ffmpeg source code notes - find codec (1)

The structure of AVCodec type contains the encapsulation of the underlying implementation of an encoder ; the definition is as follows:

typedef struct AVCodec {
    //编码器名,在编码器和解码器两个类别中分别具有唯一性;
    //如:libx264
    const char *name;
 
    //编码器实例的完整名称;
    //如:libx264 H.264/AVC/MPEG-4 AVC/MPEG-4 part 10
    const char *long_name;
 
    //当前编码器处理的媒体类型;
    enum AVMediaType type;
 
    //编码类型ID;
    enum AVCodecID id;
 
    //当前编码器所支持的能力;
    int capabilities;
 
    //支持的帧率
    const AVRational *supported_framerates; 
    //支持的图像像素格式;
    const enum AVPixelFormat *pix_fmts;    
 
    //支持的音频采样率 
    const int *supported_samplerates;  
    //支持的音频采样格式    
    const enum AVSampleFormat *sample_fmts; 
    //支持的声道布局
    const uint64_t *channel_layouts;      
    //支持的降分辨率解码;  
    uint8_t max_lowres;                     
    const AVClass *priv_class;  
 
    //支持的编码档次;      
    const AVProfile *profiles;              
 
    /*编码器实现的组件或封装名称,主要用于标识该编码器的外部实现者;
      当该字段为空时,该编码器有libavcodec库内部实现;当该字段不为空时,该编码器由硬件或操作系统
      等外部实现,并在字段保存AVCodec.nam的缩写;    
    */
    const char *wrapper_name;
 
    int priv_data_size;
 
    //实现链表
    struct AVCodec *next;
   
    int (*update_thread_context)(struct AVCodecContext *dst, const struct AVCodecContext *src);
  
    const AVCodecDefault *defaults;
 
    void (*init_static_data)(struct AVCodec *codec);
 
    int (*init)(struct AVCodecContext *);
    int (*encode_sub)(struct AVCodecContext *, uint8_t *buf, int buf_size,
                      const struct AVSubtitle *sub);
    int (*encode2)(struct AVCodecContext *avctx, struct AVPacket *avpkt,
                   const struct AVFrame *frame, int *got_packet_ptr);
    int (*decode)(struct AVCodecContext *, void *outdata, int *outdata_size, struct AVPacket *avpkt);
    int (*close)(struct AVCodecContext *);
   
    int (*send_frame)(struct AVCodecContext *avctx, const struct AVFrame *frame);
    int (*receive_packet)(struct AVCodecContext *avctx, struct AVPacket *avpkt);
   
    int (*receive_frame)(struct AVCodecContext *avctx, struct AVFrame *frame);
    void (*flush)(struct AVCodecContext *);
    
    int caps_internal;
    const char *bsfs;
 
    const struct AVCodecHWConfigInternal **hw_configs;
 
    const uint32_t *codec_tags;
} AVCodec;

1. How to find the encoder

//通过指定名称查找编码器实例; 
AVCodec *avcodec_find_encoder_by_name(const char *name);
 
//通过指定编码器ID查找编码器实例;
AVCodec *avcodec_find_encoder(enum AVCodecID id);

(a) The name is the name of the codec . I don’t know how to fill in the many extern structures defined at the beginning of the file;

The most common is extern AVCodec ff_libx264_encoder;

Click to go to:

AVCodec ff_libx264_encoder = {
    .name             = "libx264",
    .long_name        = NULL_IF_CONFIG_SMALL("libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
    .type             = AVMEDIA_TYPE_VIDEO,
    .id               = AV_CODEC_ID_H264,
    .priv_data_size   = sizeof(X264Context),
    .init             = X264_init,
    .encode2          = X264_frame,
    .close            = X264_close,
    .capabilities     = AV_CODEC_CAP_DELAY | AV_CODEC_CAP_AUTO_THREADS |
                        AV_CODEC_CAP_ENCODER_REORDERED_OPAQUE,
    .priv_class       = &x264_class,
    .defaults         = x264_defaults,
    .init_static_data = X264_init_static,
#if X264_BUILD >= 158
    .caps_internal    = FF_CODEC_CAP_INIT_CLEANUP | FF_CODEC_CAP_INIT_THREADSAFE,
#else
    .caps_internal    = FF_CODEC_CAP_INIT_CLEANUP,
#endif
    .wrapper_name     = "libx264",
};

It can be seen that the corresponding name can be filled with libx264;

To support x264, you need to configure --enable-x264 and other words when configuring;

(b) enum AVCodecID id; corresponding codec_id.h;

2. How to find the decoder

//通过名字查找指定解码器
AVCodec *avcodec_find_decoder_by_name(const char *name);
 
//通过ID查找指定解码器
AVCodec *avcodec_find_decoder(enum AVCodecID id);

Many codec structures are defined at the beginning of libavcodec/allcodecs.c; such as:

extern AVCodec ff_h264_decoder;
...
 
//音频编解码器
extern AVCodec ff_aac_encoder;
extern AVCodec ff_aac_decoder;
...
 
//PCM编解码器
extern AVCodec ff_pcm_alaw_encoder;
extern AVCodec ff_pcm_alaw_decoder;
...
 
//DPCM编解码器
extern AVCodec ff_derf_dpcm_decoder;
extern AVCodec ff_gremlin_dpcm_decoder;
...
 
//ADPCM编解码器
extern AVCodec ff_adpcm_4xm_decoder;
extern AVCodec ff_adpcm_adx_encoder;
...
 
//字幕编解码器
extern AVCodec ff_ssa_encoder;
extern AVCodec ff_ssa_decoder;
...
 
//外部库、
extern AVCodec ff_aac_at_encoder;
extern AVCodec ff_aac_at_decoder;
...
//优于libwebp
extern AVCodec ff_libx264_encoder;
 
 
//文本
extern AVCodec ff_bintext_decoder;
extern AVCodec ff_xbin_decoder;
extern AVCodec ff_idf_decoder;
 
//其他外部库
extern AVCodec ff_aac_mf_encoder;
extern AVCodec ff_ac3_mf_encoder;
...

3. Source code interpretation avcodec_find_encoder_by_name call

avcodec_find_encoder_by_name called find_codec_by_name;

//参数1,需要查找的编解码器名  参数2:回调函数指针,用于判断下面查找出来的AVCodec是否为编/解码器
static AVCodec *find_codec_by_name(const char *name, int (*x)(const AVCodec *))
{
    void *i = 0;
    const AVCodec *p;
 
    if (!name)
        return NULL;
 
    //av_codec_iterate相当于迭代器遍历所有的codec_list数组,返回值p为遍历到的编/解码器
    while ((p = av_codec_iterate(&i))) {
        if (!x(p)) //既不是编码器也不是解码器,滤过;
            continue;
        if (strcmp(name, p->name) == 0) //比较名字
            return (AVCodec*)p;
    }
 
    return NULL;//没有找到
}

The av_codec_iterate called is as follows:

//传入的是上面的整形值i的地址,二级指针,可以改传入的地址值;
const AVCodec *av_codec_iterate(void **opaque)
{
    uintptr_t i = (uintptr_t)*opaque;
    const AVCodec *c = codec_list[i];
 
    ff_thread_once(&av_codec_static_init, av_codec_init_static);
 
    if (c) //若codec_list不为NULL,继续下一个
        *opaque = (void*)(i + 1); //相当于i++
 
    return c; //返回AVCodec,可能为NULL;
}

Which calls ff_thread_once again;

4. Source code interpretation avcodec_find_encoder call process

avcodec_find_encoder calls find_codec,

static AVCodec *find_codec(enum AVCodecID id, int (*x)(const AVCodec *))
{
    const AVCodec *p, *experimental = NULL;
    void *i = 0;
 
    //看名字是重映射,实际啥都没干;
    id = remap_deprecated_codec_id(id);
 
    while ((p = av_codec_iterate(&i))) { //通过codec_list数组获取AVCodec
        if (!x(p)) //判断是否为编/解码器
            continue;
        if (p->id == id) { //比较ID
 
            //判断编/解码器是否是实验性的,优先选择非实验性的;
            if (p->capabilities & AV_CODEC_CAP_EXPERIMENTAL && !experimental) {
                experimental = p;
            } else
                return (AVCodec*)p;
        }
    }
 
    return (AVCodec*)experimental;
}

5. Source code interpretation avcodec_register role

Called ff_thread_once(&av_codec_next_init, av_codec_init_next);

static AVOnce av_codec_next_init = AV_ONCE_INIT; //标识只初始化一次
 
static void av_codec_init_next(void)
{
    AVCodec *prev = NULL, *p;
    void *i = 0;
    while ((p = (AVCodec*)av_codec_iterate(&i))) {
        if (prev)
            prev->next = p; //把所有编/解码器连接起来
        prev = p;
    }
}

6. Summary

avcodec_find_encoder_by_name to find the encoder can make developers more control over the system, but the overall compatibility is weak, because once the currently used FFmpeg does not support the specified encoder, the entire process will end with an error;

If avcodec_find_encoder is used, the caller cannot specify a specific encoder for encoding, it can only be automatically selected by the system according to the priority, so the compatibility is better;

Select according to requirements during actual development;

Supplementary Note:

#if CONFIG_OSSFUZZ
AVCodec * codec_list[] = {
    NULL,
    NULL,
    NULL
};
#else
#include "libavcodec/codec_list.c"
#endif

There is a codec_list array in the source code, and its content is in #include "libavcodec/codec_list.c", but there is no such file in the source code; but there is this file in the compiled source code; so the file is generated according to the version when compiling ; its contents are as follows:

static const AVCodec * const codec_list[] = {
    &ff_a64multi_encoder,
    &ff_a64multi5_encoder,
    &ff_alias_pix_encoder,
    &ff_amv_encoder,
    &ff_asv1_encoder,
    &ff_asv2_encoder,
    &ff_avrp_encoder,
    &ff_avui_encoder,
    ...
    ...
};

Original link: ffmpeg source code notes - find codecs (1) _ffmpeg View supported encoders

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