Perception of rigid motion in depth from the optical deformations of shadows and occlusion boundaries

Three experiments were designed to examine the abilities of observers to determine an object's 3-dimensional structure and motion from various types of optical deformations. Observers were required to discriminate whether pairs of moving ellipsoids were rotating rigidly about a single axis or nonrigidly about different axes that varied in slant. Discrimination thresholds were significantly influenced by whether the ellipsoids were intersecting or nonintersecting and whether they contained identifiable texture elements. Performance was unaffected by precession movements of the axis of rotation, by increasing the number of intersecting ellipsoids beyond 2, or by replacing the deforming silhouettes with the projected motions of cast shadows presented in isolation against a planar background. These findings indicate that observers can perceive structure from motion based on several different types of optical deformation, including the deformations of shadows and silhouettes that do not contain identifiable features on which most existing theoretical analyses are designed to operate.