[FFmpeg-trac] #9883(ffmpeg:new): FFmpeg AVIF decoder does not seem to handle transparency when decoding YUV420P

[FFmpeg]

#9883: FFmpeg AVIF decoder does not seem to handle transparency when decoding
YUV420P
-------------------------------------+-------------------------------------
             Reporter:  btdmaster    |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:  ffmpeg       |                  Version:
                                     |  unspecified
             Keywords:  transparent  |               Blocked By:
             Blocking:  #7621        |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 FFmpeg seems to ignore transparency when decoding YUV420P. For example,
 compare the following:
 {{{
 % ffmpeg -i transparency-present.avif transparency-missing.png
 % convert transparency-present.avif transparency-present.png
 Arch Linux FFmpeg 2:5.1-1
 }}}
 This can also be reproduced when printing the file with ffplay and mpv
 (which, from what I understand, also used FFmpeg) but not with firefox or
 chafa, hence suggesting FFmpeg is the culprit.

 I also found a similar limitation in libavif, though only for converting
 to yuv4mpeg2
 (https://github.com/AOMediaCodec/libavif/blob/98a453fdbb0340eaf8bd6f794359483048e72305/apps/shared/y4m.c#L435):

 {{{
 % avifdec -i transparency-present.avif -o transparency-also-missing.y4m
 Arch Linux libavif 0.10.1-2
 }}}

 FFmpeg does not seem to use libavif, but I thought that may provide some
 insight into the problem.
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/9883>
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