Make blending state explicit

[kvark]

Writing 251649e I realized how awkward our "default" values for blending are:

|descriptor|.{{GPUColorStateDescriptor/alphaBlend}} and |descriptor|.{{GPUColorStateDescriptor/colorBlend}}
are either both defaults, or |descriptor|.{{GPUColorStateDescriptor/format}} has
{{GPUTextureComponentType/"float"}} component type.

I believe the spec would be better if instead of comparing to some "default" this was an explicit "provided" versus "undefined".

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Preview | Diff

[kainino0x]

We chose this because the default values semantically represent what happens when blending is disabled. This would create a difference between unspecified/undefined and {} and { srcFactor: "one", dstFactor: "zero", operation: "add" }.

How about instead having a dfn dfn which says something like

<dfn dfn>blending is enabled</dfn> in a {{GPUBlendDescriptor}} if it is not equal to
`{ srcFactor: "one", dstFactor: "zero", operation: "add" }`

and using that?

[kvark]

We chose this because the default values semantically represent what happens when blending is disabled.

Right. Except that if your render target is non-blendable, then enabling blending is technically a UB in native APIs.

[kainino0x]

You want to reintroduce the distinction, then? (between blend-disabled and blend-enabled-but-noop)

I have no real feelings either way.

[kvark]

You want to reintroduce the distinction, then?

Yes. That's how it's done in all the native APIs. E.g. in Metal blending is an explicit flag.
I don't think it poses any harm or confusion.

[kainino0x]

Sounds good to me. Let's take it to the meeting then.

[kdashg]

Needs to spec what happens if only one but not both are undefined. (is blending enabled then?)

[kainino0x]

We could put one extra layer of dictionary in between so there's only one nullable thing.

{ blend?: { alpha: GPUBlendDescriptor, color: GPUBlendDescriptor }, ... }

[kainino0x]

Editors meeting: Agreed to add that extra layer of dictionary.

[kvark]

This is now updated with a separate struct. Please take another look!