air_field_mat

The banking hall's air, marched per fragment through one box covering the interior. Each view ray accumulates three things along its way: the dust the heated mosaic lifted, lit by whatever the sky is putting through the glass; the sun's own light wherever the ray stands inside the picture the south window throws, coloured by the pane it came through, cut by the lead came and carved by the marble counter and the mezzanine deck; and the motes themselves, a sparse lattice of points that rise and turn. Scattering is forward-biased, so a shaft seen side-on is a quiet solid wedge and the same shaft seen from inside, looking back up at the window, is blinding. Density scales with `air`, so at first light the medium contributes nothing at all and the shaft is invisible; everything animated is a function of `film_t` alone, so the same film second draws the same frame however the engine reached it.