Every site holds a rotor pointing in one of four directions. A particle leaves the centre. At each occupied site it turns that rotor a quarter-turn and follows it, and it settles at the first empty site. There is no randomness after the first frame.
Tap to restart the growth from a new centre. The same ?seed= always gives the same run.
LITRotor-router aggregation; its shape converges to a disc (Levine & Peres 2009). Every 30 steps the settled sites must equal the particles released, or the page throws.
AMBEROnly the starting rotors are seeded; the growth itself is deterministic.