Perception of lightness of glossy surfaces

How do we perceive the lightness of a region of an achromatic curved surface containing a glossy highlight? We manipulated diffuse and specular reflectance components of computer-generated displays of ellipsoid surfaces. Each contained a pattern of patches on a background of uniform reflectance. Observers indicated whether the diffuse component of a test patch's reflectance appeared higher or lower than that of other patches, which had identical reflectances. Factors affecting performance included the position of the highlight relative to the test patch, and degree of similarity of the reflectance of the patch pattern to that of the background. Results demonstrate that observers can discriminate the diffuse component of reflectance even when luminance in a region is perturbed by a highlight. Performance improves if the background's diffuse reflectance is close to that the standard patches, and also if it is higher, rather than lower, than that of the standard patches. Results suggest that visual mechanisms that are sensitive to isophotes compare intensities in neighboring regions, to assess degree of gloss. We conclude with a demonstration of induced glossiness, whereby a given block of image intensities can appear shiny or dull, depending on manipulations of intensity values in other parts of the image.