An example of the lines an illustrator would use to convey the shape of this airplane foot pedal. They observed that illustrators use edge lines, consisting of surface boundaries, silhouettes, discontinuities, and creases to separate individual parts and to suggest important features in the shape of each object. These static images represented edge lines with black lines of uniform weight. To decide which lines to draw, we started by analyzing some examples from hand drawn technical illustrations. The illustration in Figure 1 consists of just enough lines to separate individual parts and to suggest important features in the shape of each object.
Most modeling systems display only a wireframe or a shaded image.
A wireframe display is common because it can give a lot of information which is occluded by shading. However, a wireframe display of a complex model can be confusing due to the number of lines being displayed, as can be seen by comparing Figure 2 and 3 . Figure 4 demonstrates that changing which isolines are displayed can change the perception of the surface. By drawing silhouettes, surface boundaries, discontinuities, and creases, one can imitate the lines drawn in technical illustrations without being parameterization dependent. An example of these different line types is provided in Figure 5 .