[FFmpeg-trac] #11135(avcodec:new): Text outline becomes stretched depending on video aspect ratio when converting SRT to ASS subtitles

[FFmpeg]

#11135: Text outline becomes stretched depending on video aspect ratio when
converting SRT to ASS subtitles
---------------------------------+---------------------------------------
             Reporter:  omgold   |                     Type:  defect
               Status:  new      |                 Priority:  normal
            Component:  avcodec  |                  Version:  unspecified
             Keywords:           |               Blocked By:
             Blocking:           |  Reproduced by developer:  0
Analyzed by developer:  0        |
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 Summary of the bug:

 When burning SRT subtitles with the subtitles filter into a video, the
 outline/border (not the text itself) becomes stretched according to the
 aspect ratio of the video.

 I tracked the problem down to a rather odd behavior of ass, where the
 outline width in x/y directions are scaled with PlayResX/PlayResY
 parameters.

 If these are set to values match the video aspect ratio, it works fine.
 But apparently ffmpeg does not do that when converting SRT to ASS (which
 it does before burning the subtitle into the images with the subtitles
 filter).

 The problem appears both when using the filter and also when keeping the
 subtitle as a separate track (converted to ASS), but not with a separate
 track keeping it in SRT format. (With an embedded separate ASS track the
 stretching is visible with a player that uses libass, like mpv, but not
 with vlc).

 It can be worked around by converting the SRT to an ASS file with ffmpeg,
 then edit the PlayResX/PlayResY by hand and using the fixed ASS for
 desired operation with ffmpeg.

 Note, though, that the issue propagates into higher-level video editing
 software like kdenlive, which use SRT internally and do not give the user
 to ability to apply such a workaround.

 How to reproduce:
 {{{
 % ffmpeg -report -i untitled.mp4 -i test.srt -vcodec copy -scodec copy
 test-srt.mkv
 % ffmpeg -report -i untitled.mp4 -i test.srt -vcodec copy -scodec ass
 test-ass.mkv
 % ffmpeg  -i untitled.mp4 -filter subtitles=filename=test.srt test-
 embedded.mkv

 ffmpeg version 7.0.1
 built on Arch Linux
 }}}
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Ticket URL: <https://trac.ffmpeg.org/ticket/11135>
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