Depth perception
Depth perception is the ability to perceive distance to objects in the world using the visual system and visual perception. It is a major factor in perceiving the world in three dimensions.
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.
Depth perception arises from a variety of depth cues. These are typically classified into binocular cues and monocular cues. Binocular cues are based on the receipt of sensory information in three dimensions by both eyes while monocular cues can be observed with just one eye. Binocular cues include retinal disparity, which exploits parallax and vergence. Stereopsis is made possible with binocular vision. Monocular cues include relative size, texture gradient, occlusion, linear perspective, contrast differences, and motion parallax.
Monocular cues
cues provide depth information even when viewing a scene with only one eye.Motion parallax
When an observer moves, the apparent relative motion of several stationary objects against a background gives hints about their relative distance. If information about the direction and velocity of movement is known, motion parallax can provide absolute depth information. This effect can be seen clearly when riding in a car. Nearby things pass quickly, while far-off objects appear stationary. 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.When an object moves toward the observer, the retinal projection of an object expands over a period of time, which leads to the perception of movement in a line toward the observer. Another name for this phenomenon is depth from optical expansion. The dynamic stimulus change enables the observer not only to see the object as moving, but to perceive the distance of the moving object. Thus, in this context, the changing size serves as a distance cue. A related phenomenon is the visual system's capacity to calculate time-to-contact of an approaching object from the rate of optical expansiona useful ability in contexts ranging from driving a car to playing a ball game. However, the calculation of TTC is, strictly speaking, a perception of velocity rather than depth.Kinetic depth effect
If a stationary rigid figure is placed in front of a point source of light so that its shadow falls on a translucent screen, an observer on the other side of the screen will see a two-dimensional pattern of lines. But if the cube rotates, the visual system will extract the necessary information for perception of the third dimension from the movements of the lines, and a cube is seen. This is an example of the kinetic depth effect. The effect also occurs when the rotating object is solid, provided that the projected shadow consists of lines which have definite corners or end points, and that these lines change in both length and orientation during the rotation.Perspective
The property of parallel lines converging in the distance, at infinity, allows us to reconstruct the relative distance of two parts of an object, or of landscape features. An example would be standing on a straight road, looking down the road, and noticing the road narrows as it goes off in the distance. Visual perception of perspective in real space, for instance in rooms, in settlements and in nature, is a result of several optical impressions and the interpretation by the visual system. The angle of vision is important for the apparent size. A nearby object is imaged on a larger area on the retina, while the same object or an object of the same size further away is imaged on a smaller area. The perception of perspective is possible when looking with one eye only, but stereoscopic vision enhances the impression of the spatial. Regardless of whether the light rays entering the eye come from a three-dimensional space or from a two-dimensional image, they hit the inside of the eye on the retina as a surface. What a person sees, is based on the reconstruction by their visual system, in which one and the same image on the retina can be interpreted both two-dimensionally and three-dimensionally. If a three-dimensional interpretation has been recognised, it receives a preference and determines the perception.In spatial vision, the horizontal line of sight can play a role. In the picture taken from the window of a house, the horizontal line of sight is at the level of the second floor. Below this line, the further away objects are, the higher up in the visual field they appear. Above the horizontal line of sight, objects that are further away appear lower than those that are closer. To represent spatial impressions in graphical perspective, one can use a vanishing point. When looking at long geographical distances, perspective effects also partially result from the angle of vision, but not only by this. In picture 5 of the series, in the background is Mont Blanc, the highest mountain in the Alps. It appears lower than the mountain in front in the center of the picture. Measurements and calculations can be used to determine the proportion of the curvature of Earth in the subjectively perceived proportions.