Does compositingAlphaMode affect using the canvas as an image source?

[kainino0x]

In #1839 we spec the behavior of reading from WebGPU canvases from other APIs, but we need to clarify whether compositingAlphaMode affects those.

1. If compositingAlphaMode is "opaque" does that mean e.g. drawImage reads opaque values?
2. If we add compositingAlphaMode "unpremultiplied" would that mean e.g. texImage2D sees it as an unpremultiplied source?

My initial feeling is that the answers to both should be yes. In that case we should also probably rename compositingAlphaMode to something like canvasAlphaMode.

[kainino0x]

Actually I'm not sure which makes more sense.

1. Reading opaque values seems unnecessary, and may cause complications for implementations which only do premultiply/unpremultiply conversions now, and not "opaquification" conversions.
2. I think the answer to this would need to be yes, though it only matters if we actually add that mode (which we are hoping not to do). Still, the possibility may warrant changing this from compositingAlphaMode to canvasOutputAlphaMode or something.

[kainino0x]

Or two separate flags, compositeAsOpaque (which doesn't affect using it as an image source) and alphaPremultiplied (which affects both compositing and using it as an image source).

[kainino0x]

Another side note: recent additions to the spec mean that webgpu canvases can't be both composited and used as image sources at the same time. So on macOS, where we have to clear the alpha channel to get the "opaque" compositing behavior, we do have the opportunity to do so without affecting drawImage et al.)

[kainino0x]

Meeting update: This should be blocked on #1988

[Kangz]

#1988 is post-V1. What is the tacit resolution candidate for this?

[kainino0x]

We don't have a proposal yet, I just put that label to indicate that editors should discuss and look for a solution we can propose. Specifically, I want to attempt to unblock this on #1988, just define the behavior and keep the API open for future improvements.

[kainino0x]

More specifically, the spec actually says you can't use already-composited results as an image source:

• get a copy of the image contents of a context returns the contents of [[currentTexture]]
• present the context to the canvas removes the [[currentTexture]]

and we need to make sure that's OK because I'm pretty sure it resolves this issue.

[kainino0x]

If we add a way to read back the "display buffer" (presented image), then compositingAlphaMode would need to affect the result of that readback (on all platforms for consistency) because some platforms have to clear the alpha channel to 1.0 for presentation.
#2743 (comment)

[shaoboyan]

I think we have conclusion in this issue that we don't apply compositingAlphaMode to the drawing buffer content. Can we close this issue?

[kainino0x]

It's not concluded yet because I need to make a proposal for #2743. I think the solution there is going to necessitate compositingAlphaMode affecting drawImage/toDataURL/etc.