Examining technical manuals, illustrated textbooks, and encyclopedias reveals shading and line illustration conventions which are quite different than traditional computer graphics conventions. The use of these artistic conventions produces technical illustrations, a subset of non-photorealistic rendering. The illustrations in several books, e.g., [20,23], imply that illustrators use fairly algorithmic principles. Although there are a wide variety of styles and techniques found in technical illustration, there are some common themes. This is particularly true when examining color illustrations done with air-brush and pen. The following characteristics are present in many illustrations:
These illustration characteristics result from a hierarchy of priorities. The edge lines and highlights are black and white, respectively, and provide a great deal of shape information themselves. Several studies in the field of perception [2,4,6,26] have concluded that subjects can recognize 3D objects at least as well, if not better, when the edge lines (contours) are drawn versus shaded or textured images. Christou et al. [6] concluded in a perceptual study that ``a few simple lines defining the outline of an object suffice to determine its 3-D structure''(p. 712). As seen in children's coloring books, humans are good at inferring shape from line drawings. Lines can help distinguish different parts and features of an object and draw attention to details which may be lost in shading. Many illustrators use black edge lines to separate parts. Sometimes an illustrator might choose to use a white highlight line instead of a black edge line for interior silhouettes or discontinuities. Deciding which lines to draw and how to draw them is essential in imitating the conventions used in technical illustration. The above observations form only a subset of the conventions used by illustrators. We have concentrated only on the material property and shading aspects of illustration. Work done in computer graphics by Seligmann and Feiner [25] and Dooley and Cohen [7] concentrate on additional aspects of technical illustration like layout, object transparency, and line style. We have also drawn on the work of Markosian et al. [19], Elber [9], and Saito and Takahashi [24]. Markosian et al. [19] developed a real-time 3D interactive system for illustrating silhouettes and creases of non-self-intersecting polygon mesh-based models. Elber [9] provides algorithms for determining NURBS surface information by finding four types of curves: the surface boundary curves, curves along C1 discontinuities in the surface, isoparametric curves, and silhouette curves. Saito and Takahashi [24] offer convincing pictures to show how 3D models enhanced with discontinuity lines, contour lines, and curved hatching can generate images which convey shape and structure. In Section 8.3, we will discuss the rules, properties, and types of lines needed to convey shape information as accomplished by the line drawings of technical illustrators.
When shading is added, in addition to edge lines, shape information can be maximized if the shading uses colors and intensities that are visually distinct from both the black edge lines and the white highlights. This means the dynamic range available for shading may be limited. Another important characteristic used in technical illustration is the conveyance of material property. Illustrators alternate bands of light and dark to represent a metallic object, similar to the real anisotropic reflections seen on real milled metal parts. These shading conventions will be investigated in detail in Section 8.4. Illustrators rarely use shadows in an illustration. Shadows are used only when they do not obscure details in other parts and will be discussed in Section 8.5.