.\" Generated by the Allegro makedoc utility
.TH polygon_z_normal 3alleg4 "version 4.4.3" "Allegro" "Allegro manual"
.SH NAME
polygon_z_normal, polygon_z_normal_f \- Finds the Z component of the normal vector to three vertices. Allegro game programming library.\&
.SH SYNOPSIS
.B #include <allegro.h>

.sp
.B fixed polygon_z_normal(const V3D *v1, const V3D *v2, const V3D *v3);

.B float polygon_z_normal_f(const V3D_f *v1, const V3D_f *v2, const V3D_f *v3);
.SH 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.

.SH SEE ALSO
.BR cross_product (3alleg4),
.BR ex3d (3alleg4)