Background objects ordering in a scene is one of the main adaptive mechanisms of survival and behavior. Due to its importance, infants start using the innate ability of visual perception from the first seconds after the birth. They train and develop their visual abilities all the time when their eyes are open even if they do something like eating, smiling, crying etc. Together with vestibular, auditory, tactile and other synchronous senses, they quickly and successfully develop these abilities:
- depth and height estimation in the observed scene;
- range estimation to the visible objects;
- displacement and location estimation of one object in front of other objects;
- visual estimation to determine the difference and proportion of objects in the visible region.
Without these basic abilities it will be very difficult for children to master more complex mental tasks of recognition, orientation, etc, thus stereoscopic vision is completely natural for children. Early adaptation to 3D vision is an evolutionary guarantee of normal vision in adult people.
These tasks are preformed automatically by a person in order to look at an object:
- the musculoskeletal system (mostly the neck) turns the face towards the object;
- the eye muscles target both eyes simultaneously at the object;
- the cillary muscle change the lens curvature (this is called accommodation) to focus the eyes at the object so that its image would be sharp enough for further processing by the brain after it is captured by the retina;
- Two extremely refined images from the left and right eyes (called the left and right angles) are processed by the visual cortex of the brain, which synthesizes the signs of volume and motion parallax in the scene, based on the graphic difference between the angles.
These features are the basis of the stereoscopic vision (stereopsis) in humans.