
 Frustum Clipping for PovRay (Site not responding. Last check: 20071013) 
  In order to compute which polygons are outside the frustum one needs to be able to define the 4 planes making up the view frustum against which each vertex of the polygons will be tested. 
  Where vp is the view position vector, vd is the unit view direction vector, vu is the unit up vector, and right is the unit vector to the right (the cross product between vd and vu). 
  The 4 frustum planes are (vp,p0,p1), (vp,p1,p2), (vp,p2,p3), (vp,p3,p0) from which the normal of each plane can be computed. 
 local.wasp.uwa.edu.au /~pbourke/rendering/clipping (450 words) 
