ffmpeg分析系列之三(输入输出格式)

转自：http://blog.csdn.net/menuconfig/article/details/6613589

探测数据结构:

/** This structure contains the data a format has to probe a file. */
typedef struct AVProbeData {
    const char *filename;
    unsigned char *buf; /**< Buffer must have AVPROBE_PADDING_SIZE of extra allocated bytes filled with zero. */
    int buf_size; /**< Size of buf except extra allocated bytes */
} AVProbeData;

h264的探测函数:

static int h264_probe(AVProbeData *p)
{
    uint32_t code= -1;
    int sps=0, pps=0, idr=0, res=0, sli=0;
    int i;

    for(i=0; i<p->buf_size; i++){
        code = (code<<8) + p->buf[i];
        if ((code & 0xffffff00) == 0x100) {
            int ref_idc= (code>>5)&3;
            int type = code & 0x1F;
            static const int8_t ref_zero[32]={
                2, 0, 0, 0, 0,-1, 1,-1,
               -1, 1, 1, 1, 1,-1, 2, 2,
                2, 2, 2, 0, 2, 2, 2, 2,
                2, 2, 2, 2, 2, 2, 2, 2
            };

            if(code & 0x80) //forbidden bit

                return 0;

            if(ref_zero[type] == 1 && ref_idc)
                return 0;
            if(ref_zero[type] ==-1 && !ref_idc)
                return 0;
            if(ref_zero[type] == 2)
                res++;

            switch(type){
            case 1: sli++; break;
            case 5: idr++; break;
            case 7:
                if(p->buf[i+2]&0x0F)
                    return 0;
                sps++;
                break;
            case 8: pps++; break;
            }
        }
    }
    if(sps && pps && (idr||sli>3) && res<(sps+pps+idr))
        return AVPROBE_SCORE_MAX/2+1; // +1 for .mpg

    return 0;
}

视频读首部函数:

static int video_read_header(AVFormatContext *s,
                             AVFormatParameters *ap)
{
    AVStream *st;

    st = av_new_stream(s, 0);
    if (!st)
        return AVERROR(ENOMEM);

    st->codec->codec_type = AVMEDIA_TYPE_VIDEO;
    st->codec->codec_id = s->iformat->value;
    st->need_parsing = AVSTREAM_PARSE_FULL;

    /* for MJPEG, specify frame rate */
    /* for MPEG-4 specify it, too (most MPEG-4 streams do not have the fixed_vop_rate set ...)*/
    if (ap->time_base.num) {
        st->codec->time_base= ap->time_base;
    } else if ( st->codec->codec_id == CODEC_ID_MJPEG ||
                st->codec->codec_id == CODEC_ID_MPEG4 ||
                st->codec->codec_id == CODEC_ID_DIRAC ||
                st->codec->codec_id == CODEC_ID_DNXHD ||
                st->codec->codec_id == CODEC_ID_H264) {
        st->codec->time_base= (AVRational){1,25};
    }
    av_set_pts_info(st, 64, 1, 1200000);

    return 0;
}

原始地读实际的包函数:

int ff_raw_read_partial_packet(AVFormatContext *s, AVPacket *pkt)
{
    int ret, size;

    size = RAW_PACKET_SIZE;

    if (av_new_packet(pkt, size) < 0)
        return AVERROR(ENOMEM);

    pkt->pos= url_ftell(s->pb);
    pkt->stream_index = 0;
    ret = get_partial_buffer(s->pb, pkt->data, size);
    if (ret < 0) {
        av_free_packet(pkt);
        return ret;
    }
    pkt->size = ret;
    return ret;
}

原始地写包函数:

static int raw_write_packet(struct AVFormatContext *s, AVPacket *pkt)
{
    put_buffer(s->pb, pkt->data, pkt->size);
    put_flush_packet(s->pb);
    return 0;
}

h264混合器:

AVOutputFormat h264_muxer = {
    "h264",
    NULL_IF_CONFIG_SMALL("raw H.264 video format"),
    NULL,
    "h264",
    0,
    CODEC_ID_NONE,
    CODEC_ID_H264,
    NULL,
    raw_write_packet,
    .flags= AVFMT_NOTIMESTAMPS,
};

h264分离器:

AVInputFormat h264_demuxer = {
    "h264",
    NULL_IF_CONFIG_SMALL("raw H.264 video format"),
    0,
    h264_probe,
    video_read_header,
    ff_raw_read_partial_packet,
    .flags= AVFMT_GENERIC_INDEX,
    .extensions = "h26l,h264,264", //FIXME remove after writing mpeg4_probe
    .value = CODEC_ID_H264,
}

libavformat/allformats.c文件的av_register_all函数注册了h264分离器和混合器:

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

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

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

void av_register_all(void)
{
    /* 省略部分代码 */
    /* protocols */
    REGISTER_MUXDEMUX (H264, h264);
    /* 省略部分代码 */
}

把注册格式函数也贴出来吧:

/** head of registered input format linked list */
AVInputFormat *first_iformat = NULL;
/** head of registered output format linked list */
AVOutputFormat *first_oformat = NULL;

void av_register_input_format(AVInputFormat *format)
{
    AVInputFormat **p;
    p = &first_iformat;
    while (*p != NULL) p = &(*p)->next;
    *p = format;
    format->next = NULL;
}

void av_register_output_format(AVOutputFormat *format)
{
    AVOutputFormat **p;
    p = &first_oformat;
    while (*p != NULL) p = &(*p)->next;
    *p = format;
    format->next = NULL;
}