Use of interreflection and shadow for surface contact

The interaction of light with surfaces results in a number of lighting effe cts that may serve as valuable visual cues. Previous research on shadows ha s shown them to be effective in determining the three-dimensional (3-D) lay out of a scene, but interreflections have been ignored as cues for spatial layout. Interreflections as well as shadows may help to disambiguate the 3- D layout of objects by providing information about an object's contact with a surface. We generated computer images of a box on an extended textured g round plane that was either in contact with the ground or was slightly abov e the ground. Images were rendered for four conditions: (1) no shadow + no interreflection, (2) shadow only, (3) interreflection only, and (4) shadow + interreflection. A photometrically incorrect condition was also included. The participants rated the degree of contact for each image on a scale, wh ich was used to generate receiver operating characteristic (ROC) curves and a measure of sensitivity. In the images with no shadows or interreflection s, the participants performed at chance levels. Interreflections, shadows, and a combination of interreflections and shadows all resulted in high sens itivity for judging object contact, More important, information from shadow s and interreflections can be combined, resulting in near-perfect judgment of surface contact. Interreflections and shadows can be effective cues for object contact.