This latter approach is illustrated in the photograph from the California Museum of Art. The person is looking at a slide with two images, much like a viewmaster, with each image being taken from a slightly different position to allow the brain to see disparity and, thus, see a very realistic three-dimensional picture. They were created by Peter Chang and Gareth Richards in England. There are actually two sets of random dots overlayed on these images, one for each eye. What often helps is to place your nose on the screen and slowly move backwards.
Most people report seeing a three-dimensional face.
For this image instead of relaxing your eyes, cross them slowly. Eventually you will see a cone that is pointed towards you. Try one or the other or both. All of the other cues to depth are called monocular or one-eye. One such cue, the relative motion of objects at different distances, can be a powerful cue to depth but is unavailable to the painter.