Door
From DoomWiki.org
Doors in Doom only go up and down. This is because the NODES tree is prebuilt and not modified at runtime, and thus all lines must stay in the same place (when the map is viewed from above).
A closed door is constructed as a thin rectangular sector whose ceiling and floor are both at the same height as the neighboring sectors' floors. When the door is opened, its ceiling moves up at a controlled rate to four units below the lowest adjacent ceiling.
By convention, doors are 16 units thick. But any thickness will work.
The doorjambs are the walls exposed at the sides as the door opens. They are usually specified with the lower unpegged bit, which causes them to be drawn from the floor up rather than from the ceiling down as usual. When this bit is not used, the doorjambs move up and down with the door.
There are, of course, many textures designed specifically for doors and doorjambs.
The precise behavior of a door is dictated by the type field of the linedef that activates it.
Some of these linedefs require a key, and the corresponding door is thus locked. By convention, locked doors are bordered with a distinctive frame in the appropriate color.
