In technical illustration, shape information is valued above photorealism. Illustrators use conventions which don't just eliminate details, but focus on specific details. In using this level of abstraction, an illustrator can amplify the shape information to a level that realistic images can't. Based on our observation of several images like this one, illustrators seem to follow the convention of only including lines consisting of silhouettes, boundaries and discontinuities which we call edge lines.
Illustrators use a specific type of shading.
Although illustrators rarely use shadows, if they are used, they are placed where they don't occlude details. Illustrators tend to use a simplified lighting model,, one light, generally placed above and to the right of the image. These are only a subset of the conventions used by illustrators. We have only concentrated on the material property and shading aspects of illustration. Work done in computer graphics by Feiner and that of Dooley and Cohen concentrate on additional aspects of technical illustration, like layout object transparency, and line width and style.