Rotating 1d FFT to get 2D FFT?

Question

I have a blurry image with a sharp edge and I want to use the profile of that sharp edge to estimate the point spread function (PSF) of the imaging system (assuming that it is symmetric). The profile of the edge gives me the "edge spread function" (ESF) and the derivative of that gives me the "line spread function" (LSF). I am trying to follow these directions that I found in an old paper on how to convert from the LSF to the PSF:

"If we form the one-dimensional Fourier transform of the LSF and rotate the resulting curve about its vertical axis, the surface thus generated proves to be the two-dimensional fourier transform of the PSF. Hence it is merely necessary to take a two-dimensional inverse Fourier transform to obtain the PSF"

I can't seem to get this to work. The 2D FFT of a PSF-like function (for example a 2d gaussian) has lots of alternative positive and negative values, but if I rotate a 1D FFT, I get concentric rings of positive or negative values and the inverse transform looks nothing like a point-spread function. Am I missing a step or misunderstanding something? Any help would be appreciated! Thanks!

Edit: Here is some code showing my attempt to follow the procedure described

;generate x array
x=findgen(1000)/999*50-25
;generate gaussian test function in 1D
;P[0] = peak value
;P[1] = centroid
;P[2] = sigma
;P[3] = base level
P=[1.0,0.0,4.0,0.0]
test1d=gaussian_1d(x,P)

;Take the FFT of the test function
fft1d=fft(test1d)
;create an array with the frequency values for the FFT array, following the conventions     used by IDL
;This piece of code to find freq is straight from IDL documentation:  http://www.exelisvis.com/docs/FFT.html
N=n_elements(fft1d)
T=x[1]-x[0]  ;T = sampling interval
fftx=(findgen((N-1)/2)+1)
is_N_even=(N MOD 2) EQ 0
if (is_N_even) then $
freq=[0.0,fftx,N/2,-N/2+fftx]/(N*T) $
else $
freq=[0.0,fftx,-(N/2+1)+fftx]/(N*T)

;Create a 1000x1000 array where each element holds the distance from the center
dim=1000 
center=[(dim-1)/2.0,(dim-1)/2.0]
xarray=cmreplicate(findgen(dim),dim)
yarray=transpose(cmreplicate(findgen(dim),dim))
rarray=sqrt((xarray-center[0])^2+(yarray-center[1])^2)
rarray=rarray/max(rarray)*max(freq) ;scale rarray so max value is equal to highest freq    in 1D FFT

;rotate the 1d FFT about zero to get a 2d array by interpolating the 1D function to the    frequency values in the 2d array
fft2d=rarray*0.0
fft2d(findgen(n_elements(rarray)))=interpol(fft1d,freq,rarray(findgen(n_elements(rarray))))

;Take the inverse fourier transform of the 2d array
psf=fft(fft2d,/inverse)
;shift the PSF to be centered in the image 
psf=shift(psf,500,500)
window,0,xsize=1000,ysize=1000
tvscl,abs(psf) ;visualize the absolute value of the result from the inverse 2d FFT

Can you post some code? You may be confusing radially symmetric PSF with a separable PSF. — aganders3, Aug 21 '14 at 19:45
Actually, based on your description (2D FFT of a 2d Gaussian having "lots of alternative positive and negative values") I would guess you are missing an fftshift somewhere. The FFT of a 2d Gaussian should be entirely positive in the real signal, with Hermitian symmetry. Again, posting your code would help figure this out. — aganders3, Aug 21 '14 at 20:13

score 1 · Answer 1 · edited Jun 20 '20 at 09:12

I don't know IDL, but I think your problem here is that you're taking the FFT of signals that are centered, where by default the function expects 0-frequency components at the beginning of the array.

A quick search for the proper way to do this in IDL indicates the CENTER keyword is what you're looking for.

CENTER

Set this keyword to shift the zero-frequency component to the center of the spectrum. In the forward direction, the resulting Fourier transform has the zero-frequency component shifted to the center of the array. In the reverse direction, the input is assumed to be a centered Fourier transform, and the coefficients are shifted back before performing the inverse transform.

Without letting the FFT routine know where the center of your signal is, it will seem shifted by N/2. In the converse domain this is a strong phase shift that will appear as if values are alternating positive and negative.

score 0 · Answer 2 · answered Aug 22 '14 at 17:10

Ok, looks like I have solved the problem. The main issue seems to be that I needed to use the absolute value of the FFT results, rather than the complex array that is returned by default. Using the /CENTER keyword also helped make the indexing of the FFT result much simpler than IDL's default. Here is the working version of the code:

;generate x array
x=findgen(1000)/999*50-25
;generate lorentzian test function in 1D
;P[0] = peak value
;P[1] = centroid
;P[2] = fwhm
;P[3] = base level
P=[1.0,0.0,2,0.0]
test1d=lorentzian_1d(x,P)

;Take the FFT of the test function
fft1d=abs(fft(test1d,/center))

;Create an array of frequencies corresponding to the FFT result
N=n_elements(fft1d)
T=x[1]-x[0]  ;T = sampling interval
freq=findgen(N)/(N*T)-N/(2*N*T)
;Create an array where each element holds the distance from the center
dim=1000 
center=[(dim-1)/2.0,(dim-1)/2.0]
xarray=cmreplicate(findgen(dim),dim)
yarray=transpose(cmreplicate(findgen(dim),dim))
rarray=sqrt((xarray-center[0])^2+(yarray-center[1])^2)
rarray=rarray/max(rarray)*max(freq) ;scale rarray so max value is equal to highest freq     in 1D FFT
;rotate the 1d FFT about zero to get a 2d array by interpolating the 1D function to the     frequency values in the 2d array
fft2d=rarray*0.0
fft2d(findgen(n_elements(rarray)))=interpol(fft1d,freq,rarray(findgen(n_elements(rarray))))

;Take the inverse fourier transform of the 2d array
psf=abs(fft(fft2d,/inverse,/center))
;shift the PSF to be centered in the image 
psf=shift(psf,dim/2.0,dim/2.0)
psf=psf/max(psf)
window,0,xsize=1000,ysize=1000
tvscl,real_part(psf) ;visualize the resulting PSF

;Test the performance by integrating the PSF in one dimension to recover the LSF
psftotal=total(psf,1)
plot,x*sqrt(2),psftotal/max(psftotal),thick=2,linestyle=2
oplot,x,test1d

Rotating 1d FFT to get 2D FFT?

2 Answers2