Shape discrimination for 3D objects with conflicting stereo and shading cues

Abstract

We are able to discriminate the 3D shape of smooth volumetric objects from monocular information alone, using cues like shading and the boundary contour. With binocular viewing, stereo information has been shown to improve shape discrimination even when rich monocular cues are present. This study used cue conflict conditions to test the relative contributions of stereo and shading information to 3D shape perception. We generated binocular images for which the shape specified by shading in each eye’s view was different from the shape specified by binocular disparities. Observers performed shape discrimination comparing these cue conflict stimuli to stimuli with consistent shading and stereo, or to stimuli that provided stereo information but no shading. We also measured the baseline discriminability of pairs of objects using consistent cue stimuli. Subjectively, the shading variations in cue conflict stimuli often appeared as variations in surface reflectance rather than solely effects of illumination. Despite this inconsistent percept, shading had a strong influence on shape discrimination. A pair of stimuli was more likely to be judged the same when shading information was consistent than when stereo information was consistent. Shading also influenced comparisons to shapes that were defined only by stereo. The results demonstrate that shading information has a strong influence on 3D shape discrimination even when conflicting stereo information is present, and when shading is not entirely attributed to illumination effects. We further found that the effect of shading depended on baseline discriminability: shading had more influence for pairs of objects that were difficult to discriminate than for pairs that were easy to discriminate. We speculate that stereo contributes a coarse estimate of 3D shape and shading is used to perceive finer shape variations. Meeting abstract presented at VSS 2016