particle_sim

The sprite particle simulation itself: integrate one particle, resolve it against the emitter's colliders, sample its curves, and write the billboard quad it draws as.

A shader that includes it binds particles, vertices and params, includes particle_layout ahead of it, and defines world_field(p) — the distance from a point to the scene's own geometry. That one function is the difference between the update shaders: one answers from a distance field it binds, the other answers that nothing is near.