Idea: Automatic Coloring Books

I had this idea at lunch today and think it might apply to grade-school or older art-education: Software that converts a picture into a coloring book line-art drawing. Does something like this exist already? It would work like rotoscoping, except that instead of color areas, the software would make an outline. The concept would be to allow students to explore the color regions of objects (like a still life) in front of them without being excessively concerned about their ability to accurately depict shape.

Don Markstein's Toonopedia Glossary: Rotoscope