A monocular cue is a visual cue for depth perception that only requires one eye. People with vision loss in one eye can still rely on these cues to navigate the world, although their depth perception will be impaired. Some examples include motion parallax, interposition, and linear perspective. Many of these cues can be seen in works of art, where artists rely on visual tricks to add depth and texture to visual scenes so viewers feel like they are looking at a three dimensional environment.
One example of a monocular cue is size and height differential. People rely on known data about the relative size of objects to orient themselves; a small car is interpreted as further away, for example, relying on what is known about car size. Likewise, even if the exact size or height of an object is not known, surrounding objects can be used for general reference. Two trees of the same type and shape but different sizes will be perceived at different distances on the assumption that the larger tree is closer, for instance.
Linear perspective, the tendency of distant lines to appear to converge, is an important monocular cue for depth perception. The position of objects relative to those lines can be judged as well. The apparent convergence of train tracks at the horizon is an example. Motion parallax, the tendency of distant objects to move more slowly when people are in motion, is another of the monocular cues people use to determine the position of objects in the environment. A person on a train may see a distant mountain for several minutes or hours, while a power pole whizzes by in seconds. That person knows that the mountain is further away.
Other monocular cues include texture gradient, where textures appear more detailed and precise when they are closer, along with atmospheric perspective. Distant objects can appear fuzzy, pale, or otherwise different due to atmospheric interruptions like dust, and these visual distortions can provide clues about the distance of objects. Interposition is another monocular cue; the eyes assume that if an object overlaps another, the overlapped object is further away.
Each monocular cue can help the brain interpret the image projected onto the retinas. Although the world is three dimensional, the eyes actually see in two dimensions, and the brain relies on visual cues to provide three dimensional feedback. Other depth perception cues are binocular, requiring both eyes to fix the position of objects in the environment.