To generate two-dimensional images of three-dimensional solid objects at high speed, an object is defined within a three-dimensional universe hierarchically subdivided into a plurality of discrete volumes of uniform size and similar orientation. The three-dimensional universe is represented by an octree...http://www.google.co.uk/patents/US4694404?utm_source=gb-gplus-sharePatent US4694404 - High-speed image generation of complex solid objects using octree encoding
