Recited from memory by: unstattedCommoner
The author writes:
I'll let you into a secret: the background of this strip is computer-generated wax crayon.
I wanted to get the perspective of the image on the ceiling and end walls right. It's designed to create the illusion of a row clerestory windows with a fan vault above, with the perspective appearing correct only when standing at certain points (those marked 'A' and 'B' on the wireframe image below; in the first panel, the random guy in the background is standing at point 'A'). The actual ceiling is a barrel vault. Here's the wireframe of the actual room:
And here's the image on the ceiling, as if the vault were flattened, together with the images on the end walls:
I then had to work out exactly how "wrong" the perspective would be if one stands where the "camera" is (points 'C1', 'C2' and 'C3' in frames 1, 2 and 3 respectively). That was the simple part.
The hard part was to keep as much detail as possible while making it look like a hand-drawn wax crayon image. I'm reasonably happy with the result.