The lightmaps of a level are computed with all doors shut, so the light of a room stays in that room, even when its door opens.

D2X-XL now has light portals. When a door opens, the opening starts to glow like a window: the light of the room behind it falls through the opening and lights the floor, the walls, the robots and your ship in front of it. It has the color of the light behind the door, gets brighter as the door slides open and fades out again when the door closes.

The opening is a window and not a lamp: the light gets weaker with distance as if the lamps of the room themselves were shining through the door, and it only falls into the space in front of the opening.

It also follows what happens behind the door. Fire a flare in the room, or shoot out a lamp there, and the light in the corridor changes with it. And it carries on through further doors.

All this works for doors, for destructible and cloaking walls and for walls that are opened or removed by triggers; a door with holes in its texture lets some light through even when it is shut.