Sleeping shotgun guy in MAP02 (Doom II)

In MAP02 of Doom II, after going through the first door, the player must get past a sergeant in order to press the switch that opens the next part of the level. However, this sergeant 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 troopers in the same room before reaching the sergeant. The action of killing them usually wakes up the sergeant (who is still sensitive to sound).

The same error can occur, in reverse, on E1M5 of Doom. If the player walks slowly up the left side of the first staircase, he may awaken the Imp in the far northeast corner of the room directly in front of him, 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; }

This code causes the engine to think that any coordinate lies on a linedef, when the linedef is horizontal and the thing's x is the same as the y coordinate of a BSP divider line's origin, which is always a vertex.

In the MAP02 case, the sergeant is at (1200,1232) and one of the vertices behind him 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 sergeant]] (file info) on MAP02: Underhalls