KidBridges

PERSPECTIVE #1: Using Depth Cues
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How do we see things "in depth"? How do we decide if things are near or far away from us? Close together or far apart? Understanding how our vision works can help us to understand how to depict a 3-dimensional object on a 2-dimensional surface (like a piece of paper) in a realistic way.

The three dimensions in the everyday physical world are height (H), width (W), and depth (D). To find the volume of a rectilinear 3-dimensional object, we multiply W x H x D. When we say an object is 2-dimensional, we mean that it has height and width, but that its depth is so small in proportion to the other two dimensions that we are going to disregard it.

When we look at a 3-dimensional object, the visual information about that object is recorded on a 2-dimensional (flat) "screen" called the retina. There is a retina at the back of each of our eyes. Our brain uses visual "depth cues," together with our own experience of the world, to translate the 2-dimensional images on the retina back into three dimensions.

Linear perspective is just one of the depth cues which our brain uses. It is based on the fact that parallel lines that recede into the third dimension look like converging lines to the human eye. Think about train tracks going off into the distance, for example. The parallel sides of the track will seem to meet at the horizon.

The Statue of Liberty looks small due to its distance away from the bridge.

The Statue of Liberty looks small due to its distance away from the bridge.

Now think about how you might look at a friend walking towards you on the street, for example, and figure out how far away that person is. You know that a person looks bigger close up than far away. That's actually because closer things take up more space on the screen/retina in your eye. You might also have noticed that more distant things look higher up in your field of vision than nearby things.

A nearby object also may partly cover up an object that is farther away. This depth cue is called "interposition".

If you only had one eye, depth cues like linear perspective, relative size and position, and interposition would still work. They are "monocular cues." Other cues are called "binocular cues" because they only work with two eyes. This is because the image of the same object on each of your two retinas is slightly different (disparate).

To prove this, hold a pencil about 6 inches in front of your nose. Close one eye and then the other. What happens? The difference you see is called "binocular disparity". Your brain uses it to help calculate the distance of the pencil from your nose. If the pencil gets too close to your nose, the binocular disparity will be so large that your brain won't be able to blend the two separate images into one, and you will "see double".

Try threading a needle with one eye closed, and you'll see just how much binocular disparity helps us to locate things in depth and to coordinate our eyes with our hands. Do you think you could be a great baseball or basketball player if you only had one eye?

Artists are able to create the illusion of depth by using linear perspective, interposition, and other monocular depth cues. By studying how vision works in the real world, they have learned to create images that mimic 3-dimensional reality. With a little practice, you can do it too.