[Numpy-discussion] real_fft2d

Scott Rifkin scott.rifkin at yale.edu
Tue Feb 17 12:07:03 CST 2004

I'd like to take the 2d fourier transform of an image, remove some of the 
lower frequency signal, take the inverse 2d fourier transform, and then 
reconstruct the image.  I'm trying to use the python imaging library to do 
the image part and FFT for the fourier transform part.

When I take the real_fft2d of a matrix (integer valued in this
case-UInt16), I get a new matrix of the same size with complex entries.  
In this implementation of the fourier transform, how is this resulting 
matrix arranged?  There seem to be a few different conventions out there 
for what, say, entry [0,0] represents.  Which one does FFT use?

If I want to remove signal from wavelengths longer than a certain interval 
(say anything over 200 pixels), do I need to process the result from 
real_fft2d more or can I do the removal directly on that matrix and then 
take the inverse directly?

Is there a good reference for the FFT package or the FFTPACK library out 

Thanks much,
Scott Rikin

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