# caustic The pool of light a flask of liquid throws on a bench: a warped interference net gathered to a crest A curved wall full of liquid is a bad lens with a moving surface, so what lands on the bench is an ellipse with a bright caustic web crawling inside it. Three plane waves are taken over a warped domain — on a straight grid they make a lattice, on a warped one they make the wandering net a real surface throws — gathered to a sharp crest and clipped to the ellipse so nothing square ever shows. Additive: it is light arriving, not a surface standing there. `focus` walks the ellipse across the quad, `phase` drives the churn, and `color` is read off the solution's own concentration.