whatA pool in the sun: parallel light falls on a surface made of five seeded travelling waves, each ray bends once by Snell's law and lands on the floor. No ray knows about any other. The ink draws where the light gathers; drag across the pool and your ripples refract too.

whyThe claim: the dancing web on every pool floor is an ENVELOPE -- the creases where neighbouring rays fold over each other and land together, pinching into cusps. Flatten the waves with the slider and the web dissolves into even light: measured here, the brightest patch of floor falls from tens of times the average brightness to about one. Raise the waves and the folds condense out of nothing. Focus is geometry, not intent.

howForty thousand rays per frame in the paraxial approximation: each lands displaced by the surface gradient times the optical lever, and lands are binned into a density field rendered as halftone ink. The stats line reports the brightest bin against the mean, which is the claim's number. One integer seeds the sea state, so the same seed dances the same dance.