Perceiving shape from shading.

Our visual experience of the world is based on two-dimensional images: nat patterns of varying light intensity and color faIling on a single plane of cells in the retina. Yet we come to perceive solidity and depth. We can do this because a number of cues about depth are available in the retinal image: shading, perspective, occlusion of one object by another and stereoscopic disparity. In some mysterious way the brain is able to exploit these cues to recover the three-dimensional shapes of objects. Of the many mechanisms employed by the visual system to recover the third dimension, the ability to exploit shading is probably the most primitive. One reason for believing this is that in the natural world many animals have evolved pale undersides, presumably to make themselves less visible to predators. "Countershading" compensates for the shading effects caused by the sun shining from above and has at least two benefits: it reduces the contrast with the background and it "flattens" the animal's perceived shape. The prevalence of countershading in a variety of species, including many fishes, suggests that shading may be a crucial source of information about three-dimensional shape. Painters, of course, have long ex-