GLTFLoader: Remove uv2 attribute when not being used

[mrdoob]

Description

This is a quick workaround until we improve the current uvset code (coming up soon in my todo list!)

Some gltf models may have second uv set but the aoMap may not use it, resulting in broken models.

Before	After

/cc @elalish @donmccurdy

[mrdoob]

@elalish

Meh... This breaks webgl_loader_gltf_compressed.
I'm afraid I'll have to leave it for r141 (or a point release if we figure out the fix).

[donmccurdy]

Probably these are the cases to consider:

aoMap.texCoord	uv2 provided	result
uv2	✔	ok
uv2		invalid
uv1	✔	rewrite and warn
uv1		rewrite

The only official glTF material texture that three.js allows to use uv2 is .aoMap, and so when AO would otherwise break, we'll copy uv to uv2 even if that requires overwriting an existing uv2 attribute. If there's no aoMap present, an unused uv2 will not be removed or overwritten.

[donmccurdy]

I think this would do it:

if ( material.aoMap ) {

	if ( material.aoMap.userData._gltfTexCoord === 0 && geometry.attributes.uv !== undefined ) {

		if ( geometry.attributes.uv2 !== undefined ) {

			console.warn( 'THREE.GLTFLoader: Removed unused uv2 attribute.' );

		}

		geometry.setAttribute( 'uv2', geometry.attributes.uv );

	}

}

[elalish]

Any hope of merging this one since multi-UVs are delayed? Seems like Don's answer is a good one.

[mrdoob]

#25721 solved this.