Monocular Depth Cues

The phenomenon of perceiving all three dimensions of the world and all objects in such a way as to discern their depth and distance is known as depth perception. The ability to perceive depths is an extremely important function as the proper performance of it helps a person to move about appropriately in relation to his/her immediate surroundings and by discerning the distance of various objects from oneself in such a surrounding. Depth perception is performed on the basis of three different depth 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 depth cues are those cues that 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 media various interactions of different parts of the human eye. Monocular depth cues are what we are going to discuss here.

What are Monocular Depth Cues?

As mentioned above, 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.

Accommodation
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 (read the Buzzle article how does the human eye work to know more). This stretching of the ciliary muscles causes the eye lens to expand along length and width and become thinner to do so. This mechanism is effective only for perceiving objects within 2 meters of oneself.

Aerial Perspective
We all 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. This makes the foreground appear more graphically prominent than the background. Therefore, objects that have different contrasts with the background appear to have differing levels of depth.

Blurring
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 from visual limits, the more blurred it appears, giving an idea of distance.

Size Cues
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 gaged even in the absence of information regarding the absolute size of both objects.

Motional Depth
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. One of the typical and most common kinetic monocular depth cues examples is when we drive in a busy road and adjust the speed of our vehicles in accordance with the surrounding stationary or moving objects to avoid collision. Another cue, motional parallax, is witnessed when the 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 and 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.

Peripheral Vision
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.

Occlusion
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.

Perspective
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 above 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. Hope that cleared any doubts you had with regards to monocular depth perception.