This sketch implements the activator/inhibitor model for the patterns on certain seashells described in the 1992 SIGGRAPH paper "Modeling Seashells" by Deborah R. Fowler, Hans Meinhardt, and Przemyslaw Prusinkiewicz. Link to paper.
The original BASIC implementation was a helpful reference in understanding how the differential equations are simulated. It's a little unusual, the simulation only happens on a line, but every few iterations, the line is added as a row of the image to simulate a growing shell.
Example output for presets:
OLIVIA_PORPHYRIA - (the Olivia porphyria shell described in the paper)
LUMPY_STRIPES:
BLOCKY:
