Encode VideoFrame with alpha

[akre54]

Hi there - I'm trying to encode / mux a video with alpha (the inverse of #377). I can get an RGBA VideoFrame but the resulting video file doesn't have transparency.

See my renderer code here: https://gist.github.com/akre54/e93ab2ce27999aecb109e38085f2e7a7

I've also tried using the examples from these repos:

https://github.com/w3c/webcodecs/tree/main/samples/video-decode-display
https://github.com/tidoust/media-tests/tree/main

Do I understand correctly that encoding with alpha isn't supported in any current browsers? Or is there something I'm missing. Thanks!

[Djuffin]

Chromium doesn't support encoding frames with alpha yet.

[akre54]

Okay thank you for the fast response. Am I understanding correctly that this is an implementation question rather than a required change to the spec?

Are there any workarounds available, or is this a wontfix?

[Djuffin]

Correct. The spec seems to have all the parts needed, but nobody's implemented yet.
https://www.w3.org/TR/webcodecs/#dom-encodedvideochunkmetadata-alphasidedata
https://www.w3.org/TR/webcodecs/#dom-videoencoderconfig-alpha

Are there any workarounds available, or is this a wontfix?

I can't think of any workarounds other than manually encoding alpha channel with a separate encoder.

[akre54]

Okay I'll give that a shot and possibly open a crbug. Thanks for your help