Light — Science & Magic, 6th Edition

REVEALING SHAPE AND CONTOUR 77

DEPTH CLUES

Why does the second picture look more three-dimensional than the rst? A

look at the drawings gives us two immediate answers. e rst is perspective

distortion: some edges of the cube seem to be longer than others, and some

seem to be shorter, even though we know they are all the same length. e

corners all appear to join at dierent angles, even though we know they are

all 90 degrees.

Besides perspective distortion, there is a second clue that our brain uses

to perceive depth: tonal variation. As we just saw in Figure 5.2, a dierence

in the various tones in a scene also helps our brains to perceive, or “see,”

depth.

Notice that these visual clues are so powerful that the brain perceives

depth ...