Stained Glass Day

The banking hall of the Farmers & Merchants Union Bank, Rye County — a windowless brick cube 19 m long with one enormous leaded window in its south wall, shut for thirty years, the vault standing open and empty and demolition numbers chalked on the columns. Over ninety-nine seconds the sun crosses that window and drags a seven-metre coloured picture of it across the west wall, down onto the mosaic floor, over the counter and out along the east end. Nobody comes; swallows nesting in the mezzanine capitals are the only fast thing in a film whose subject moves at fifteen degrees an hour.

The film's other subject is the air. It opens in a room so clear you can see exactly where the light lands and nothing at all of where it travels. By the third act the window's light is a solid green-gold wedge you could put your hand into. Then the sun leaves the window, the beam goes out, and the dust does not settle — the last shot is the first shot's station, height and lens exactly, with no colour in it and an atmosphere you can see turning.

What it is made of

• The room — 741 entities: mosaic floor, coffered ceiling, cornice and terracotta frieze, 13 engaged piers, 5 columns, mezzanine deck and 68 balusters, marble counter with 48 real teller bars, the open vault, and the 63-light leaded window with its rosette and one missing pane.
• The light — a spot 140 m out along the sun's own ray, with a compute shader painting the window's coloured image into the light's clip space every frame, so the leads stay sharp at any throw and the picture is foreshortened by the sun's real obliquity. Everything the beam crosses cuts it.
• The air — a marched participating medium over the whole interior, heat shimmer over sunlit stone, and thirty years of settled dust on the mosaic with the clean arc the day swept through it.
• The colony — fourteen barn swallows, each one's whole day a pure function of film time.
• The cut and the score — eighteen set-ups and a ten-source mix, both pure functions of the same clock, so seeking to any second reproduces that second exactly and the film records frame-for-frame.

The clock

One rendered frame is one 1/24 s film step, advanced by exactly one component. The film is identical at 60 fps and at 4 fps, and two full playthroughs land on the same frame.

Known gaps

Three beats in the shot list are not in the room. At this latitude and this window height the sun never puts its picture on the counter top, never climbs the north wall onto the balustrade, and never reaches the vault's face at any hour of the day — those three lenses were re-aimed at where the light actually goes, and the comments in demo.sequence say so at each one. The noon patch reads as light on a mosaic rather than as a legible picture of the window.