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Contours

Contours (or silhouettes) are a basic part of non-photorealistic volume rendering. They're easy to make, and they help emphasize the shapes of objects.

They're usually make with a view-dependent environment map, where you find the dot product between the view vector and the surface normal, and if it falls within some fixed range, then you color the surface differently.

Here's the result of doing that. The problem here is that the thickness of the contour varies, depending on the shape of the object.