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. Christou et al. 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. We have also drawn on the work of Markosian et al.