Concave

Concave simply means curving inward or hollow at one side. If a brush was concave a straight line to enter and exit the shape two or more times. This can only happen when brushes are concave (the opposite of concave is convex). Concave brushes are not allowed in the BSP format. Hammer and the compile tools will report concave brushes so you can repair them (by reshaping solids) if your map is still open. However, Hammer will not be able to load concave brushes from a saved map with, if Hammer cannot fix them when the map is loaded, they will be ignored.

Note how the left and middle brushes are convex (legal) while the right brush is concave (illegal).

External Links

• Concave objects (Vadear.com)
• Convex and concave polygons (Wikipedia)