Line_Mirror — turns a one-sided line into a mirror.
Line_Mirror ();
Line_Mirror does not take any parameters.
Line_Mirror cannot be used from inside a script. It must be placed on a line. However, you can use SetLineSpecial to place Line_Mirror on a line or remove it as desired while the level is played.
When you place the Line_Mirror special on a line, that line will not be drawn. Instead, the area in front of the line will be reflected behind the line to create a mirror. This special must only be used on one-sided lines.
There must be enough void space behind the mirror so that if you were to paste a mirror image of the area in front of the mirror into the void space, it would not intersect with any part of the map. If you don't have enough void space behind the mirror, then it will be possible for the player to see the area of the map behind the mirror depending on where he stands. If you have no void space at all behind the mirror, it will not be able to reflect anything.
Note that it is possible for a player to disable mirrors by setting the r_drawmirrors cvar to false, so you should still use a reasonable mid texture on a mirror line in case the player has turned off mirrors.
ZDoom 1.23 beta 19