Stereograms

Let's explore one of the strongest optical illusions ever: stereograms, pictures that create a sense of three-dimensionality from two-dimensional images, such as 3D movies but at a glance. Warning: It is possible that after seeing this collection of stereograms you feel headache, you stay squint for a while and/or get hit by a motorcycle slightly out of focus...

What is a stereogram?

A stereogram is a very specific image that contains two "layers" of information: the one with colors and shapes, as any image, and another with a depth that is encoded in the first layer. This second layer tells your eyes how far away set focus on each point of the image, and can be hidden in random dots, patterns, shapes or textures.

When viewing a 2-dimensional image, as in reading this text, both eyes focus on a single spot and move together over the surface, crossing over the same point of the plane. In order to see a stereogram, however, the eyes focus on a point in space behind or in front of that plane, intersecting at that point and not on the plane. The result is that certain pixels seem closer than others, creating a sense of 3-dimensionality.

In order to reveal the illusion, we need some technique...

How to view a stereogram?

There are many techniques to achieve this, but all have the same goal: to focus not on the surface of the screen but beyond. These are the most common ways to accomplish this:

• Zoom out. This is the easiest method for beginners: put your face close to the screen and try to see through it as if it was not there (without getting squint). Then, move your head away slowly until your eyes focus naturally the 3D image. You can practice this with any three-dimensional object to acclimatize your sight.


• See through. A more aggressive technique that involves forcing the eyes to see "behind" the image, focusing the infinity (or blurring) without moving the eyes... and then gently moving them elsewhere to find a pattern that wipes out for "fine tuning". A distance of 1.5 - 2 feet is usually ideal.


• Relax your eyes. This is the fastest, safer and most effective for those who already have certain experience with stereograms. Simply take a look in the direction of the image without focusing on anything, staring, like daydreaming. The three-dimensional image will reveal by itself naturally and almost instantaneously.

Stereograms

These are examples of the best stereograms, some of my own creation and others found on the web (click to view in full screen):