Motion parallax

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File:Parallax Example.svg

A simplified illustration of the parallax of an object against a distant background due to a perspective shift. When viewed from "Viewpoint A", the object appears to be in front of the blue square. When the viewpoint is changed to "Viewpoint B", the object appears to have moved in front of the red square.

File:Parallax.gif

This animation is an example of parallax. As the viewpoint moves side to side, the objects in the distance appear to move more slowly than the objects close to the camera.

Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις (parallaxis), meaning "alteration".

Nearby objects have a larger parallax than more distant objects when observed from different positions, so parallax can be used to determine distances.

Parallax also affects optical instruments such as binoculars, microscopes, and twin-lens reflex cameras which view objects from slightly different angles. Many animals, including humans, have two eyes with overlapping visual fields to use parallax to gain depth perception; this process is known as stereopsis.

Visual perception

Because the eyes of humans and other highly evolved animals are in different positions on the head, they present different views simultaneously. This is the basis of stereopsis, the process by which the brain exploits the parallax due to the different views from the eye to gain depth perception and estimate distances to objects.

Animals also use motion parallax, in which the animal (or just the head) moves to gain different viewpoints. For example, pigeons (whose eyes do not have overlapping fields of view and thus cannot use stereopsis) bob their heads up and down to see depth.

See also

• Disparity
• Binocular vision
• Depth perception
• Distance perception
• Form and shape perception
• Monocular vision
• Motion perception