Compound image compression based on unified LZ and hybrid coding

Abstract

This study proposes a unified LZ and hybrid coding (ULHC) method for compound image and video compression of visually lossless quality and high compression ratio. The method is macroblock-based for ultra-low coding latency and compatibility with the conventional and most popular hybrid video coding standards such as H.264 and MPEG-2. First, each macroblock is coded by two tools: (i) gzip, a popular lossless LZ coding tool, modified to be macroblock-oriented and to be seamlessly unifiable with a lossy hybrid coding tool; (ii) H.264, an advanced lossy hybrid coding tool. Then rate-distortion optimisation is used to select either the modified gzip or H.264 as the final coding. To seamlessly unify these two coding tools for maximum quality and high compression ratio in ULHC, the modified gzip uses the most recent reconstructed and specially serialised macroblock data as the dictionary. Experimental results show that for images and videos composed of natural or synthesised picture, text and graphics, the proposed method provides higher peak signal-to-noise ratio and better subjective quality than H.264 at the same bitrate, and also achieves much higher compression ratio than gzip without any visual quality loss. In fact, ULHC can achieve partial-lossless and partial-near-lossless coding with a high compression ratio.