Cafe Wave Illusion
by Jonah Kagan
made with processing.js
Press the spacebar to change the ordering of the colors in each row. Changing the order of the colors will change the direction of the perceived motion as it relates to the direction of the arcs in each row. You can also shift some of the rows horizontally using the left/right arrow keys.
What's Going On
This illusion is a variation on 4-element motion illusions such as the rotating snakes illusion. It can also create an effect similar to the cafe wall illusion.
Each row in the image consists of a repeated pattern of arcs. One might expect that the illusory motion might correspond with the direction of the arcs, since they clearly seem to "point" in a certain direction. This variation shows that the illusory motion effect, which is caused by the color pattern, is much stronger than any bias generated by the direction of the arcs. For an explanation of why the color pattern creates illusory motion, see the page on 4-element motion illusions.
When the arcs line up vertically in a certain way (as they do in the initial configuration when the page loads) a tilt illusions is also produced. The horizontal gray dividing lines appear to tilt like the grout lines do in the cafe wall illusion. This may be caused by the same mechanism as the cafe wall illusion. If the blue and black arcs are grouped as one dark element, and the green and white arcs are grouped as one light element, then we see a pattern similar to the cafe wall illusion, in which the darks and lights on adjacent rows are slightly staggered, creating the illusion of tilt.