Sleeping shotgun guy in MAP02 (Doom II)

In MAP02 of Doom II, after going through the first door, the player must get past a shotgun guy in order to press the switch that opens the next part of the level. However, this shotgun guy ignores the player and does not "wake up" as monsters usually do when spotting a player.

Usually, the bug is not noticed, as the player must confront two zombiemen in the same room before reaching the shotgunner. The action of killing them usually wakes up the shotgun guy (that is still sensitive to sound).

The same error can occur in reverse; one example is in E1M5 of Knee-Deep in the Dead. If the player moves slowly up the left side of the first staircase, he may awaken the imp in the far northeast corner of the room directly ahead, even though there is a solid wall between the two locations.

Technical
The behavior occurs due to a bug in Doom's line of sight code. The code which causes it is located in p_sight.c, in the function P_DivlineSide.

if (!node->dy) {       if (x==node->y) return 2; if (y <= node->y) return node->dx < 0; return node->dx > 0; }

By comparing the wrong coordinates, this code causes the engine to think that any coordinate lies on a BSP node partition line, when the partition line is horizontal and the thing's x is the same as the y coordinate of the node line's origin, which is always a vertex. This can cause the sidedness decision to be incorrect, either indicating that the enemy is on the same side of the node line as player or a different side than the player, when the opposite is actually true.

In the MAP02 case, the shotgun guy is at (1200,1232) and one of the vertices behind it is at (1232,1200). In the E1M5 case, the imp is at (672,1264) and one of the linedefs south of it has a vertex at (400,672).

Demo files

 * [[Media:D202blnd.lmp|The sleeping shotgun guy]] (file info) in MAP02: Underhalls