[FFmpeg-trac] #10486(undetermined:new): Concat with a single input stream is changing the number of frames in the output stream

[FFmpeg]

#10486: Concat with a single input stream is changing the number of frames in the
output stream
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             Reporter:  abingham     |                    Owner:  (none)
                 Type:  defect       |                   Status:  new
             Priority:  normal       |                Component:
                                     |  undetermined
              Version:  unspecified  |               Resolution:
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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Comment (by abingham):

 This is a greatly distilled example from a video compositing system I'm
 writing. Internally, it assumes that `concat` with one (or more) inputs
 will produce a stream with a number of frames equal to the sum of the
 input frames. When this assumption was violated (as demonstrated in this
 issue), it caused some problems.

 Perhaps my assumption about concat is incorrect. If so, I'd appreciate
 some guidance (docs, etc.) on what to expect from concat so that I can
 compensate for its behavior. My sense is that my assumption is indeed
 incorrect, and that I should expect rounding issues from concat (e.g.
 because it converts between frames numbers and timestamps or something).

 Or perhaps this is a bug in concat. I'm not really sure.
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Ticket URL: <https://trac.ffmpeg.org/ticket/10486#comment:1>
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