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The basic property of the colormap which we would like to control is how the light level, or luminance, changes througout the colormap.

In many cases we want it to increase monotonically, like this [first example]; in other cases we might want it to stay fixed [second example]. Its pretty unlikely, though, that we want it to be going up and down [third example], like in the standard rainbow colormap.

The reason to care about the pattern of luminance variation is that perceptual psychology tells us that luminance plays a fundamental role in how we perceive details and shapes in images.