Pictorial Depth

PICTORIAL DEPTH CUES. When combined, they can create a powerful illusion of depth.

l. Linear perspective. This cue is based on the apparent convergence of parallel lines in the environment. If you stand between two railroad tracks, they appear to meet near the horizon. Since you know they are parallel, their convergence implies great distance.

2. Relative size. If an artist wishes to depict two objects of the same size at different distances, the artist makes the more distant object smaller. Films such as Star Wars and Return of the Jedi created sensational illusions of depth by rapidly changing the image size of planets, space stations, and starships.

3. Light and shadow. Most objects in the environment are lighted in ways that create clear patterns of light and shadow. Copying such patterns of light and shadow can give a two dimensional design a three-dimensional appearance.

4. Overlap. (Also known as interposition ) . It is a depth cue that occurs when one object partially blocks another object. Hold your hands up and have a friend try to tell from across the room which is nearer. Relative size will give the answer if one hand is much nearer to your friend than the other. But if one hand is only slightly closer than the other, your friend may have difficulty--until you slide one hand in front of the other. Overlap then removes any doubt.

5. Texture gradients. Changes in texture also contribute to depth perception. If you stand in the middle of a cobblestone street, the street will look coarse near your feet. However, its texture will get smaller and finer if you look into the distance.

6. Aerial perspective. Smog, fog, dust, and haze add to the apparent distance of an object. Because of aerial perspective, objects seen at great distance tend to be hazy, washed-out in color, and lacking in detail. This is true even in clear air, but it is increasingly the case in our mechanized society. As a matter of fact, aerial haze is often most noticeable when it is missing. If you have traveled the wide open spaces of states such as Colorado or Wyoming, you may have seen mountain ranges that looked only a few miles away, and then were shocked to find that you were actually viewing them through 50 miles of crystal-clear air.

7. Relative motion. Relative motion, also known as motion parallax , can be seen by looking out a window and moving your head from side to side. Notice that objects near you appear to move a sizable distance as your head moves. In comparison, trees, houses, and telephone poles at a greater distance appear to move slightly in relation to the background. Distant objects like hills, mountains, or clouds don't seem to move at all. Not really a pictorial cue except in movies, television, or animated cartoons. However, when it is present, depth is almost always perceived. Much of the apparent depth of a good movie comes from the relative motion of objects captured by the camera. People who can only see with one eye depend heavily on motion parallax. Often, they make frequent head movements to exaggerate parallax and improve depth perception.

Pictorial depth cues are not entirely universal. Some cultures use only selected pictorial cues to represent depth. People in these cultures may not easily recognize other cues (Deregowski, 1972). For example, researcher William Hudson tested members of remote tribes who do not use relative size to show depth in drawings. These people perceive simplified drawings as two-dimensional designs. They do not assume, as we do, that a larger image means that an object is closer.

How do the depth perception cues relate to daily experience? Like the bodily depth cues, we constantly use the pictorial cues to gauge depth and judge distances. Cues of both types also combine to produce an intriguing illusion. When the moon is on the horizon, it tends to look as large as a silver dollar. When it is directly overhead, it looks like a dime, very much smaller than it did earlier the same evening. Contrary to what some people believe, the moon's image is not magnified by the atmosphere. If you take a photograph of the moon and measure its image, you will find that it is not larger on the horizon. But the moon looks larger when it's low in the sky. This is because the apparent distance of the moon is greater when it is near the horizon than when it is overhead. When the moon is overhead there are few depth cues around it. In contrast, when you see the moon on the horizon, it is behind houses, trees, telephone poles, and mountains. These objects add numerous depth cues, which cause the horizon to seem more distant than the sky overhead (Dember & Warm, 1980).

The apparent distance hypothesis. The moon will immediately appear to shrink when depth cues are eliminated. It is directly related to changes in accommodation (Iavecchia et al., 1983; Roscoe, 1985). Extra depth cues near the horizon cause the eyes to focus on a more distant point than they do when you look overhead. Such changes in accommodation appear to provide the brain with a "yardstick" for judging the size of images, including that of the moon.