In human vision each eye records the same scene under slightly different viewing angles. The difference in view between left and right eyes is called parallax and depends inversely on the distance of an object from the eyes. Through daily training this differential vision is continuously combined in the brain with our experience for close and far distances. This processing enables us to estimate distances and finally leads to a single virtual-stereo impression. In the subsequent text I use the term "mono-vision" for situations in which a scene or an object is viewed by one single sensor, and "stereo-vision" for cases where the object is seen by combining a pair of images with a parallax displacement (stereo-pair images).

In stereo-pair images the left and right image do not entirely match, but show double contours of most objects. This is essential for spatial vision because the displacement of the contours contains information about the distance of the object from the eye. The stereo-effect can be induced from such superpositions when anaglyphs are constructed and then watched with red-green glasses (anaglyph glasses). An anaglyph image is obtained when each image of a stereo-pair is shaded in different transparent colors (e.g., red and green) and then superposed. When observed with corresponding anaglyph glasses, a stereo impression emerges. The effect is explained by the fact that the eye watching through the red glass recognizes only the red portion of the image (the green image is filtered out and becomes black), while the other eye watching through the green glass recognizes the slightly displaced green portion of the image (the red image becomes extinct). This differential vision is neuronally combined to a three-dimensional perception. Anaglyph glasses are widely available for example from www.3d-brillen.de/.