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Abstract

To solve the ill-posed problem of shape-from-shading, the visual system often relies on prior assumptions, for example, that the illumination is from above or that the viewpoint is from above. Here we demonstrate that a third prior assumption is used, namely that the surface is globally convex. Using unfamiliar randomly corrugated
surfaces, we find that performance in a local shape discrimination task is significant higher when the surfaces are globally convex than when they are globally concave. The results are surprising because the qualitative global shape of the surfaces are perceptually unambiguous. The results thus generalize findings such as the hollow
potato illusion (Hill H. Bruce V. 1994) which considered global shape perception only.