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Green Image
Issue No. 02 - February (1993 vol. 15)
ISSN: 0162-8828
pp: 166-170
<p>It is shown that shaded images that cannot have originated from a uniformly illuminated, smooth continuous surface with uniform albedo exist. The typical condition where this occurs is when a dark area (corresponding to a region of high gradient) is surrounded by a lighter region (with low gradient). For this to correspond to a real surface, it must be established that there is a local extremum or area of lower gradient inside the dark region. This, in turn, will show up as either a light area in the image or an orientation discontinuity in the surface (thus violating either intensity or smoothness constraints). The impossibility of a shaded image can be established by counting the number of extrema inside a region corresponding to an isolated surface patch.</p>
shaded images; local extremum; lower gradient; light area; orientation discontinuity; isolated surface patch; image recognition

B. Horn, A. Yuille and R. Szeliski, "Impossible Shaded Images," in IEEE Transactions on Pattern Analysis & Machine Intelligence, vol. 15, no. , pp. 166-170, 1993.
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