The cafe wall illusion has the dramatic effect of making a straight line appear slanted:
That’s right, the line is precisely horizontal. It was created by Akiyoshi Kitaoka, one of the world’s foremost authorities on visual illusion, who is also a wonderful artist. In addition to the hundreds of other illusions he’s created, he’s posted an entire page of illusions all based on the cafe wall effect.
But why does the line appear to be slanted? It must have something to do with the juxtaposition of light and dark patches, but what exactly? Take a look at this more elaborate illusion which relies on the cafe wall phenomenon:
This illusion was the subject of a study by Rob van Lier and Árpád Csathó. In the background between the rectangles (yes, they really are all rectangles) is a gradient which smoothly changes from white to black. The brightness of the background gradient, combined with the color of the rectangle’s border, determines whether we see a rectangle or some sort of trapezoid. Now watch what happens when the picture is put in motion:
The rectangles appear to change shape, as if they are rippling in the wind. In fact, all that is happening is that the gradient behind the rectangles is moving horizontally across the picture. The juxtaposition of the different shades of gray with a light or dark line appears to be causing the illusion. To make the phenomenon even simpler, take a look at this animation:
As before, the only thing that’s changing is the background. The two rectangles on the left appear to change size as the color changes, but the rectangles in the center and on the right do not. The interior color of the size-changing rectangles is the same, but the border color is different. Notice that the two middle rectangles are the same color as the two on the left, but they don’t seem to change size. The two on the right have the same border color as the two on the left, but the color inside changes with the background, and they don’t change size.
Focusing in on the rectangles that seem to change, when the rectangle with the dark border has a dark background, it seems bigger. The rectangle with the light border seems bigger on a light background.
Returning to the original animation, when the shading of the background varies constantly, then one side of a rectangle can have a dark background, while the other has a light background. Depending on whether the border of the rectangle is light or dark relative to its inside color, this will make one side of the rectangle appear bigger than the other — so now the rectangle appears to be a trapezoid.
Van Lier and Csathó’s study broke the illusion down further, and in separate experiments they varied the shading of the inside, outside, and border of rectangles, showing how the relative brightness of each of these components affects the perception of the illusion.
What makes this phenomenon really interesting is when rectangles are combined in creative ways. In the cafe wall illusion, for example, the placement of the blocks next to a line makes the same line appear “darker” or “lighter” depending on what it’s next to:
In the hands of an artist like Kitaoka, there’s no limit to what can be done with such a phenomenon:
van Lier, R. and Csathó, Á. (2006). Dancing shapes: A comparison of luminance-induced distortions. Perception (35), 775-798.