{\em fpsnr255} computes the additive factor to get the '255' PSNR from the 
'max-min' PSNR, where
\index{Peak Signal to Noise Ratio}
\[
\mbox{PSNR}_{\mbox{max-min}} = 
10 \times \log_{10} \frac{(\max_{x,y}f(x,y) - \min_{x,y}f(x,y))^{2}}{MSE}
\]
and 
\[
\mbox{PSNR}_{255} = 10 \times \log_{10} \frac{255^{2}}{MSE}
\],
for $f$ the given image and $MSE$ the mean square error between this original
image and a reconstructed image.

This module allows to get the '255' PSNR from PSNR results obtained by
the module \verb+fmse+, when it has not been called with the option \verb+-p+.

Use the \verb+-n+ option when \verb+fmse+ has been called with the 
\verb+-n+ option.
