RENDERING, SHADING AND COLOURING
By introducing hidden line removal we have already taken one step away from wire-frame drawings towards being able to realistically model and display 3-D objects. Perhaps the biggest step down that road comes when attempting to "colour in" our simple line drawings. The various algorithms for rendering, the process of applying lighting, colouring, shadow and texture to an object or scene in order to obtain a realistic image, are all based to a greater or lesser extent on the study of the physical properties of light. In this unit we shall examine various properties of light and the way it interacts with objects and develop some simple mathematical models of its behaviour. It is worth setting the following discussion in the context of our system as developed so far. Currently our 3d model is made up of surfaces, each of which we represent on the screen by drawing its outline. If we wanted to shade each polygon ("colour it in") what color would we use? What we basically are trying to achieve in this chapter is to derive a method for calculating that colour. Figure 3.16 shows the difference between a wire-frame representation and a simple rendered version