The phenomenon of perceiving all three dimensions of the world as well as all the objects in such a way as to discern their depth and distance, is known as depth perception.
This ability is an extremely important function, as the proper performance of this helps a person to move about appropriately in relation to his/her immediate surroundings. Depth perception is performed on the basis of three different cues - monocular, binocular, and inferred depth cues.
Monocular depth cues are those cues that are arrived at, when the subject views his surroundings with just one eye. Binocular ones are picked up when the subject perceives his/her surroundings with both eyes.
Inferred depth cues are the additional cues, which are provided by the brain on the basis of the cues gathered by monocular or binocular vision, via various interactions of different parts of the human eye.
What are Monocular Depth Cues?
As mentioned here, cues of monocular depth are those that arise when the world is perceived from only one eye. As such, the following depth perception cues are at work when you take a monocular survey of your surroundings.
The focal length changes to accommodate the perception of distant objects as a result of the stretching of the ciliary muscles when you concentrate your vision on objects that are far away.
This stretching of the ciliary muscles causes the eye lens to expand along length and width, while becoming thinner to do so. This mechanism is effective only for perceiving objects within 2 meters of oneself.
We know that the atmosphere scatters light rays. This phenomenon causes far away objects to appear less prominent due to lower chromatic saturation and decreased luminance contrast. So the foreground appears more graphically prominent than the background and objects that have different contrasts with the background appear to have differing levels of depth.
Blurring occurs as a result of limited depth focusing capacity of the human eye. When an object is way beyond clear visual periphery, its outlines appear blunted and the farther an object is from visual limits, the more blurred it appears, giving an idea of distance.
This includes cues given by relative size and familiar size. Familiar size cue is when a previous knowledge of the perceived object's size and dimensions is often used by the brain, to process information about the absolute dimensions of it, by combining it with information gathered by the current variations in the visual angles of the retina.
As per relative size cues principle, if two objects are relatively of the same size, then the relative depth of both can be gauged, even in the absence of information regarding the absolute size of both objects.
When in motion, an object appears smaller as it goes farther away, and it appears bigger as it comes closer. This is known as kinetic depth perception and this information is used by the brain to assess collision time at a given velocity.
A typical kinetic monocular depth cues example is when we drive on a busy road and adjust the speed of our vehicles as per the surrounding stationary or moving objects, to avoid collision. Another cue, motional parallax, is seen when a spectator infers the relative distance of a moving object, based upon its apparent relative motion against the background.
Light, Shade and Texture Gradient
The angle in which light falls or illuminates an object and the way it is reflected by the object, influences its depth perception. For instance, a shadow creates an illusion of increased depth, and this technique is often used in paintings, makeup, or photography, to create a perception of increased depth.
The exact texture of an object is clearer from close distances than from far away. For instance, a gravelly rock would appear smoother from a longer distance.
The partial vision of objects outside the immediate visual periphery, those things that we can somewhat see from the tail of our eyes when we are focusing on something else, greatly influences the three-dimensional perception of an object.
When the sight of an object is blocked, whether fully or partially, by other objects, an idea of relative distance can still be formed, by formulating a comparative degree of nearness of the same.
The roundness of the Earth and the structure of our eyes, make it appear that parallel lines converge together, when they are infinitely long. This perspective allows our eyes to break an object into two parts to reconstruct an idea of relative distance of both the parts.
This is especially true in case of large objects or landscapes, that go on till they create a horizon-like phenomenon.
The given cues, when brought together, define how the world looks when seen with just one eye. The binocular cues of convergence, stereopsis and shadow stereopsis, are absent here.