Depth perception

Depth sensation is the corresponding term for non-human animals, since although it is known that they can sense the distance of an object, it is not known whether they perceive it in the same way that humans do.

Some animals that lack binocular vision due to their eyes having little common field-of-view employ motion parallax more explicitly than humans for depth cueing (for example, some types of birds, which bob their heads to achieve motion parallax, and squirrels, which move in lines orthogonal to an object of interest to do the same[6]).

[8] A related phenomenon is the visual system's capacity to calculate time-to-contact (TTC) of an approaching object from the rate of optical expansion – a useful ability in contexts ranging from driving a car to playing a ball game.

This prior knowledge can be combined with information about the angle it subtends on the retina to determine the absolute depth of an automobile in a scene.

Some painters, notably Cézanne, employ "warm" pigments (red, yellow and orange) to bring features forward towards the viewer, and "cool" ones (blue, violet, and blue-green) to indicate the part of a form that curves away from the picture plane.

When humans try to focus on distant objects, the ciliary muscles relax, allowing the eye lens to become thinner, which increases the focal length.

This effect, although it is usually eliminated from both art and photos by the cropping or framing of a picture, greatly enhances the viewer's sense of being positioned within a real, three-dimensional space.

Animals that have their eyes placed frontally can also use information derived from the different projections of objects onto each retina to judge depth.

By using two images of the same scene obtained from slightly different angles, it is possible to triangulate the distance to an object with a high degree of accuracy.

It is stereopsis that tricks people into thinking they perceive depth when viewing Magic Eyes, autostereograms, 3-D movies, and stereoscopic photos.

As happens with the monocular accommodation cue, kinesthetic sensations from these extraocular muscles also help in distance and depth perception.

Isaac Newton proposed that the optic nerve of humans and other primates has a specific architecture on its way from the eye to the brain.

Bernhard von Gudden showed that the OC contains both crossed and uncrossed retinal fibers, and Ramon y Cajal[26] observed that the grade of hemidecussation differs between species.

[28][page needed] In other words, that the number of fibers that do not cross the midline is proportional to the size of the binocular visual field.

[29] Thus, the general hypothesis was for long that the arrangement of nerve fibres in the optic chiasm in primates and humans has developed primarily to create accurate depth perception, stereopsis, or explicitly that the eyes observe an object from somewhat dissimilar angles and that this difference in angle assists the brain to evaluate the distance.

According to the EF hypothesis, stereopsis is evolutionary spinoff from a more vital process: that the construction of the optic chiasm and the position of eyes (the degree of lateral or frontal direction) is shaped by evolution to help the animal to coordinate the limbs (hands, claws, wings or fins).

The reverse is true for the left hand, the processing of visual, tactile information, and motor command – all of which takes place in the right hemisphere.

That OC architecture will provide short nerve connections and optimal eye control of the crocodile's front foot.

In conclusion, the EF hypothesis does not reject a significant role of stereopsis, but proposes that primates' superb depth perception (stereopsis) evolved to be in service of the hand; that the particular architecture of the primate visual system largely evolved to establish rapid neural pathways between neurons involved in hand coordination, assisting the hand in gripping the correct branch[31] Most open-plain herbivores, especially hoofed grazers, lack binocular vision because they have their eyes on the sides of the head, providing a panoramic, almost 360°, view of the horizon – enabling them to notice the approach of predators from almost any direction.

However, most predators have both eyes looking forwards, allowing binocular depth perception and helping them to judge distances when they pounce or swoop down onto their prey.

Matt Cartmill, a physical anthropologist and anatomist at Boston University, has criticized this theory, citing other arboreal species which lack binocular vision, such as squirrels and certain birds.

He also uses this hypothesis to account for the specialization of primate hands, which he suggests became adapted for grasping prey, somewhat like the way raptors employ their talons.

Photography utilizes size, environmental context, lighting, textural gradience, and other effects to capture the illusion of depth.

[33] Stereoscopes and Viewmasters, as well as 3D films, employ binocular vision by forcing the viewer to see two images created from slightly different positions (points of view).

Trained artists are keenly aware of the various methods for indicating spatial depth (color shading, distance fog, perspective and relative size), and take advantage of them to make their works appear "real".

The viewer feels it would be possible to reach in and grab the nose of a Rembrandt portrait or an apple in a Cézanne still life—or step inside a landscape and walk around among its trees and rocks.

Cubism was based on the idea of incorporating multiple points of view in a painted image, as if to simulate the visual experience of being physically in the presence of the subject, and seeing it from different angles.

The radical experiments of Georges Braque, Pablo Picasso, Jean Metzinger's Nu à la cheminée,[36] Albert Gleizes's La Femme aux Phlox,[37][38] or Robert Delaunay's views of the Eiffel Tower,[39][40] employ the explosive angularity of Cubism to exaggerate the traditional illusion of three-dimensional space.

The subtle use of multiple points of view can be found in the pioneering late work of Cézanne, which both anticipated and inspired the first actual Cubists.

Cubism, and indeed most of modern art is an attempt to confront, if not resolve, the paradox of suggesting spatial depth on a flat surface, and explore that inherent contradiction through innovative ways of seeing, as well as new methods of drawing and painting.