Finds the Z component of the normal vector to three vertices.
Finds the Z component of the normal vector to the specified three
vertices (which must be part of a convex polygon). This is used mainly in
back-face culling. The back-faces of closed polyhedra are never visible
to the viewer, therefore they never need to be drawn. This can cull on
average half the polygons from a scene. If the normal is negative the
polygon can safely be culled. If it is zero, the polygon is perpendicular
to the screen.
However, this method of culling back-faces must only be used once the X and
Y coordinates have been projected into screen space using persp_project()
(or if an orthographic (isometric) projection is being used). Note that
this function will fail if the three vertices are co-linear (they lie on
the same line) in 3D space.
The following projects include source code containing this keyword: