centos man page for polygon_z_normal_f

polygon_z_normal(3alleg4)					  Allegro manual					 polygon_z_normal(3alleg4)

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 per-
       pendicular 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(3alleg4), ex3d(3alleg4)

Allegro 							   version 4.4.2					 polygon_z_normal(3alleg4)