What do you see?
Click 'Play' to start the animation.
The canvas above looks like a rolling cicle. Would you believe that all points are only moving in a
straight line? Try following one circle with your eye and you'll see it only moves back and forth. if
you're having trouble seeing it try enableing the 'Show Line' checkbox or set the number of points to
one and increase it one by one until the illusion apears.
Explanation
This isn't really an optical illusion, it's more of a counterintuitive fact. For a very specific type of hypotrochoid the shape it traces is a line. The hypotrochoid in question is refered to as a Tusi couple. It's the shape the originates from a circle rolling inside another circle where the inner circle is exactly half the size of the outher circle. If you trace any point on the inner circle as it rolls, the resultant shape is a line.
Desmos
A version of this illusion made in Desmos.
Tusi couple in desmos
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