Both rectangles moove at the same speed, right?
Because of the contrast in the rectangles being yellow and blue, and the background has white and black, it looks like the rectangles are alternately stepping, but they both move together the entire time.
That's really cool! Do you understand why it has that effect?
Well yes, I get that, the question is why does the eye or the brain respond in such a bizarre way to those colors?
Great, I can get Jens to explain it to me.
I do not see the illusion...
Make the blue darker and the yellow lighter; does that work?
It is easier for us to see the lighter yellow move across the black than the darker blue move across the black, and it is easier for us to see the darker blue move across the white than the lighter yellow move across the black.
That really makes it clearer what the source of the illusion is!
Ah that sounds plausible. Thanks!