@vi2labs: Concave or convex? It depends entirely on what your eye locks onto. At first glance, you see the outer surface. Shift your focal depth, and the whole geometry flips inside out into the hollow interior. The real glitch happens when you try to reverse the spin. Force the perspective shift, and the direction completely snaps the other way like a psychedelic optical trip. Once you see both sides at once, the motion breaks entirely.