Ambiguity of Shading and Stereo Contour

Imagine a smooth surface patch, bounded by a closed contour which is observed stereoscopically. The interior of the patch presents an interpretation problem because of its lack of matchable features. Shading inside the patch is a potentially rich source of information. Its exploitation is facilitated by the stereoscopically matched contour, which supplies boundary conditions on surface shape. Interpretation of the shape of the patch from shading and the stereo contour, is subject to certain ambiguities. The well known Necker-like inversion ambiguity of the surface, with corresponding reflection of the source, is generally capable of being resolved by the stereo contour. But there are three other forms of ambiguity which can occur. The first relates to source position; it may be determined entirely, or else restricted to a set of positions. The second ambiguity is of the contour strip the contour labelled with depth and surface orientation which, for general source position, can undergo just a single, global inversion. The third is of the surface patch itself, given source and contour strip: it is generally determined, but bistable ambiguity may be induced by the presence of two or more points of maximal intensity.