game.stage.beam

The shaft of a spotlight standing in the air of a tent, drawn as an additive cone shell over the scene's own scattering.

It shades itself: what a beam looks like is decided by how much air the eye is looking through, not by how the shell is lit. A ray that grazes the wall travels a long way inside the shaft; one that goes straight through it crosses almost nothing — so the term is (1 - |dot(N, V)|)^edge, which gives a hollow shell the soft bright rim and dim middle a real beam has from every angle, and keeps it from reading as a translucent sheet across a wide frame.

dust is how much of the shaft is motes turning in it and drift how fast they fall. UV.y comes from game.stage.tentGeometry.cone: 0 at the lamp, 1 at the pool, which is what falloff fades along.