FFmpeg AVIF decoder does not seem to handle transparency when decoding YUV420P

[btdmaster]

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.

[btdmaster]

Demonstrates AVIF transparency

[Balling]

We know this, apply this if you are in a hurry. ​https://patchwork.ffmpeg.org/project/ffmpeg/patch/20220630210434.1551769-2-vigneshv@google.com/

Series: ​https://patchwork.ffmpeg.org/project/ffmpeg/list/?series=6946