[Libav-user] av_frame_get_best_effort_timestamp(): PTS or DTS?

[wm4]

On Sat, 29 Mar 2014 00:49:18 +0100
Lucas Soltic <lucas.soltic at orange.fr> wrote:

> Le 28 mars 2014 à 22:04, wm4 <nfxjfg at googlemail.com> a écrit :
> 
> > On Fri, 28 Mar 2014 21:19:55 +0100
> > Lucas Soltic <lucas.soltic at orange.fr> wrote:
> > 
> >> Hello!
> >> 
> >> Does av_frame_get_best_effort_timestamp() gives a PTS or DTS?
> >> 
> >> At first I would have thought it's a PTS, because it's impossible for decoding to correctly happen if DTS does not exist in the packet, but then I read that the packet DTS may be AV_NOPTS_VALUE (DTS being a NOPTS..??!). If that is correct, is there a reliable way to know the DTS too (in the core meaning of DTS, even if the file does not set it)?
> > 
> > Some files have PTS and DTS mixed (mpeg mostly), some files have PTS
> > only (e.g. Matroska), some files have DTS only (e.g. AVI). The general
> > solution is to use DTS if PTS is set to NOPTS.
> > 
> > Here's how av_frame_get_best_effort_timestamp() is calculated:
> > 
> > https://git.videolan.org/?p=ffmpeg.git;a=blob;f=libavcodec/utils.c;h=b43f67540e834d45058f9d694cc6c090d1b4c27d;hb=HEAD#l1995
> > 
> > Looks complicated, but is rather simple. As I understand, this is
> > mostly a workaround if PTS or DTS are set incorrectly. 
> > 
> > 
> > This function takes AVFrame.pkt_pts and AVFrame.pkt_dts as inputs. It
> > returns what will be used as AVFrame.best_effort_timestamp. What this
> > function does is:
> > 
> > 1. count how often it happens that PTS or DTS are not monotonic
> >   (normally, the DTS _and_ PTS should not decrease after the decoder;
> >   this is not true _before_ the decoder, but the decoder should
> >   reorder them so that they're monotonic, and some broken files might
> >   violate this)
> > 2. if PTS is more often wrong than DTS, use the DTS as output timestamp
> > 
> > Libav does not have the AVFrame.best_effort_timestamp field. Butthe
> > Libav command line tools (avplay, avconv) calculates it the same way:
> > 
> > https://git.libav.org/?p=libav.git;a=blob;f=cmdutils.c;h=69a11bdd4fb33ccdc1420e15edb7c47c8ce84098;hb=HEAD#l1431
> > 
> > And that's why I would recommend just copying the code. It's trivial
> > anyway, at least once you understand it, and your project will be
> > compatible to both FFmpeg and Libav.
> > _______________________________________________
> > Libav-user mailing list
> > Libav-user at ffmpeg.org
> > http://ffmpeg.org/mailman/listinfo/libav-user
> 
> Thanks for all your answers :)
> 
> So... to sum it up, some formats have DTS or PTS or both. I guess no file has none?

I guess it's not impossible that some files have no or just broken
timestamps. But with "normal" files this usually doesn't happen.

> Then if I want to seek, I saw that some demuxers seek by PTS, others by DTS, depending on whether AVFMT_SEEK_TO_PTS is set.
> 
> If the format only has DTS, will AVFMT_SEEK_TO_PTS never be set?
> If the format only has PTS, will AVFMT_SEEK_TO_PTS always be set?
> If AVFMT_SEEK_TO_PTS is set, are the PTS provided by the format reliable?
> If AVFMT_SEEK_TO_PTS is not set, are the DTS provided by the format reliable?

I don't think you should try to conclude anything from these flags.
They're probably mostly for the libavformat internal seek code.