Selects all faces whose outline has any concave corners when viewed from the front. Any faces detected by this operation will have at least 4 vertices, since a triangle is always convex. Concave faces aren't always a topology issue, but may be undesirable in some circumstances.

Due to numerical imprecision, a small tolerance value is used to avoid false positives on linear edges. This creates the possibility of false negatives for very small amounts of concavity, but under normal circumstances, this amount should be too small to be perceptible as anything other than a rounding error.