[FFmpeg-devel] About guess_correct_pts / AVFrame.best_effort_timestamp

[Peter Ross]

On Wed, Feb 16, 2011 at 05:47:15PM +0000, M?ns Rullg?rd wrote:
> Martin Storsj? <martin at martin.st> writes:
> 
> > On Wed, 16 Feb 2011, M?ns Rullg?rd wrote:
> >
> >> Nicolas George <nicolas.george at normalesup.org> writes:
> >> 
> >> > L'octidi 28 pluvi?se, an CCXIX, M?ns Rullg?rd a ?crit?:
> >> >> AVI doesn't have timestamps, so the first decoded frame has PTS 0, and
> >> >> the following ones increment it by 1/framerate.  Simple.
> >> >
> >> > Are you saying that Mr X should put a special case in his code for AVI?
> >> 
> >> No, he should put a special case for no PTS on an input frame:
> >> 
> >> pts = frame.pkt_pts != AV_NOPTS_VALUE? frame.pkt_pts : prev_pts + 1/framerate;
> >> prev_pts = pts;
> >> 
> >> This code will always work.
> >
> > Wouldn't this break for VFR-streams? That is, shouldn't it prefer 
> > frame.pkt_dts over the calculated pts, if pkt_dts is present (and 
> > reliable, assuming that all guessing code is disabled).
> 
> 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.

Re your first sentence. It depends on the codec. MPEG-4 includes timestamps
within the codec payload (AVPacket.data). So it is possible to decode a
VFR MPEG-4 stream with a container format lacking PTS and DTS.

-- Peter
(A907 E02F A6E5 0CD2 34CD 20D2 6760 79C5 AC40 DD6B)
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