FFmpeg h264_probe函数剖析

函数调用
avformat_open_input
s->iformat->read_header(s)

函数说明
/**

• Read the format header and initialize the AVFormatContext
• structure. Return 0 if OK. 'avformat_new_stream' should be
• called to create new streams.
  /
  int (read_header)(struct AVFormatContext *);

从read_header函数调用，去探测h264的码流格式，流程是怎么样的有待研究
代码说明
static int h264_probe(AVProbeData *p)
{
uint32_t code = -1;
int sps = 0, pps = 0, idr = 0, res = 0, sli = 0;
int i, ret;
int pps_ids[MAX_PPS_COUNT+1] = {0};
int sps_ids[MAX_SPS_COUNT+1] = {0};
unsigned pps_id, sps_id;
GetBitContext gb;

for (i = 0; i + 2 < 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[] = {
             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) {
            if (!(code == 0x100 && !p->buf[i + 1] && !p->buf[i + 2]))
                res++;
        }

        ret = init_get_bits8(&gb, p->buf + i + 1, p->buf_size - i - 1);
        if (ret < 0)
            return 0;

        switch (type) {
        case 1:
        case 5:
            get_ue_golomb_long(&gb);
            if (get_ue_golomb_long(&gb) > 9U)
                return 0;
            pps_id = get_ue_golomb_long(&gb);
            if (pps_id > MAX_PPS_COUNT)
                return 0;
            if (!pps_ids[pps_id])
                break;

            if (type == 1)
                sli++;
            else
                idr++;
            break;
        case 7:
            skip_bits(&gb, 14);
            if (get_bits(&gb, 2))
                return 0;
            skip_bits(&gb, 8);
            sps_id = get_ue_golomb_long(&gb);
            if (sps_id > MAX_SPS_COUNT)
                return 0;
            sps_ids[sps_id] = 1;
            sps++;
            break;
        case 8:
            pps_id = get_ue_golomb_long(&gb);
            if (pps_id > MAX_PPS_COUNT)
                return 0;
            sps_id = get_ue_golomb_long(&gb);
            if (sps_id > MAX_SPS_COUNT)
                return 0;
            if (!sps_ids[sps_id])
                break;
            pps_ids[pps_id] = 1;
            pps++;
            break;
        }
    }
}
ff_tlog(NULL, "sps:%d pps:%d idr:%d sli:%d res:%d\n", sps, pps, idr, sli, res);

if (sps && pps && (idr || sli > 3) && res < (sps + pps + idr))
    return AVPROBE_SCORE_EXTENSION + 1;  // 1 more than .mpg

return 0;

}

参考
http://blog.csdn.net/leixiaohua1020/article/details/44064715
http://blog.csdn.net/hustbin/article/details/7631238

     本文转自fengyuzaitu 51CTO博客，原文链接：http://blog.51cto.com/fengyuzaitu/2043501，如需转载请自行联系原作者