[FFmpeg-trac] #11140(avfilter:new): Subtitles blended into chroma-subsampled video contain unintended stripes

[FFmpeg]

#11140: Subtitles blended into chroma-subsampled video contain unintended stripes
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             Reporter:  Oneric       |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:  avfilter     |                  Version:  git-
             Keywords:  subtitles    |  master
  ass                                |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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 I’m forwarding this from https://github.com/libass/libass/issues/816
 I was able to reproduce this myself using ffmpeg
 719e46f54c0f3ea557503ec4cd5618332484f78a but it also occured on older
 versions too; if this is a regression then not a too recent one.


 When subs are blended by the subtitles/ass videofilter the sub overlay
 bitmaps appear to get truncated to the video format’s chroma plane
 resolution individually. This can cause issues e.g. with neighbouring sub
 bitmaps whose border lies within a subsampled chroma pixel which will be
 visible as jarring stripes. Those stripes are not present in video players
 or authoring tools.

 One possible fix is to first blend all subtitles into a single non-
 subsampled RGBA surface and then convert this overlay as a whole to the
 video’s format and blended onto the video frame.

 Atm users can and must workaround stripes by first forcing the input video
 to be converted to yuv444 or similar and then later convert back to the
 desired format, e.g. yuv420
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Ticket URL: <https://trac.ffmpeg.org/ticket/11140>
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