OpenGL Shading Language
Author(s): Randi J. Rost, Bill Licea-Kane.
In Book: Chapter 18 - Non-photorealistic Shaders, pp. 507--532, Addison-Wesley, 3rd,
2009.
[BibTeX] 
Abstract:
A significant amount of computer graphics research has been aimed at
achieving more and more realistic renditions of synthetic scenes. A longtime
goal has been to render a scene so perfectly that it is indistinguishable
from a photograph of the real scene, a goal called PHOTOREALISM. With the
latest graphics hardware, some photorealistic effects are becoming possible
in real-time rendering.
This quest for realism is also reflected in graphics APIs such as OpenGL. The
OpenGL specification defines specific formulas for calculating effects such as
illumination from light sources, material properties, and fog. These formulas
attempt to define effects as realistically as possible while remaining relatively
easy to implement in hardware, and they have duly been cast into silicon by
intrepid graphics hardware designers.
But the collection of human art and literature shows us that photorealism
is not the only important style for creating images. The availability of lowcost
programmable graphics hardware has sparked the growth of an area
called NON-PHOTOREALISTIC RENDERING, or NPR. Researchers and practitioners
in this field are attempting to use computer graphics to produce a wide
range of artistic effects other than photorealism. In this chapter, we look at
a few examples of shaders whose main focus is something other than generating
results that are as realistic as possible.