2-D fast Fourier transform

`Y = fft2(X)`

`Y = fft2(X,m,n)`

`Y = fft2(`

returns
the two-dimensional
Fourier transform of a matrix using a fast Fourier transform
algorithm, which is equivalent to computing `X`

)`fft(fft(X).').'`

.
If `X`

is a multidimensional array, then `fft2`

takes
the 2-D transform of each dimension higher than 2. The output `Y`

is
the same size as `X`

.