INVESTIGATING SHAPE-FROM-SHADING ILLUSIONS USING SOLID OBJECTS

Recent growth in the shape-from-shading psychophysics literature has b een paralleled by an increasing availability of computer graphics hard ware and software, to the extent that most psychophysical studies in t his area now employ computer lighting algorithms, The most widely used of these algorithms in shape-from-shading psychophysics is the Phong lighting model. This model, and other shading models of its genre, pro duce readily interpretable images of three-dimensional scenes. However , such algorithms are only approximations of how light interacts with real objects in the natural environment, Nevertheless, the results fro m psychophysical experiments using these techniques have been used to infer the processes underlying the perception of shape-from-shading in natural environments, It is important to establish whether this subst itution is ever valid. We report a series of experiments investigating whether two recently reported illusions seen in computer-generated, P hong shaded images occur for solid objects under real illuminants, The two illusions investigated are three-dimensional curvature contrast a nd the illuminant-position effect on perceived curvature, We show that both effects do occur for solid objects, and that the magnitude of th ese effects are equivalent regardless of whether subjects are presente d with ray traced or solid objects. Copyright (C) 1996 Elsevier Scienc e Ltd.