[Bug 735450] vp8dec: Does not support alpha channel

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=735450
  GStreamer | gst-plugins-good | 1.2.4

--- Comment #2 from ashleighbcthomas at gmail.com 2014-08-28 09:10:49 UTC ---
I only know that FFMpeg supports the encoding of VP8 with alpha: 

http://ffmpeg.org/pipermail/ffmpeg-devel/2013-May/143461.html

That post may shed some light on how it's actually encoded, it appears the
alpha is encoded separately with the VP8 codec and then muxed into the
BlockAdditional element of the container, as per the spec: 

http://wiki.webmproject.org/alpha-channel

Note: The spec talks specifically about the webm container

Chrome supports the decoding and playback of VP8 alpha videos in the webm
format, but I don't know of anything that decodes the alpha in the matroska
format correctly.

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