This module applies a multiplicative mask on a Fimage $u(x,y)$, called 
the Hamming window. 
\index{Hamming window}
If $u$ is defined on $[-a,a] \times [-b,b]$, the result is given by
$$u'(x,y) = u(x,y) . \left( \alpha + (1-\alpha) \cos \frac{\pi x}{a} \right)
\left( \alpha + (1-\alpha) \cos \frac{\pi y}{b} \right),$$
where $\alpha = 0.54$.
Applying this operator to a non-periodic image before computing its Fourier
Transform results in a better frequency estimation, since the high energy
of the vertical and horizontal frequencies (due to the border's gap of 
the image) are cancelled.
\index{Fourier Transform}

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NB: Calling this C-subroutine with $out=in$ is possible