Everybody’s seen Kanizsa’s triangle:
It’s a simple illusory figure illusion, first reported by Kanizsa(1). The illusion is likely caused by the processes that the visual system uses to separate figures from their ground(2), but to date there doesn’t appear to be any consensus about exactly how these processes cause the perception of illusory figures (here’s a list of several competing explanations).
Recently, researchers in the Human Technology Laboratories have begun producing 3D versions of the Kanizsa triangle:
These figures are created by starting with the original Kanizsa triangle, and adding three regions with different shades (luminances), which converge in the center of the illusory triangle (see the explanation here. Here’s what it looks like with the three little Pac-Man figures reversed, so that the contours of the triangle no longer appear (from here):
As if a 3D version wasn’t enough, two members of the HT Lab team, Pietro Guardini & Luciano Gamberini, won second place in this year’s Best Visual Illusion of the Year contest with moving 3D Kanizsa triangles (Kanizsa pyramids, I guess). You can see the moving pyramid here . All they do to make it look like the pyramid (which isn’t actually there, remember!) is moving, is move the edges of the different luminance regions around. Man, our visual system is easily fooled.
1Kanizsa G. (1955). Quasi-perceptional margins in homogenously stimulated fields. Rivista di Psicologia, 49, 7-30.
2Liinasuo, M. Rovamo, J., & Kojo, I. (1997). Effects of spatial configuration and number of fixations on kanizsa triangle perception. Investigative Ophthalmology & Visual Science, 38, 2554-2565.