[FFmpeg-devel] [RFC] Non monotonically increasing timestamps

[Mike Scheutzow]

Stefano Sabatini wrote:
>  On date Monday 2011-03-21 18:26:36 +0100, Stefano Sabatini encoded:
> > The problem shows up tipically when copying with -vcodec copy, when
> >  transcoding I suppose DTS is recomputed and so it never results in
> >  duplicated DTSs.
> >
> > So I'm asking if someone can suggest which is a good / the best way
> >  to fix the DTS duplication problem.
> >
> > A simple solution may consists of simply dropping the packets with
> >  the duplicated DTS, which may result in video corruption (e.g. in
> >  the abovementioned case, where the packet is a key packet), or fix
> >  the dts somehow.
> >
> > A full-fledged solution may involve the implementation of a
> > packet-level filtering API (which would be useful also for ffprobe,
> >  e.g. for selecting only particular packets, e.g.
> > type=A|V|S/stream=X/PTS>X etc.).
>
>  Well, I investigated more with the issues. A possible fix may require
>  adding a check in ffmpeg (see attached), which issues a warning and
>  fixes the DTS/PTS accordingly (BUT: maybe this should be moved into
>  the library, and enable the check according to some flag).
>
>  But then I discover that there is a problem while reading, in certain
>  cases ffmpeg generates incorrect DTSes.
>
>  Precisely when we have this situation: pkt1: PTS1=DTS1 pkt2:
>  DTS=AV_NOPTS_VALUE
>
>  in this case FFmpeg (libavformat/utils.c:compute_pkt_fields) sets:
>  DTS2 = pkt.dts = st->last_IP_pts
>
>  which corresponds to PTS1=DTS1, so we have DTS2 == DTS1 and ffmpeg
>  fails when remuxing the file.
>
>  Note that the attached code doesn't prevent the error for remuxed
>  MPEG-PS streams (with -*codec copy), indeed the MPEG-PS demuxer
>  sometimes doesn't read the DTS written to the output, DTS is set to
>  AV_NOPTS_VALUE and we're back to the abovementioned situation.
>
>  A possible solution: add an AVStream.last_IP_dts, and replace the
>  above instruction with: DTS2 = pkt.dts = st->last_IP_dts+1;
>
>  and/or add sanitizing code: dts = FFMAX(st->last_IP_dts+1, dts);
>
>  Comments?

Stefano,

I added some preliminary investigation to the roundup page. I'm not sure 
that this particular sample is worth the effort to fix.

I have no specific comment on your patch right now.


Mike Scheutzow