The footstep illusion & more optical tricks

…four squares, two of them light yellow and two of them dark blue, move horizontally at constant speed across stationary, vertical black and white stripes. Each square has the same width as two stripes, so that its front and back edges always lie on the same color (black or white). As Anstis found, the squares appear to speed up and slow down in alternation, depending upon their local contrast. When the dark blue squares lie on white stripes, they have high contrast (dark vs. white) and they appear to speed up momentarily. When they lie on black stripes, they have low contrast (dark vs. black) and they appear to slow down. The opposite is true for the light yellow squares.

Consequently, the squares appear to go faster and slower in alternation, like a pair of walking feet. So it was called the ‘‘footsteps illusion…’’ the contrast hugely alters the apparent speed in real time. For instance, the two squares in each row seem to be alternately closer together and further apart, although their actual separation is always constant.

This optical illusion is called the footstep illusion. It’s demonstrated above in a video by Hind Sight Grafx for the Morris Museum in Morris Township, New Jersey, and is explained on Akiyoshi Kitaoka’s site and in his 2015 paper with University of California San Diego psychology professor Stuart Anstis, above.

You can see another version of the illusion in Kitaoka’s tweet, originally shared above before it was removed from YouTube.

A demo of the foot-step illusion (Anstis) pic.twitter.com/HhJdxcNvRR

— Akiyoshi Kitaoka (@AkiyoshiKitaoka) August 31, 2017

Professor Akiyoshi Kitaoka studies and makes visual illusions in the psychology department at Kyoto‘s Ritsumeikan University. In the illusion above, he demonstrates the footstep illusion. It’s explained on his site and in his 2015 paper with University of California San Diego psychology professor Stuart Anstis:

…four squares, two of them light yellow and two of them dark blue, move horizontally at constant speed across stationary, vertical black and white stripes. Each square has the same width as two stripes, so that its front and back edges always lie on the same color (black or white). As Anstis found, the squares appear to speed up and slow down in alternation, depending upon their local contrast. When the dark blue squares lie on white stripes, they have high contrast (dark vs. white) and they appear to speed up momentarily. When they lie on black stripes, they have low contrast (dark vs. black) and they appear to slow down. The opposite is true for the light yellow squares.

Consequently, the squares appear to go faster and slower in alternation, like a pair of walking feet. So it was called the ‘‘footsteps illusion…’’ the contrast hugely alters the apparent speed in real time. For instance, the two squares in each row seem to be alternately closer together and further apart, although their actual separation is always constant.

Kitaoka also tweets his printed experiments with multi-dimensional illusions that seem to move while you look at them.

The heart appears to move. pic.twitter.com/UMA7iz3tuR

— Akiyoshi Kitaoka (@AkiyoshiKitaoka) August 29, 2017

作り方 pic.twitter.com/RmobtdRigL

— Akiyoshi Kitaoka (@AkiyoshiKitaoka) August 31, 2017

The inset appears to move. pic.twitter.com/2Rh66pEzHW

— Akiyoshi Kitaoka (@AkiyoshiKitaoka) August 29, 2017

穴が開いています。 pic.twitter.com/Wl3NFPgUa5

— Akiyoshi Kitaoka (@AkiyoshiKitaoka) August 31, 2017

"Hatching" (2011): An "egg" appears to move. pic.twitter.com/lQJjIw6pKE

— Akiyoshi Kitaoka (@AkiyoshiKitaoka) August 29, 2017

See more of his illusions at ritsumei.ac.jp/~akitaoka.

Next: The Impossible Rooftop & Folding Ladder Illusions and the Ambiguous Cylinder Illusion by Kokichi Sugihara.

Plus: How to draw a floating / levitating cube.