Rendered points showing the determination of OR positions. The NOR and SOR must lie along the upper (opening away from the observer) and lower (opening toward the observer) medium gray tubes, respectively. One possible location of the rings could be the intersection of these tubes with the CS hourglass, shown in panels (a)–(b). Panels (c)–(d) show the inner edge of the belt (light gray), which is a good fit to the SOR.