“…Some of these theories state the minimal conditions which are necessary to recover a unique structure from general 3-D rigid motion (Huang and Lee, 1989;Longuet-Higgins & Prazdny, 1980;Ullman, 1979) or more constrained motion (Hoffman & Bennett, 1985Hoffman & Flinchbaugh, 1982;Longuet-Higgins, 1982;Webb & Aggarwal, 1981). Other theories describe how the instantaneous 2-D motion limits, but does not uniquely specify, the possible 3-D interpretations (Koenderink & van Doorn 1975, Todd & Bressan, 1990Ullman, 1983) or similarly how 2 discrete views limit the possible interpretations of 3-D shape (Bennett, Hoffman, Nicola & Prakash, 1989;Huang & Lee, 1989, Koenderink & van direction just grazes an object's surface (see Figure 1). These points (the critical set) map onto the viewplane to form a profile ; hence for each possible viewing direction there is a critical set and its corresponding profile.…”