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Ridge and Valley emphasis: Idea

Okay, last section.

Creases, or sharp edges on a surface, are either ridges or valleys. Emphasizing them is a basic part of standard technical illustration.

We do this in volume rendering by working in the two-dimensional space of the principal curvature magnitudes, shown again here.

Following convention, we'll select these regions, the ridges, and lighten them, and these regions, the valleys, will be darkened.