Condensation

a fogged window over a snowy street at night: drag across the glass to wipe the condensation off, and it fogs back over at the rate the humidity sets.

key insight: the fog is a texture the gpu keeps between frames, red for fog and green for the wet film a wipe leaves. a stroke is a list of segments stamped into it, so the glass remembers every wipe and the composite only has to read it.

the street is one photograph, blurred once at load for the fogged side, and every bead and running drop is a lens sampled from the sharp copy.