Provided by: allegro4-doc_4.4.2-4_all

**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>fixedpolygon_z_normal(constV3D*v1,constV3D*v2,constV3D*v3);floatpolygon_z_normal_f(constV3D_f*v1,constV3D_f*v2,constV3D_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(3alleg4),ex3d(3alleg4)