[FFmpeg-devel] About guess_correct_pts / AVFrame.best_effort_timestamp

[Martin Storsjö]

On Wed, 16 Feb 2011, M?ns Rullg?rd wrote:

> Martin Storsj? <martin at martin.st> writes:
> 
> > On Wed, 16 Feb 2011, M?ns Rullg?rd wrote:
> >
> >> A VFR stream obviously needs timestamps.  I'm not aware of any
> >> VFR-capable container providing only DTS.  In fact, I'm not aware of
> >> _any_ container providing only DTS.  Even if such a container did exist,
> >> DTS is not PTS.  PTS can, of course be derived from DTS, decoder delay,
> >> and frame reordering, but that is not what any code in ffmpeg does, at
> >> least not correctly.
> >
> > I'm not sure if any such formats exist though, I'm just trying to make 
> > sure my knowledge of the subject is right.
> >
> > As far as I understand, DTS'es shouldn't be reordered? The definition I 
> > saw is that whatever comes out from an ideal, instanteous decoder, when 
> > fed with a packet at time DTS, should be displayed at that particular 
> > time.
> 
> I suggest to anyone who wants to understand timestamps to get a copy of
> the MPEG2 systems spec (ITU-T H.222.0, free download).  It explains how
> it all works in great detail.

Downloaded a copy, thanks for the tip. I'll browse it later.

> > The new AVFrame.pkt_dts should track this right, as far as I know. As in, 
> > it isn't reordered along with the PTS, but it is set to the DTS of the 
> > AVPacket that triggered this frame to be emitted.
> 
> pkt_dts is apparently set to the DTS of the AVPacket passed to
> avcodec_decode_video2(), making it rather pointless.

Earlier it would probably be kinda pointless, yes. For -mt, however, it 
would be useful if you previously used AVPacket.dts directly, AFAIK.

> > So given this, I could either use AVFrame.pkt_pts, if set, or 
> > AVFrame.pkt_dts if that exists instead, and if that doesn't exist either, 
> > I'd have to revert to calculating it from the frame rate.
> 
> This works under the assumption that the decoded picture buffer has a
> constant size.  If this is not true for some codec, using input DTS as
> output PTS will not work.  VP8 is an example of a codec where this is
> not true.

Hmm, ok, thanks for the explanations.

// Martin