He has a case study of improving a computer graphics animation by lowering the contrast of the shading and adding black lines to indicate direction. Tufte feels that this principle is so important that he devotes an entire chapter to it in his book Visual Explanations . Based on observing several illustrations, surfaces with little or no curvature are generally flat or Phong-shaded in technical illustrations. Illustrators apply different conventions to convey metallic surface properties, especially if the object has regions of high curvature like an ellipsoid. In the next chapter I will discuss the algorithms used for shading in computer graphics and why it is inadequate for technical illustration.
Illustrators combine edge lines with a specific type of shading.
Shading in technical illustration brings out subtle shape attributes and provides information about material properties.