[FFmpeg-devel] Issue with native WebP muxer in ffmpeg

Urvang Joshi urvang at google.com
Thu Oct 1 22:02:02 CEST 2015

Hi ffmpeg folks,
This is based on a report on WebP issue tracker:

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).

But according to the WebP container spec:

Alpha (L): 1 bit
Set if any of the frames of the image contain transparency information

This needs to be corrected. I took a dig at this, but it seemed non-trivial:
ffmpeg needs to find out if there is "at least one frame containing
transparency" -- this could be through (i) an ALPH chunk or (ii) inside a
VP8L chunk. While, detecting case (i) is easy due to the presence of
ALPH_FLAG in VP8X chunk, the same is not true for (ii) as in that case VP8X
chunk may not be present.

Can someone think of a good solution?


More information about the ffmpeg-devel mailing list