GLTFLoader: Clone node associations.

[nkrajina]

Description

GLTFLoader doesn't correctly assign the nodes association when multiple
nodes reference the same mesh. The problem occurs in _loadNodeShallow
where nodes property is added to the shared object describing the node/mesh
association.

For example, in the CesiumMilkTruck model nodes 0 (Wheels) and 2 (Wheels.001)
reference the same mesh 0, but resulting associations for both three.js meshes are
{meshes: 0, primitives: 0, nodes: 2}.

A simple solution is to clone the association object if there are multiple references to
the same mesh.

[Mugen87]

GLTFLoader doesn't correctly assign the nodes association when multiple
nodes reference the same mesh.

What are the consequences of this? Does that means certain glTF asset can't be loaded properly? If so, can you share one so it's possible to reproduce the issue?

[nkrajina]

GLTF assets will load properly, but the object returned by loader.parse() will have incorrect data in parser.associations map when multiple GLTF nodes map to the same GLTF mesh.

I need a way to reference an arbitrary point in the GLTF model with animations, so I can compute where it is positioned at each animation frame. The point can be uniquely specified by the following:

• index of the node in the GLTF
• index of the primitive in the node's mesh
• indices of vertices of the triangle in the primitive
• barycentric coordinates of the point inside the triangle
• optional index of the instance if instancing is used (ignored in the rest of this discussion)

Node and primitive indices should suffice to uniquely determine the generated three.js mesh, from which further computations can be done. But since the nodes property in the associations map is incorrectly stored, incorrect mesh will be addressed.

[donmccurdy]

Let's go ahead with this, it sounds like a worthwhile fix to me.

I have been somewhat regretting the complexity of how GLTFLoader unpacks the scene and manages associations – this is a maintenance challenge but I don't have free personal time to work on it, which would need some careful testing. For the changes in output that we probably need to make eventually, see:

• #30091
• #29768