[FFmpeg-devel] About guess_correct_pts / AVFrame.best_effort_timestamp
Wed Feb 16 19:55:46 CET 2011
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.
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