Sparse Smoke Simulations in Houdini

High resolution fluid simulations are commonly used in the visual effects industry to convincingly animate smoke, steam, and explosions. Traditional volumetric fluid solvers operate on dense grids and often spend a lot of time working on empty regions with no visible smoke. We present an efficient sparse fluid solver that effectively skips the inactive space. At the core of this solver is our sparse pressure projection method based on unsmooth aggreggation multigrid that treats the internal boundaries as open, allowing the smoke to freely move into previously-inactive regions. We model small-scale motion in the air around the smoke with a noise field, justifying the absence of reliable velocities in the inactive areas.