Cooperativity in the perception of surface shape and reflectance

Abstract

The luminance pattern in an image is a function of several different scene characteristics. The primary ones are surface shape, surface reflectance, and the pattern of incident illumination. Faced with the task of generating and maintaining an accurate percept of these scene characteristics, our visual systems must somehow be able to parcel out the different causes to changes in image luminance. This requires some degree of interaction in the estimation of shape, reflectance, and illumination. We used two juxtaposed, equivalent luminance gradients as a stimulus for studying this interaction. When the stimulus appears flat, the perceived lightness of the two patterns is consistent with the assumption of a smooth illuminant over a step change in reflectance. This is not, however, the only plausible percept, as a similar luminance pattern can be generated by natural shading of two smoothly curved surfaces with equivalent reflectances. We added several different shape cues to the stimulus in support of the latter interpretation, including occluding contours, texture, and stereo, and studied their effect on perceived lightness.