Overview

Pyro is the built-in Houdini solver for simulating smoke, dust and explosion effects. There are two main modes of the Pyro solver, Dense and Sparse.

Dense Simulation

simulates the entire container, useful as the velocity values are maintained even in areas with no visible smoke

Sparse Simulation

only simulates the active areas of the container, i.e; areas with smoke. Supposedly faster as it only performs computations on areas of interest. Comes with some limitations and caveats.

Notes on Sparse Solving

Regions of smoke that are not connected are invisible to each other until they are close enough to merge
The container will not keep up with fast moving fluids by default, unless Expand by Velocity is enabled
This means that by default, simulations with high velocity fields will look very different in Sparse and Dense mode
Sparse solving is especially suited for simulations like missile trails, where you would otherwise have to simulate a very large region in Dense mode