One of the most used effects in games is toon shading, which is also known as celluloid (CEL) shading. It is a non-photorealistic rendering technique that makes 3D models appear flat. Many games use it to give the illusion that the graphics are being hand-drawn rather than being 3D modeled. You can see, in the following diagram, a sphere rendered with a toon Shader (left) and a Standard Shader (right):
Achieving this effect using just surface functions is not impossible, but it would be extremely expensive and time consuming. The surface function, in fact, only works on the properties of the material, not its actual lighting condition. As toon shading requires us to change the way light reflects, we need to create our custom lighting model instead.
