Qualitative shape from active shading

It is shown how to actively compute qualitative shape properties of a surface directly from image intensities without first quantitatively reconstructing the surface. The approach diverges from classical active shape-from-shading in that two different types of lighting conditions (a diffuse source and a point source) are used, rather than two instances of a single type (a point source). There is no attempt to compute a dense depth map of the surface. Rather, the presence of certain qualitative geometric features is detected. In particular, a shape property, surface aperture, is introduced, and it is shown that under diffuse lighting there is typically a spatial correspondence between the minima of surface aperture and the minima of image intensity. Thus, the minima in surface aperture can be located. Different possible causes of the aperture minima can subsequently be distinguished by actively illuminating the surface with a point source positioned in the viewing direction