Behind the Scenes of a Random Dot Stereogram
Maria Shea Terrell, Robert Terrell · American Mathematical Monthly · 1994
What are all those people staring at? What do they hope to see in those finely textured posters, beyond the almost random display of little flowers or paint splotches? Look deeper into the picture. Look beyond the surface. Cross your eyes. Blur your vision. Not since Rubic's Cube has a puzzle or game engaged and fascinated the public as have the currently faddish 3D illusions on display in bookstores and shopping malls across the country. For many the exercise ends in frustration, seeing nothing beyond the surface images. Could it be a case of mass hypnosis or The Emperor's New Clothes? For those who see the three dimensional images pop into view, there is surprise, pleasure, and wonder. Having had experiences of both types, we sympathize with those who have decided they have better things to do than stare off into space. But in fact knowing how the pictures are created, understanding the elementary geometry and optics of stereo- graphic vision, makes it much easier to see the images. We begin with the simplest example. Stare at the two dots below. If you relax your eyes you will probably see four dots. Try to merge the two middle images so that you see three dots. Once you've done that, you've seen your first, not so random, random dot stereogram. If after a few minutes you find that you cannot make yourself see three dots, you may be one of the 2% of the population which is stereoblind.