[FFmpeg-trac] #5789(undetermined:new): Fast in-place replacement of embedded subtitles with ffmpeg

[FFmpeg]

#5789: Fast in-place replacement of embedded subtitles with ffmpeg
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             Reporter:  forthrin     |                     Type:
               Status:  new          |  enhancement
            Component:               |                 Priority:  normal
  undetermined                       |                  Version:
             Keywords:               |  unspecified
             Blocking:               |               Blocked By:
Analyzed by developer:  0            |  Reproduced by developer:  0
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 Let's say you have a 10 GB video file with embedded subtitles, and there
 is an error in the subtitles.

 You extract subtitles with `ffmpeg -i in.mp4 in.srt` and quickly edit
 them, but to get subtitles back in the `.mp4` the whole file must be
 rewritten, which takes forever even with `-codec copy`.

 Since subtitles are 0.001% of of a modern video file, has `ffmpeg`
 developers considered an in-place replacement of subtitles, ie. use
 `fseek` in the same file and simply overwrite the previous data?

 This would obviously require that the new data was smaller than the old
 one, but `ffmpeg` could even reserve a few extra K of zeroes when
 initially writing a full file to accommodate for this eventuality.

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Ticket URL: <https://trac.ffmpeg.org/ticket/5789>
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