where hard edges are delineated, soft edges are blurry transitions (more area than edge), and lost edges are merely implied. Good pen & ink work uses many lost edges.
I love this paper for the systematic approach it applies to a problem that all drawing teachers face. At the heart of the problem is a simple paradox... that a line can somehow symbolise light, texture and even colour. This paper is not perfect, but makes a good stab at the problem.
