Copied from https://gitlab.freedesktop.org//gstreamer/gst-plugins-bad/-/merge_requests/2200
This is a draft and a request for comment. I'd like to render transparent video onto transparent window from now on. So I'm asking if this is a fine behaviour or should we absolutly make this opt-in through a property.
As for the implementation, for now we simply make the background sub-surface fully transparent, but this wastes a layer, so the final implementation will properly remove the background surface (which is used to draw the black borders). I'm adding this video to show what it looks like:
Another note, I notice that Wayland and X11 requires pre-multiplied alpha content, but GStreamer always produces non-premultiplied alpha (unless it was accidently premultiplied, e.g. wpe). I have for now ignored this issue, as I believe it will be better solved through a seperate series of MR.