Given two images $u$ and $v$, this module computes the optimal 
nondecreasing contrast change $g$ that minimizes the $l^2$ norm of $v-g(u)$,
and returns rhe image $out = g(u)$.
\index{contrast!change}

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This contrast change is found in two steps: first, the mononicity condition
is relaxed and a raw contrast change $r$ (which may not be nondecreasing) 
is found; then, $g$ is obtained from $r$ by isotonic regression.
Please refer to~\cite{moisan:modelling} for more details.

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As usual, $g$ is returned (if requested with the \verb+-g+ option)
as a 2-Flist that may be used with \verb+fcontrast+.
Note that $r$ (which may be returned using \verb+-r+ option) is not
a valid contrast change (it is not monotone) 
and should not be used with \verb+fcontrast+.
