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Summary
The values indexed s (saturation) control the transition saturated colors→white (resp. gray scale), i.e. intermediate values → infinity.
The values indexed h (value/valenz) control the transition black→bright, i.e. zero → nonzero.
Parameter a controls where the transition takes place: a is just the radius of the circle dividing the two regions (dark/bright, saturated/gray, etc.)
Parameter b controls how sharp the transition is: b small = soft, b large = sharp.
The following images all show the range [-10,10]×[-10,10] and use (radius of rainbow) and (radius of black disc).