There is a line on the inside and outside of the circles where the colors go in and out of sync with the circle as the arrows change directions - it snaps between trailing by half the length of the color segments, going being ahead by the same amount, or in sync.<p>When the outer edge of a part of the circle is trailing and the inner edge is ahead, the circle appears to move outwards; reverse this and it appears to move inwards. The parts where the circle appears to be moving up/down/left/right have one side set to the 'outwards' motion, the opposite side set to the 'inwards' motion, and the other sides in sync.<p>The human visual cortex has so, so many edge cases like this. It's wonderful!
More than you wanted to know about the Reversed Phi illusion:<p><a href="https://en.wikipedia.org/wiki/Phi_phenomenon" rel="nofollow">https://en.wikipedia.org/wiki/Phi_phenomenon</a><p><a href="https://jov.arvojournals.org/article.aspx?articleid=2121864" rel="nofollow">https://jov.arvojournals.org/article.aspx?articleid=2121864</a><p><a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6661801/" rel="nofollow">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6661801/</a>
This needs a memetic hazard warning! I had just eaten, and the linked illusion caused me instant nausea/motion sickness, which has not gone away after hiding the browser tab.
A bit disappointed that the arrows are not what is driving this (see other comments).<p>But would it be possible to have a "semantic optical illussion"? Was any such thing discovered?
Either the compression is changing the circles a bit, or they do in fact move - I’ve held a piece of paper covering all but a slight edge of a circle (obscuring everything else) and I can see the edge expanding/moving ever so slightly.<p>Still not enough to call BS on the illusion, but it might actually help “augment” it a bit.
If you like optical illusions, I recommend visiting <a href="http://illusionoftheyear.com/" rel="nofollow">http://illusionoftheyear.com/</a> when you have some spare time.
This is fake. Cover both the arrows and you’ll still see them move in the exact direction that the arrow suggests when you uncover it.<p>The movement is of 1px at best but it’s there.