About Marbling
Marbling is the craft of floating ink on water, nudging it into a pattern, then pressing a sheet on top to capture it. Two big traditions feed it. Suminagashi is the Japanese version, going back to the 12th century, sumi ink spread on plain water. Ebru is the Turkish and Persian one, dense colors floated on a thickened bath. Both share the same trick: you can't really plan a marbled pattern. You drop, you swirl, the water gives you something back.
If you've seen the swirly endpapers inside an old book, that's marbling. Same with the patterns on those vintage Italian notebooks. It's one of those crafts that looks impossible until you watch a master do it for ten seconds and realise it's just dropping and dragging.
How It Works
The whole simulation runs on an honestly insane bit of math from Aubrey Jaffer, worked out across a few papers in the early 2000s. The drops and the comb wakes you see here are the exact mathematical model of how ink behaves on water, not a cheap approximation. Every shape is provably correct. That blew my mind a little.
Drops are the cleanest part. When a circle of paint of radius r lands at point C, every existing point P moves outward by sqrt(1 + r²/d²). The old colors get pushed into a thin ring around the new one. Drop again into the same spot and you get nested rings, which is the basis of every marbled pattern you've ever seen.
Combs work on the same principle. Each tine pulls the paint along your stroke direction, with a falloff to either side. That's why a single drag through a row of drops turns them into the classic feathery wakes.
How to Use It
Pick a color from the toolbar (or just press 1, 2, or 3). Click the water to drop ink. The active color is the one with the dark ring around it. Click an active swatch a second time to change its color.
Three drop sizes, three comb tines. Smaller drops let you build up layered detail. The combs each pull a different number of wakes per stroke. Slower drags give cleaner curves, fast ones get more chaotic.
Try this: drop three or four colors right on top of each other, then drag a comb left to right once, then top to bottom once. That's the classic non-pareil pattern from old book endpapers.
Sources and Reading
Suminagashi
on Wikipedia.
Paper marbling
on Wikipedia.
Aubrey Jaffer's
marbling page, with the math that does all the heavy lifting
under the hood.