Door

[[Image:E1M7 start.png|thumb|A characteristic [[Knee-Deep in the Dead|E1]] door in map 7

Surfaces: BIGDOOR4 Doorjambs: DOORTRAK Ceiling flat: FLAT20 Type: Unlocked, stays open]] Doors in Doom, Heretic and Strife only go up and down. This is because the NODES tree is prebuilt and not modified at runtime, and thus all linedefs must stay in the same place (when the level is viewed from above). However, Hexen introduces more flexible doors that can be made using polyobjects.

A closed door is constructed as a 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 (or doortracks) 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 many textures designed specifically for doorjambs, a common one being DOORTRAK. Some door textures also have a special cosmetic purpose: EXITDOOR indicates the final door of the map and DOOR3 is the main texture for an entrance "door", a wall that is only textured to look like a door. These two types of door are used in Doom and (less frequently) in Doom II.

The precise behavior of a door is dictated by the type field of the linedef that activates it. Doors themselves can be given a variety of types that determine, for instance, whether the doors stay open or close four seconds after reaching their destination heights. For unlocked closing doors, one can also choose if the player is able to activate them again while they are in motion or in their up position. Some of the linedef types require a key, and the corresponding door is thus locked. Generally, locked doors are bordered with a distinctive frame or have an alternative sign in the appropriate color.

Doom II presents fast doors that move in a higher speed in comparison with the original ones and have their own sound effects. It also adds a group of linedef types that allow door switches to be "locked". When the player has the correct key, a switch can be activated to open a fast door permanently. The feature is only used in Ultimate Doom and Final Doom.

A door does no damage to the player or monsters if it closes upon their head. However, in Doom or Hexen a closing door will crush corpses into a pool of blood. This applies to many monster types but not all. Items dropped by monsters and trapped beneath a closing door vanish and are removed from play.

Sliding doors
In the early stages of Doom's development cycle, doors were supposed to have the additional option of sliding open horizontally to reflect the original door behavior in Wolfenstein 3D. The alpha versions of Doom still contained graphics for doors that were designed for horizontal movement. The option was never completed and dropped entirely for retail. Remnants of this code are still present in the source code but commented out; the feature can be enabled via additional code or by applying patches.

A patch for Chocolate Doom is available here: https://soulsphere.org/random/slidoor.zip. A video demonstration is also available at YouTube.

The sliding door is a cosmetic feature only; it most closely resembles, and acts as, a non-passable linedef. Weapons can fire right through a horizontal door. No source port to date has re-enabled the sliding doors. It was, however, used by Rogue Entertainment as the basis for the special sliding and portal-style animated doors used in Strife.

EDGE and its successor 3DGE both have native sliding door code, between linetypes 442-453 via Lines.DDF. Those line types are also used with 3DGE's BSP conversion of Wolfenstein 3D's map format natively.

Trivia

 * Two sector types, 10 and 14, offer door behavior that is independent of linedef types. The former makes its host sector close when thirty seconds have elapsed from the level startup. The latter commands its sector to open when the player has been in the level for five minutes. An example of a type 10 sector is found in MAP27: Monster Condo where a secret compartment in the northern hall (with the super shotgun) becomes blocked for good after half a minute.
 * E2M3: Refinery is a special map in usage of door textures. It is the only stock level to have EXITDOOR texture depicting an entrance door. The locked blue door also has skull frames although the key itself is a card, not a skull. A similar oddity can be met in MAP24: The Chasm. Both may be meant as signs of Hell's influence warping the locations.