Qualitative shape from shading, highlights, and mirror reflections

The human visual system has a remarkable ability to perceive three-dimensional (3-D) surface shape from shading and specular reflections. This paper presents two experiments that examined the perception of local qualitative shape under various conditions. Surfaces were rendered using standard computer graphics models of matte, glossy, and mirror reflectance and were viewed from a small oblique angle to avoid occluding contour shape cues. The subjects' task was to judge whether a marked point on each surface lay on a local hill or valley. In the first experiment, performance was lower for glossy surfaces than matte surfaces, which is contrary to findings in previous studies of quantitative shape. For mirror surfaces, performance was high despite the absence of occluding contours, and performance was increased when the environment map was brighter in the upper hemisphere as in a natural environment. The second experiment examined how subjects resolve a depth-reversal shape ambiguity where surfaces can be either upward or downward facing. An upward-facing surface prior that is known to exist for matte surfaces was also found to exist for glossy and mirror surfaces. Subjects relied entirely on this prior to resolve the depth-reversal ambiguity for matte and glossy surfaces, but relied on perspective cues as well for mirror surfaces.