Why does the burst follow the grid lines in thin gas but spread as a ring in thick gas? At the default background-density of 8, the bright disc spreads mainly along the four grid directions. In the expanding ring, density along the axes is about twice the density along the diagonals.
Try this Choose the Vacuum Burst preset (background-density 0). With nothing to collide with, the disc splits into four copies of itself that fly apart along the axes at one patch per tick. A patch with all four channels full holds no head-on pair, so the disc never scatters.
About this model Where it ends up
Hardy, Pomeau and de Pazzis's lattice gas (1973), the HPP model: particles on a square grid move in four directions, and the only collision turns a head-on pair by 90°, conserving particle number and momentum. With enough collisions a density pulse travels like sound; with too few, the grid's four directions show through.
Each patch of a 129 × 129 torus has four channels (up, down, left, right), each empty or holding one particle. Setup fills each channel with probability background-density percent and fills all four inside a disc of radius blob-radius at the center; both are read only at setup. Each tick, all at once: a patch holding exactly an up-down pair or exactly a left-right pair swaps it for the other pair, then every particle moves one patch in its direction. density is the number of full channels, 0 to 4.
This is the plain HPP rule, with no rest particles and no randomness after setup, so the square lattice's anisotropy is on display. Frisch, Hasslacher and Pomeau's hexagonal lattice gas (1986) was introduced to fix it.
