BatchedMesh: Enable per-instance opacity

[vanruesc]

Description

Related: #28151

This PR adds support for per-instance opacity to BatchedMesh. The current implementation of BatchedMesh already uses colors with 4 components and it supports sorting. The only thing missing is to use the alpha information in the shader.

I've removed the opacity option from the example because it no longer made sense with per-instance transparency.

This contribution is funded by Cozy Giant

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	355.82 84.56	355.71 84.53	-113 B -25 B
WebGPU	621.5 172.57	621.5 172.57	+0 B +0 B
WebGPU Nodes	620.1 172.33	620.1 172.33	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	488.06 119.45	487.94 119.42	-113 B -27 B
WebGPU	691.97 187.85	691.97 187.85	+0 B +0 B
WebGPU Nodes	641.77 175.02	641.77 175.02	+0 B +0 B

[vanruesc]

Puppeteer doesn't like my screenshot - probably because of too many random values 🤔

I'll try changing the materials used in the demo back to MeshNormalMaterial when I find some time.

[gkjohnson]

Just added some comments - at a high level I don't have an issue with the idea but I think there are some architectural complexities with vertex colors that make this more difficult.

Puppeteer doesn't like my screenshot - probably because of too many random values 🤔

It looks like the uploaded screenshot doesn't include any transparency. Here's the comparison downloaded from the CI artifacts:

Commited	CI

[vanruesc]

It looks like the uploaded screenshot doesn't include any transparency.

My bad, I forgot to build the lib before generating the screenshot 🤦‍♂️

[gkjohnson]

A few more comments but generally this is looking like the right direction to merge. To summarize the changes from what I can tell:

• the vertex color varying is now always set to a vec4
• vertex colors multiply in just the rgb channel if a vec3 (USE_COLOR). Otherwise all four channels are multiplied if using RGBA (USE_COLOR_ALPHA).
• BatchedMesh colors always multiply in vec4 rgba
• InstancedMesh colors always multiply in vec3 rgb

[vanruesc]

Open question

Should batchedMesh.setColorAt(color, index) set alpha to 1 if a Color object is supplied?

👍 for yes | 👎 for no

Will be reevaluated after #32742 is done.