BatchedMesh: add per-instance setOpacity

[Kikedao]

Description

This PR adds per-instance setOpacity and getOpacity to BatchedMesh.

I created this PR inspired by these two threads from the forum:
https://discourse.threejs.org/t/batchedmesh-per-geometry-opacity/61284
https://discourse.threejs.org/t/batchedmesh-set-per-instance-opacity/64170

I tested it in the project I'm working on and it seems to work wonders with absolute perfect performance.

If you are fine with this implementing setColorAt shouldn't be too difficult, and with that the InstancedMesh and BatchedMesh APIs would finally converge :).

Here is a video capture of it animating individually the opacity of all instances in the BatchedMesh with the help of gsap (the links you see in the graph are tube geometries all used in the same BatchedMesh):

2024-04-17.23-47-32_1_4.mp4

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

Filesize dev	Filesize PR	Diff
674.7 kB (167.2 kB)	675.9 kB (167.3 kB)	+1.2 kB

🌳 Bundle size after tree-shaking

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

Filesize dev	Filesize PR	Diff
454.1 kB (109.6 kB)	454.6 kB (109.7 kB)	+472 B

[Kikedao]

Worth mentioning that the material used in the BatchedMesh of the video is as simple as this:

this.three.material = new THREE.MeshLambertMaterial({ 
    color: 0xffffff,
    transparent: true,
    vertexColors: true
});

[Mugen87]

TBH, I don't like the idea of moving material properties to BatchedMesh as suggested. This will only blow up the API of BatchedMesh and transform the implementation to a hard to maintain code base. As implied in this comment by @gkjohnson, it's seems viable to investigate a more generalized approach.

[Kikedao]

I get your point, yet here is how I see it:
The problem with BatchedMesh is that the only way to be able to 'set' any kind of per-instance property is to use textures. BatchedMesh implementation is already doing that for its setMatrixAt method to be able to position the instances, encoding the per-instance matrices in a texture.
The let down for me was discovering that BatchedMesh doesn't have the same API as InstancedMesh for setting things per-instance:

• InstancedMesh has setMatrixAt and setColorAt
• BatchedMesh has setMatrixAt and setVisibleAt (yet setVisibleAt is a bit special...)

So my take is to just converge both classes APIs so they support at least setMatrixAt, setColorAt, and as I suggest in this PR setOpacityAt, which I think should be added to InstancedMesh too for completeness.
I find being able to set these three properties per-instance in both classes is the minimum you would expect to be able to manipulate them a little bit and control how the instances are displayed for a lot of usages.

At least I think we should add setColorAt, it makes no sense that InstancedMesh has it and not BatchedMesh.
If you agree BatchedMesh should have a setColorAt method just as InstancedMesh does I can create a new PR to add it, just want to know if you agree so I don't do it for nothing (then we can discuss if setOpacityAt would be also useful to be added to both classes).

[Mugen87]

It feels consistent to align both APIs and add setColorAt() to BatchedMesh. setColorAt() should be RGB only though (so no alpha).

@gkjohnson Are you okay with that?

[gkjohnson]

It feels consistent to align both APIs and add setColorAt() to BatchedMesh
...
@gkjohnson Are you okay with that?

I'll defer to you but my feeling is that this is all already possible with shader overrides, etc as my prior demo shows. I'd rather see some thought put into what a more extensible API would look like to support per-object material properties. Even with InstancedMesh users were quickly asking for other material setting support if I recall correctly.

If you'd just like to just add setColorAt to align with InstancedMesh I won't block it. I don't think I support adding more property support like opacity, though, without considering some more robust solutions to this issue.

[Kikedao]

Thx for your feedback @Mugen87 and @gkjohnson, I will then close this PR and add just setColorAt for BatchedMesh in a new one.