Multiple interpretations of a pair of images of a surface

It is known that if two optical images of a visually textured surface, projected from finitely separated viewpoints, allow more than one 3D interpretation, then the surface must be part of a quadric passing through the two viewpoints. It is here shown that this quadric is either a plane or a ruled surface of a type first considered by Maybank in a study of ambiguous optic flow fields; and that in the latter case, three is the maximum number of distinct interpretations that the two images can sustain.