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(3alleg4),
ex3d(3alleg4)