All of the methods of depth perception discussed so far
rely on different views from different eyes. However there are other more subtle
ways to make a two dimensional image "feel" three dimensional. These
methods, called ambiguous depth cues, are the methods used by artists to
make their pictures appear more realistic. The methods are called ambiguous
because there is more than one way to interpret them, and the brain prefers to
see them as three dimensional. While the other methods of depth perception occur
in the eye, these methods occur in the brain. The primary forms of ambiguous
depth cues are size, brightness, shadows, perspective, the use of the straight
line, and obstruction.
Size: As an object gets farther away it
is focused to a smaller area on the retina. As a result, we generally associate a smaller object with
being further away.
Brightness: A light source illuminates
objects closer to it much more than objects further away. When we see
different shades of brightness we assume different distances from a light
source.
Color can have a very similar affect. Changing colors may create the
illusion of fog and make a piece of a picture appear further away.
Shadows: While a light source illuminates
an object, it will not illuminate objects behind it. Shadows make one
object appear to be in front of another, and the shadow size determines
the depth between them.
Perspective: As we look at the horizon,
straight lines appear to converge. By converging lines in a picture, it
may make them appear to move further away.
A sculpture making great use of perspective is Adelbert Ames' famous
room, built in 1946 and based on the ideas of L. Hermann. The room is constructed in a distorted way so that lines don't seem to converge as they should. As someone moves further away in the room,
they appear to be getting smaller instead of further away. In the image to the left, both children are actually the same height.
The Straight Line: By curving a
"straight" line, a two dimensional picture may appear to be
bending into three dimensions.
Obstructions: Because we can not see
around corners, an object will block your view of everything behind it. In
a picture, the object that blocks your line of site must be in front.
