Hierarchical multi-stage classified vector quantization

Two advanced vector quantization schemes were proposed. These schemes made use of the spatial correlation of different block size in different hierarchical levels. In the top level, the image is partitioned into large block size and coded by a small codebook and every index of the codeword acts as the classification index of the succeeding level. In the next level, the differential image is partitioned into smaller blocks, and coded by a number of subcodebooks. These hierarchical, multi-stage and classified coding methods extend the coverage of codewords, thus improved the visual quality, while without penalty in compression ratio