TSL instancePositionNode

[PoseidonEnergy]

Related issue: #32465

Description

This PR adds instancePositionNode to NodeMaterial to allow for accessing or setting positionLocal immediately after it is transformed by the instance matrix.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	357.49 84.84	357.49 84.84	+0 B +0 B
WebGPU	622.71 172.91	622.95 172.95	+247 B +38 B
WebGPU Nodes	621.31 172.67	621.56 172.71	+247 B +43 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	490.47 119.9	490.47 119.9	+0 B +0 B
WebGPU	693.82 188.21	694.07 188.24	+247 B +30 B
WebGPU Nodes	643.2 175.31	643.45 175.34	+247 B +34 B

[sunag]

I'm thinking of adding a .transformNode so we can handle the vertex transformations, this should also be fix the #32872, so we can remove the .toStack() syntax as well, I was talking to @brunosimon a few days ago about this.

I'm still undecided about the names... but the concept should be similar

import { transform, instance, skinning } from ...

material.transformNode = Fn( ( { object } ) => {

	let finalTransform = transform;

	// transform is a struct or object with position,normal,tangent,bitangent

	if ( object.isSkinnedMesh ) finalTransform = skinning( finalTransform );
	if ( object.isInstancedMesh ) finalTransform = instance( finalTransform );
	// ...

	return finalTransform ;

} )();

or maybe only

import { instance, skinning } from ...

material.transformNode = Fn( ( { object } ) => {

	if ( object.isSkinnedMesh ) skinning( object );
	if ( object.isInstancedMesh ) instance( object );
	// ...

} )();

In this sense, we have access to any stage of vertex transformation, like, position, normal, etc.

[PoseidonEnergy]

@sunag Looks good to me. However, if I call this:

if ( object.isInstancedMesh ) finalTransform = instance( finalTransform );

Does this create a second InstanceNode in addition to the one that already gets created in NodeMaterial.js? See this code snippet:

three.js/src/materials/nodes/NodeMaterial.js

Lines 832 to 836 in 399951c

	if ( ( object.isInstancedMesh && object.instanceMatrix && object.instanceMatrix.isInstancedBufferAttribute === true ) ) {
	instancedMesh( object ).toStack();
	}

I'm worried about unnecessary creation of extra InstancedBufferAttribute objects and everything that goes along with the instance matrix. Would calling instance(object) double-apply the instance matrix in the shader?

[brunosimon]

That would be so useful and make learning a lot easier.
Does positionLocal still makes sense?
Shouldn't it be something like positionTransformed after this node is applied?

[PoseidonEnergy]

That would be so useful and make learning a lot easier. Does positionLocal still makes sense? Shouldn't it be something like positionTransformed after this node is applied?

In my opinion, positionLocal should already be the position of an instance after the matrix has been applied. This is because it is identical to positionGeometry when accessed before the transformation. Simply moving the material.positionNode property to insert shader code after the transform would not result in a loss of functionality. If a developer wants to know what positionLocal was before it was transformed, they can just use positionGeometry. So, introducing another node like positionTransformed would just be taking the place of what positionLocal should be in the first place.