air_floor

Thirty years of dust lying on the mosaic, and the clean arc the day's light walked through it. The film is a thin grey surface over the tesserae, lit by the engine exactly as the mosaic under it is lit, so the coloured patch crossing the floor crosses the dust with it. Where the patch has already been, convection took the dust up and it has not come back: the floor there reads as the tile's own colour, with a ridge of swept dust standing along each edge of the arc. The arc's centre line and half width arrive as two cubics fitted to where the window's picture actually lands through the day, and front says how far east the sweep has reached.