"Apparent Ridges for Line Drawing"
Line drawing - the question is, where do you put the lines? Various answers - bounding contour (silhouette), occluding contours (arm in front of body), discontinuities of surface normal(e.g. in a cube) This paper describes a rule that yields a rich set of lines."...Human observers are highly sensitive to line-like and edge-like features, i.e. points of high luminance variation. A given object, will tend to have such features and their locations will be stable across different choices of BRDF and illumination..."
This is their rule: Draw a line when the surface normal is changing at a locally maximal rate with respect to image position. View dependent curvature means that the first and second derivatives of the normal are calculated with respect to a screen plane istead of the surface. As a result, the directional derivative with respect to a vector on the screen plane will be different depending on where (on the object surface) this vector is being projected to.
Apparent ridges give better results than Canny edge detectors and suggestive contours, where better = what the author claims is more visually pleasing.