modelling
In art, the depiction of a three-dimensional form either through the sculpture of a soft substance, or the use of light, shadow, and colour in painting and drawing.
In painting and drawing modelling gives a two-dimensional surface the impression of three-dimensional solidity, and the subject matter roundness and weight. The term is most often used with reference to representations of the human figure.

http://en.wikipedia.org/wiki/Chiaroscuro