Linedef

Linedefs are what make up the 'shape' (for lack of a better word) of a map. Every linedef is between two vertices and contains one or two sidedefs (which contain wall texture data). There are two major purposes of linedefs. The first is to divide the map into sectors, and the second is to trigger action specials.

Any area of a map directly behind a one-sided linedef is void space (which cannot be occupied except by using the idclip cheat code). A two-sided linedef is needed (to separate the two sectors) any time there is a change in (1) the height or texture of the floor or ceiling, (2) the light level, or (3) the sector tag or type.

There are three ways to trigger an action: Which of these applies depends on the linedef type number, which also specifies what particular action will occur.
 * 1) walkover
 * 2) "use" (or "push") (with the space bar, by default)
 * 3) shoot (with an impact weapon)

The specified action usually will take place in the sector or sectors that have the same tag number as the linedef. Exceptions to this include local doors (which act on the sector on the other side of the line), special effects that apply to the linedef itself, and the exit-level actions.

Most actions can be specified as either once-only or repeating (by using different linedef types).

Linedef Flags
Linedefs contain a two-byte (16 bit) field reserved for various flags. Flags are as follows: