How It Works

When you look at something, two separate motor activities take place. The first action is to adjust the muscle tension which deforms the lens of the eye and sharpens the focus. The second action is the coordination of the two eyes to point to the same point in space. Each eye now has a picture of the object being viewed. Because of the disparity in viewpoints, there are discrepancies between the two views. These discrepancies form the stereoscopic image. The greater the discrepancy, the greater the diversion from the flat plane of focus, which lies at a set distance from the viewer at the point of intersection. Every discrepancy must be interpreted as nearer or farther, because the difference is greater along the intersecting lines of vision according to distance, depending on if it seems a view from the left or a view from the right.
If you look 'beyond' a target object, or 'before' it, you will get a dual image, or double vision of that object. Each image is slightly different. If you have two target objects, and you look before or beyond the objects, there is a point at which the two images will intersect and appear as a single object.		 If those two objects are properly formatted as views of the same object with a slight disparity in viewpoints, the united objects will take on the appearance of a single binocular view in full stereoscopic 3D imagery of the object pictured.
Random dot stereograms apply a pattern to a picture. When you focus beyond or before the image, the patterns overlap, and discrepancies in the images hidden within the patterns emerge as points closer to or farther away than the plane of focus, which is the regular pattern without discrepancies.
These butterflies inside of geometric forms will illustrate the principle. If you look beyond or before the actual images, you will see four images instead of three.
The two middle images will be sharp and clear, while the ones on either end are less solid in appearance. If you look beyond the images, then the butterfly on the left will float before the form and the one on the right will float behind it. If you cross your eyes to look before the image, then the one on the right will float closer and the one on the left will float further away.
Because the images are not on a harmonic frequency with the background, the geometric forms will appear to stand off from the background when you relax focus, and just be confusing when you cross focus.