Discriminating Rigid from Nonrigid Motion

Theoretical investigations of structure from motion have demonstrated that an ideal observer can discriminate rigid from nonrigid motion from two views of as few as four points. We report three experiments that demonstrate similar abilities in human observers In one experiment 4 of 6 subjects made this discrimination from two views of four points the remaining subjects required five points. Accuracy in discriminating rigid from nonrigid motion depended on the amount of nonrigidity in the nonrigid structure. Our measure on nonrigidity was based on the variance of the interpoint distances over views. The ability to detect a rigid group dropped sharply as noise points points not part of the rigid group were added to the display. We conclude that human observers do extremely well in discriminating between nonrigid and fully rigid motion, but do quite poorly at segregating points in a display on the basis of rigidity.