Another excerpt from "The Tao of Figure Drawing" by Tom Byrd:
In the next class, Prof Rosen has the students do a different warm-up exercise, which he calls "visual tracking." The resultant drawing he calls a "space cage." "We are drawing not the figure as it is, but the figure as it is perceived." This could also be described as "drawing the process of perception," and at first it is a difficult concept for some students to follow through on.
As the students gather around, he has me pose and illustrates what he wants. As he views me through his aperture, he calls out where his eye is tracking as he draws a matching line. "My eye rests on his beard, moves to his left elbow, across to his penis, down to the right big toe, back to the right heel, up to the left knee, and so forth."
After the students do this exercise, he has them turn their easels around for criticism. Some have ended up with an outline of at least part of my figure. One clearly shows the shape of my head and nose. This is not what Rosen wants. "The eye doesn't work that way," he says. "It makes straight lines, not curved ones. If you are sensitive to how your eye sees a curve, you will find that it tracks back and forth along it, checking one side, then the other in a zig-zag, a series of straight lines. There is no way, if you are honest in doing this exercise, that you will end up outlining the head, or any other part of the body."
After the final attempt at this exercise, Rosen allows the students to begin filling in the figure, working over the palimpsest of the visual tracking exercise. The illustrations in this post show first poor visual tracking, showing the outline of the figure. The other two show a pose and a very successful exercise with that pose.