[FFmpeg-trac] #8721(undetermined:new): -f concat hardcode input at 25fps. Impossible to set to something else.

[FFmpeg]

#8721: -f concat hardcode input at 25fps. Impossible to set to something else.
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             Reporter:  grisk         |                     Type:  defect
               Status:  new           |                 Priority:  normal
            Component:  undetermined  |                  Version:  3.4.6
             Keywords:                |               Blocked By:
             Blocking:                |  Reproduced by developer:  0
Analyzed by developer:  0             |
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 Summary of the bug:
 There is no option that let you change the input of the command -f concat
 -i "{txtPath}" to something other than the default 25fps.

 Here are my tests and the results:
 # -r before "-f concat"                       : Override all duration from
 file. Its just like using a static fps
 # -vf "fps=X"                                 : Fake final fps, it will
 keep very few frames in the final output.
 # -r after -i "{txtPath}"                     : It appear to work, but its
 start to have problems after 60fps.
 # -filter:v "fps={fps},setpts={speed}*PTS"    : It encode correctly after
 the video is encoded at 25fps. So if there is a duration smaller than
 25fps, it will delete those frames.

 Attached is a python script that make it very easy to test it.

 How to reproduce:
 {{{
 % ffmpeg -y -f concat -i "{txtPath}" -c:v libx264 -crf 1 -pix_fmt yuv420p
 -r 120 "{out}"
 % ffmpeg -y -r 120 -f concat -i "{txtPath}" -c:v libx264 -crf 1 -pix_fmt
 yuv420p"{out}"
 % ffmpeg -y -f concat -i "{txtPath}" -c:v libx264 -crf 1 -pix_fmt yuv420p
 -filter:v "fps=120,setpts={speed}*PTS" "{out}"
 % ffmpeg -y -f concat -i "{txtPath}" -c:v libx264 -crf 1 -pix_fmt yuv420p
 -vf "fps=120" "{out}"

 ffmpeg 4.2.2-1ubuntu1~18.04.york0
 built on ubuntu0.18.04.1
 }}}
 Patches should be submitted to the ffmpeg-devel mailing list and not this
 bug tracker.

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Ticket URL: <https://trac.ffmpeg.org/ticket/8721>
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