GLTFLoader: Add createNodeMesh hook

[takahirox]

Background

We added invokeAll createNodeAttachment hook in GLTFLoader #19892. It's a good hook for adding extra objects to a node object, like light.

But as I wrote in #19892 (comment) this hook can't be used for suppressing core spec meshes (and cameras). Some extensions want to suppress loading core spec objects (and instead want to load objects defined in the extensions). eg. materials unlit extension

Actually I needed this capability in node as I was writing MSFT_lod extension plugin. So I would like to add a new hook point for node. I know non KHR_ extensions are not really our glTF loader extensibility mechanism targets, but it would be good if we can add a new hook point without messing the core code.

InvokeOne loadNode() hook may be one of the options similar to loadMaterial or others. But core loadNode() does a lot common works for example make a group for multiple objects and setting transform. I think extensions need the same ones.

Suggestion

So I would like to suggest to extract the part creating node meshes and add an invokeOne hook for it, named createNodeMesh hook. The common works for node still be usable for the extensions using the new hook. And the extensions using the new hook doesn't have an effect to the extensions using createNodeAttachment like light extension. I think it's good for most of the cases.

[takahirox]

Sorry I know you seem busy but friendly ping @donmccurdy

[donmccurdy]

Looks good to me!

[takahirox]

Sorry, I found a problem in this implementation after the review has been done. Plugins' createNodeMesh() should be called even if core nodeDef.mesh is undefined. e.g. GPU instancing extension mesh without fallback in core. I would update soon.

Nit: EXT_mesh_gpu_instancing doesn't define a behavior for a node with the extension that doesn't also have a mesh so the example above may not be good, anyways.

[takahirox]

Updated

[mrdoob]

Probably better to move it to next cycle so we have time to test.

[takahirox]

It seems this PR had conflicts in examples/js/loaders/GLTFLoader.js made by #21588. But examples/jsm/loaders/GLTFLoader.js had no conflicts so I ran npm run build-examples and now no conflicts but there are lot of diffs in examples/js/loaders/GLTFLoader.js. I haven't looked into the examples builder script but do the diffs look ok? I think if I trust the script, it should be fine.

[mrdoob]

We have no other option than trusting the script... 😅

[mrdoob]

Thanks!