Vertices are culled when the face orientation determined by the dot product is the same as the face specified by CullFace. When all of the vertices of a polygon are culled, then the polygon may be culled. The eye direction is undefined if the modelview matrix is singular or nearly singular. This operation in effect projects the z axis in eye coordinates back into object space. If the projection matrix or DepthRange settings cause the z axis in window coordinates to be misaligned with the z axis in eye coordinates, this extension should not be used.
Vertex culling is performed independently of face culling.
Polygons on the silhouettes of objects may have both front and back facing vertices.