WebGLRenderer: Add support for GLTF opaque alpha_mode

[mrdoob]

Related issue: #22414 #22424

Description

I think this is how GLTF's OPAQUE ALPHA_MODE is supposed to behave.

(Ab)using blending doesn't feel right though.
We should be able to reuse NormalBlending, but we need a way to trigger the #define OPAQUE in the shader.

material.alpha = false
material.disableAlpha = false
???

/cc @donmccurdy @WestLangley @elalish

[mrdoob]

Another option that my younger-self proposed was:

material.format = THREE.RGBFormat;

[mrdoob]

I think for the time being I'm going to go with material.format = THREE.RGBFormat.
I want to get this out of the way now that I've my head around it, but would love to hear any other naming ideas for it.

[donmccurdy]

I'm OK with .format here, but perhaps .outputFormat would be a little clearer?

I do not really like the name .alpha as a boolean, too easily confused with .transparent or .opacity.

[mrdoob]

I'm OK with .format here, but perhaps .outputFormat would be a little clearer?

Maybe yeah...

Although GLTF is currently confusing. Getting transmission to work in alpha: true contexts is incompatible with alpha_mode opaque...

three.js/src/renderers/shaders/ShaderChunk/output_fragment.glsl.js

Lines 2 to 9 in ee46b28

	#ifdef OPAQUE
	diffuseColor.a = 1.0;
	#endif
	// https://github.com/mrdoob/three.js/pull/22425
	#ifdef USE_TRANSMISSION
	diffuseColor.a *= transmissionAlpha + 0.1;
	#endif

[donmccurdy]

I guess I think of glTF's alphaMode as specifying how the material's alpha value (0–1) or texture values should be interpreted, and little more than that. It doesn't prohibit the snippet above, so long as alpha components in the base color texture are being ignored for opaque materials.

[mrdoob]

I see. That makes sense.

[marcatec]

I think for the time being I'm going to go with material.format = THREE.RGBFormat.
I want to get this out of the way now that I've my head around it, but would love to hear any other naming ideas for it.

Setting material.format = THREE.RGBFormat has the following side effect (e.g. in the editor):
Trying to change the opacity (and tranparent value) of an opaque material of an imported glTF file doesn't work anymore.
material.format should be set to THREE.RGBAFormat to make it work..

[mrdoob]

Yeah... I'm going to continue working on this issue this month. material.format was not the right solution.

[donmccurdy]

/cc #22598 (comment)