Using Curves

From ZBrush Info

When a Shader has a curve modifier, such as the ReflectCurve modifier, a compressed version of the curve appears below the numeric modifier in the palette.

To open the full curve, click it. The result will be a larger graph with which you can work, as shown below:

To understand how modifier curves work, let's first look at an example that uses the DiffuseCurve to produce two different effects:

The curves indicate how strongly the effect is to modified, from the center of the effect to its outer edge, with the intial curves being the "standard" effect. The center of the effect is not necessarily the center of the object; for example, the center of a Diffuse effect are those parts of an object directly facing (and hence most strongly effected by) a light.

In the case of second of the above examples, the Diffuse effect is not modified at the center of the effect, which means that part of the sphere is lit normally. The effect then rises through the next 1/4 of the graph, making the area around the center brighter than it otherwise would be. A fall in the curve below its default value causes a darker 'ring' around the center of the effect. Finally, a rise in the curve that brings it more closely into alighment with the default curve makes for a brighter ring, and then finally the edge tapers off into darkness as the light source no longer affects that part of the sphere.

In the case of Noise, the graph creates a fractal pattern across the entire document, making an object's surface appear irregular:

Interesting, 'cartoonish' effects can be achieved by using straight line segments in the DiffuseCurve: