heat_shimmer

Hot air above a flame. The card samples the lit opaque scene standing behind its own fragment twice — once straight and once at a coordinate pushed around by two decorrelated turbulence fields scrolling upward — and composites the displaced read over the straight one through a rising plume mask. Nothing is added to the frame except a faint forward-scattered tint of the flame's own colour; what makes the wall behind the burner come apart is where each of its pixels is fetched from.

Its clock is per-instance lane 0 (shader_data[0].x), written every film frame by demo/stages/air.module, so the shimmer advances on film time rather than on the engine's wall clock.

strength is the displacement in screen fractions at the plume's widest; detail the turbulence frequency across the card; speed how fast the column rises; foot/crown/throat the plume's shape; coverage how much of the straight scene the bent read replaces; tint the forward-scattered glow, alpha being its weight.