Teleporter

A teleporter moves players and monsters from one location on the map to another, almost instantly, with a flash of green fog and a whoosh noise.

Conventionally, a teleporter is constructed as a 64-unit square with a distinctive floor texture; however, this is not a technical requirement. All sides of a teleporter usually teleport to the same place, but again this is not required: some levels attempt to confuse the player by sending him/her to different places depending on which side the teleporter is entered from.

Doom teleporters do not conserve momentum and after the player teleports, there is a delay of 18 tics (about half of a second) before he can move again. Monsters are not affected by this.

Barrels will also teleport if they cross a teleport line, e.g. recoiling from a shot.

If a monster or player is on the landing spot at the time of teleporting, monsters will not teleport but players will, killing whatever was on the landing spot. This is known as a telefrag. A player will self-telefrag if a voodoo doll is on the landing spot. In vanilla implementations, moreover, monsters can telefrag each other or the player on MAP30 of any WAD (some source ports allow the player to disable this exception or apply it to every map).

Teleporters figure prominently in Doom's background story: it was an unexpected side effect of UAC's experiments with teleportation that opened the gates of Hell.

Technical
In classic Doom, there are four linedef types for teleporting. These implement the combinations of one-time versus repeating and players or monsters versus monsters only. All of them are of the walk-over variety. Teleporter linedefs are direction-sensitive, which is necessary for cases where the landing area is another teleporter, so that the player can walk off the landing spot without teleporting again.

The linedef's tag specifies the destination sector: the lowest-numbered sector with the same tag value. Within that sector, there must be a teleport landing Thing, which specifies the coordinates and angle the player or monster will have after teleporting.

Boom introduced some extended teleporter line types, including switch-activated teleporters and silent teleporters. The latter preserve player orientation, position, momentum and height so that the teleport may not be noticed; this can be used to simulate rooms over rooms and for other special effects.

Teleport landings themselves are always invisible; the engine creates a separate object for the fog sprite (TFOG, also drawn at the departure point after the character disappears). The corresponding sound sample is DSTELEPT.

Teleporter traps
Areas where the player may be tempted with pick-up items such as powerful weapons, health, or armor, or necessary progression points such as switches and keys, may often be setup to open a door inside a remote area connected to the map by a "sound tunnel," a small thin sector through which sound can propagate, or through areas remotely joined into the same sector, within which are waiting many monsters who will then teleport close to the player or elsewhere into the map one at a time. This is often referred to as a teleporter trap. A few notable examples occur in Doom itself on E1M9: Military Base, in Doom II's MAP16: Suburbs, in the No Rest for the Living expansion's, and in countless PWAD levels.