Creative Control in PB Fluid Simulations

Fluid simulations — water, fire, smoke — are governed by partial differential equations. The state of the fluid at any frame depends on every frame that came before it. So if an artist wants to tweak one moment, they have to alter the initial conditions and recompute the entire sequence. Iteration is painfully slow.

I built a method that lets artists steer a simulation at any point in time without recomputing what came before. The controls were sketches and brushes drawn directly on a paused frame — treated as constraints the running solver had to respect.

The hard part was on the math side: how do you inject those constraints into a running solver without breaking conservation laws? Naive approaches leak mass or energy and the simulation falls apart. I had to build the constraint injection carefully enough that the fluid stayed physically plausible after each edit.

The other half was making that injection feel like drawing. Most physics tools ask artists to learn the math; I wanted the math to feel like a brush.