Stereopsis

Stereopsis is a process in visual perception leading to perception of the depth or distance of objects. It comes from two Greek roots, stereo meaning solidity, and opsis meaning vision or sight. That means it could refer to any sort of visual depth perception, but since about the 1960s it has come to refer to depth perception from binocular vision, requiring two eyes. Prior to then, it was often referred to as "binocular stereopsis".Depth from stereopsis arises from the slightly different positions each eye occupies on the head, a form of parallax. A stereoscope is a device by which each eye can be presented with different images, allowing stereopsis to be stimulated with two pictures, one for each eye. This has lead to various crazes for stereopsis, usually prompted by new sorts of stereoscopes. In Victorian times it was the prism stereoscope (allowing stereo photographs to be viewed), in the 1950s it was red-green glasses (allowing stereo movies to be viewed), in the 1970s it was polarizing glasses (allowing coloured movies to be viewed), and in the 1990s it was Magic Eye pictures (autostereograms). Magic Eye pictures did not require a stereoscope, but relied on viewers using a form of free fusion so that each eye views different images.