1. A method for shading polygon surfaces in a rendering system comprising the steps of: providing a polygon within the scene database to be shaded a polygon coordinate system defined by three normalized vectors which represent the X, Y and Z axes; providing a light source defining a three-dimensional light source direction vector; rotating the three-dimensional light source direction vector to correspond with the polygon coordinate system in order to provide a rotated three-dimensional light source direction vector; and converting the rotated three-dimensional light source direction vector into a two-dimensional vector whose length is proportional to the angle between the polygon Z vector and the three-dimensional light source direction vector.