Depth perception

Depth perception is the visual ability to perceive the world in three dimensions. It is a trait common to many higher animals. Depth perception allows the beholder to accurately gauge the distance to an object.

Depth perception is achieved through a variety of mechanisms:

Binocular vision/Parallax - By using two images of the same scene taken from slightly different angles, it is possible to triangulate the distance to an object with a high degree of accuracy. This is the major mechanism for depth perception.
Color vision - Correct interpretation of color, and especially lighting cues, allows the beholder to determine the shape of objects.
Perspective - The property of parallel lines converging at infinity allows us to reconstruct the relative distance of two parts of an object, or of landscape features.
Relative size - An automobile that is close to us seems larger than one that is far away; our visual system exploits the relative size of similar (or familiar) objects to judge distance.
Distance fog - Due to light scattering, objects that are a great distance away seem hazier to the eye; the visual system is attuned to this effect.
Occlusion - Occlusion (blocking the sight) of objects by other objects is a clue, albeit a weak one, for judging relative distance. It only allows the beholder to create a "ranking" of nearness, and does not give any insight as to actual distances. In the absence of color vision (as at night) or binocular vision (as with one-eyed creatures - test it!) occlusion often serves as the method of last resort for providing rudimentary depth perception.

These functions are carried out automatically by the visual cortex; however, they are still a sort of guesswork by the brain, and so the beholder may fall victim to optical illusions that result in mistakes in depth perception.