 must be buffered. previous frame blocks into the current frame This introduces a buffering and computational delay that according to the motion estimate pixel displacement. can be very noticeable in the case of real-time video The motion compensator uses the threshold criteria communications. to decide which blocks will be encoded as prediction Video compression techniques treating visual error blocks using motion vectors and which blocks information in accordance with human visual system will only be spatially encoded. models have recently been introduced. These methods are termed "second-generation or object-based," and The generalized model does not address some video attempt to achieve very large compression ratios by im- compression system details such as the bit-stream syntax itating the operations of the HVS. The HVS model has (which supports different application requirements), or also been incorporated into more traditional video com- the specifics of the encoding algorithms. These issues are pression techniques by reflecting visual perception into dependent upon the video compression system design. various aspects of the coding algorithm. HVS weightings Alternative video encoding models have also been the have been designed for the DCT AC coefficients quan- focus of current research. Three-dimensional (3D) video tizer used in the MPEG encoder. information can be compressed directly using VQ or 3D Digital video compression is currently enjoying tre- wavelet encoding models. VQ encodes a 3D block of mendous growth in part due to the great advances in pixels as a codebook index that denotes its "closest or VLSI, ASIC, and microcomputer technology in the last nearest neighbor" in the minimum squared or absolute decade. The realtime nature of video communications error sense. However, the VQ codebook size grows on necessitates the use of general purpose and specialized the order as the number of possible inputs. Searching the high performance hardware devices. In the near future, codebook space for the near