AQ2World.com

Gamedesign FAQ wrote:What are Detail brushes and when should I use them?
Detail brushes are used to help speed up the compile time by "hiding" those brushes from the compilers. Since the QBSP compiler breaks up a brush if another brush intersects it, this means that a lot of brushes may be created. Detail brushes can be used for any brushes that do not block the player's line of site.

This is taken from a text file included with the gamex86.dll source code. The author is John Carmack.

The basic idea is that you should make all of the brushes that aren't major occluders into detail brushes. This lets the vis program ignore all of the little protrusions that never really block sight. Because of the factorial order of the vis algorithm, it is pretty easy to get a 10x speedup in vis time by making the non-essential brushes detail. This also gives a method to fight some of the errors that still occur in bsps. If polygons are dissapearing in the game, but they come back with "r_novis 1" set, then there is a vis error. This can usually be fixed by making more of the brushes between the problem viewpoint and the problem polygon into detail brushes.