Stereograms allow you to see a three-dimensional shape while looking at a two-dimensional image. Below is a wallpaper stereogram. If you hold the image close and allow your eyes to unfocus, the rows of animals should appear to hover at three different depths, with the leopards in the foreground, the parrots in the middle ground, and the gorillas in the background. If you then slowly move your face further away from the image, the 3D effect is quite vivid. (If you cross your eyes instead of allowing them to unfocus, then the gorillas will be in the foreground instead.)
The Magic Eye series of books popularized stereograms in the 1990s. Hold the image below close to your eyes and allow your eyes to unfocus slightly until an image pops into view.
Do you see the tricycle? Once it is in view, you can even move your head slightly to see the 3D image seem to hover in place. Notice the illusion of depth, with certain parts of the trike in the foreground and certain parts in the background. The image above is one type of random dot stereogram, which takes advantage of a chaotic background to allow the mask image to be repeated while still blending in. The 3D effect is created when your eyes combine the two images. Here’s another:
The stereograms on this page are autostereograms, which means that they are contained in a single image and do not require a stereoscope to view. Another type of stereogram consists of two images taken from slightly different angles, viewed through a stereoscope so that each eye sees only one image. Cross-view stereograms are also fun.
Autostereograms can be viewed wall-eyed or cross-eyed. The stereograms above are designed to be viewed wall-eyed, by allowing your vision to focus on a point behind the image. If they are viewed cross-eyed, by focusing on a point in front of the image, then the effect will be reversed. So instead of seeing a 3D tricycle or airplane, you will see a tricycle or airplane-shaped hole.
Stereograms of moving images are also possible. Here is an autostereogram music video:
How to Make a Random Dot Stereogram
The tricycle and airplane stereograms above were created using easystereogrambuilder.com. Generally, a stereogram that shows different levels of depth will require software that can determine the location of each pixel using a depth map. However, it is also possible to create simple random dot stereograms using image manipulation software such as Photoshop or Gimp. Here, I will show you how to create the stereogram below. Unlike the images above, this stereogram is designed to be viewed cross-eyed, and you will probably not want the image as close to your eyes. The red dots are included to help you lock the image into place. Cross your eyes so that each red dot becomes two dots, and then allow the dots closest to the center to overlap and lock into a single red dot. At that point, you should see a 3D image appear below the dot.
To create a random dot stereogram like this, first you will need a picture of random dots. The easiest thing to do is take a picture of a TV screen when it is not tuned to a channel. Open the image in an image manipulation program such as Photoshop or Gimp. You may want to increase the contrast so that you have only black and white, and no grey. If you wish to use color, you can use the Blend tool to create a color gradient in another image or layer, then use the Select by Color tool to select the white space in the random dot image and paste the color gradient into the white space. (You may also simply use the image below if you wish.) Take note of the width of the image and the midpoint. You will be creating two images, one on the left and one on the right.
Next, choose a silhouette image that does not contain too much detail and is not wider than one-half the width of your background image. I used this skull silhouette, which I filled in green to help distinguish it from the background. I then pasted it as a new layer on the left half of the background. On the background layer, I used the Paintbrush tool to place a red dot directly above the skull.
Next, while on the background layer, use the Select by Color tool to select the red dot. Then switch to the skull layer, and with the Select by Color tool in Add mode, add the green skull to the selection. Then hide or delete the skull layer, as you no longer need it. While on the background layer, copy the selection to the clipboard. You will have selected the red dot and a skull-shaped portion of the random dot background. Note the vertical position of the initial red dot, so that you can paste the selection on the right half of the image at the same vertical position. You now have two side-by-side images, and you have created a stereogram. When you cross your eyes to bring the red dots together, the skull will appear in 3D.
Magic Eye Books
If you enjoy stereograms, there is no better source than the Magic Eye books, which launched the stereogram craze of the 1990s. You can click on the image to the right and use Amazon’s Look Inside feature to view some of the stereograms presented in the Magic Eye books.