The "walkers" are moving in a straight line at the same speed, but appear to be stepping along.
What: This interactive version of the “stepping feet” optical illusion, where both the yellow and blue walkers appear to be stepping across the screen but are in fact moving in a straight line at the same speed.
The effect relies on your brain’s perception of contrast, and the way it exaggerates motion (surprise! It exaggerates motion)
How To: This illusion is initially set up in the traditional way, with yellow and blue, which allows you to see that the “walkers” are moving uniformly if you look at them individually, but also see the effect of the illusion when you don’t concentrate particularly hard on just one.
If you are having trouble experiencing the effect in any way, you can set the walker colors to be the same as the background. This will highlight the key mechanic of the effect, which is the contrasting colors creating a different perception of speed. [2]
You can click anywhere on the visual box to add a border around the walkers, which will completely break the effect by removing the clear contrasting colors. Click again and the border is removed.
There are several variables to play with that can drastically alter how the effect is perceived. Changing the background spacing and walker sizes will change the sizes of their steps, though I find that it’s also easy to break the illusion in different ways. If the contrast is too low the effect gets a lot less strong.
I also found that if I add a lot of walkers, the effect starts to fall apart, perhaps due to the number of straight lines amongst the many walkers compared to when there are two competing with each other.
Explain it: As I touched on in the “How To” section, the effect is created by the contrasting colors creating a different perception of speed [2], these different “perceived” speeds then create different “perceived” motions, an effect observed fairly recently in the grander scheme of things [3] (in 1982)
There is a super cool real-life experience of the contrast effect, described by Stuart Anstis in their 2003 paper [4], paraphrased here:
n foggy conditions, drivers experience a contrast effect where 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 perceive other cars as less visible. The reduced contrast causes other vehicles to appear slower than they actually are.
[1] Explaining the footsteps, belly dancer, Wenceslas, and kickback illusions, This 2006 journal has some great diagrams showing the perceived motion and effects your eyes perceive.
[2] Second-Order Footsteps Illusions, Another great paper explaining the effect in more detail, with an explicit focus on this “second-order” effect.
[4] Moving objects appear to slow down at low contrasts, by Stuart Anstis in 2003, discussion the effect when driving in foggy weather.
Another Interactive Example of the Contrast Effect on Optical Toys.
I've researched these optical illusions in my spare time but am clearly not any kind of expert and my explainations are pretty smooth brained, if you find something mis-cited, earlier examples, or general mistakes please new let me know via toymaker@toms.toys, be kind!