Peter N. Belhumeur David J. Kriegman Alan L. Yuille Shadows, Shading, and Projective Ambiguity. 132-154 1999 conf/shape/1999 Shape, Contour and Grouping in Computer Vision http://link.springer.de/link/service/series/0558/bibs/1681/16810132.htm db/conf/shape/shape1999.html#BelhumeurKY99