The effects of smooth occlusions and directions of illumination on the visual perception of 3-D shape from shading.

Human observers made local orientation judgments of smoothly shaded surfaces illuminated from different directions by large area lights, both with and without visible smooth occlusion contours. Test-retest correlations between the first and second halves of the experiment revealed that observers' judgments were highly reliable, with a residual error of only 2%. Over 88% of the variance between observers' judgments and the simulated objects could be accounted for by an affine correlation, but there was also a systematic nonaffine component that accounted for approximately 10% of the perceptual error. The presence or absence of visible smooth occlusion contours had a negligible effect on performance, but there was a small effect of the illumination direction, such that the response surfaces were sheared slightly toward the light source. These shearing effects were much smaller, however, than the effects produced by changes in illumination on the overall pattern of luminance or luminance gradients. Implications of these results for current models of estimating 3-D shape from shading are considered.