"Quake" was the first true-3D game to use a special map design system that preprocessed and pre-rendered some elements of the 3D environment, so as to reduce the processing required when playing the game on the 50–75 MHz CPUs of the time. The 3D environment in which the game takes place is referred to as a "map", even though it is three-dimensional in nature rather than a flat 2D space. The map editor program uses a number of simple convex 3D geometric objects known as "brushes" that are sized and rotated to build the environment. The brushes are placed and oriented to create an enclosed, empty, volumetric space, and when the design is complete, the map is run through the rendering preprocessor. The preprocessor is used to locate two types of empty space in the map: the empty space enclosed by brushes where the game will be played and the other empty space outside the brushes that the player will never see. The preprocessor then strips away the back faces of the individual brushes, which are outside the game space, leaving only a few polygons that define the outer perimeter of the enclosed game space.