Alpha channel support in VP9

[AUsernameTooMany]

My specific use case is using vpx_codec_decode to decode a WEBM video file that has an embedded alpha channel.

Does VP9 support an alpha channel?  Is the YUVA420p video format viable in the libvpx library?  There is a little bit of code that references the alpha channel, but it doesn't appear to do anything. (VPX_PLANE_ALPHA, VPX_IMG_FMT_HAS_ALPHA)

Is there a special flag that I need to set before compiling the VPX library in order to enable alpha channel support?

Ian Seale

Relic Entertainment

[James Zern]

Hi,

Those flags are legacy and as you mentioned aren't used within the
library. VP9+alpha is the same as VP8 (method 1) [1][2]. Alpha+dummy
UV is encoded as a separate stream, you'll need 2 decoder instances to
reconstruct the YUV and alpha.

[1] http://wiki.webmproject.org/alpha-channel
[2] https://developers.google.com/web/updates/2013/07/Alpha-transparency-in-Chrome-video