The Active Brush is a special type of brush. It is the only brush actor in the Unreal Engine that does not actually influence the final environment in any way. It is a go-between tool used to create other brushes. The Active Brush is highlighted in red, and never actually has any solidity.
It can be well described like a mould used to create a brick. The mould itself does not get used when building a wall, but rather is used to create the bricks that will finally be used to make the wall.
When a CSG Operation is performed, the Active Brush is used, and the properties that it has (size, location, rotation etc) are used to create a new brush in it"s place. The Active Brush can then be used to create the next brush through the next CSG Operation. However, not all properties can be set in the Active Brush. For example, the Active Brush does not contain information on the texture to be placed onto the brush surfaces (This is not entirely true, but for the time being, assume that it only contains information as to the physical shape, size and location of the brush).
Unlike other brushes in the Unreal Editor, the Active Brush can be changed on the fly, with new size settings, location information and as soon as a CSG Operation is performed, these settings are used to create the new brush. While the Active Brush does start as a cube, but can be any shape at all. It is not limited in any way. Through the use of the Brush Building Tools in the Unreal Editor, almost any shape can be made through the Active Brush.
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