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.

In vanilla Doom, there are four linedef types for teleporting. These implement the combinations of one-time versus repeating and players plus monsters versus monsters only. All of them are of the walk-over variety.

The tag field of the teleporter linedef 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 specific coordinates and angle the player or monster will have after teleporting.

If a monster or player is on the teleport 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.

Teleporter linedefs are direction sensitive. This is necessary for when the landing spot is another teleporter, so that the player can walk off the landing spot without teleporting again.

Normally, teleporters are 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 tricky levels have teleporters which take players to different places depending on which side the teleporter is entered from.

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.

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 other special effects.