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Folds and cuts: how shading flows into edges

We consider the interactions between edges and intensity distributions in semi-open image neighborhoods surrounding them. Locally this amounts to a kind of figure-ground problem, and we analyze the case of smooth surface occluding one another. Techniques from differential topology permits a classification of edges based on what we call folds and cuts. Intuititively, folds arise when a surface "folds" out of sight, which in turn may "cut" another surface from view. The classification depends on tangency between an edge tangent map and a shading flow field. Examples are included.