Hiroyuki Kumazawa

Progressive coding of color-quantized images

Progressive coding of color-quantized images, a technique that allows a progressive increase in the number of image colors transmitted, is described. In most color-quantized image coding techniques, the representative colors are assigned indices that bear no logical relationship to them. In this paper, algorithms that assign indices logically related to the representative colors are presented and it is shown that bitplane transmission with the indices assigned in this manner allows progressive transmission of color-quantized images. Two coding algorithms are proposed: runlength coding of independent bitplanes and predictive coding based on the correlation between neighboring pixels and on interbitplane correlation. The coding efficiencies of the two methods for various kinds of color-quantized images are evaluated. Predictive coding can reduce entropy by 10 percent for photographic images and by 30 percent for computer graphic images.