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What: This interactive version of the “stepping feet” optical illusion presents a scenario where both the yellow and blue walkers seem to be stepping across the screen. However, in reality, they are moving in a straight line at the same speed. The illusion depends on your brain's perception of contrast and the way it amplifies motion (yes, it amplifies motion).

How To: Initially, this illusion is set up in the traditional style with yellow and blue. If you look at the “walkers” individually, you can see that they are moving uniformly. But when you don't focus too intently on just one, you can observe the effect of the illusion. If you're having difficulty experiencing the effect, you can set the walker colors to match the background. This will emphasize the key mechanism of the effect, which is that the contrasting colors create a different perception of speed. You can click anywhere on the visual box to add a border around the walkers. This will completely disrupt the effect by eliminating the distinct contrasting colors. Click again, and the border will be removed. There are several variables you can manipulate that can significantly change how the effect is perceived. Altering the background spacing and walker sizes will change the sizes of their steps. However, I've noticed that it's also easy to break the illusion in various ways. If the contrast is too low, the effect becomes much weaker. I also discovered that if I add a large number of walkers, the effect starts to deteriorate, perhaps because of the numerous straight lines among the many walkers compared to when there are only two competing with each other.

Explain it: As I mentioned in the “How To” section, the effect is generated by the contrasting colors creating a different perception of speed. These different “perceived” speeds then result in different “perceived” motions, an effect that was observed relatively recently in the broader context (in 1982). There's a really cool real - life example of the contrast effect described by Stuart Anstis in their 2003 paper, and here's a summary: In foggy conditions, drivers experience a contrast effect. The brightness difference between their car and the surroundings is lower compared to a sunny day. This leads to an illusion where drivers underestimate their speed and think other cars are less visible. The reduced contrast makes other vehicles appear slower than they actually are.