HOM is also an acronym for the Hall Of Mirrors effect, the bane of 3d computer game level designers. A HOM is when somehow a map ends up with a kind of hole, or a place where the geometry is messed up or too complicated for the game engine to deal with correctly. Visually, it appears as a sort of hall of mirrors; reflections of reflections of reflections of the immediate area. Often times it slows down the level considerably.

HOM is a major problem for mappers in the Quake games due to the 'additive' way those levels are made. In Quake, Quake 2, Quake 3: Arena, and related games, levels are made by adding chunks of geometry. If one misses a little area which opens up into the surrounding void, a HOM results.

In Unreal engine games it is a little easier to avoid HOM because levels can be constructed using both additive and subtractive techniques. This means that structure can be both added and subtracted, so it is easier to avoid the sort of leaks one finds in Quake engine games.

Going completely subtractive does not remove the risk of HOM though, as Jedi Knight had a level system which was completely subtractive, and HOM was still an issue. Amusingly, one of the multiplayer levels for the Jedi Knight expansion pack, 'Mysteries of the Sith', used HOM on purpose for its interesting visual effect. In an area of this particular city level there was a spaceport where there were a few ships landed. Underneath the grating on the ground there was a HOM, which looked really interesting. Kind of a fans stuck in a hall of mirrors look.

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