How do they do that?
Your visual system is tuned to certain spatial frequencies, so you tend not to notice image content of frequency higher or lower. I did an experiment and created my own version of this optical illusion. I started with these images.
Then I applied a lowpass filter to YES, and a highpass filter to NO. That gave me these images.
Adding them together gives this image...
Yes, your visual system acts as a filter (as does every optical system).
Cool.
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