

 
Manual Reference Pages  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.
CONTENTS
Synopsis
Description
See Also
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
backface culling. The backfaces 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 backfaces 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 colinear (they lie on
the same line) in 3D space.
SEE ALSO
cross_product(3),
ex3d(3)
Allegro  POLYGON_Z_NORMAL (3)  version 4.4.2 
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