CATEGORICAL LOCAL-SHAPE PERCEPTION

How well do observers perceive the local shape of an object from its s haded image? This problem was addressed by first deriving a potential representation of local solid shape. The descriptor of local shape, ca lled shape characteristic, provides a viewpoint-independent continuum between hyperbolic (saddle-shaped) and elliptic (egg-shaped) points. T he ability of human observers to make categorical judgments of local s olid shape was then studied, This question was investigated by using a smooth 'croissant: a simple object made of two connected regions of e lliptic and hyperbolic points. Observers decided whether the surface w as locally elliptic or hyperbolic at various points on the object. The task was natural, and the observers could reliably partition the shad ed image of the object into two regions, one elliptic and one hyperbol ic. The ability of observers to perform this partition shows that they can, at least implicitly, localize the parabolic curves on a surface. This ability to locate the parabolic curve could in turn be exploited for other purposes, for instance to segment an object into its parts.