VP9 Alpha channel isn't decoded

[Zipdox]

Important Information

• mpv 0.34.1
• Debian GNU/Linux bookworm/sid x86_64
• Installed with apt

Reproduction steps

Make sure your system supports transparency rendering. On Linux you might need a build of mesa with these patches.

Refer to the example files. Play these two files with the following commands. The gif will play transparently but the WebM will not.

mpv --alpha=yes travolta.webm

mpv --alpha=yes Nikki.gif

Expected behavior

The WebM plays with transparency.

Actual behavior

The WebM plays with a black background.

Log file

mpv_transparency.log

Sample files

• travolta.webm: https://0x0.st/ouvI.webm
• Nikki.gif: https://0x0.st/ouvl.gif

[Zipdox]

It appears that forcing the decoder using --vd=libvpx-vp9 works as a workaround, though it would be nice if this is not necessary.

[Zipdox]

Yeah I was aware that FFmpeg does this too, and you need to specify the decoder to decode alpha. Maybe MPV could detect alpha channel and force the decoder of necessary.

[TeeMoYan]

Can you play alpha webm?

[Zipdox]

Can you play alpha webm?

Only if I install the mesa patches and force the encoder like I said.