[FFmpeg-devel] About guess_correct_pts / AVFrame.best_effort_timestamp

[Måns Rullgård]

Peter Ross <pross at xvid.org> writes:

> On Wed, Feb 16, 2011 at 09:15:07PM +0000, M?ns Rullg?rd wrote:
>> Peter Ross <pross at xvid.org> writes:
>> 
>> > 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).
>> 
>> Where?
>
> MPEG-4 encodes timebase and PTS within the VOL and VOP headers
> respectively. The PTS information is used by the decoder to calculate
> B-frame motion vectors. Its integral to the codec. You can find more
> information in ISO/IEC 14496-2; search for 'vop_time' and 'time_base'.

Those are timecodes, not PTS.  They provide a temporal reference for
direct mode blocks in B-frames, but they have no relation to PTS used
for presentation timing.

-- 
M?ns Rullg?rd
mans at mansr.com