In almost all illustrations, lines are drawn in black. Occasionally,
if the illustration incorporates shading, another convention may apply
in which some interior lines are drawn in white, like a
highlight. This technique may be the representation of the real white
highlights as can be seen on edges of the mechanical part in
Figure 3.7. By using this convention, lines
drawn in black and white suggest a light source, and denote the model's
orientation. For example, Figure 3.8 shows how
an artist may use white for interior lines, producing a highlight.
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| Figure 3.7: This photograph of a metal object shows the anisotropic reflections and the white edge highlights which illustrators sometimes depict. |
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| Figure 3.8: Illustrators sometimes use the convention of white interior edge lines to produce a highlight [25]. |