1polygon_z_normal(3)             Allegro manual             polygon_z_normal(3)
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NAME

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

10       #include <allegro.h>
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13       fixed polygon_z_normal(const V3D *v1, const V3D *v2, const V3D *v3);
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15       float polygon_z_normal_f(const V3D_f *v1, const V3D_f *v2, const  V3D_f
16       *v3);
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DESCRIPTION

19       Finds  the Z component of the normal vector to the specified three ver‐
20       tices (which must be part of a convex polygon). This is used mainly  in
21       back-face culling. The back-faces of closed polyhedra are never visible
22       to the viewer, therefore they never need to be drawn. This can cull  on
23       average  half  the polygons from a scene. If the normal is negative the
24       polygon can safely be culled. If it is zero, the polygon is perpendicu‐
25       lar to the screen.
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27       However, this method of culling back-faces must only be used once the X
28       and  Y  coordinates  have  been  projected  into  screen  space   using
29       persp_project()  (or if an orthographic (isometric) projection is being
30       used). Note that this function will fail if the three vertices are  co-
31       linear (they lie on the same line) in 3D space.
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SEE ALSO

35       cross_product(3), ex3d(3)
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39Allegro                          version 4.4.3             polygon_z_normal(3)
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