After reading this article you will learn about the monocular and binocular cues for interpretation of the perception of depth.
Some of the monocular cues are described below:
If one object is superimposed on another object and if this object partially blocks the other object, the object in front, which superimposes, is perceived as closer and the object which is lying behind, or being superimposed, will appear to be farther away. In Figure 7.8, it is shown that when the mountains are superimposed on the sun, the sun appears to be farther than the mountains.
2. Linear Perspective:
When two parallel lines are extended into a distance they seem to come together at a point. If you stand on a platform in a railway station and look at the tracks, you can see that they appear to converge at a point. This gives you a fair idea of how far the rails are extending.(See Fig.7.9).
3. Aerial Perspective:
Objects that are far away have a blurred outline and give hazy appearance. The loss of distinctness, fading of colours, loss of original shape and size of the stimuli-with increasing distance, is called aerial perspective.
An object that is on a higher horizontal plane seems to be farther away than an object on a lower plane as in fig.7.10.
5. Texture Gradient:
An object that is close to us often appears to have detailed texture. As the distance increases, the texture becomes finer until finally the original texture becomes very difficult to distinguish. A man standing in the fields, for example, will be able to distinguish the uneven grass blades at his feet. But as he looks into the distance the grass cannot be distinguished into separate blades and the whole field appears to be even. If he looks even further he sees a smooth, continuous green carpet.
7. Movement Parallax:
While travelling in a train or bus one must have noticed that the trees and electric poles that are close to the track or the road seem to flash past the windows very rapidly, while huts, building and other objects that are farther away seem to move relatively less rapidly. The differences in the speeds at which the objects move across the retina provide an important cue to distance and depth.
It would seem from the description of most of these cues that they have to be learned, and also that depth perception itself is something that develops through visual experience. However, there have been a series of studies, starting from the famous Gibson and Walk experiments on the visual cliff, which emphasise the innate nature of depth perception.
Perception of depth and distance in binocular vision is made possible by cues like the convergence of the eyes and the difference between the retinal images on the two eyes which is known as retinal disparity.
When we look at objects which are fairly close to us, our eyes tend to converge, that is, they turn slightly inward. These eye movements provide a cue to distance or give a fair estimation as to how far the object is. If the object is very close, like the tip of your nose, then in such a case the eyes will be unable to converge and two separate images are perceived if you strain to see it.
However, if the object is more than thirty feet away the eyes become more or less parallel and convergence does not occur. Thus, the convergent and parallel positions of the eyes help human beings to perceive distance and depth.
2. Retinal Disparity:
Since the eyes are approximately two and half inches apart, each eye has a slightly different view of the world. The difference between the two images that the eyes receive is known as retinal disparity. The left eye receives more information about the left side of an object, the right eye more about the right side.
You can easily prove to yourself that each of your eyes receives a different image. You can close one eye and line up a finger with some vertical line, like the edge of a door. Then, when you open the eye and close the other one, your finger will appear to have moved a considerable distance.
However, when you look at the finger with both the eyes the two different images merge into one and you see only one finger. This stereoscopic vision obtained from merging of two retinal images makes the perception of depth and distance more accurate, particularly in tasks that require precision like threading a needle.