polygon_z_normal - Man Page

#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);

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.

cross_product(3), ex3d(3)

cross_product(3), ex3d(3), exscn3d(3), exzbuf(3).

The man page polygon_z_normal_f(3) is an alias of polygon_z_normal(3).