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Clarify behavior of out-of-gamut canvas#4476

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Merged

kainino0x merged 4 commits intogpuweb:mainfromkainino0x:premultiplied-nit

Apr 12, 2024

Merged

Clarify behavior of out-of-gamut canvas#4476

kainino0x merged 4 commits intogpuweb:mainfromkainino0x:premultiplied-nit

Apr 12, 2024

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kainino0x commentedJan 31, 2024•
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It's OK if out-of-gamut intermediate values are written to the canvas, it only matters what's there when it gets presented.

Also move the existing explanation of the undefined behavior to a note,~~and expand it with an example.~~

Clarify behavior of out-of-gamut canvas

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It's OK if out-of-gamut intermediate values are written to the canvas,it only matters what's there when it gets presented.

kainino0x added the copyeditingPure editorial stuff (copyediting, *.bs file syntax, etc.) label

Jan 31, 2024

kainino0x added this to theMilestone 0 milestone

Jan 31, 2024

kainino0x requested a review fromjimblandy

January 31, 2024 22:48

kainino0x added the for webgpu editors meeting label

Feb 12, 2024

clarify note with example

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kainino0x commentedMar 11, 2024

@jimblandy revised, PTAL

not the only way it can fail

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github-actionsbot commentedMar 11, 2024•
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Previews, as seen when thisbuild job started (fe175dc):
WebGPU_{webgpu.idl |Explainer |Correspondence Reference}
WGSL_{grammar.js |wgsl.lalr.txt}

kainino0x commented

Mar 11, 2024

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spec/index.bs Outdated

		{{GPUCanvasAlphaMode/"premultiplied"}}, {{PredefinedColorSpace/"srgb"}},
		{{GPUTextureFormat/"rgba8unorm"}} canvas, the color `[0.51, 0, 0, 0.5]` represents
		<code><a funcdef>color</a>(srgb 1.02 0 0 / 0.5)</code>,
		however upon presentation it could be internally unpremultiplied before color

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kainino0xMar 11, 2024

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It's probably not actually likely that a browser would unpremultiply like this, as basically everything prefers to operate in premultiplied space. I don't think this is the only case under which bad things would happen. It was the easiest to explain, but could it mislead people to believe this is actually a likely scenario?

The example is not critical, but it's very helpful to demonstrate what we're talking about. Maybe it would be possible to show other examples as well of hypothetical compositing+display pipelines?

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kainino0x commentedMar 18, 2024

I thought about this further, and I think there are some cases that this doesn't allow that we need to allow. I'm going to tentatively close this and open a new PR that does that.