EXPERIMENTAL EVALUATION OF PSYCHOPHYSICAL DISTORTION METRICS FOR JPEG-ENCODED IMAGES

Two experiments for evaluating psychophysical distortion metrics in Jo int Photographic Experts Group (JPEG) encoded images are described. Th e first is a threshold experiment, in which subjects determined the bi t rate or level of distortion at which distortion was just noticeable. The second is a suprathreshold experiment in which subjects ranked im age blocks according to perceived distortion. The results of these exp eriments were used to determine the predictive value of a number of co mputed image distortion metrics. It was found that mean-square-error i s not a good predictor of distortion thresholds or suprathreshold perc eived distortion. Some simple pointwise measures were in good agreemen t with psychophysical data; other more computationally intensive metri cs involving spatial properties of the human visual system gave mixed results. It was determined that mean intensity, which is not accounted for in the JPEG algorithm, plays a significant role in perceived dist ortion.