Effects of spatial correlations and global precedence on the visual fidelity of distorted images

This paper presents the results of three psychophysical experiments designed to investigate the effects of spatial correlations and the disruption of global precedence on the visual fidelity of natural images. The first two experiments used a psychophysical scaling paradigm in which subjects placed distorted images along a linear, one-dimensional "distortion axis" where physical distance corresponded to perceived distortion. In the first experiment, images were distorted at fixed levels of total distortion contrast in various fashions chosen to provide a reasonable representation of commonly encountered distortions. In the second experiment, images were simultaneously distorted with both structural distortion and additive white noise. Additionally, a third experiment was performed in which visual dissimilarities between all pairs of images were measured for the images used in second experiment. Results revealed that structural distortion generally gave rise to the greatest perceived distortion among the types of distortions tested; and, at highly suprathreshold distortion contrasts, additive white noise gave rise to the least perceived distortion. Furthermore, although structural distortion and noise appear to correspond to two separate perceptual dimensions, the addition of low-contrast white noise to an image which already contained structural distortion decreased the perceived distortion of the image, despite the increase in the total contrast of the distortions. These findings suggest that a measure of visual fidelity must take into account the spatial correlation between the distortions and the image, masking imposed by the images, the perceived contrast of the distortions, and the cross masking effects which occur between multiple types of distortions.