Incorrect override amounts when using binary expressions

[mattrossman]

When an expression is set to binary with isBinary: true, and these expressions also have blended overrides (e.g. overrideLookAt: "blend"), their weight is blended continuously instead of discretely when the corresponding expression group (e.g. gaze expression) has nonzero weight. I expect binary expressions to maintain the sharp 0.5 threshold whether using "blend" overrides or not.

I believe this stems from the overrideBlinkAmount / overrideLookAtAmount / overrideMouthAmount calculations not taking into account the isBinary flag, e.g.:

three-vrm/packages/three-vrm-core/src/expressions/VRMExpression.ts

Lines 61 to 69 in 7a35f7e

	public get overrideLookAtAmount(): number {
	if (this.overrideLookAt === 'block') {
	return 0.0 < this.weight ? 1.0 : 0.0;
	} else if (this.overrideLookAt === 'blend') {
	return this.weight;
	} else {
	return 0.0;
	}
	}

Changing the expression in the 'blend' case to this.isBinary ? (this.weight <= 0.5 ? 0.0 : 1.0) : this.weight as is done elsewhere in the applyWeight logic produces the expected discrete blending behavior.

I wonder if this also affects the 'block' override mode, as the binary expression will begin blocking as soon as it has nonzero weight, even though it doesn't visually apply that weight until after the 0.5 weight threshold. So perhaps the 'block' case should look more like:

const actualWeight = this.isBinary ? (this.weight <= 0.5 ? 0.0 : 1.0) : this.weight;
return 0.0 < actualWeight ? 1.0 : 0.0;

[0b5vr]

Woow, that is a factor I haven't considered in the spec layer; thank you.
Let me consider the interaction between them, I'm not sure your PR is working perfect or not.

[mattrossman]

A comparison of binary expressions with texture transform binds and "blend" lookAt overrides:

Before	After
CleanShot.2024-08-30.at.10.36.18.mp4	CleanShot.2024-08-30.at.10.38.13.mp4

Notice before how when lookLeft is applied, texture transforms on the blink expression apply smoothly instead of a sharp binary change, which breaks the use case of expression textures.

[0b5vr]

Thank you for the detailed explanation! We are currently discussing the behavior with the VRM spec team. I'm 90% confident that we can fix it in the desired way, but we must ensure that the behavior can be described as a spec without confusion before we apply this in our implementation.