When all of the vertexes of a polygon are culled, then the polygon is culled. The eye direction used for vertex culling is determined by the culling eye position. When the w component of the position is non-zero, the position is local and the eye direction at each vertex is computed by subtracting the vertex position from the eye position. When the w component of the position is zero, the position is non-local and the is used as the eye direction for all vertexes. The culling eye position is specified by the CullParameter command.
Vertex culling is performed independently of face culling.
Polygons on the silhouettes of objects may have both front and back facing vertexes. Since polygons are culled only when all of their vertexes are culled, face culling may have to be used in addition to vertex culling in order to correctly cull silhouette polygons.