light_shaft

The beam a lamp cuts through vapour, drawn on a card standing in the beam. A shaft is the vapour in the light seen edge-on, so the card is brightest where the beam is thickest through the eye's line, breaks into slabs where the vapour drifting across it is denser, and dies at the beam's edges instead of ending on a line. Drawn additive: scattered light adds to what is already behind it.

uv.y runs from the lamp at 0 to where the beam lands at 1. mouth is the beam's width at the lamp against its width at the far end, core how much of the width stays flat before the sides fall away, reach and falloff how far it carries, grain/drift/mottle the vapour crossing it, and lip how quickly the card stops showing its own edge at the lamp.

Lane 0 (shader_data[0]) carries (film seconds, beam strength 0..1, -, -), written every film frame by demo/stages/air.module.