This module is no longer required for the 2D Fast Fourier Transform 
index{Fourier Transform}
({\sf fft2d} module), but can be useful to speed-up computations, though
the more efficient module {\sf fft2dshrink} should be prefered in general.
It extracts from an input image the largest centered part whose dimensions 
are power of $2$.
\index{crop!an image}
For example, an original image of size $600 \times 300$ will be shrinked to 
size $512 \times 256$, by extracting the part $[44,555]\times [22,277]$.

\vskip 0.5cm

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