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polygon_z_normal(3)		Allegro	manual		   polygon_z_normal(3)

       polygon_z_normal,  polygon_z_normal_f  -	 Finds	the Z component	of the
       normal vector to	three vertices.	Allegro	game programming library.

       #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

       Finds  the Z component of the normal vector to the specified three ver-
       tices (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 perpendicu-
       lar 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.

       cross_product(3), ex3d(3)

Allegro				 version 4.4.2		   polygon_z_normal(3)


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