What do your eyes do to see a magic eye picture?
I know how to see it, and I know how the brain is interpreting the signal, and I know how they make them. What I want to know is what physically is my eye doing to see it? Am I changing the focal length? Crossing them slightly?
There are two ways of looking at those “Magic Eye” images. In one you’re crossing your eyes slightly. In the other I think you’re un crossing them a little. In either case, your two eyes are actually looking at slightly different parts of the same scene as if they were looking at the same thing, and the slight differences between the two views duplicate the parallax you get from seeing real 3-D objects, making your brain interpret the scene as 3D.
The two ways are not identical. It depends upon how the artist intended the scene to be viewed. One wat gives you the intended view, the other seems “inside out” (a “pseudoscopic” view). One of the cleverest “Magic Eye” renderings I ever saw was a 3-D variation on the old 2-D “Faces or Vase” illusion. Depending upon which of the two ways you looked at it, you either saw a 3-D Vase in the middle, or two 3-D Faces in profile on the outside.
You can see similar (although not terribly interesting) views by holding a slightly stretched spring or Slinky close to your face, or looking at bathroom tiles.
Most Magic Eye pictures are designed to be viewed with eyes uncrossed. In fact, I don’t think I’ve ever seen one for crossed eyes. Perhaps some people find it easier to cross their eyes than the reverse?
I have a book of Magic Eye pictures, and each has a symbol to indicate whether it is meant to be viewed with eyes crossed or uncrossed (diverged?). I find it easier to uncross my eyes when I am not wearing my glasses (I’m nearsighted) and easier to cross them when I am wearing them.