[FFmpeg-user] WebM alpha plane decode

[Vin Kemp]

Hi everyone, this is my first post to the list.

I need to decode WebM videos with an alpha channel and after looking
around, this is the only conclusive and most recent piece of information
I've found:
https://ffmpeg.org/pipermail/ffmpeg-user/2014-January/019280.html

It says that the alpha plane is not available in the decoder. I've verified
this programmatically (custom build of ffmpeg with latest libvpx) and from
ffmpeg utility and both return a YUV420P pixel format. VLC also plays back
the video without the alpha. The sample video I'm using is:
http://simpl.info/videoalpha/video/dancer1.webm

I believe the pixel format returned should be AV_PIX_FMT_YUVA420P for this
video, and the returned AVPicture to have 4 entries (YUV+A) instead of 3
(YUV). In a simple test, I passed AV_PIX_FMT_YUVA420P to avpicture_alloc()
and was able to get the linesize[] and data[] filled correctly for a YUVA
picture. So it seems like the decoder itself is not handling the alpha
plane as specified here:
http://wiki.webmproject.org/alpha-channel

I'm wondering if someone is interested/able to implement this, or he/she
could point me to where the implementation needs to happen and general
advice on how to go about it. Many thanks in advance!

VK