[FFmpeg-trac] #7843(undetermined:closed): FFmpeg not outputting to spec MKV files

[FFmpeg]

#7843: FFmpeg not outputting to spec MKV files
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             Reporter:               |                    Owner:
  alexrichmond                       |
                 Type:  defect       |                   Status:  closed
             Priority:  normal       |                Component:
                                     |  undetermined
              Version:  unspecified  |               Resolution:  duplicate
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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Comment (by mkver):

 1. My initial assessment was of course based upon the command line from
 the initial post (which involved a transport stream and a private option
 of the libzvbi_teletextdec decoder, hence my presumption that you are
 actually copying DVBSUB subtitles despite you claiming your subtitles to
 be text subtitles).
 2. Nevermind. It is essentially still the same issue:\\
  a) Your new sample is weird: It only uses SimpleBlocks and only the video
 track contains keyframes; even the audio blocks are not marked as
 keyframes. mkvalidator complains a lot about this file and my patch
 doesn't change it (as it doesn't affect whether SimpleBlocks are
 considered keyframes or not).\\
  b) Remuxing this file (there is actually no need to transcode the video
 at all, just drop it) creates a file where the subtitles are written in
 BlockGroups with a duration of zero. Not good, actually a bug, but not
 spec-incompliant. And mkvalidator's error message is bonkers, of course.
 3. Btw: If you remux your sample with a current version of mkvmerge, your
 text subtitles will end up in BlockGroups with proper durations, but also
 ReferenceBlock elements which marks them as non-keyframes (I already
 opened an issue for this:
 https://gitlab.com/mbunkus/mkvtoolnix/issues/2541). If you remux
 mkvmerge's output with FFmpeg, you'll end up with a normal text subtitle.
 (But it's probably best not to let current FFmpeg touch the DVB subtitles:
 It will explicitly set its duration to zero and mkvmerge will keep it that
 way.)
 4. Last not least, it seems that fix_sub_duration doesn't do anything
 here. Which would be a ticket of its own.

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Ticket URL: <https://trac.ffmpeg.org/ticket/7843#comment:7>
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