Tweaking stylized light and shade
Author(s): Ken Anjyo, Shuhei Wemler.
Proceedings: ACM SIGGRAPH2004 Sketches,
2004.
[BibTeX] 
Abstract:
Light and shade in non-photorealistic rendering, such as for digital
cel animation, should be symbolic, rather than realistic, in order to
depict the scene in a simple, yet impressive way. This sketch
presents a new method of creating and editing the stylized light
and shade in real-time by direct manipulation for an on-screen
surface to be cartoon-shaded.
The manipulation simply means dragging the shaded area
including highlight on the surface. The drag operations then offer
an intuitive, interactive tool not only in changing the light source,
but also in designing the stylized highlight shape. Our prototype
system includes the algorithms in this method along with a realtime
cartoon shader, and is built up with programmable graphics
hardware.