[FFmpeg-trac] #4297(undetermined:new): video sync errors and missing stream after clipping h264 files

[FFmpeg]

#4297: video sync errors and missing stream after clipping h264 files
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             Reporter:  seahorse41   |                    Owner:
                 Type:  defect       |                   Status:  new
             Priority:  important    |                Component:
              Version:  git-master   |  undetermined
             Keywords:  h264 video   |               Resolution:
  sync clip                          |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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Comment (by seahorse41):

 In the real projects I'm working on, (not just this test video), I also
 see the bug I'm trying to report.
 I finally see a pattern. When a h264 video has larger I-frame intervals,
 like 2 or 2.5 seconds, if I try to do just the clipping '''-ss # -t # -c
 copy''' (before or apart from any other work, like
 crop/resize/watermark/etc,) then I see mis-matched aud/vid sync way too
 often, as I rarely request the video to be cut at exactly the I-frame
 position.
 The same happens after '''ffmpeg -c:v h264'''  Any subsequent attempts to
 clip the video results in messed up aud/vid sync. The key is when the
 I-frame interval is other than 0.5 or 1.00 second, the usual clipping
 locations.

 Ok I've now worked this through enough times to see this may not be a true
 bug after all, but a limitation on the I-frame location relative to the
 desired clipping spots.

 Maybe the only realistic improvement would be to add a warning that the
 first x.x seconds were lost before the first I-frame was found.

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Ticket URL: <https://trac.ffmpeg.org/ticket/4297#comment:3>
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