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Stereogram Concepts
When we look at an object, each eye sends a slightly different image to the brain, which fuses both images into a single image. The slight differences between the images received from each eye allows the brain to create the sense of depth.

As we focus on a point, both eyes come together to focus towards the point. The closer the object, the more the eyes have to come together (converge). As the object comes closer to our face we can feel the tightening in the muscles of our eyes as they attempt to converge to stay in focus.

Conversely, as an object moves away from us our eyes relax and and the eyes spread apart (diverge).

The following diagram shows this concept. The line S represent a pane of glass placed between our eyes and the points being viewed. The distances a and b represent the separation between the rays of light (at the image plane) coming from points A and B, respectively.

From this diagram you can see that the farther away the point, the greater is the eye separation. In this example, Point A is farther away than point B and so its separation a is greater than separation b.

The key to creating a stereogram is to create an image whose points are placed with a separation corresponding to the distance where the points would be found on an image that is placed behind the image. Then, the image is viewed by allowing the eyes to focus where the object would be found, not on the image itself.

For example, in the diagram above, if the image consisted of exactly two red points and and two yellow points, the person viewing the image would see a single red point and a single yellow point, each positioned behind the image as shown in the diagram.

By adding many points to the image, suitably separated and corresponding to the complete image of the object found behind the image plane, a complete 3D image can be seen when the eyes are correctly focussed behind the image plane.