the volumetric form of an object. As was mentioned in the discussion of
hatching and cross-hatching, it is necessary for artists to develop the
interior definition of objects in order for the viewer to recognize that such
objects exist in real three-dimensional space, the space we ourselves occupy.
For example, a human face is not as flat as a pancake, and therefore should
be depicted, as in Michelangelo’s Head of Satyr, with hills and
valleys simulated by a careful distribution of light areas and dark areas.