[FFmpeg-trac] #4293(avformat:new): avformat: optionally write an approximate duration ahead-of-time if possible

[FFmpeg]

#4293: avformat: optionally write an approximate duration ahead-of-time if
possible
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             Reporter:  11rcombs  |                     Type:  enhancement
               Status:  new       |                 Priority:  wish
            Component:  avformat  |                  Version:  git-master
             Keywords:            |               Blocked By:
             Blocking:            |  Reproduced by developer:  0
Analyzed by developer:  0         |
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 This would be useful when writing a format like MKV or MP3 that has a
 duration in the header that we currently avoid writing until the entire
 file is finished, for cases where we're writing to an un-seekable output
 (like a pipe), or where the output is going to be read before it's
 finished being written. Having a duration, even if it may be somewhat
 inaccurate, can improve behavior in some players. For ffmpeg.c, this value
 would be based on the corresponding input streams' durations. The easiest
 way to implement this would probably be setting AVFormatContext::duration,
 but I wouldn't be surprised if there were cases where setting the
 individual AVStream::durations would make more sense. We could also send
 it as metadata, either at the format or stream level.

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Ticket URL: <https://trac.ffmpeg.org/ticket/4293>
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