The walls of a sector map as CauldronGPU.Sectors.Surfaces surfaces, one quad per section of each side, keyed {:picture, id} and facing into the side's sector.

A one-sided line shows its middle from floor to ceiling. A two-sided line shows its upper between the two ceilings, its lower between the two floors, and its middle once, not repeated, within the opening. Pictures follow Doom's pegging:

• middle of a one-sided line: top at the ceiling; :lower_unpegged, bottom at the floor
• upper: bottom at the lower ceiling; :upper_unpegged, top at the higher ceiling
• lower: top at the higher floor; :lower_unpegged, placed as if hung from the side's own ceiling
• middle of a two-sided line: top at the lower ceiling; :lower_unpegged, bottom at the higher floor

x_offset and y_offset move the picture along and down the wall. Where both sectors' ceilings are in sky, the upper is left out. Lines running along the map's y axis are drawn 16 light levels brighter and along its x axis 16 darker.

CauldronGPU.Sectors.Walls.surfaces(map, pictures, MapSet.new([sky_flat]))