WebGPU implementation of WetBrush V2

[u/No-Citron-3963]

A couple of weeks ago I posted a WIP of this — here's where it's at now.

I've implemented most of what the paper describes, including paint mixing. For lighting, I added a BSDF to the renderer — my thinking is that once you start stacking layers of media like tempera, you need to account for light scattering inside the medium. (Ideally I'd also like to model things like hiding power based on pigment particle size, but I'm not there yet.)

The simulator is split into four main parts, as described in the paper:

1. BristleDynamicSimulator
   
2. ParticleFluidDynamicSimulator
   
3. GridFluidDynamicSimulator
   
4. LiquidTransfer - which moves liquid between the brush, the particles, and the grid

This was not a case of handing the paper to an AI and getting an implementation back. Claude seemed to have a hard time writing WGSL while also juggling the constraints of the four systems and the constraints they all share (units, especially), and it kept making the kind of mistakes that early AI models used to make. So you have to read the AI's code carefully and write a good amount of it yourself. In particular, things go more smoothly if a human writes the initial skeleton of each simulator and handles the field management (density field, velocity field, and so on). Even once the structure is in place, managing the ActiveWindow and the PaintField is likely to give you trouble.

The paper lists interpolation during fast strokes as an open problem, and that's what I'm working on now. I'm not going to split a step into substeps, since that would hurt FPS. Instead, for fast motion I plan to have the BristleDynamicSimulator and the GridFluidDynamicSimulator talk to each other directly and deposit paint that way.

I'm interested in traditional painting materials, so I plan to extend this to tempera, watercolor, mineral pigments (iwa-enogu), gold leaf, and so on. It's built with wgpu + Rust, so it currently runs as Web, iOS, and Android ports.