Monocular Cues (Psychological Cues)

Monocular cues to depth perception are effective when objects are viewed with only one eye. These cues are often used deliberately by artists to create a sense of depth within two-dimensional paintings, which is why they are also referred to as pictorial cues. Several important monocular cues help us judge distance and depth on two-dimensional surfaces.

Relative size: the size of the retinal image an object casts allows us to judge its distance, based on our own past and present experience with similar objects. As objects move farther away, their retinal image becomes progressively smaller; correspondingly, we tend to perceive an object as being farther away when it appears smaller, and closer when it appears bigger.

Interposition, or overlapping: this cue occurs whenever some portion of one object is covered by another object. The object that is partly covered (the overlapped object) is judged to be farther away, while the object doing the covering appears nearer.

Linear perspective: this reflects the phenomenon by which distant parallel lines appear to converge, or come closer together, compared with the same lines viewed nearer at hand. For example, parallel lines such as railway tracks appear to converge toward a single vanishing point on the horizon as distance increases; the more the lines appear to converge, the farther away that part of the scene appears to be.

Aerial perspective: the air around us contains microscopic particles of dust and moisture, which make distant objects look comparatively hazy or blurry. This is called aerial perspective. For example, distant mountains often appear bluish because of the scattering of blue light within the atmosphere, whereas the very same mountains are perceived as being closer when the surrounding atmosphere is especially clear.

Light and shade: in the presence of light, some parts of an object are highlighted while other parts fall into shadow and appear darker. These highlights and shadows supply us with useful information about an object's distance and three-dimensional form.

Relative height: larger objects are generally perceived as being closer to the viewer, and smaller objects as being farther away. When we expect two objects to actually be the same size and they turn out not to look the same size, the object that looks larger of the two is perceived as nearer, and the one that looks smaller is perceived as farther away.

Texture gradient: this represents the phenomenon by which a visual field containing a greater density of repeating texture elements (such as individual stones in a wall, or blades of grass) is seen as being farther away -- the density of these texture elements increases steadily as we look farther into the distance. …