Slide 43 & 44 of 52

Technical illustrators use a different technique to communicate whether or not an object is 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.

Interestingly this visual convention is even used for smooth metal objects.

To simulated a metallic object, we map a set of twenty stripes of varying intensity along the parametric axis of minimum curvature. The stripes are random intensities between 0.0 and 0.5 with the stripe closest to the light source direction being overwritten with white. Between the stripe centers, the colors are linearly interpolated.

The metal-shaded object is more obviously metal than the Phong-shaded image.

We note that our banding algorithm is very similar to the technique Lance Williams applied to a clear drinking glass using image processing.