code/_onclick/hud/plane_master.dm

This system works by exploiting BYONDs color matrix filter to use layers to handle emissive blockers.

Emissive overlays are pasted with an atom color that converts them to be entirely some specific color. Emissive blockers are pasted with an atom color that converts them to be entirely some different color. Emissive overlays and emissive blockers are put onto the same plane. The layers for the emissive overlays and emissive blockers cause them to mask eachother similar to normal BYOND objects. A color matrix filter is applied to the emissive plane to mask out anything that isn't whatever the emissive color is. This is then used to alpha mask the lighting plane.