b) Division between the highlights and the shadows: As is shown in Fig.7ˈthe angle, produced by the Normal of the spot in the surface and the line of the spot to the light source, can be used to show the relationship between the spot and the light source. When the angle is over 90°, the surface is deviated from the light source, and it is in shadow. The dividing line between the highlights and the shadows can be determined by accounting the Normal of the spot in the surface and the angle of the light source. Different angle intervals can be used by colors in different brightness, so that the polychrome drawing can be achieved.