The requirement for dealing with digital images is increasing rapidly. On one hand, because of recent technological advances, the conversion of photographs, print text, fine scanner, and other media into digital form is easily accomplished. On the other hand, direct cquisition of digital images is also becoming more common as sensors and related electronics improve. Magnetic resonance imaging(MRI) and computed tomography(CT) produce images directly in digital form. Computer-generated images are becoming an additional source of digital data. Computer graphics has found applications in areas like advertising, entertainment and scientific visualization. Polyhedral models are the primary representation for 3D objects, which are very important for video-games, virtual reality and fly-through applications which involve complex computer aided disign models and thus need particularly ast and effective rendering, usually hardware assisted. A polyhedral model is defined as a set of vertices (geometry) and the relation between these positions (connectivity), and sometimes mixed with color, normal and texture information (photometry).

There is a common feature for both digital images and 3D objects in polyhedral models: they occupy huge chunk of space if not by some smart storage format; they limit the network throughput or may cause traffic conjestion when transmitted over current bandwidth-constrained network. So compression and encoding is a must.