register all media file container muxers in ffmpeg

最新推荐文章于 2024-05-26 20:51:29 发布

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本文介绍FFmpeg中如何注册与使用mov和3gp等媒体封装器(muxers)，包括注册过程、数据结构及查找指定封装器的方法。通过源代码示例详细解释了这些封装器的工作原理。

1. muxers such mov and 3gp

libavformat/movenc.c:

#if CONFIG_MOV_MUXER
MOV_CLASS(mov)
AVOutputFormat ff_mov_muxer = {
    .name              = "mov",
    .long_name         = NULL_IF_CONFIG_SMALL("QuickTime / MOV"),
    .extensions        = "mov",
    .priv_data_size    = sizeof(MOVMuxContext),
    .audio_codec       = AV_CODEC_ID_AAC,
    .video_codec       = CONFIG_LIBX264_ENCODER ?
                         AV_CODEC_ID_H264 : AV_CODEC_ID_MPEG4,
    .write_header      = mov_write_header,
    .write_packet      = mov_write_packet,
    .write_trailer     = mov_write_trailer,
    .flags             = AVFMT_GLOBALHEADER | AVFMT_ALLOW_FLUSH | AVFMT_TS_NEGATIVE,
    .codec_tag         = (const AVCodecTag* const []){
        ff_codec_movvideo_tags, ff_codec_movaudio_tags, 0
    },
    .priv_class        = &mov_muxer_class,
};
#endif
#if CONFIG_TGP_MUXER
MOV_CLASS(tgp)
AVOutputFormat ff_tgp_muxer = {
    .name              = "3gp",
    .long_name         = NULL_IF_CONFIG_SMALL("3GP (3GPP file format)"),
    .extensions        = "3gp",
    .priv_data_size    = sizeof(MOVMuxContext),
    .audio_codec       = AV_CODEC_ID_AMR_NB,
    .video_codec       = AV_CODEC_ID_H263,
    .write_header      = mov_write_header,
    .write_packet      = mov_write_packet,
    .write_trailer     = mov_write_trailer,
    .flags             = AVFMT_GLOBALHEADER | AVFMT_ALLOW_FLUSH | AVFMT_TS_NEGATIVE,
    .codec_tag         = (const AVCodecTag* const []){ codec_3gp_tags, 0 },
    .priv_class        = &tgp_muxer_class,
};
#endif

2. so how to register these muxers such as mov and 3gp?

in libavformat/allformats.c:

#define REGISTER_MUXER(X, x)                                            \
    {                                                                   \
        extern AVOutputFormat ff_##x##_muxer;                           \
        if (CONFIG_##X##_MUXER)                                         \
            av_register_output_format(&ff_##x##_muxer);                 \
    }

#define REGISTER_DEMUXER(X, x)                                          \
    {                                                                   \
        extern AVInputFormat ff_##x##_demuxer;                          \
        if (CONFIG_##X##_DEMUXER)                                       \
            av_register_input_format(&ff_##x##_demuxer);                \
    }

#define REGISTER_MUXDEMUX(X, x) REGISTER_MUXER(X, x); REGISTER_DEMUXER(X, x)

also in libavformat/allformats.c

void av_register_all(void)
{
    static int initialized;

    if (initialized)
        return;
    initialized = 1;

    avcodec_register_all();

    /* (de)muxers */
    REGISTER_MUXER   (A64,              a64);
    REGISTER_DEMUXER (AAC,              aac);
    REGISTER_MUXDEMUX(AC3,              ac3);
......
    REGISTER_MUXDEMUX(MOV,              mov);
    REGISTER_MUXER   (MP2,              mp2);
    REGISTER_MUXDEMUX(MP3,              mp3);
    REGISTER_MUXER   (MP4,              mp4);

all the registered muxers are save in the below single linked list:

static AVOutputFormat *first_oformat = NULL;

linked by the below field of AVOutputFormat:

struct AVOutputFormat *next;

get the target AVOutputFormat(a muxer) according to muxer name, for example mov:

m_pFormatCtx->oformat = av_guess_format("mov", NULL, NULL);

AVOutputFormat *av_guess_format(const char *short_name, const char *filename,
                                const char *mime_type)
    /* Find the proper file type. */
    fmt_found = NULL;
    score_max = 0;
    while ((fmt = av_oformat_next(fmt))) {
        score = 0;
        if (fmt->name && short_name && match_format(short_name, fmt->name))
            score += 100;
        if (fmt->mime_type && mime_type && !strcmp(fmt->mime_type, mime_type))
            score += 10;
        if (filename && fmt->extensions &&
            av_match_ext(filename, fmt->extensions)) {
            score += 5;
        }
        if (score > score_max) {
            score_max = score;
            fmt_found = fmt;
        }
    }
    return fmt_found;

in the above example, only short_name is not null. both filename and mime_type are null