This module implements the simple sharpen filter\index{filter!sharpen}, 
\index{deblurring}
exactly similar to the one of xv (John Bradley's shareware). 
It is a linear filter
defined on a 3x3 neighborhood by
$$ B(x,y) = 
\frac{\displaystyle A(x,y) - p/100 \cdot \frac 19 \sum_{-1\leq i,j\leq 1}A(x+i,y+j)}{1-p/100},$$
and $A(x,y)$ on the boundary of the domain.


