the three-dimensional perception of an object received when viewing two flat perspective images of the object. A stereoscopic effect is produced when the following main conditions are met: each eye must see only a single image; the images must be positioned relative to the eyes in such a way that the corresponding (from conjugate points) lines of sight intersect; and any differences in the scale of the images must not exceed 16 percent.
Distinctions are made between direct, inverse, and zero stereoscopic effects. A direct effect corresponds to the actual spatial position of the points of an object and arises when the left and right images are viewed by, respectively, the left and right eyes. Reversing the positions of the images gives rise to an inverse stereoscopic effect, and a rotation of the images by 90° will create a zero effect. Stereoscopes facilitate the creation of a stereoscopic effect.