InstancingMesh: Added per instance color support

[mrdoob]

This allows doing this:

const color = new THREE.Color( 0xff0000 );
const mesh = new THREE.InstancedMesh( geometry, material, 10 );
mesh.setColorAt( 0, color );

[donmccurdy]

@mrdoob note that this already works, without GLSL or renderer changes, as shown in this example:

three.js/examples/webgl_instancing_scatter.html

Lines 101 to 105 in b5c272c

	blossomGeometry.setAttribute( 'color', new THREE.InstancedBufferAttribute( color, 3 ) );
	blossomMaterial.vertexColors = true;
	stemMesh = new THREE.InstancedMesh( stemGeometry, stemMaterial, count );
	blossomMesh = new THREE.InstancedMesh( blossomGeometry, blossomMaterial, count );

Maybe .setColorAt(...) could rely on the same method?

[mrdoob]

I looked at that example but I'm not able to follow the code.

And then we have this other one:
https://threejs.org/examples/?q=inst#webgl_instancing_modified

I think we need a simpler example.

[mrdoob]

Does the approach in webgl_instancing_scatter support instanceColor + vertexColors?

[donmccurdy]

Does the approach in webgl_instancing_scatter support instanceColor + vertexColors?

No, it is relying on the fact that existing shader code for the color attribute can be given an instance attribute instead of a vertex attribute. So you can only use one or the other.

I think we need a simpler example.

I think most of the complexity has to do with turning a Mesh into InstancedMesh+InstancedBufferGeometry. calling .setAttribute( 'color', new THREE.InstancedBufferAttribute( color, 3 ) ); is not complicated. In my opinion https://threejs.org/examples/?q=inst#webgl_instancing_modified should be changed to do something that can only be done by editing the shader, like different UV offsets for each instance to give the effect of different textures on each.

[mrdoob]

No, it is relying on the fact that existing shader code for the color attribute can be given an instance attribute instead of a vertex attribute. So you can only use one or the other.

What does GLTF think about this?

[donmccurdy]

To support EXT_mesh_gpu_instancing we need the ability to put custom InstancedBufferAttributes on InstancedMesh, as both of these examples do. But glTF doesn't specify any particular meaning of those attributes, except the instance transform. So, support for per-instance colors is not required for glTF: having color+instanceColor, or just color, would both be fine.

[mrdoob]

As far as I can see, this PR doesn't conflict with that usage, correct?

The intent of this PR is to make a nicer API for per-instance colors.

[Mugen87]

Just for my understanding: This PR allows to use vertex colors and a color value per instance right?

In any event, I like the idea of defining the instance color via setColorAt() without working with buffer attributes.

[mrdoob]

This PR allows to use vertex colors and a color value per instance right?

This PR allows setting a color per-instance that multiplies the geometry vertex colors (if enabled) which multiplies the material color, etc

[donmccurdy]

As far as I can see, this PR doesn't conflict with that usage, correct?

I don't see any confilct, no. :) Worst case, we could always change the internal implementation of setColorAt later.

[mrdoob]

Thanks doob!

[LasseD]

I am happy to see color being added to instancing.

Is there a reason for not including alpha, or at least the possibility of including the alpha channel?

[Mugen87]

@LasseD Instancing and transparency are tricky, see #20431.

[LasseD]

Thanks @Mugen87 . This is what I feared. Thanks for the link, it provides good insight.