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Monocular vision is vision in which each eye is used separately. By using the eyes in this way, as opposed by binocular vision, the field of view is increased, while depth perception is limited. The eyes are usually positioned on opposite sides of the animal's head giving it the ability to see two objects at once. The word monocular comes from the Greek root, mono for one, and the Latin root, oculus for eye.
Monocular vision impairment refers to having no vision in one eye with adequate vision in the other.
Monopsia is a medical condition in humans who cannot perceive three-dimensionally even though their two eyes are medically normal, healthy, and spaced apart in a normal way. Vision that perceives three-dimensional depth requires more than parallax. In addition, the resolution of the two disparate images, though highly similar, must be simultaneous, subconscious, and complete. (After-images and "phantom" images are symptoms of incomplete visual resolution, even though the eyes themselves exhibit remarkable acuity.) A feature article in The New Yorker magazine published in early 2006 dealt with one individual in particular, who, learning to cope with her disability, eventually learned how to see three-dimensional depth in her everyday life. Medical tests are available for determining monoptic conditions in humans.
Monocular cues provide depth information when viewing a scene with one eye.
- Motion parallax - When an observer moves, the apparent relative motion of several stationary objects against a background gives hints about their relative distance. If information about the direction and velocity of movement is known, motion parallax can provide absolute depth information. This effect can be seen clearly when driving in a car nearby things pass quickly, while far off objects appear stationary. Some animals that lack binocular vision due to wide placement of the eyes employ parallax more explicitly than humans for depth cueing (e.g. some types of birds, which bob their heads to achieve motion parallax, and squirrels, which move in lines orthogonal to an object of interest to do the same).1
- Depth from motion - One form of depth from motion, kinetic depth perception, is determined by dynamically changing object size. As objects in motion become smaller, they appear to recede into the distance or move farther away; objects in motion that appear to be getting larger seem to be coming closer. Using kinetic depth perception enables the brain to calculate time to crash distance (aka time to collision or time to contact - TTC) at a particular velocity. When driving, we are constantly judging the dynamically changing headway (TTC) by kinetic depth perception.
- 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 - If two objects are known to be the same size (e.g., two trees) but their absolute size is unknown, relative size cues can provide information about the relative depth of the two objects. If one subtends a larger visual angle on the retina than the other, the object which subtends the larger visual angle appears closer.
- Familiar size - Since the visual angle of an object projected onto the retina decreases with distance, this information can be combined with previous knowledge of the objects size to determine the absolute depth of the object. For example, people are generally familiar with the size of an average automobile. This prior knowledge can be combined with information about the angle it subtends on the retina to determine the absolute depth of an automobile in a scene.
- Aerial perspective - Due to light scattering by the atmosphere, objects that are a great distance away have lower luminance contrast and lower color saturation. In computer graphics, this is called "distance fog". The foreground has high contrast; the background has low contrast. Objects differing only in their contrast with a background appear to be at different depths. The color of distant objects are also shifted toward the blue end of the spectrum (e.g., distance mountains). Some painters, notably Cezanne, employ "warm" pigments (red, yellow and orange) to bring features forward towards the viewer, and "cool" ones (blue, violet, and blue-green) to indicate the part of a form that curves away from the picture plane.
- Accommodation - This is an oculomotor cue for depth perception. When we try to focus on far away objects, the ciliary muscles stretches the eye lens, making it thinner. The kinesthetic sensations of the contracting and relaxing ciliary muscles (intraocular muscles) is sent to the visual cortex where it is used for interpreting distance/depth.
- Occlusion (also referred to as interposition) - Occlusion (blocking the sight) of objects by others is also a clue which provides information about relative distance. However, this information only allows the observer to create a "ranking" of relative nearness.
- Peripheral vision - At the outer extremes of the visual field, parallel lines become curved, as in a photo taken through a fish-eye lens. This effect, although it's usually eliminated from both art and photos by the cropping or framing of a picture, greatly enhances the viewer's sense of being positioned within a real, three dimensional space. (Classical perspective has no use for this so-called "distortion", although in fact the "distortions" strictly obey optical laws and provide perfectly valid visual information, just as classical perspective does for the part of the field of vision that falls within its frame.)
- Texture gradient - Suppose you are standing on a gravel road. The gravel near you can be clearly seen in terms of shape, size and colour. As your vision shifts towards the distant road the texture cannot be clearly differentiated.
- Ferris, S. H. (1972). Motion parallax and absolute distance. Journal of experimental psychology, 95(2), 258--63.
- O’Shea, R. P., Blackburn, S. G., & Ono, H. (1994). Contrast as a depth cue. Vision Research, 34, 1595-1604.
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