Qualitative shape from active shading

We show how to actively compute qualitative shape properties directly from image intensities that is, without having to rst \reconstruct the surface". Our approach diverges from classical active shape from shading [1] in two important ways. First, we do not attempt to compute a dense depth map of the surface. We rather detect the presence of certain qualitative geometric features. Second, we use two di erent types of lighting conditions rather than two di erent instances of a single type (the point source). Our two types of lighting conditions are a di use source and a point source positioned at the camera. The major result of this paper is that concave orientation discontinuities and smooth valleys share the same shading signatures. Under di use conditions, both produce local minima in the image intensity. Under the point source condition, both produce local maxima in the image intensity. One can detect these features by alternating between the lighting conditions. 2