"The main stereotype that the Doom 3 Engine suffers from, is that it can only render small, cramped inside corridors. This in fact is completely untrue, because the engine is quite capable of rendering big, detailed outside worlds. You will see the outside capabilities of the engine in Quake 4 and it will dramatically change people's perception on what the Doom 3 Engine can do. The Doom 3 engine is more suitable for inside environments though, because of its BSP (Binary Space Partitioning) system for optimizing the graphics engine. The Doom 3 Engine also supports larger textures, which in the future will allow the textures to look even better. The Source Engine is pretty much the opposite. It is more suited for outdoor environments and its inside environments don't look nearly as lifelike or detailed as Doom 3's. Both of these things are debatable and it is pretty much a toss-up for this category"