Stereograms are optical illusions that make you see flat images as 3D ones, without the need for auxiliary equipment. They are based on the binocular vision principle. Stereograms become visible in 3D by shifting the focus either in front of the flat image or behind it. Seeing them requires a bit of practice, but after you manage it the first time, it will be easier.
There are several types of stereograms:
Stereoscopic images: in normal view, they look like two identical photos placed side by side. By focusing in front of the image, the two photos would overlap, and the 3D image will be visible in the center. Here below you can see an example of stereoscopic imaging. In order to visualize the 3D image, you need to cross your eyes and focus in front of the screen. You can guide your view by looking at the two dots above the image and shift your focus until you see three dots. Then, your 3D image will appear in the center of the picture.
(photo: courtesy NASA/JPL-Caltech)
Autostereograms: they hide one single image which is not visible when focusing normally, but only when looking through it, in what could be called “parallel focus”. The image and the background are made of repetitive patterns, the one that forms the 3D hidden image being slightly different that the rest of the background. Looking like this, behind the image, is called “the diverging method” and it seems to be easier to achieve that the “cross-eyed method”. Here are a few examples of autostereograms, generated with an online application:
Anaglyph images: these are images that need to be visualized with special glasses, in order to achieve the 3D effect. Random dot stereograms: when viewed normally, they look like autostereograms, they seem to be just random patterns without any shape or meaning. However, by using one of the viewing methods described above, the 3D scene they hide becomes visible. Wiggle stereograms: they are the easiest to see, and they are in fact animated images that create the 3D sensation because items in the picture that are supposed to be nearer the observer are moving more than images that are further away.
How To See Stereograms
This chapter covers the basics of viewing the autostereograms and the stereoscopic images, but for convenience purposes, we are going to generically call them “stereograms”. There are two methods for revealing the 3D images to the brain: the diverging method and the cross-eyed method.
The Diverging Method
For achieving this, you’ll have to train your brain to refuse changing the vision focus when the items you’re watching changes position. The best exercise is to start by looking at a stereogram on your computer screen from very close distance. Then try to relax your eyes and you’ll see that the image becomes blurry. Let it be like that, and start moving backwards, slowly, trying not to change your focus. The sensation is similar to the one you have when you think of something and you just look “through” objects and people around you. Keeping this slow backward motion, at a point you should already see the 3D image. Once you manage this, you’ll see that it’s very easy to keep the image in sight and even look at it from many angles, play with it, move it closer or further away. It’s truly amazing what our brain is capable of with the right stimulation.
The Cross-Eyed Viewing Method
This method supposes that the eyes focus in front of the image. One way of achieving it is to put your finger on your nose, watch it with your both eyes, then start moving it away, in front of the nose. In the same time, the stereogram on the computer screen will be in your peripheral view, so you’ll be able to see it. If you have problems with the binocular vision, you may not be able to succeed. You can attempt it without wearing your glasses, but if the difference between the left eye and the right eye is too big, the brain might not be able to compensate that. Nonetheless, you still can enjoy watching 3D stereograms by using the parallel or diverging method.
