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Can anyone explain how this illusion works? How does our brain mess it up? It occurs even with one eye closed. I can kind of imagine how it would work with a symmetrical object, but this floors me. Here's my theory. Imagine a straight, diamond shaped pole. As it turns, it will widen and narrow. With the elongated points at zero and 180 degrees, it is narrow, at 90 and 270 it is fat. Imagine it starts with the long points at 90 and 270, and imagine it spinning first clockwise, then CCW. Now think about the individual frames of the point that starts at 270. When it spins CCW from 270 halfway to 225, the frame at that point will be a particular wideness. At 220 it will be slightly thinner. Now think about when the point swings CW from 270 to 315. The wideness will be the same as at 225. It is exactly the same frame. At the frame at CW 320 is identical to CCW 220. It only works if the object is in silhouette.
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Wikipedia
Btw, this is a great illusion. love it.