Variable-rate multi-stage vector quantization for image coding

A hierarchical approach to encoding of images using vector quantization (VQ) is described which allows VQ of large blocks with a tolerable computational complexity and permits progressive image reconstruction at bit rates as low as 0.3 bit/pixel. The technique used, multi-stage hierarchical VQ (MSHVQ), is a successive approximation approach for quantization that is based on recursive decomposition of larger image blocks into smaller subblocks. The encoder of MSHVQ consists of several stages, each of which encodes the quantization error generated from the previous stage. Digital decimation and interpolation techniques are used to convert between different vector dimensions in the higher level stages and to reduce the otherwise highly noticeable blocking effect in the reconstructed image. Variable-rate encoding increases the compression efficiency by allocating greater resolution to high-detail regions and provides a perceptually more consistent image quality throughout all areas of the decoded image. Experimental results show that MSHVQ yields a significantly improved image quality over conventional VQ of equivalent complexity