[FFmpeg-devel] Match source video timestamp

[Eran Kornblau]

> Hi all,
> 
> I'm trying to transcode some video file (MP4/h264 baseline vfr/AAC) with ffmpeg, and I would like to have the 
> frame timestamps in the transcoded file perfectly match the source video. This is in order to have them both
> play together adaptively without issues (from my experience, DASH is very sensitive to timestamp continuity,
> a difference as small as a couple of frames between renditions causes playback to fail)
> 
> After trying all sorts of parameters (-vsync 0/2, -copytb, with/out -r, -video_track_timescale) without success,
> I found that the reason for the mismatched timestamps is that ffmpeg uses 1/frame_rate as the time base
> of the encoder. With cfr this is probably ok, but with vfr this quantizes the timestamps and they come out
> different than the source.
> 
> Just for testing, I made this change:
> root at pa-front-vod-stg2 FFmpeg# git diff
> diff --git a/ffmpeg.c b/ffmpeg.c
> index 3b91710..9cba0d5 100644
> --- a/ffmpeg.c
> +++ b/ffmpeg.c
> @@ -3351,7 +3351,7 @@ static int transcode_init(void)
>                  enc_ctx->time_base      = (AVRational){ 1, enc_ctx->sample_rate };
>                  break;
>              case AVMEDIA_TYPE_VIDEO:
> -                enc_ctx->time_base = av_inv_q(ost->frame_rate);
> +                enc_ctx->time_base = ist->st->time_base; //av_inv_q(ost->frame_rate);
>                  if (!(enc_ctx->time_base.num && enc_ctx->time_base.den))
>                      enc_ctx->time_base = ost->filter->filter->inputs[0]->time_base;
>                  if (   av_q2d(enc_ctx->time_base) < 0.001 && video_sync_method != VSYNC_PASSTHROUGH
> 
> And I am able to get a perfect match (=identical stts atom) between the source & transcoded videos.
> This is the command line I'm using (the -r is probably meaningless...):
> ffmpeg -threads 1 -i input.mp4 -c:v libx264  -subq 2 -qcomp 0.6 -qmin 10 -qmax 50 -qdiff 4 -coder 0 -x264opts stitchable -vprofile baseline -force_key_frames source -pix_fmt yuv420p -b:v 400k -s 640x480 -r 25.174 -g 86400 -aspect 640:480 -c:a copy -map_chapters -1 -map_metadata -1  -f mp4 -flags +loop+mv4 -cmp 256 -partitions +parti4x4+partp8x8+partb8x8 -trellis 1 -refs 1 -me_range 16 -keyint_min 20 -sc_threshold 0 -i_qfactor 0.71 -bt 100k -maxrate 400k -bufsize 1200k -rc_eq 'blurCplx^(1-qComp)' -level 30  -vsync 2 -threads 4  -y output.mp4
> 
> I'm thinking about adding variables for this -video_timescale / -audio_timescale (following the convention of 
> movenc's video_track_timescale) that will get these values:
> 1.	0 (default) - the existing behavior - 1/frame rate for video, 1/sampling rate for audio
> 2.	-1 - match the input stream (as in the patch above)
> 3.	>0 - fixed value (e.g. when passing 1000 it will use 1/1000)
> 
> Does this make sense? Any other ideas for solving this?
> 
> Thank you!
> 
> Eran
>
As I got no feedback on the above... sending a patch with my suggestion for this

Thanks,

Eran
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