Support model vertex colors

[appgurueu]

Problem

Most model file formats, and all the formats we support (.b3d, .obj, .gltf / .glb, .x), support vertex colors, however we do not. For nodes, we overwrite colors with lighting information. For objects, we mostly overwrite colors with all white.

It is possible to bake colors into textures, which is why colors are not strictly necessary. However, it would still be nice to have colors for a couple reasons:

• Models with colors just work, no extra steps, just grab the file.
• Color-only mesh buffers are easy to optimize (e.g. by batching or instancing) since no textures need to be bound in-between. (Array textures offer a decent alternative though.)
• Colors offer an interesting target for morph animations. See also Add glTF morph animation support (PoC) #16096.

For the last point specifically, I have a dream: That one day, we will be able to render an animated hue-manatee in Luanti. Generally I think vertex color animations are quite an interesting and flexible tool, e.g. for "shine" or "glow" effects.

Solutions

Colors are already there as a vertex attribute, we just need to refrain from overwriting them; we should make sure the readers set them properly.

As for node lighting, I see two options. One is to add new vertex attributes for lights. This is probably the cleanest option and if I recall correctly, would also allow the resumption of #14343. The other is to just premultiply.

Alternatives

Textures. Even the hue-manatee is possible using an appropriately animated texture, but it won't be smooth without SSCSM, and even with SSCSM it would be wasteful (as you'd not be doing the coloring in hardware / need many textures to swap), or you'd have to hackily use morph animation of texture coordinates.

Additional context

The .x and .b3d readers seem to read colors properly and the .obj reader sets colors based on materials (this might need tweaking). The gltf reader at the moment does not read colors, but this is easily implemented.

[kromka-chleba]

I hope this is directly needed for implementing KHR_texture_transform and enabling UV animations?
I believe we discussed this on IRC. UV animations are a standard way of doing animated textures in Blender for things like animated faces and such. That hue-manatee could be implemented simply by seamlessly animating an UV by drivers on an armature.

edit: I can even model this thing and rig it as a reward for making this happen lol

[appgurueu]

I hope this is directly needed for implementing KHR_texture_transform and enabling UV animations?

It is not. I was not intending to create the impression that vertex colors help with your use case, they just seemed like a low hanging fruit and possibly interesting target for morph animation to me. It's possible that there are better alternatives.

I believe we discussed this on IRC.

We did, but it's been a while so forgive me if I forgot something.

That hue-manatee could be implemented simply by seamlessly animating an UV by drivers on an armature.

Interesting! How would that work? From the gltf perspective, I know that there are some ways to animate UVs (with morph animation or by animating the transforms), but how would that get you the change in hue without needing something like a hundred copies of the texture (e.g. vertical frames)?

I took a look at the gltf file you shared back then (https://files.catbox.moe/cbwps2.zip), and it uses the KHR_texture_transform extension, but I don't see how it would animate texture coordinates: It seems to use neither morph animation, nor KHR_animation_pointer?

I've opened another issue for texcoord animation, let's continue the discussion there: #16748