Lambert's Cosine Law
Johann Heinrich Lambert (1728-1777) stated the law:
Flux per unit solid angle leaving a surface in any direction is proportional to the cosine of the angle between that direction and the normal to the surface.
A surface that obeys Lambert's law is called lambertian; ``flat'' paint produces such a surface. The albedo of the surface is the fraction of incoming light that is reflected. Illumination is spread across the surface proportional to the cosine of the angle with the surface normal. A reflective surface is specular (mirror-like).
The cosine law can be solved to determine the surface normals from observed intensity, thus recovering object shape from shading. Assumptions must be added to constrain the problem enough to make it mathematically solvable.
Non-planar objects may be represented as generalized cylinders.