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
       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(3), ex3d(3)



Allegro                          version 4.2.3             polygon_z_normal(3)
