Tanaka (relief shaded) contours in R

Tanaka K. 1950. The relief contour method of representing topography on maps. Geographical Review 40(3) 444-456.

A simple and exact method is available […]. If the tip of a drawing pen […] is filed down so that its thickness is the maximum thickness required in a contour line, and if […] the pen-point edge is kept at a fixed orientation […] parallel to the assumed horizontal direction of the incident light, then the thickness of the contour line will vary with the cosine of the angle. (1950, page 449)