The Gooch Lighting Model
"A Non-Photorealistic Lighting Model For Automatic Technical Illustration"
I've recently been toying with the Gooch et al. (1998) non-photorealistic lighting model. Unfortunately, the nature of the project does not permit me to post any of the "real" results quite yet, but some of the tests have a nice look to them all on their own.
My implementation takes a normal map and colour map, e.g.:
This is the result from those inputs: