Depth Cues: Creating the Illusion of Depth - Flash 3D: Animation, Interactivity, and Games - page 18

This sense of depth is a result of stereoscopic vision (Figure 2.1). Our eyes are spaced
apart, which produces a slightly different image on each retina. To demonstrate this,
hold this topic in front of you. Take turns closing one eye. Notice the difference in what
you see from the left eye to the right. Your left eye will see the left side of the topic,
while the right eye will reveal the right side of the topic.
Figure 2.1 Stereoscopic vision—each eye produces a different image.
Our brain interprets this retinal difference and then merges the two images into a single
three-dimensional image. The resulting image allows us to perceive depth and estimate
distance. The difference between each retinal image is a direct result of the depth of
the objects that we are looking at—the closer the object, the greater the difference in
each retinal image. Distant objects, such as mountains, are so far away that our eyes
produce essentially the same image, making depth imperceptible.
Stereoscopic vision isn't the only way our brain interprets depth. Similar to a camera
lens, our eyes adjust themselves to bring something into focus. Try the following
experiment. Hold your finger in front of you. Position it about six inches from your face
and focus on it. What happens to the objects in the distance? They are blurry and out
of focus. Our brain uses the eye's focal adjustment to determine the distance of the
object from ourselves. We perceive the finger to be closer to us based on the level of
detail our eyes adjusted to.
Finally, knowing an object's relative size in relation to other objects helps the brain
perceive depth. Take a look at Figure 2.2. We perceive the skyscrapers in the picture