Night 053Rauch

Night fifty-three, a piece night, and the rotation pointed at a simulation. The stable-fluids idea had waited in the backlog since the first week. Rauch is the German word for smoke.

The solver is Jos Stam’s from 1999, the one that made fluids affordable for pictures. Velocity lives on a coarse grid, dye on a finer one, both as half-float textures on the GPU. Each frame the velocity is carried along itself by looking backwards, a curl term puts back the small swirls that step blurs away, a pressure field is relaxed by two dozen Jacobi sweeps so the flow neither piles up nor tears, and the dye rides the corrected flow. Walls are a mask the visitor paints: inside it the velocity is zero and the pressure solve treats the wall as a mirror, which is what makes smoke go around a stone rather than through it.

Four scenes. Schornsteine opens the piece: chimneys at seeded positions along the bottom edge, their smoke lifted by buoyancy. Tinte is still water where the pointer is the only source. Windkanal blows from the left past three round walls, and streets of eddies form behind them. Wirbel starts with one ring of velocity that breaks into curls. Wind, buoyancy, vorticity and palette are in the link; the walls and the smoke are not, because a mask is too big for a hash and a scene is meant to be stirred, not stored.

Claude Opus 5.5 built it from my spec in two rounds. The first came back with every check passing and nine departures from the spec, each with a reason, among them a relaxation strip on the upwind edge because a mere force took half a minute to spin the tunnel up. I accepted all nine. The second round came from my review: my own phone threshold had classed a 1280 by 800 laptop as a phone and cut its grid and pressure sweeps, the reduced-motion scene opened as an empty sky, and a double tap on the scene button would have zoomed an iPhone. All fixed.

A detail nobody asked for: the floor is closed and the other three edges open, so smoke leaves at the top and the wind tunnel flows through rather than piling up at the far wall. Safari and Firefox are untested, as usual.