[FFmpeg-trac] #5431(undetermined:new): ffmpeg's native WebP muxer always sets ALPHA_FLAG to be true in VP8X chunk

[FFmpeg]

#5431: ffmpeg's native WebP muxer always sets ALPHA_FLAG to be true in VP8X chunk
-------------------------------------+-------------------------------------
             Reporter:  triune       |                     Type:  defect
               Status:  new          |                 Priority:  important
            Component:               |                  Version:  git-
  undetermined                       |  master
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 Summary of the bug:

 Looks like ffmpeg's native WebP muxer always sets ALPHA_FLAG to be true in
 VP8X chunk even for a completely opaque image: e.g. an animated image
 where none of the frames have transparency info:
 https://github.com/FFmpeg/FFmpeg/blob/master/libavformat/webpenc.c#L110

 But according to the WebP container spec:
 https://developers.google.com/speed/webp/docs/riff_container?hl=en#extended_file_format

 <Quote>
 Alpha (L): 1 bit
 Set if any of the frames of the image contain transparency information
 ("alpha").
 <Unquote>

 Bug information aggregated from:
 http://ffmpeg.org/pipermail/ffmpeg-devel/2015-October/179955.html
 https://bugs.chromium.org/p/webp/issues/detail?id=266

 How to reproduce:
 {{{
 % ffmpeg -i video.mp4 -c:v libwebp -loop 0 anime.webp

 ffmpeg version N-79190-ga5f5b8b Copyright (c) 2000-2016 the FFmpeg
 developers
   built with gcc 4.8 (Ubuntu 4.8.4-2ubuntu1~14.04.1)
   configuration: --enable-libopus --enable-libvpx --enable-gpl --enable-
 libx264 --extra-libs=-ldl --enable-libmp3lame --disable-yasm --enable-
 libfdk_aac --enable-nonfree --enable-libwebp --enable-filter=drawtext
 --enable-libfreetype
   libavutil      55. 19.100 / 55. 19.100
   libavcodec     57. 33.100 / 57. 33.100
   libavformat    57. 29.101 / 57. 29.101
   libavdevice    57.  0.101 / 57.  0.101
   libavfilter     6. 40.102 /  6. 40.102
   libswscale      4.  0.100 /  4.  0.100
   libswresample   2.  0.101 /  2.  0.101
   libpostproc    54.  0.100 / 54.  0.100
 }}}

 animated webp images generated by ffmpeg with this defect present are
 linked here:
 https://storage.googleapis.com/bucketami/00245%3D07576%3D07576%3D07576.webp
 https://storage.googleapis.com/bucketami/00247%3D06534%3D06534%3D06534.webp
 https://storage.googleapis.com/bucketami/femto.webp
 https://storage.googleapis.com/bucketami/VFP3.webp

--
Ticket URL: <https://trac.ffmpeg.org/ticket/5431>
FFmpeg <https://ffmpeg.org>
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