In Eternity actions are caused to happen in the game thru linedef types. Eternity has four kinds of linedef types:
- Regular - the linedef types that were already in DOOM II v1.9
- Extended - linedef types not in DOOM II v1.9 but less than 8192 in value
- Parameterized - linedef types also less than 8192 in value, but with arguments accessible only from ExtraData
- Generalized - linedef types over 8192 in value that contain bit fields that independently control the actions of the sector affected.
Parameterized specials are subject to a completely separate system for determining how they are activated and what can activate them that is far more flexible than that used by normal lines.
A linedef with non-zero linedef type is called a linedef trigger. Normal, extended, and generalized linedef triggers are always activated in one of three ways - using the first sidedef of the linedef, walking over the linedef, or shooting the linedef with an impact weapon (fists, chainsaw, pistol, shotgun, double barreled, or chaingun). Watch out that unless the PASSTHRU flag is enabled, any non-zero linedef type will block any farther switchtype linedefs. This is a common mapping error on maps tested on, but not designed specifically for ports that ignore it.
Parameterized line specials' activation methods are controlled by flags in the ExtraData linedef extflags field. A parameterized line can be activated in multiple ways simultaneously, and can also be activated in ways not available for normal lines, including being used from the back side and impacted by projectiles. The extflags field also controls what classes of objects can perform the activation (monsters, missiles, and players), whether or not the special activates from the second side of the line, and whether or not the line special is repeatable. See the ExtraData documentation for full information.
Linedefs activated by using them come in two varieties. A manual linedef affects the sector on the second sidedef of the line pushed. A switched linedef affects all sectors that have the same tag field as the linedef that was pushed.
Nearly all switched, walkover, and gun linedefs operate on the sectors with the same tag as that in the tag field of the linedef. Most parameterized specials take the tag of sectors to act on as their first parameter, freeing the line tag for use as a line identifier.
Some linedefs are never activated per se, but simply create or control an effect through their existence and properties, usually affecting the sectors sharing the linedef's tags. There are also a few special case like line-line teleporters and exit linedefs that do not affect sectors.
Some linedefs are only triggerable once, others are triggerable as many times as you like.
Triggering types are denoted by the letters D, S, W, and G for manual(push), switched, walkover, and gun. Their retriggerability is denoted by a 1 or R following the letter. So the triggering types for linedefs are:
D1 DR S1 SR W1 WR G1 GR
Often linedef actions depend on values in neighboring sectors. A neighboring sector is one that shares a linedef in common, just sharing a vertex is not sufficient.
In Doom only one action on a sector could occur at a time. Eternity supports one floor action, one ceiling action, and one lighting action simultaneously.
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