There is no spoon
A Brush can be one of several flavours:
The chart on the right is (probably) the correct way of envisioning the types of brush, but the one on the left may be of more use to the mapper since the majority of brushes used are addtive or subtractive.
These either remove solid matter from the world or add it back in, depending on their CSG operation: additive or subtractive.
Brush order is important in determining the result of complex mixes of types. For example, if you subtract a room shape, add things back in, decide you want the room to be larger and subtract a larger brush, it will remove all solid matter, including that created by your additive brushes. You would need to move the new large room shape to first in brush order, or use vertex editing on the original room brush to enlarge it.
These add material into the world much like additive solid brushes. They have a different effect on the BSP and when used judiciously reduce polycount and node count, thus optimizing complex geometry. They can also create invisible brushes that block the player around glass and masked textures, known as Invisible Collision Hulls. See the Semisolid page for more.
A nonsolid brush behaves as though each poly were a single sheet. Note that the differences in adding a single sheet as Solid / Semisolid / Nonsolid are not strictly determined: see sheet for more this.
Examples of nonsolid use:
- a panorama in a skybox
- a complex zone portal, eg the cube in UT's DM-HyperBlast, the healing area in DM-Healpod
Changing brush solidity
The solidity of any brush can be changed in the editor by right-clicking the brush and choosing Solidity. See Brush Context Menu.