time_base of ffmpeg transcode

来源：互联网发布：杉杉来了知乎编辑：程序博客网时间：2024/06/08 03:35

ffmpeg-2.2.2

ffmpeg -f mpegts -i film.ts -c:a libfaac -c:v libx264 -f mp4 ff.mp4

1. video

1.1 get frame_rate in transcode_init

Line: 2497

           if (codec->codec_type == AVMEDIA_TYPE_VIDEO) {
                if (ost->filter && !ost->frame_rate.num)
                    ost->frame_rate = av_buffersink_get_frame_rate(ost->filter->filter);

.......................................................

}

=> ost->frame_rate = {num = 24000, den = 1001}

so far, (initializing value)

enc->time_base = {0, 1}

enc->pkt_timebase = {0, 0}

ost->st->time_base = {1, 90000}

1.2 get codec->time_base in transcode_init

Line: 2531

case AVMEDIA_TYPE_VIDEO:
codec->time_base = av_inv_q(ost->frame_rate);

codec->time_base = {1001, 24000}

1.3 get st->time_base and enc->pkt_timebase

=>avformat_write_header =>mov_write_header =>

for (i = 0; i < s->nb_streams; i++) {
AVStream *st= s->streams[i];
MOVTrack *track= &mov->tracks[i];

...........................................................................................

track->enc = st->codec;
track->st = st;

..............................................................................................

            if (mov->video_track_timescale) {
                track->timescale = mov->video_track_timescale;
            } else {
                track->timescale = st->codec->time_base.den;    // track->timescale = 24000;
                while(track->timescale < 10000)
                    track->timescale *= 2;
            }

....................................................................................................

avpriv_set_pts_info(st, 64, 1, track->timescale);

=>st->time_base = {1, 24000}

=>st->codec->pkt_timebase = {1, 24000}

.......................................................................................................

}

而如果： ffmpeg -f mpegts -i film.ts -c:a libfaac -c:v libx264 -f mpegts ff.ts

st->codec->time_base = {1001, 24000}

则在 mpegts_write_header =>

for(i = 0;i < s->nb_streams; i++) {
st = s->streams[i];
avpriv_set_pts_info(st, 33, 1, 90000);

=>st->time_base = {1, 90000}

=>st->codec->pkt_timebase = {1, 90000}

..........................................................................

}

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