[FFmpeg-user] WebM alpha plane decode

Vin Kemp vinkemp at gmail.com
Fri Sep 12 12:40:19 CEST 2014

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:

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:

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:

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!


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