In this paper we give a complete characterization of critical configurations for projective reconstruction with any number of points and views. A set of cameras and points is said to be critical if the projected image points are insufficient to determine the placement of the points and the cameras uniquely, up to a projective transformation. For two views, the critical configurations are well-known. In this paper it is shown that a configuration of n 3 cameras and m points is critical if all points and cameras lie on the intersection of two distinct ruled quadrics. Contrary to the two-view case, which in general allows two ambiguous solutions, there is a family of ambiguous reconstructions for the n-view case. Conversely, it is shown that (except for minimal cases) for any critical configuration, all the points and cameras lie on the intersection of two ruled quadrics.
Fredrik Kahl, Kalle Åström, Richard I.