1948580 - If CanvasContext configuration is invalid, WebGPU requires to give fallback black bitmap

Status: NEW

(Core :: Graphics: WebGPU, enhancement, P1)

Description

[Sotaro Ikeda [:sotaro]]

test of Bug 1940072 failed by not handling it.

Comment 1

[Sotaro Ikeda [:sotaro]]

chromium implementation GPUCanvasContext::TransferToImageBitmap().

Comment 2

[Jamie Nicol [:jnicol]]

Fixing this is fairly straightforward. We just need to create a source surface and initialise its pixels to opaque black in CanvasContext::GetSurfaceSnapshot(). But I'm confused by the spec.

The test expects the pixels to have an alpha value of 1.0, but the relevant part of the spec isn't so clear:

Content timeline steps:
    1. Let snapshot be a transparent black image of the same size as context.canvas.
    2. Let configuration be context.[[configuration]].
    3. If configuration is null:
        Return snapshot.
    ...
    6. Let alphaMode be configuration.alphaMode.
    7. If alphaMode is "opaque":
           Clear the alpha channel of snapshot to 1.0.
           Tag snapshot as being opaque.
           Note: If the [[currentTexture]], if any, has been destroyed (for example in "Expire the current texture"), the alpha channel is unobservable, and implementations may clear the alpha channel in-place.
       Otherwise:
           Tag snapshot with alphaMode.

Clearly 6 and 7 mention clearing the alpha channel to 1.0. But this canvas is unconfigured, so shouldn't we be returning a transparent black (ie alpha == 0.0) snapshot as per points 1-3?

Clearly chrome disagrees as they unconditionally use an alpha mode of kOpaque, and therefore clear the alpha to 1.0, when the canvas is unconfigured.

There is a comment in the test saying since the canvas is unconfigured we should use the default alpha mode of "opaque". This default is specified here, but I don't see how that overrides step 3 of the above.

Comment 3

[Teodor Tanasoaia [:teoxoy]]

I think that's a recent change https://github.com/gpuweb/gpuweb/pull/4738, it's likely that the CTS and chrome hasn't been updated in this regard.

Comment 4

[Jamie Nicol [:jnicol]]

Filed https://github.com/gpuweb/cts/issues/4259