What we interpret as lines in optical reality are actually the contrasts between and among surfaces and tonal differences, as illustrated to the right.
The choice of how one shades or renders these surfaces is ultimately a matter of fitness to purpose, style, and creativity.
The methods and styles for shading in surfaces can range from the lightest gestural suggestions to all the details and gradations of photorealistic rendering, and with everything in between.
It is possible to suggest tonal differences and three-dimensional form by manipulating the weight of line alone. By making some lines darker and others lighter, highlights and shadows can be implied.
Surfaces tones can be indicated by dots, lines, hatching patterns, or virtually any kind of disturbance that sets off one area against another. In sketching, sometimes a simple squiggled line will not only be adequate but also most effective.
Using parallel lines is one way to get a surface. But notice the optical ...