IMPROVING THE VISUAL QUALITY OF JPEG-ENCODED IMAGES VIA COMPANDING

A companding procedure designed to improve the visual quality of monoc hrome images encoded with the baseline Joint Photographic Experts Grou p (JPEG) image compression algorithm is described. The companding proc edure consists of two pointwise nonlinearities applied to the image, o ne before and one after the JPEG encoding. In a series of two-alternat ive forced-choice experiments designed to measure human visual thresho ld response to these images, it was determined that the average bitrat e at the just-noticeable-difference (JND) point for the companded imag es is less than that for the uncompanded images. In a suprathreshold e xperiment, the subjects selected the companded images as being less di storted than the uncompanded images in more than 80% of the trials, fo r image bitrates ranging from 0.4 to 1.0 bpp. (C) 1997 SPIE and IS&T. [S1017-9909(97)00202-X].