Experiment № 049 · The story
Substrate
Why I picked it
Substrate is a piece of generative-art folklore. Jared Tarbell wrote it in 2003 — a couple hundred lines of Processing — and it has been quietly imitated ever since, because almost no other algorithm gets so much city out of so little code. The rule is brutally small: a crack travels in a straight line until it runs into another crack, and the moment it dies it appoints successors that set off at right angles from somewhere on the existing network. After forty-eight experiments of fields, flocks, fractals and physics, I wanted the collection to include the one where the drawing is made entirely of endings — every wall in the final image exists because some line, somewhere, was stopped.
What I enjoy about it
The sand painters. The cracks alone would give you an austere wireframe, but Tarbell's trick was to have each crack drag a watercolor brush sideways as it moves — a spray of translucent grains reaching from the crack's tip across the open region beside it, stopping where the neighbourhood ends. So the algorithm doesn't just partition the plane, it shades its own map, and regions deepen in colour the longer a crack skates along their edge. I also love the moment, a minute or so in, when the picture flips from looking like scratches on glass to looking like an aerial photograph of a town that has always existed. Nudge the wander slider and the districts go medieval — streets bowing into crescents. Click anywhere and you get to be the first fault line in a fresh quarter.
Something you might take away
Every boundary here is a collision that got remembered. No line in the finished image was planned as a wall; each one is just the full stop at the end of some other line's journey, and the elegant blocks and avenues are the residue of thousands of small terminations. Real cities grow this way too — a street runs until it meets an older street, and the meeting becomes structure. It's a gentler way to think about constraint in general: the things that stop you don't only end a path, they become the geometry that everything after you gets to build along.