# Using Ellipses to Define Surface Contours

Ellipses are also used to define the curvature of a surface. The key is to vary in specific ways the minor axes of the ellipses that describe that surface. A change in minor axis from one area to another indicates a change in the contour of the surface.

The greater the variation in the ellipses' minor axes, the more changes in surface contour are indicated. Study the illustrations shown here and notice how the minor axes have been handled to describe both flat and curved surfaces.

In this illustration the ellipses have been arranged to create the illusion of a flat plane. As these shapes recede toward the background, their minor axes diminish in size at a constant rate. Compare this rate of change with that in the illustration at right.

Here a curved surface is indicated. In the foreground, where part of the surface is nearly parallel to the picture plane, the changes in minor axes are gradual. As the surface curves away from the picture plane, the ellipses' minor axes diminish rapidly.

Ellipses have been arranged to create the illusion of a horizontal plane that recedes in space, then curves upward into a vertical plane. This is accomplished by making the minor axes progressively smaller, then larger.

The ellipse pattern creates the illusion of a large, horizontal cylinder.

The texture of this tree trunk is an example of how ellipses describe a curved contour.

Just as the ellipses here indicate a flat plane, so too do the bases of the tree trunks, which are circular and therefore fit into imaginary ellipses appropriate for circles seen at that angle and distance.

Here the ellipses and the bases of the trees are arranged to create tne impression that they rest on a curving (cylindrical) surface.