Bad seg angle

A bad seg angle being displayed in Doom II's MAP09: The Pit

A bad seg angle is the cause of a rendering error that occurs in Doom when the angle stored in a seg (a linedef segment) is incorrect.

Bug ramifications

On June 8, 2018, it was discovered that BSP, DoomBSP, ZenNode and ZokumBSP all incorrectly compute the angles when a seg is split. This error is therefore also present in the original Doom games and most of the maps created for vanilla and many of the source ports. Some source ports do not utilize the pre-computed seg angle, and are therefore immune to this error.

Explanation

During the node building process, when a partitioning line intersects a seg, a vertex is created at the intersection, and the seg is split in two at that vertex. Due to limited coordinate precision, the new vertex can be slightly off from the actual intersection, which will affect the angles of the new segs. If the angle is not recomputed to fit the new vectors formed, rendering errors can occur. A good example of this is at the star structure near the exit on Doom II MAP09. When some of the sides of the star are viewed at an angle where the line of sight is almost parallel to the seg, the bad angle stored for this seg will lead to a rendering error. Unlike slime trails, this also causes wall textures to be drawn incorrectly.

Similar bugs

This was previously thought to be a type of slime trail, but it is now known to be a distinct error caused by incorrectly coded tools. It is not a precision error, unlike slime trails.

Fixed utilities

ZokumBSP released a beta version the same day that fixes this issue. There is also a script written in omgifol that can fix the bad angles in a WAD file to remove this error.