Shading of Metal Objects

  Illustrators use a different technique to communicate whether or not an object is made of metal. In practice illustrators represent a metallic surface by alternating dark and light bands. This technique is the artistic representation of real effects that can be seen on milled metal parts, such as those found on cars or appliances. Milling creates what is known as ``anisotropic reflection.'' Lines are streaked in the direction of the axis of minimum curvature, parallel to the milling axis. Interestingly, this visual convention is used even for smooth metal objects [15,18]. This convention emphasizes that realism is not the primary goal of technical illustration.

To simulate a milled object, we map a set of twenty stripes of varying intensity along the parametric axis of maximum curvature. The stripes are random intensities between 0.0 and 0.5 with the stripe closest to the light source direction overwritten with white. Between the stripe centers the colors are linearly interpolated. An object is shown Phong-shaded, metal-shaded (with and without edge lines), and metal-shaded with a cool-warm hue shift in Figure 10. The metal-shaded object is more obviously metal than the Phong-shaded image. The cool-warm hue metal-shaded object is not quite as convincing as the achromatic image, but it is more visually consistent with the cool-warm matte shaded model of Section 4.2, so it is useful when both metal and matte objects are shown together. We note that our banding algorithm is very similar to the technique Williams applied to a clear drinking glass using image processing [25].