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Sometimes a purely static image will appear to be moving because of some design feature.

"Illusory movement" image showing a checkerboard-like pattern in concentric rings (alternating half-white, half-light-gray rectangles with half-black, half-dark-gray rectangles)

That’s not what I’m asking about.

I’m asking about an illusion that arises only through real movement involving patterned objects. This movement can be supplied by the movement of one object with respect to another; or the movement can be supplied the changing point-of-view of an observer moving past two objects that are stationary with respect to each other.

For example, sometimes as you pass a fenced yard along the north edge, you look at the north face of the fence, and through the gaps in the fence you can see the west face of the fence. As you pass, the two faces seem to move with respect to each other (a phenomenon known as parallax – also not exactly what I’m asking about) and something weird seems to happen. This weird thing – exactly what I’m talking about – seems to be a pattern of light and dark with contours different from those of the real fence. You can sometimes see curves where you know all of the real objects are straight, and if you just stop moving for a moment the illusion also stops and you see things as they are.

I believe that I’ve had a similar experience with hubcaps and with circular fans, which can appear to spin backwards or at the wrong speed.

I would be interested in any clear name or description of this experience, and further examples if you've got 'em.

  • The general term is optical illusion. Not sure if there's a specific one for this sub-type of optical illusion. – Dan Bron Dec 8 '17 at 17:42
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    Are you asking about Moiré patterns? – Jim Dec 8 '17 at 17:57
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    Hub caps appearing to spin backwards is the result of aliasing due to low sampling rates (camera shutter speed in this case) – Jim Dec 8 '17 at 17:59
  • @Jim I think Moire patters is the term I was seeking; I'll have to read more about them. That said, I'm not sure I follow this remark about camera shutter speed. I was not thinking of a recorded motion picture. I was thinking of things seen directly with the naked eye. – Chaim Dec 8 '17 at 19:45
  • Ceiling fans may appear to spin backwards because lights normally operate at 50 or 60 hertz; frame rates of motion pictures are even lower. Unless your ceiling has lines painted on it extending outwards from the fan, only the part with looking through fences is about Moiré patterns. Hubcaps and fans are just examples of the Wagon-wheel effect - be it continuous illumination or not. – Mazura Dec 9 '17 at 0:16
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You may be thinking of moiré patterns. These are interference patterns produced by the interaction of two or more fixed patterns. The fixed patterns can either be static or moving relative to each other.

Moiré patterns were quite a large part of the op art {not to be confused with pop art} movement of the 1960s.

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The Cambridge Dictionary offers kaleidoscopic. Its definition and bouquet of thesaurus connections:

quickly changing from one thing to another

Thesaurus: synonyms and related words

Changing frequently

butterfly capricious changeable elasticity fickle flighty fluidity inconsistent inflammable mercurial mutable patchy protean shifting unpredictable variable volatile yo-yo zigzag

The Merriam-Webster definition takes us in a similar direction:

Definition of kaleidoscope 1 : an instrument containing loose bits of colored material (such as glass or plastic) between two flat plates and two plane mirrors so placed that changes of position of the bits of material are reflected in an endless variety of patterns 2 : something resembling a kaleidoscope: such as a : a variegated changing pattern or scene

a kaleidoscope of colors

b : a succession of changing phases or actions

a kaleidoscope of changing fashions

c : a diverse collection

Another possible direction, suggested by the fence-post and hubcap images described, might be stroboscopic (see the M-W definition ) which could describe the apparent motion of an object viewed in a rapidly intermittent fashion.

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