polygon_z_normal(3) Allegro manual polygon_z_normal(3)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 ver‐
tices (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 perpendicu‐
lar 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 ALSOcross_product(3), ex3d(3)Allegro version 4.4.2 polygon_z_normal(3)
