NAME
polygon_z_normal, polygon_z_normal_f - Finds the Z component of the
normal vector to three vertices. Allegro game programming library.
SYNOPSIS
#include <allegro.h>
fixed polygon_z_normal(const V3D *v1, const V3D *v2, const V3D *v3);
float polygon_z_normal_f(const V3D_f *v1, const V3D_f *v2, const V3D_f
*v3);
DESCRIPTION
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.
SEE ALSO
cross_product(3alleg), ex3d(3alleg)