Material Retouching is a way to control a shaded area by using Material Nodes with Vertex Color. It is useful if you didn't get desired results in a tiny shaded area of a model. Now you could remake the model, but remaking takes time. Another way would be to use post processing as typified by use of the Composite Node to give a 2D effect, although this creates a partially incorrect 3D look. Or you can adjust shading of the model with Material Retouching in less time. So let's see how Material Retouching works.
First, while in Vertex Paint mode, we will paint two shading control areas, one in red and one in blue. Paint your bright areas in red, paint your shadow areas in blue, and paint unimportant areas in black.
Vertex Color painting example. Second, create the following nodes from the original Material as shown in the following image.
"Material Retouch" Material Node. Note: To extract Red or Blue from the Vertex Colors you use an RGBCurve Node. To extract Red, you use a value of zero in the green and blue channels of the RGBCurve node. Now you can render your scene and continue to adjust your Vertex Color painting until you achieve your desired look.
Figure 3 :
Left:normal shading of Suzanne. Right:"Concave Retouch" shading of Suzanne. Lighting is provided by a sun lamp. This method is based on colors because Blender 2.45 doesn't support shader influence controls. Notice that Material Retouching only affects colors, not Ray Traced or Buffered Shadows.
Locally Controllable Stylized Shading (English)
Locally Controllable Stylized Shading (Japanese)
Test of Toon shading retouch on 3D (Japanese,XSI)