFFSET(probesize2), AV_OPT_TYPE_INT64, {.i64 = 5000000 }, 32, INT64_MAX, D},
{"formatprobesize", "number of bytes to probe file format", OFFSET(format_probesize), AV_OPT_TYPE_INT, {.i64 = PROBE_BUF_MAX}, 0, INT_MAX-1, D},
{"packetsize", "set packet size", OFFSET(packet_size), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, 0, INT_MAX, E},
{"fflags", NULL, OFFSET(flags), AV_OPT_TYPE_FLAGS, {.i64 = AVFMT_FLAG_FLUSH_PACKETS }, INT_MIN, INT_MAX, D|E, "fflags"},
{"flush_packets", "reduce the latency by flushing out packets immediately", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_FLUSH_PACKETS }, INT_MIN, INT_MAX, E, "fflags"},
{"ignidx", "ignore index", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_IGNIDX }, INT_MIN, INT_MAX, D, "fflags"},
{"genpts", "generate pts", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_GENPTS }, INT_MIN, INT_MAX, D, "fflags"},
{"nofillin", "do not fill in missing values that can be exactly calculated", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_NOFILLIN }, INT_MIN, INT_MAX, D, "fflags"},
{"noparse", "disable AVParsers, this needs nofillin too", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_NOPARSE }, INT_MIN, INT_MAX, D, "fflags"},
{"igndts", "ignore dts", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_IGNDTS }, INT_MIN, INT_MAX, D, "fflags"},
{"discardcorrupt", "discard corrupted frames", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_DISCARD_CORRUPT }, INT_MIN, INT_MAX, D, "fflags"},
{"sortdts", "try to interleave outputted packets by dts", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_SORT_DTS }, INT_MIN, INT_MAX, D, "fflags"},
{"keepside", "don't merge side data", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_KEEP_SIDE_DATA }, INT_MIN, INT_MAX, D, "fflags"},
{"latm", "enable RTP MP4A-LATM payload", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_MP4A_LATM }, INT_MIN, INT_MAX, E, "fflags"},
{"nobuffer", "reduce the latency introduced by optional buffering", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_NOBUFFER }, 0, INT_MAX, D, "fflags"},
{"seek2any", "allow seeking to non-keyframes on demuxer level when supported", OFFSET(seek2any), AV_OPT_TYPE_INT, {.i64 = 0 }, 0, 1, D},
{"bitexact", "do not write random/volatile data", 0, AV_OPT_TYPE_CONST, { .i64 = AVFMT_FLAG_BITEXACT }, 0, 0, E, "fflags" },
{"analyzeduration", "specify how many microseconds are analyzed to probe the input", OFFSET(max_analyze_duration2), AV_OPT_TYPE_INT64, {.i64 = 0 }, 0, INT64_MAX, D},
{"cryptokey", "decryption key", OFFSET(key), AV_OPT_TYPE_BINARY, {.dbl = 0}, 0, 0, D},
{"indexmem", "max memory used for timestamp index (per stream)", OFFSET(max_index_size), AV_OPT_TYPE_INT, {.i64 = 1<<20 }, 0, INT_MAX, D},
{"rtbufsize", "max memory used for buffering real-time frames", OFFSET(max_picture_buffer), AV_OPT_TYPE_INT, {.i64 = 3041280 }, 0, INT_MAX, D}, /* defaults to 1s of 15fps 352x288 YUYV422 video */
{"fdebug", "print specific debug info", OFFSET(debug), AV_OPT_TYPE_FLAGS, {.i64 = DEFAULT }, 0, INT_MAX, E|D, "fdebug"},
{"ts", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_FDEBUG_TS }, INT_MIN, INT_MAX, E|D, "fdebug"},
{"max_delay", "maximum muxing or demuxing delay in microseconds", OFFSET(max_delay), AV_OPT_TYPE_INT, {.i64 = -1 }, -1, INT_MAX, E|D},
{"start_time_realtime", "wall-clock time when stream begins (PTS==0)", OFFSET(start_time_realtime), AV_OPT_TYPE_INT64, {.i64 = AV_NOPTS_VALUE}, INT64_MIN, INT64_MAX, E},
{"fpsprobesize", "number of frames used to probe fps", OFFSET(fps_probe_size), AV_OPT_TYPE_INT, {.i64 = -1}, -1, INT_MAX-1, D},
{"audio_preload", "microseconds by which audio packets should be interleaved earlier", OFFSET(audio_preload), AV_OPT_TYPE_INT, {.i64 = 0}, 0, INT_MAX-1, E},
{"chunk_duration", "microseconds for each chunk", OFFSET(max_chunk_duration), AV_OPT_TYPE_INT, {.i64 = 0}, 0, INT_MAX-1, E},
{"chunk_size", "size in bytes for each chunk", OFFSET(max_chunk_size), AV_OPT_TYPE_INT, {.i64 = 0}, 0, INT_MAX-1, E},
/* this is a crutch for avconv, since it cannot deal with identically named options in different contexts.
 * to be removed when avconv is fixed */
{"f_err_detect", "set error detection flags (deprecated; use err_detect, save via avconv)", OFFSET(error_recognition), AV_OPT_TYPE_FLAGS, {.i64 = AV_EF_CRCCHECK }, INT_MIN, INT_MAX, D, "err_detect"},
{"err_detect", "set error detection flags", OFFSET(error_recognition), AV_OPT_TYPE_FLAGS, {.i64 = AV_EF_CRCCHECK }, INT_MIN, INT_MAX, D, "err_detect"},
{"crccheck", "verify embedded CRCs", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_CRCCHECK }, INT_MIN, INT_MAX, D, "err_detect"},
{"bitstream", "detect bitstream specification deviations", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_BITSTREAM }, INT_MIN, INT_MAX, D, "err_detect"},
{"buffer", "detect improper bitstream length", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_BUFFER }, INT_MIN, INT_MAX, D, "err_detect"},
{"explode", "abort decoding on minor error detection", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_EXPLODE }, INT_MIN, INT_MAX, D, "err_detect"},
{"ignore_err", "ignore errors", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_IGNORE_ERR }, INT_MIN, INT_MAX, D, "err_detect"},
{"careful",    "consider things that violate the spec, are fast to check and have not been seen in the wild as errors", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_CAREFUL }, INT_MIN, INT_MAX, D, "err_detect"},
{"compliant",  "consider all spec non compliancies as errors", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_COMPLIANT }, INT_MIN, INT_MAX, D, "err_detect"},
{"aggressive", "consider things that a sane encoder shouldn't do as an error", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_AGGRESSIVE }, INT_MIN, INT_MAX, D, "err_detect"},
{"use_wallclock_as_timestamps", "use wallclock as timestamps", OFFSET(use_wallclock_as_timestamps), AV_OPT_TYPE_INT, {.i64 = 0}, 0, INT_MAX-1, D},
{"avoid_negative_ts", "shift timestamps so they start at 0", OFFSET(avoid_negative_ts), AV_OPT_TYPE_INT, {.i64 = -1}, -1, 2, E, "avoid_negative_ts"},
{"auto",                "enabled when required by target format",    0, AV_OPT_TYPE_CONST, {.i64 = -1 }, INT_MIN, INT_MAX, E, "avoid_negative_ts"},
{"disabled",            "do not change timestamps",                  0, AV_OPT_TYPE_CONST, {.i64 =  0 }, INT_MIN, INT_MAX, E, "avoid_negative_ts"},
{"make_zero",           "shift timestamps so they start at 0",       0, AV_OPT_TYPE_CONST, {.i64 =  2 }, INT_MIN, INT_MAX, E, "avoid_negative_ts"},
{"make_non_negative",   "shift timestamps so they are non negative", 0, AV_OPT_TYPE_CONST, {.i64 =  1 }, INT_MIN, INT_MAX, E, "avoid_negative_ts"},
{"skip_initial_bytes", "set number of bytes to skip before reading header and frames", OFFSET(skip_initial_bytes), AV_OPT_TYPE_INT64, {.i64 = 0}, 0, INT64_MAX-1, D},
{"correct_ts_overflow", "correct single timestamp overflows", OFFSET(correct_ts_overflow), AV_OPT_TYPE_INT, {.i64 = 1}, 0, 1, D},
{"flush_packets", "enable flushing of the I/O context after each packet", OFFSET(flush_packets), AV_OPT_TYPE_INT, {.i64 = 1}, 0, 1, E},
{"metadata_header_padding", "set number of bytes to be written as padding in a metadata header", OFFSET(metadata_header_padding), AV_OPT_TYPE_INT, {.i64 = -1}, -1, INT_MAX, E},
{"output_ts_offset", "set output timestamp offset", OFFSET(output_ts_offset), AV_OPT_TYPE_DURATION, {.i64 = 0}, -INT64_MAX, INT64_MAX, E},
{"max_interleave_delta", "maximum buffering duration for interleaving", OFFSET(max_interleave_delta), AV_OPT_TYPE_INT64, { .i64 = 10000000 }, 0, INT64_MAX, E },
{"f_strict", "how strictly to follow the standards (deprecated; use strict, save via avconv)", OFFSET(strict_std_compliance), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, D|E, "strict"},
{"strict", "how strictly to follow the standards", OFFSET(strict_std_compliance), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, D|E, "strict"},
{"strict", "strictly conform to all the things in the spec no matter what the consequences", 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_STRICT }, INT_MIN, INT_MAX, D|E, "strict"},
{"normal", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_NORMAL }, INT_MIN, INT_MAX, D|E, "strict"},
{"experimental", "allow non-standardized experimental variants", 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_EXPERIMENTAL }, INT_MIN, INT_MAX, D|E, "strict"},
{"max_ts_probe", "maximum number of packets to read while waiting for the first timestamp", OFFSET(max_ts_probe), AV_OPT_TYPE_INT, { .i64 = 50 }, 0, INT_MAX, D },
{NULL},
};

#undef E
#undef D
#undef DEFAULT
#undef OFFSET

#endif /* AVFORMAT_OPTIONS_TABLE_H */

AVCodecContext

AVFormatContext 中的AVClass定義位於libavcodec\options.c中。是一個名稱爲av_codec_context_class的靜態結構體。

例如如下所看到的。

static const AVClass av_codec_context_class = {
    .class_name              = "AVCodecContext",
    .item_name               = context_to_name,
    .option                  = avcodec_options,
    .version                 = LIBAVUTIL_VERSION_INT,
    .log_level_offset_offset = offsetof(AVCodecContext, log_level_offset),
    .child_next              = codec_child_next,
    .child_class_next        = codec_child_class_next,
    .category                = AV_CLASS_CATEGORY_ENCODER,
    .get_category            = get_category,
};

從源碼可以看出：
（1）class_name
該AVClass名稱是「AVCodecContext」。
（2）item_name
item_name指向一個函數context_to_name ()，該函數定義例如如下所看到的。

static const char* context_to_name(void* ptr) {
    AVCodecContext *avc= ptr;

    if(avc && avc->codec && avc->codec->name)
        return avc->codec->name;
    else
        return "NULL";
}

從函數的定義可以看出。假設AVCodecContext中的Codec結構體不爲空。則返回Codec的name，不然返回「NULL」。
（3）category
option字段則指向一個元素個數極多的靜態數組avcodec_options。該數組單獨定義於libavcodec\options_table.h中。當中包括了AVCodecContext支持的所有的AVOption。由於該數組定義實在是太多了，在這裏僅貼出它前面的一小部分。

/*
 * 雷霄驊
 * leixiaohua1020@126.com
 * 中國傳媒大學/數字電視技術
 * http://blog.csdn.net/leixiaohua1020
 *
 */

#ifndef AVCODEC_OPTIONS_TABLE_H
#define AVCODEC_OPTIONS_TABLE_H

#include <float.h>
#include <limits.h>
#include <stdint.h>

#include "libavutil/opt.h"
#include "avcodec.h"
#include "version.h"

#define OFFSET(x) offsetof(AVCodecContext,x)
#define DEFAULT 0 //should be NAN but it does not work as it is not a constant in glibc as required by ANSI/ISO C
//these names are too long to be readable
#define V AV_OPT_FLAG_VIDEO_PARAM
#define A AV_OPT_FLAG_AUDIO_PARAM
#define S AV_OPT_FLAG_SUBTITLE_PARAM
#define E AV_OPT_FLAG_ENCODING_PARAM
#define D AV_OPT_FLAG_DECODING_PARAM

#define AV_CODEC_DEFAULT_BITRATE 200*1000

static const AVOption avcodec_options[] = {
{"b", "set bitrate (in bits/s)", OFFSET(bit_rate), AV_OPT_TYPE_INT, {.i64 = AV_CODEC_DEFAULT_BITRATE }, 0, INT_MAX, A|V|E},
{"ab", "set bitrate (in bits/s)", OFFSET(bit_rate), AV_OPT_TYPE_INT, {.i64 = 128*1000 }, 0, INT_MAX, A|E},
{"bt", "Set video bitrate tolerance (in bits/s). In 1-pass mode, bitrate tolerance specifies how far "
       "ratecontrol is willing to deviate from the target average bitrate value. This is not related "
       "to minimum/maximum bitrate. Lowering tolerance too much has an adverse effect on quality.",
       OFFSET(bit_rate_tolerance), AV_OPT_TYPE_INT, {.i64 = AV_CODEC_DEFAULT_BITRATE*20 }, 1, INT_MAX, V|E},
{"flags", NULL, OFFSET(flags), AV_OPT_TYPE_FLAGS, {.i64 = DEFAULT }, 0, UINT_MAX, V|A|S|E|D, "flags"},
{"unaligned", "allow decoders to produce unaligned output", 0, AV_OPT_TYPE_CONST, { .i64 = CODEC_FLAG_UNALIGNED }, INT_MIN, INT_MAX, V | D, "flags" },
{"mv4", "use four motion vectors per macroblock (MPEG-4)", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_4MV }, INT_MIN, INT_MAX, V|E, "flags"},
{"qpel", "use 1/4-pel motion compensation", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_QPEL }, INT_MIN, INT_MAX, V|E, "flags"},
{"loop", "use loop filter", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_LOOP_FILTER }, INT_MIN, INT_MAX, V|E, "flags"},
{"qscale", "use fixed qscale", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_QSCALE }, INT_MIN, INT_MAX, 0, "flags"},
#if FF_API_GMC
{"gmc", "use gmc", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_GMC }, INT_MIN, INT_MAX, V|E, "flags"},
#endif
#if FF_API_MV0
{"mv0", "always try a mb with mv=<0,0>", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_MV0 }, INT_MIN, INT_MAX, V|E, "flags"},
#endif
#if FF_API_INPUT_PRESERVED
{"input_preserved", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_INPUT_PRESERVED }, INT_MIN, INT_MAX, 0, "flags"},
#endif
{"pass1", "use internal 2-pass ratecontrol in first  pass mode", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_PASS1 }, INT_MIN, INT_MAX, 0, "flags"},
{"pass2", "use internal 2-pass ratecontrol in second pass mode", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_PASS2 }, INT_MIN, INT_MAX, 0, "flags"},
{"gray", "only decode/encode grayscale", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_GRAY }, INT_MIN, INT_MAX, V|E|D, "flags"},
#if FF_API_EMU_EDGE
{"emu_edge", "do not draw edges", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_EMU_EDGE }, INT_MIN, INT_MAX, 0, "flags"},
#endif
{"psnr", "error[?] variables will be set during encoding", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_PSNR }, INT_MIN, INT_MAX, V|E, "flags"},
{"truncated", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_TRUNCATED }, INT_MIN, INT_MAX, 0, "flags"},
#if FF_API_NORMALIZE_AQP
{"naq", "normalize adaptive quantization", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_NORMALIZE_AQP }, INT_MIN, INT_MAX, V|E, "flags"},
#endif
{"ildct", "use interlaced DCT", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_INTERLACED_DCT }, INT_MIN, INT_MAX, V|E, "flags"},
{"low_delay", "force low delay", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_LOW_DELAY }, INT_MIN, INT_MAX, V|D|E, "flags"},
{"global_header", "place global headers in extradata instead of every keyframe", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_GLOBAL_HEADER }, INT_MIN, INT_MAX, V|A|E, "flags"},
{"bitexact", "use only bitexact functions (except (I)DCT)", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_BITEXACT }, INT_MIN, INT_MAX, A|V|S|D|E, "flags"},
{"aic", "H.263 advanced intra coding / MPEG-4 AC prediction", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_AC_PRED }, INT_MIN, INT_MAX, V|E, "flags"},
{"ilme", "interlaced motion estimation", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_INTERLACED_ME }, INT_MIN, INT_MAX, V|E, "flags"},
{"cgop", "closed GOP", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_CLOSED_GOP }, INT_MIN, INT_MAX, V|E, "flags"},
{"output_corrupt", "Output even potentially corrupted frames", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_OUTPUT_CORRUPT }, INT_MIN, INT_MAX, V|D, "flags"},
{"fast", "allow non-spec-compliant speedup tricks", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG2_FAST }, INT_MIN, INT_MAX, V|E, "flags2"},
{"noout", "skip bitstream encoding", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG2_NO_OUTPUT }, INT_MIN, INT_MAX, V|E, "flags2"},
{"ignorecrop", "ignore cropping information from sps", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG2_IGNORE_CROP }, INT_MIN, INT_MAX, V|D, "flags2"},
{"local_header", "place global headers at every keyframe instead of in extradata", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG2_LOCAL_HEADER }, INT_MIN, INT_MAX, V|E, "flags2"},
{"chunks", "Frame data might be split into multiple chunks", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG2_CHUNKS }, INT_MIN, INT_MAX, V|D, "flags2"},
{"showall", "Show all frames before the first keyframe", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG2_SHOW_ALL }, INT_MIN, INT_MAX, V|D, "flags2"},
{"export_mvs", "export motion vectors through frame side data", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG2_EXPORT_MVS}, INT_MIN, INT_MAX, V|D, "flags2"},
{"me_method", "set motion estimation method", OFFSET(me_method), AV_OPT_TYPE_INT, {.i64 = ME_EPZS }, INT_MIN, INT_MAX, V|E, "me_method"},
{"zero", "zero motion estimation (fastest)", 0, AV_OPT_TYPE_CONST, {.i64 = ME_ZERO }, INT_MIN, INT_MAX, V|E, "me_method" },
{"full", "full motion estimation (slowest)", 0, AV_OPT_TYPE_CONST, {.i64 = ME_FULL }, INT_MIN, INT_MAX, V|E, "me_method" },
{"epzs", "EPZS motion estimation (default)", 0, AV_OPT_TYPE_CONST, {.i64 = ME_EPZS }, INT_MIN, INT_MAX, V|E, "me_method" },
{"esa", "esa motion estimation (alias for full)", 0, AV_OPT_TYPE_CONST, {.i64 = ME_FULL }, INT_MIN, INT_MAX, V|E, "me_method" },
{"tesa", "tesa motion estimation", 0, AV_OPT_TYPE_CONST, {.i64 = ME_TESA }, INT_MIN, INT_MAX, V|E, "me_method" },
{"dia", "diamond motion estimation (alias for EPZS)", 0, AV_OPT_TYPE_CONST, {.i64 = ME_EPZS }, INT_MIN, INT_MAX, V|E, "me_method" },
{"log", "log motion estimation", 0, AV_OPT_TYPE_CONST, {.i64 = ME_LOG }, INT_MIN, INT_MAX, V|E, "me_method" },
{"phods", "phods motion estimation", 0, AV_OPT_TYPE_CONST, {.i64 = ME_PHODS }, INT_MIN, INT_MAX, V|E, "me_method" },
{"x1", "X1 motion estimation", 0, AV_OPT_TYPE_CONST, {.i64 = ME_X1 }, INT_MIN, INT_MAX, V|E, "me_method" },
{"hex", "hex motion estimation", 0, AV_OPT_TYPE_CONST, {.i64 = ME_HEX }, INT_MIN, INT_MAX, V|E, "me_method" },
{"umh", "umh motion estimation", 0, AV_OPT_TYPE_CONST, {.i64 = ME_UMH }, INT_MIN, INT_MAX, V|E, "me_method" },
{"iter", "iter motion estimation", 0, AV_OPT_TYPE_CONST, {.i64 = ME_ITER }, INT_MIN, INT_MAX, V|E, "me_method" },
{"extradata_size", NULL, OFFSET(extradata_size), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX},
{"time_base", NULL, OFFSET(time_base), AV_OPT_TYPE_RATIONAL, {.dbl = 0}, INT_MIN, INT_MAX},
{"g", "set the group of picture (GOP) size", OFFSET(gop_size), AV_OPT_TYPE_INT, {.i64 = 12 }, INT_MIN, INT_MAX, V|E},
{"ar", "set audio sampling rate (in Hz)", OFFSET(sample_rate), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, A|D|E},
{"ac", "set number of audio channels", OFFSET(channels), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, A|D|E},

AVFrame

AVFrame 中的AVClass定義位於libavcodec\options.c中。是一個名稱爲av_frame_class的靜態結構體。

例如如下所看到的。

static const AVClass av_frame_class = {
    .class_name              = "AVFrame",
    .item_name               = NULL,
    .option                  = frame_options,
    .version                 = LIBAVUTIL_VERSION_INT,
};

option字段則指向一個元素個數極多的靜態數組frame_options。frame_options定義例如如下所看到的。

static const AVOption frame_options[]={
{"best_effort_timestamp", "", FOFFSET(best_effort_timestamp), AV_OPT_TYPE_INT64, {.i64 = AV_NOPTS_VALUE }, INT64_MIN, INT64_MAX, 0},
{"pkt_pos", "", FOFFSET(pkt_pos), AV_OPT_TYPE_INT64, {.i64 = -1 }, INT64_MIN, INT64_MAX, 0},
{"pkt_size", "", FOFFSET(pkt_size), AV_OPT_TYPE_INT64, {.i64 = -1 }, INT64_MIN, INT64_MAX, 0},
{"sample_aspect_ratio", "", FOFFSET(sample_aspect_ratio), AV_OPT_TYPE_RATIONAL, {.dbl = 0 }, 0, INT_MAX, 0},
{"width", "", FOFFSET(width), AV_OPT_TYPE_INT, {.i64 = 0 }, 0, INT_MAX, 0},
{"height", "", FOFFSET(height), AV_OPT_TYPE_INT, {.i64 = 0 }, 0, INT_MAX, 0},
{"format", "", FOFFSET(format), AV_OPT_TYPE_INT, {.i64 = -1 }, 0, INT_MAX, 0},
{"channel_layout", "", FOFFSET(channel_layout), AV_OPT_TYPE_INT64, {.i64 = 0 }, 0, INT64_MAX, 0},
{"sample_rate", "", FOFFSET(sample_rate), AV_OPT_TYPE_INT, {.i64 = 0 }, 0, INT_MAX, 0},
{NULL},
};

可以看出AVFrame的選項數組中包括了「width」，「height」這類用於視頻幀的選項，以及「channel_layout」，「sample_rate」這類用於音頻幀的選項。

各類組件特有的AVClass

除了FFmpeg中通用的AVFormatContext，AVCodecContext，AVFrame這類的結構體以外，每種特定的組件也包括本身的AVClass。

如下舉例幾個。

LibRTMP

libRTMP中依據協議類型的不一樣定義了多種的AVClass。

由於這些AVClass除了名字不同以外。其餘的字段如出一轍，因此AVClass的聲明寫成了一個名稱爲RTMP_CLASS的宏。

#define RTMP_CLASS(flavor)\
static const AVClass lib ## flavor ## _class = {\
    .class_name = "lib" #flavor " protocol",\
    .item_name  = av_default_item_name,\
    .option     = options,\
    .version    = LIBAVUTIL_VERSION_INT,\
};

然後定義了多種AVCLass：

RTMP_CLASS(rtmp)
RTMP_CLASS(rtmpt)
RTMP_CLASS(rtmpe)
RTMP_CLASS(rtmpte)
RTMP_CLASS(rtmps)

這些AVClass的option字段指向的數組是同樣的，例如如下所看到的。

static const AVOption options[] = {
    {"rtmp_app", "Name of application to connect to on the RTMP server", OFFSET(app), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC|ENC},
    {"rtmp_buffer", "Set buffer time in milliseconds. The default is 3000.", OFFSET(client_buffer_time), AV_OPT_TYPE_STRING, {.str = "3000"}, 0, 0, DEC|ENC},
    {"rtmp_conn", "Append arbitrary AMF data to the Connect message", OFFSET(conn), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC|ENC},
    {"rtmp_flashver", "Version of the Flash plugin used to run the SWF player.", OFFSET(flashver), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC|ENC},
    {"rtmp_live", "Specify that the media is a live stream.", OFFSET(live), AV_OPT_TYPE_INT, {.i64 = 0}, INT_MIN, INT_MAX, DEC, "rtmp_live"},
    {"any", "both", 0, AV_OPT_TYPE_CONST, {.i64 = -2}, 0, 0, DEC, "rtmp_live"},
    {"live", "live stream", 0, AV_OPT_TYPE_CONST, {.i64 = -1}, 0, 0, DEC, "rtmp_live"},
    {"recorded", "recorded stream", 0, AV_OPT_TYPE_CONST, {.i64 = 0}, 0, 0, DEC, "rtmp_live"},
    {"rtmp_pageurl", "URL of the web page in which the media was embedded. By default no value will be sent.", OFFSET(pageurl), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC},
    {"rtmp_playpath", "Stream identifier to play or to publish", OFFSET(playpath), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC|ENC},
    {"rtmp_subscribe", "Name of live stream to subscribe to. Defaults to rtmp_playpath.", OFFSET(subscribe), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC},
    {"rtmp_swfurl", "URL of the SWF player. By default no value will be sent", OFFSET(swfurl), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC|ENC},
    {"rtmp_swfverify", "URL to player swf file, compute hash/size automatically. (unimplemented)", OFFSET(swfverify), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC},
    {"rtmp_tcurl", "URL of the target stream. Defaults to proto://host[:port]/app.", OFFSET(tcurl), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC|ENC},
    { NULL },
};

Libx264

Libx264的AVClass定義例如如下所看到的。

static const AVClass x264_class = {
    .class_name = "libx264",
    .item_name  = av_default_item_name,
    .option     = options,
    .version    = LIBAVUTIL_VERSION_INT,
};

當中option字段指向的數組定義例如如下所看到的。這些option的使用頻率仍是比較高的。

static const AVOption options[] = {
    { "preset",        "Set the encoding preset (cf. x264 --fullhelp)",   OFFSET(preset),        AV_OPT_TYPE_STRING, { .str = "medium" }, 0, 0, VE},
    { "tune",          "Tune the encoding params (cf. x264 --fullhelp)",  OFFSET(tune),          AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE},
    { "profile",       "Set profile restrictions (cf. x264 --fullhelp) ", OFFSET(profile),       AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE},
    { "fastfirstpass", "Use fast settings when encoding first pass",      OFFSET(fastfirstpass), AV_OPT_TYPE_INT,    { .i64 = 1 }, 0, 1, VE},
    {"level", "Specify level (as defined by Annex A)", OFFSET(level), AV_OPT_TYPE_STRING, {.str=NULL}, 0, 0, VE},
    {"passlogfile", "Filename for 2 pass stats", OFFSET(stats), AV_OPT_TYPE_STRING, {.str=NULL}, 0, 0, VE},
    {"wpredp", "Weighted prediction for P-frames", OFFSET(wpredp), AV_OPT_TYPE_STRING, {.str=NULL}, 0, 0, VE},
    {"x264opts", "x264 options", OFFSET(x264opts), AV_OPT_TYPE_STRING, {.str=NULL}, 0, 0, VE},
    { "crf",           "Select the quality for constant quality mode",    OFFSET(crf),           AV_OPT_TYPE_FLOAT,  {.dbl = -1 }, -1, FLT_MAX, VE },
    { "crf_max",       "In CRF mode, prevents VBV from lowering quality beyond this point.",OFFSET(crf_max), AV_OPT_TYPE_FLOAT, {.dbl = -1 }, -1, FLT_MAX, VE },
    { "qp",            "Constant quantization parameter rate control method",OFFSET(cqp),        AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, INT_MAX, VE },
    { "aq-mode",       "AQ method",                                       OFFSET(aq_mode),       AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, INT_MAX, VE, "aq_mode"},
    { "none",          NULL,                              0, AV_OPT_TYPE_CONST, {.i64 = X264_AQ_NONE},         INT_MIN, INT_MAX, VE, "aq_mode" },
    { "variance",      "Variance AQ (complexity mask)",   0, AV_OPT_TYPE_CONST, {.i64 = X264_AQ_VARIANCE},     INT_MIN, INT_MAX, VE, "aq_mode" },
    { "autovariance",  "Auto-variance AQ (experimental)", 0, AV_OPT_TYPE_CONST, {.i64 = X264_AQ_AUTOVARIANCE}, INT_MIN, INT_MAX, VE, "aq_mode" },
    { "aq-strength",   "AQ strength. Reduces blocking and blurring in flat and textured areas.", OFFSET(aq_strength), AV_OPT_TYPE_FLOAT, {.dbl = -1}, -1, FLT_MAX, VE},
    { "psy",           "Use psychovisual optimizations.",                 OFFSET(psy),           AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, 1, VE },
    { "psy-rd",        "Strength of psychovisual optimization, in <psy-rd>:<psy-trellis> format.", OFFSET(psy_rd), AV_OPT_TYPE_STRING,  {0 }, 0, 0, VE},
    { "rc-lookahead",  "Number of frames to look ahead for frametype and ratecontrol", OFFSET(rc_lookahead), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, INT_MAX, VE },
    { "weightb",       "Weighted prediction for B-frames.",               OFFSET(weightb),       AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, 1, VE },
    { "weightp",       "Weighted prediction analysis method.",            OFFSET(weightp),       AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, INT_MAX, VE, "weightp" },
    { "none",          NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_WEIGHTP_NONE},   INT_MIN, INT_MAX, VE, "weightp" },
    { "simple",        NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_WEIGHTP_SIMPLE}, INT_MIN, INT_MAX, VE, "weightp" },
    { "smart",         NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_WEIGHTP_SMART},  INT_MIN, INT_MAX, VE, "weightp" },
    { "ssim",          "Calculate and print SSIM stats.",                 OFFSET(ssim),          AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, 1, VE },
    { "intra-refresh", "Use Periodic Intra Refresh instead of IDR frames.",OFFSET(intra_refresh),AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, 1, VE },
    { "bluray-compat", "Bluray compatibility workarounds.",               OFFSET(bluray_compat) ,AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, 1, VE },
    { "b-bias",        "Influences how often B-frames are used",          OFFSET(b_bias),        AV_OPT_TYPE_INT,    { .i64 = INT_MIN}, INT_MIN, INT_MAX, VE },
    { "b-pyramid",     "Keep some B-frames as references.",               OFFSET(b_pyramid),     AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, INT_MAX, VE, "b_pyramid" },
    { "none",          NULL,                                  0, AV_OPT_TYPE_CONST, {.i64 = X264_B_PYRAMID_NONE},   INT_MIN, INT_MAX, VE, "b_pyramid" },
    { "strict",        "Strictly hierarchical pyramid",       0, AV_OPT_TYPE_CONST, {.i64 = X264_B_PYRAMID_STRICT}, INT_MIN, INT_MAX, VE, "b_pyramid" },
    { "normal",        "Non-strict (not Blu-ray compatible)", 0, AV_OPT_TYPE_CONST, {.i64 = X264_B_PYRAMID_NORMAL}, INT_MIN, INT_MAX, VE, "b_pyramid" },
    { "mixed-refs",    "One reference per partition, as opposed to one reference per macroblock", OFFSET(mixed_refs), AV_OPT_TYPE_INT, { .i64 = -1}, -1, 1, VE },
    { "8x8dct",        "High profile 8x8 transform.",                     OFFSET(dct8x8),        AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, 1, VE},
    { "fast-pskip",    NULL,                                              OFFSET(fast_pskip),    AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, 1, VE},
    { "aud",           "Use access unit delimiters.",                     OFFSET(aud),           AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, 1, VE},
    { "mbtree",        "Use macroblock tree ratecontrol.",                OFFSET(mbtree),        AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, 1, VE},
    { "deblock",       "Loop filter parameters, in <alpha:beta> form.",   OFFSET(deblock),       AV_OPT_TYPE_STRING, { 0 },  0, 0, VE},
    { "cplxblur",      "Reduce fluctuations in QP (before curve compression)", OFFSET(cplxblur), AV_OPT_TYPE_FLOAT,  {.dbl = -1 }, -1, FLT_MAX, VE},
    { "partitions",    "A comma-separated list of partitions to consider. "
                       "Possible values: p8x8, p4x4, b8x8, i8x8, i4x4, none, all", OFFSET(partitions), AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE},
    { "direct-pred",   "Direct MV prediction mode",                       OFFSET(direct_pred),   AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, INT_MAX, VE, "direct-pred" },
    { "none",          NULL,      0,    AV_OPT_TYPE_CONST, { .i64 = X264_DIRECT_PRED_NONE },     0, 0, VE, "direct-pred" },
    { "spatial",       NULL,      0,    AV_OPT_TYPE_CONST, { .i64 = X264_DIRECT_PRED_SPATIAL },  0, 0, VE, "direct-pred" },
    { "temporal",      NULL,      0,    AV_OPT_TYPE_CONST, { .i64 = X264_DIRECT_PRED_TEMPORAL }, 0, 0, VE, "direct-pred" },
    { "auto",          NULL,      0,    AV_OPT_TYPE_CONST, { .i64 = X264_DIRECT_PRED_AUTO },     0, 0, VE, "direct-pred" },
    { "slice-max-size","Limit the size of each slice in bytes",           OFFSET(slice_max_size),AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, INT_MAX, VE },
    { "stats",         "Filename for 2 pass stats",                       OFFSET(stats),         AV_OPT_TYPE_STRING, { 0 },  0,       0, VE },
    { "nal-hrd",       "Signal HRD information (requires vbv-bufsize; "
                       "cbr not allowed in .mp4)",                        OFFSET(nal_hrd),       AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, INT_MAX, VE, "nal-hrd" },
    { "none",          NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_NAL_HRD_NONE}, INT_MIN, INT_MAX, VE, "nal-hrd" },
    { "vbr",           NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_NAL_HRD_VBR},  INT_MIN, INT_MAX, VE, "nal-hrd" },
    { "cbr",           NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_NAL_HRD_CBR},  INT_MIN, INT_MAX, VE, "nal-hrd" },
    { "avcintra-class","AVC-Intra class 50/100/200",                      OFFSET(avcintra_class),AV_OPT_TYPE_INT,     { .i64 = -1 }, -1, 200   , VE},
    { "x264-params",  "Override the x264 configuration using a :-separated list of key=value parameters", OFFSET(x264_params), AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE },
    { NULL },
};

Libx265

Libx265的AVClass定義例如如下所看到的。

static const AVClass class = {
    .class_name = "libx265",
    .item_name  = av_default_item_name,
    .option     = options,
    .version    = LIBAVUTIL_VERSION_INT,
};

當中option字段指向的數組定義例如如下所看到的。

static const AVOption options[] = {
    { "preset",      "set the x265 preset",                                                         OFFSET(preset),    AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE },
    { "tune",        "set the x265 tune parameter",                                                 OFFSET(tune),      AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE },
    { "x265-params", "set the x265 configuration using a :-separated list of key=value parameters", OFFSET(x265_opts), AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE },
    { NULL }
};

官方代碼中有關AVClass和AVOption的演示樣例

官方代碼中給出了一小段演示樣例代碼，演示了怎樣給一個普通的結構體加入AVOption的支持。

例如如下所看到的。

typedef struct test_struct {
    AVClass  *class;
    int      int_opt;
    char    str_opt;
    uint8_t  bin_opt;
    int      bin_len;
} test_struct;

static const AVOption test_options[] = {
  { "test_int", "This is a test option of int type.", offsetof(test_struct, int_opt),
    AV_OPT_TYPE_INT, { .i64 = -1 }, INT_MIN, INT_MAX },
  { "test_str", "This is a test option of string type.", offsetof(test_struct, str_opt),
    AV_OPT_TYPE_STRING },
  { "test_bin", "This is a test option of binary type.", offsetof(test_struct, bin_opt),
    AV_OPT_TYPE_BINARY },
  { NULL },
};

static const AVClass test_class = {
    .class_name = "test class",
    .item_name  = av_default_item_name,
    .option     = test_options,
    .version    = LIBAVUTIL_VERSION_INT,
};

AVClass有關的API

與AVClass相關的API很是少。

AVFormatContext提供了一個獲取當前AVClass的函數avformat_get_class()。它的代碼很是easy，直接返回全局靜態變量av_format_context_class。

定義例如如下所看到的。

const AVClass *avformat_get_class(void)
{
    return &av_format_context_class;
}

相同。AVCodecContext也提供了一個獲取當前AVClass的函數avcodec_get_class()。它直接返回靜態變量av_codec_context_class。定義例如如下所看到的。

const AVClass *avcodec_get_class(void)
{
    return &av_codec_context_class;
}

至此FFmpeg的AVClass就基本上分析完畢了。

下篇文章詳細分析AVOption。

雷霄驊leixiaohua1020@126.comhttp://blog.csdn.net/leixiaohua1020

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