I was looking at this photo of a Koosh ball: https://en.wikipedia.org/wiki/Koosh_ball#/media/File:KOOSH.png <https://en.wikipedia.org/wiki/Koosh_ball#/media/File:KOOSH.png> from about a foot away from my 27" computer screen. Closing one eye, I moved my head in a very small circle while remaining a foot away: just a little to the right, then above, then to the left, then below (rinse, lather, repeat) of dead center on the Koosh ball — and while doing so, have my gaze cycle around a circle on the screen centered at the center of the ball's image. (The screen circle is roughly the nearest screen point to my head at a given moment.) The result: a persuasive illusion that the image is changing slightly due to parallax, that is, depending on the varying positions of the eye as it moves in a circle. Question: Is this a known illusion? Does it generalize beyond Koosh balls? —Dan