This module computes $f(r)$, the modulus of the Fast Fourier Transform $v$
\index{Fourier Transform}
of a complex image $u$, averaged over the frequency direction $\theta$, that 
is
$$f(r) = \frac 1{2\pi}\int_0^{2\pi} |v(r e^{i\theta})|\,d\theta.$$
The discretization of the Fsignal $f$ can be set manually
with the \verb+-s+ option.
Note that the frequency $\xi=r e^{i\theta}$ 
is normalized in cycles per pixel, so that
$\hat u$ is defined on $[-1/2,1/2]^2$ even for non-square images.
Since $f$ decreases very quickly, it is often better to take
its logarithm (base 10) with the \verb+-l+ option.


