Detection of chromatic and luminance distortions in natural scenes

Abstract

A number of studies have measured visual thresholds for detecting spatial distortions applied to images of natural scenes. In one study, Bex (2010) measured sensitivity to sinusoidal spatial modulations of image scale. Here we measure sensitivity to sinusoidal scale distortions applied to the chromatic, luminance or both layers of natural scene images. We first established that sensitivity does not depend on whether the undistorted comparison image was of the same or of a different scene. Next we found that when the luminance but not chromatic layer was distorted, performance was the same irrespective of whether the chromatic layer was present, absent or phase scrambled; in other words the chromatic layer, in whatever form, did not affect sensitivity to luminance-layer distortion. However when the chromatic layer was distorted, sensitivity was higher when the luminance layer was intact compared to when absent or phase-scrambled. These detection threshold results complement the appearance of periodic distortions of image scale: when the luminance layer is visibly distorted, the scene appears distorted, but when the chromatic layer is visibly distorted, there is little apparent scene distortion. We conclude that (a) observers have an in-built sense of how a normal image of a natural scene should appear, and (b) the detection of distortion in, as well as the apparent distortion of natural-scene images is mediated predominantly by the luminance not chromatic layer