[FFMPEG源码]Format格式探测

Version:3.4.6

ffmpeg对于格式的探测其实是两步：

• read_probe
• extensions

参考分析路径

./libavformat
  |---- utils.c
        |---- avformat_open_input
        |---- init_input
  |---- format.c
        |---- av_probe_input_buffer2
        |---- av_probe_input_format3
  |---- avformat.h - wtvdec/paf/mgsts/jvdec.c
        |---- read_probe

详情

static int init_input(AVFormatContext *s, const char *filename,
                      AVDictionary **options)
{
    //...
    return av_probe_input_buffer2(s->pb, &s->iformat, filename,
                                 s, 0, s->format_probesize);
}

int av_probe_input_buffer2(AVIOContext *pb, AVInputFormat **fmt,
                          const char *filename, void *logctx,
                          unsigned int offset, unsigned int max_probe_size)
{
    //...
    //把AVFormatContext和具体的AVInputFormat联系起来了
     *fmt = av_probe_input_format2(&pd, 1, &score);
    //...
}

FFmpeg中实现探测的函数是av_probe_input_buffer2和av_probe_input_format3

* format.c - av_probe_input_format3

/**
 * 遍历所有的Demuxer
 * 是优先使用demuxer的read_probe函数，然后参考文件名的后缀名来关联某个AVInputFormat
 */
    fmt = NULL;
    while ((fmt1 = av_iformat_next(fmt1))) {
        if (!is_opened == !(fmt1->flags & AVFMT_NOFILE) && strcmp(fmt1->name, "image2"))
            continue;
        score = 0;
        if (fmt1->read_probe) {
            score = fmt1->read_probe(&lpd);
            if (score)
                av_log(NULL, AV_LOG_TRACE, "Probing %s score:%d size:%d\n", fmt1->name, score, lpd.buf_size);
            if (fmt1->extensions && av_match_ext(lpd.filename, fmt1->extensions)) {
                switch (nodat) {
                case NO_ID3:
                    score = FFMAX(score, 1);
                    break;
                case ID3_GREATER_PROBE:
                case ID3_ALMOST_GREATER_PROBE:
                    score = FFMAX(score, AVPROBE_SCORE_EXTENSION / 2 - 1);
break;
                case ID3_GREATER_MAX_PROBE:
                    score = FFMAX(score, AVPROBE_SCORE_EXTENSION);
                    break;
                }
            }
        } else if (fmt1->extensions) {
            if (av_match_ext(lpd.filename, fmt1->extensions))
                score = AVPROBE_SCORE_EXTENSION;
        }
        if (av_match_name(lpd.mime_type, fmt1->mime_type)) {
            if (AVPROBE_SCORE_MIME > score) {
                av_log(NULL, AV_LOG_DEBUG, "Probing %s score:%d increased to %d due to MIME type\n", fmt1->name, score, AVPROBE_SCORE_MIME);
                score = AVPROBE_SCORE_MIME;
            }
        }
        if (score > score_max) {
            score_max = score;
            fmt       = fmt1;
        } else if (score == score_max)
            fmt = NULL;
    }
    if (nodat == ID3_GREATER_PROBE)
        score_max = FFMIN(AVPROBE_SCORE_EXTENSION / 2 - 1, score_max);
    *score_ret = score_max;

    return fmt;