Toon shading (also called cel shading) is a non-photorealistic rendering technique that is intended to mimic the style of shading often used in hand-drawn animation. There are many different techniques that are used to produce this effect. In this recipe, we'll use a very simple technique that involves a slight modification to the ambient and diffuse shading model.

The basic effect is to have large areas of constant color with sharp transitions between them. This simulates the way that an artist might shade an object using strokes of a pen or brush. The following image shows an example of a teapot and torus rendered with toon shading:

The technique presented here involves computing only the ambient and diffuse components of the typical ADS shading model, and quantizing the cosine term of the diffuse component. In other words, the value of the dot product normally used in the diffuse term is restricted to a fixed number of possible values. The following table illustrates the concept for four levels:

In the preceding table, s is the vector toward the light source and n is the normal vector at the surface. By restricting the value of the cosine term in this way, the shading displays strong discontinuities from one level to another (see the preceding image), simulating the pen strokes of hand-drawn cel animation.