Quote:
Originally Posted by Krystian Galaj
The silhouette itself could be turning in either direction. However, if it's turning to the right, then we're looking at the silhouette from above and can see the shadow on the ground, even if the ground is invisible. However, the shadow is rendered in a way consistent with us looking from below the floor at silhouette turning to the left. Unfortunately, then we'd see no shadow - so the picture makes no sense either way.
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It's an
illusion - it's not rotating at all, being a two dimensional image. The point is that we can
perceive it to be rotating clockwise or anti-clockwise and this appears to be bi-stable - it seems to just flip between the two possibilities, though having disambiguated to the image to be moving in one direction it is sometimes quite difficult to "get it" to go in the opposite direction.