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alpha channel not working for .webm #3256
I haven't extensively tested this. alpha channel works correctly on .gif, .png, but not .webm (VP8) I have not tested on other video codecs because I lack any examples with alpha channel.
An example video can be found here https://raw.githubusercontent.com/photonstorm/phaser-examples/master/examples/assets/video/alpha-webm.webm
It is incorrectly rendered as yuv420p, without the default checkerboard background.
I used a recent version of mpv
default settings, no scripts.
Doesn't seem like FFmpeg recognizes VP8 with embedded alpha channels at all. Not mpv's bug.
If FFmpeg were to correctly recognize/decode and export the video as YUVA420P instead of YUV420P, mpv should display it correctly. (But I haven't tested it, is there anything else that uses a YUVA format?)
I can't even find a specification of this crap. Unless someone does, this is a WONTFIX.
VP8 doesn't support alpha, and the alpha plane is in a BlockAdditional element attached to each packet. There's no specification for the format of this data, and ffmpeg's VP8 decoder does not use it.
Once again I'm amazed by the unfathomable dumb crap Google produced. They also got in patches in ffmpeg for this, but only for encoding. Fuck Google.