How do people see in three dimensions?

 

The eyes of an average adult human are 6.4 cm apart. This inequality of position causes the eyes to see different images of the surroundings. The two images are processed by the brain and merged into one image, in which depth is perceived.

If one could capture the images that fall on the retina, one would see that the images overlap partly. The part that coincides most is the part at which the person is looking. The remaining parts of the image will not coincide. The closer an object is to the eyes, the greater the difference in the position of the objects is. This phenomenon is illustrated by the following figure:

Note: the illustration does not show the size of the objects, only their position. In reality, the blue cube would be in front of the red cube in the right-eye perspective. The person in this example looks at the black square, the eyes turned slightly inward, so that each eye looks directly at the block. This phenomenon is called 'convergence' and occurs in normally functioning eyes when a person looks at something.

The figure shows that the red cube appears to be much further away from the black cube to the right eye than to the left eye. For the green cube, this difference is much smaller, although it is equidistant to the black cube. The difference in position is largest in the blue cube, because it is closest to the eyes. This apparent change in the relative position of an object is called 'parallax'. Nearby objects have a larger parallax than more distant objects when observed from different positions. The parallax of different objects is processed by the brain and provides stereopsis, the ability to perceive depth by the use of two eyes. During displacement of perspective, nearby objects appear to move more quickly than objects at a greater distance. This results in a lower perception of depth as the distance increases. From about 10 meters, people can not judge depth on the basis of stereopsis, but they are dependent on other clues to estimate depth.

Depth clues

In the previous section it was stated that people hardly see depth beyond 10 meters. However, most people have little difficulty estimating that a building which is 40 meters away is a lot further away than a car which is 20 meters away. There are many situations in which people are fairly well able to estimate depth beyond 10 meters. This is due to several characteristics of an image, through which the relative position and shape of objects can be derived. Although some of them are obvious, for completeness they will all be named.

Overlap

If a non-transparent object is in front of another object, it will hide that object in whole or in part. In the figure below, the trees hide part of the mountains, which suggests that the trees are standing in front of the mountains.

Relative size

This clue, like most depth clues, depends upon the acquired expectation pattern of the brain. People learn that objects that are far away appear smaller than objects at close range. In addition, people remember the general size of certain objects. By combining this information, people are able to estimate the distance of familiar objects. Looking at the figure above, most people will make the assumption that the trees on the left are about the same size as the trees on the right. Based on this assumption, it will be concluded that the trees on the right are further away.

Linear perspective

When looking at the road in the figure above, you can see that it seems to be getting narrower. Usually it can be assumed that this is not actually so.

Aerial perspective

The mountains in the background have a blue glow, caused by the intervening atmosphere. Due to light scattering by the atmosphere, objects that are a great distance away have lower luminance contrast and lower color saturation. 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

Lighting and shading

The way that light falls on an object and reflects off its surfaces, and the shadows that are cast by objects provide an effective cue for the brain to determine the shape of objects and their position in space.

Texture gradient

Fine details on nearby objects can be seen clearly, whereas such details are not visible on faraway objects. Texture gradients are grains of an item. Compare the two trees in the image below. The one on the right has less detailed texture than the one on the left.

Motion parallax

As mentioned earlier, nearby objects appear to move more quickly than distant objects. This clue is especially important if you look with one eye only or at a two-dimensional image like television.

Wouter wognum (2008)