window_gobo

Paints the south window's coloured image into one layer of the shared feature-texture array, in the projector spot light's own clip space, so the light carries a picture of the glass instead of a flat cone. Each texel takes the ray it stands for, walks it to the window's plane and evaluates the leaded grid, the rosette, the cobalt band and the missing pane where the ray lands — so the leads stay sharp at any throw and the image is drawn under the exact obliquity the sun has at that film second.