As we move about, the spatial relations between our eyes and objects in our environment change in a systematic way. Corresponding changes occur in the spatial relations between the images of these objects that are projected onto our retinas. Motion parallax and structure from motion describe two ways in which relative motion between the eye and objects and surfaces in the environment can be informative about the shapes of objects and about their layout in the environment. Motion parallax refers to the inverse relation between projected speed and viewing distance, for objects moving at the same three-dimensional (3-D) speed. Structure from motion refers to the changing distances between the projections of objects, or features on objects, that occur when objects rotate relative to the line ...