canvas read as an image source alpha mode behavior update

[shrekshao]

https://github.com/gpuweb/gpuweb/pull/2905

[shrekshao]

https://www.w3.org/TR/webgpu/#GPUCanvasAlphaMode

(GPUCanvasAlphaMode) selects how the contents of the canvas will be interpreted when read, when rendered to the screen or used as an image source (in drawImage, toDataURL, etc.)

We have reftests for "canvas read when rendered to the screen", we still need reftests for "canvas read when used as an image source."

Operations include drawImage, toDataURL, texImage2D, toBlob etc.

[shrekshao]

Now it seems best to expand the readbackFromWebGPUCanvas.spec.ts with

• x= src canvas
  • type=
  • canvas element not in dom
  • canvas element in dom
  • OffscreenCanvas from transferControlToOffscreen from canvas not in dom
  • OffscreenCanvas from transferControlToOffscreen from canvas in dom
  • OffscreenCanvas from new OffscreenCanvas
  • alphaMode= {opaque, premultiplied}
• x= dst canvas
  • method=
  • 2d drawImage,
  • toDataURL,
  • toBlob,
  • createImageBitmap,
  • WebGL texImage2D,
  • WebGL texSubImage2D,
  • transferToImageBitmap
  • convertToBlob
  • ... and probably some others.
  • alphaMode= {opaque, premultiplied}

[kainino0x]

Finally tested and filed https://github.com/gpuweb/gpuweb/issues/3295

[kainino0x]

@shrekshao this issue is still marked "started". Do you remember if it was done or was there still some work to do?

[shrekshao]

Seems that the TODOs on kAllCanvasTypes are still missing

• canvas element not in dom
• canvas element in dom
• offscreen canvas from transferControlToOffscreen from canvas not in dom
• offscreen canvas from transferControlToOffscreen from canvas in dom
• offscreen canvas from new OffscreenCanvas

[kainino0x]

I'll unmark it as "Started" and unassign you for now, but feel free to reassign yourself if you end up working on it