   Implementing clip planes
  How do you implement clip planes in D3D(9)?  I have a surface patch displayed on screen, but I want it to only show within a bounding box on x, y.  I would like to clip the patch so that it remains within this box.  It's possible to clip in software of course by working out the edge quads that intersect the 4 bounding planes and interpolating between their vertices, drawing thinner quads around the edges, but I was kind-of hoping there may be a simpler solution.Any ideas?   