There are several technical methods to make realistic three-dimensional images. Spatial vision is based on the fact that the eyes receive two clearly different perspective pictures. The brain calculates the angle between the two directions to the object to realize the depth effect in our imaginations. Two perspectively correlated pictures are easy to produce with the help of a camera or computer graphics. The problem is to present the images to each eye separately. Two ways to achieve this are red/green glasses and polarized glasses. But the method of cross-view rays is a simple trick to achieve a real 3D impression without glasses. The images are arranged so that the one for the right eye is on the left and vice versa. You must squint so that the right eye looks at the left image and the left eye looks at the right image. To do this, look at one finger half the distance between eyes and screen.