The main concept is that the quantity, thickness and spacing of the lines will affect the brightness of the overall image and emphasize forms creating the illusion of
volume. Optically, brighter (less hatched) areas appear closer and darker (more hatched) areas appear further away, thus the number, spacing, and thickness of hatch lines can create the illusion of depth and volume on a two-dimensional surface like drawing paper or a printed image. Hatching lines should always follow (i.e. wrap around) the form. By increasing quantity, thickness and closeness, a darker area will result. An area of shading next to another area which has lines going in another direction is often used to create
contrast. Line work can be used to represent colors, typically by using the same type of hatch to represent particular
tones. For example, red might be made up of lightly spaced lines, whereas green could be made of two layers of
perpendicular dense lines, resulting in a realistic image. Crosshatching is the technique of using line to shade and create value.
Variations ==Representation of materials==