Invalid brush

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An invalid brush is a brush that cannot exist within Hammer or that cannot be processed by vbsp to form valid in-game geometry.

Hammer provides a built-in error checker that will find most invalid brushes. Press Alt+P or navigate to Check for problems under the Map menu to launch it.

Invalid brushes will often appear to change between saving and reloading maps due to Hammer's automatic fixing process (which also causes a shape to appear to change when clipped).

Causes

There are three categories of problems that cause invalid brushes:

Concave Shape

See concave for a description of concave shapes. To fix a concave shape, either adjust the vertices until it is no longer concave, or use the clip tool to split the brush into two or more brushes.

Floating vertices

Floating vertices are basically any vertices with fewer than three adjacent faces. Without those three adjacent faces, it is impossible for a brush to enclose an area. If the brush shape is not continuous and has holes then it cannot be defined by the algorithms used in Hammer. Fixing it requires merging vertices to cover the holes.

Improperly defined faces

An improperly defined face refers to any face that is not flat or has no surface area. All points on a face should exist in the same two dimensional plane. The plane can be at any rotation, but they need to exist on the same plane. It determines what planes are possible by sampling three sequential points from the face. If one or more vertices don’t line up with the rest it will cause errors. The face ends up unable to determine which two dimensional plane it exists in as sampling different points finds different planes. Fixing it requires the definition of new edges to give each face its own plane, or manipulation of the vertices to line up.

Note:This only applies to quadrilaterals or greater as a faces existence is defined by three points, it is impossible for a triangular face to be improperly defined, hence their preference in complex situations. Faces with two or one side cannot exist via the definition of a polygon.