Everything you ever wanted to know about: DOORS!
------------------------------------------=====-

Ok, so what is a door?  A door is basically a sector that has it's ceiling
raise and/or lower.  A player or monster can only walk through the middle
part of any sector (between the floor and the ceiling), and so can only walk
through a door when the ceiling is up (door is open).  A door that is closed 
will have the ceiling height equal to the floor height.  

There are several types of doors.  Most are activated by walking up to 
them and pressing the spacebar.  These types can only be activated on 
the right side of the line, and open the door sector on the left side of 
the line.  Thus, no trigger id is needed to tie the line to a sector.

Another similar door type is the shoot-to-open type.  The only 
difference from the last type is that you shoot the line instead.

A third type is a switch-activated door.  With this type, you need to 
use a trigger id to link the switch-line to the door sector.  This 
allows the switch to open a door, near or far away.  It also allow you 
to open multiple doors with this one switch.

Lastly, there is a type similar to the switch-activated door that is 
activated by walking over a line (player or monster).  This type usually 
has the invisible flag set on the line, so that you don't see it on the 
auto-map, unless something else about the line would make it visible 
(floor or ceilings height differences, for example).  Lines of this type 
are sometimes activated by rockets and fireball.  However, this isn't 
always reliable.

Rules for doors:

1) 2-sided lines along the sector parimeter:

        a) Must have a 'door' linetype.  Usually linetype #1 for normal 
           doors.

        b) Must have the impassable line flag cleared and the
           2-sided/transparent flag set.

        c) Should have '-' for a middle texture.  Whatever texture you 
           wish to use for a door should be used for the upper texture.

        d) Must have the right sidedef facing outward.  If you have a
           texture on the out-facing sidedef but none on the in-facing
           sidedef, then DMapEdit will automatically insure that the right
           sidedef points outward.

2) Any single-sided lines along the sector parimeter:

        a) Should have "DOORTRAK" for the middle texture, or something 
           similar.

        b) Must have the 2-sided/transparent flag cleared, like any 
           single-sided line should have.

        c) Should have the lower texture unpegged flag set, so that the 
           "DOORTRAK" texture doesn't scroll.

Of course, the most common door configuration is a rectangle:

              Sector #2

    -------+------C------+-------
           |             |
     Void  A  Sector #1  B  Void
           |             |
    -------+------D------+-------

              Sector #3

Lines A & B are single sided lines, and lines C & D are 2-sided.  The 
actual door is sector #1.

Doors are normally 64 or 128 units long, and 16 units wide.  This is pretty
much the standard id's maps set.  You are not restricted to this in any way,
however.  You may make whatever type of door you feel like creating.