Bad seg angle

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

=Bug ramifications= June 8th 2018 it was discovered that DoomBSP, ZenNode and BSP all incorrectly compute the angles when a seg is split. This error is therefor also present in the original Doom games. Many source ports do not utilize the precomputed seg angle, and are therefor immune to this error.

=Explanation= When a seg is split, the angle of the new segs can be slightly different from the original seg. 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 2 map09. When some of the sides of the star is viewed at an angle where the line of sight is almost parallell 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.

=Fixed utilities= ZokumBSP released a beta version the same day that fixes this issue. There's also a script written in omgiflol that can fix the bad angles in a wad file to remove this error.