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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 4.4.2             polygon_z_normal(3)

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