Geometry of shadows

Shadows provide a strong source of information about the shapes of sm faces. We analyze the local geometric structure of shadow contours on piecewise smooth surfaces. Particular attention is paid to intrinsic shadows that is, shadows created on a surface by the surface's own shape and placement relative to a Intrinsic shadow contours provide useful information about the direction of the light source and the qualitative shape of the underlying surface. We analyze the invariants relating surface shape and light-source direction to the shapes and singularities of intrinsic shadow contours. The results suggest that intrinsic shadows can be used to directly infer illuminant tilt, qualitative global surface structure, and, at intersections with surface creases, the concavity/convexity of a surface. We show that the results obtained for paint sources of light generalize in a straightforward way to extended Light sources, under the assumption that light sources are convex. (C) 1997 Optical Society of America.