cpercep.c: The CPercep Functions v0.9: 2002-02-10
  Adam D. Moss: adam@gimp.org <http://www.foxbox.org/adam/code/cpercep/>

  This code module concerns itself with conversion from a hard-coded
  RGB colour space (sRGB by default) to CIE L*a*b* and back again with
  (primarily) precision and (secondarily) speed, oriented largely
  towards the purposes of quantifying the PERCEPTUAL difference between
  two arbitrary RGB colours with a minimum of fuss.

  Motivation One: The author is disheartened at the amount of graphics
  processing software around which uses weighted or non-weighted
  Euclidean distance between co-ordinates within a (poorly-defined) RGB
  space as the basis of what should really be an estimate of perceptual
  difference to the human eye.  Certainly it's fast to do it that way,
  but please think carefully about whether your particular application
  should be tolerating sloppy results for the sake of real-time response.

  Motivation Two: Lack of tested, re-usable and free code available
  for this purpose.  The difficulty in finding something similar to
  CPercep with a free license motivated this project; I hope that this
  code also serves to illustrate how to perform the
  R'G'B'->XYZ->L*a*b*->XYZ->R'G'B' transformation correctly since I
  was distressed to note how many of the equations and code snippets
  on the net were omitting the reverse transform and/or were using
  incorrectly-derived or just plain wrong constants.

  TODO: document functions, rename erroneously-named arguments