Well I got a bit obsessed with how the illusion worked. Here’s a screen capture of the first frame in the animation. Notice how the 2d picture doesn’t actually show if she’s facing toward or away from the viewer. Your mind fills it in automatically. Now notice how you can force your mind to reverse the direction. That’s how you can see the rotation in the other direction from the one you hone in on automatically. Ah. I feel better now. dancer_frame.gif