The Unreal Editor uses Constructive Solid Geometry (CSG), which is a concept (or design plan) for building a three dimensional world. This concept uses brushes, which are like building blocks for the final three-dimensional environment (or level depending on choice of terminology).
A Brush is the name given to a 3D shape, which is used to define what a player can see, where they can walk, and how the world is presented to them. These brushes are often small parts of the entire environment, and are used most commonly as individual segments of the entire environment.
In the Unreal Editor a "blank" or empty environment is actually a totally solid cube. This means that the first brushes that are used are subtractive brushes. This means that they subtract the solid space within the environment, leaving a void that players can move within.
Constructing a room that a player can walk within is very simple with CSG. Remembering that the world starts as a huge solid (filled) shape, it is a simple matter of creating a cube (or any other 3D shape) and subtracting it. This will leave a void in the shape of the brush that was used, and the player will be able to move within that area.
This method of construction also means that to create a simple room, it only takes a single subtracted brush, rather than creating a floor, 4 walls and a ceiling (assuming that a cube is made). The brush then has 6 surfaces (faces) that can be individually textured to suit the environment.
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