This modules extracts the sub-region $[X_1,X_2]\times [Y_1,Y_2]$ 
of the input image. Hence, the extracted region has a size of 
$(X_2-X_1+1) \times (Y_2-Y_1+1)$.
If $X_2$ or $Y_2$ are negative or null, then they
are considered relatively to the corresponding 
last possible coordinate. For example, $X_2=0$ means that the sub-region 
will end exactly at the last column of the input image, while $Y_2=-10$
will crop 10 lines from the input image. If the sub-region is
not included in the input image domain, the $b$ value is used in
missing areas.

\medskip

With the \verb+-r+ option, $X_2$ and $Y_2$ specify the
size of the region instead
of the ending coordinates, yielding the extracted region
$[X_1,X_1+X_2-1]\times [Y_1,Y_1+Y_2-1]$.

\medskip

If a $bg$ image is specified, then the upleft corner of the
extracted region is placed at position $(X_c,Y_c)$ of this 
background image.

