GLTFLoader: Vertex color alpha support

[mrdoob]

Is your feature request related to a problem? Please describe.

Now that we support vertex colors with alpha (#20975, #21530) how should GLTFLoader handle it?

Describe the solution you'd like

I can see that it's supposed to be supported:

https://github.com/KhronosGroup/glTF/tree/master/specification/2.0

But should the loader automatically set transparent: true in the material when we find a VEC4 attribute?

@donmccurdy @takahirox

[donmccurdy]

That's exciting! :)

But should the loader automatically set transparent: true in the material when we find a VEC4 attribute?

No, the logic for setting transparent: true can remain as it is now. The A component should have no effect if the glTF material does not use one of alphaMode: BLEND or alphaMode: MASK.

[takahirox]

And we don't hardcode accessor type for vertex color attribute but we refer to accessor.type to determine the type (vec3, vec4, or so on) so vec4 vertex color attribute should already be supported.

So, my understanding is vertex color alpha + glTF loader should be workable even without any change in the loader if Three.js core supports it, shouldn't it?

[mrdoob]

Sounds good!

[mrdoob]

I guess it'd be nice to have a new gltf model test.

Right now they only have this one:
https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/VertexColorTest

[mrdoob]

/cc @emackey