Stereoscopy refers to a technique for creating or enhancing the illusion of depth in an image by presenting two offset images separately to the left and right eye of the viewer. Both of these 2-D offset images are then combined in the brain to give the perception of 3-D depth.

To see the image in “3D” you stare into the screen so that your eyes are aimed (more or less) parallel. As you stare into the screen you will get double vision as each eye sees the L & R images separately. When you are staring in at the right distance the middle two images will overlap and become 3D. The outer two images will remain and will still be 2D.