How to select a particular portion of an image and take Fourier transform

Hi,

I have a problem in the following code:

% Binary Hologram & Its Corresponding FT

mx=20; my=0;

[x,y]=meshgrid(-1:2/127:1, -1:2/127:1); circ=sqrt(x.^2+y.^2)<1; H=circ.*(cos((x*pi*mx)+(y*pi*my)))>0;

figure(1) imagesc(H); colormap gray; axis image; axis off

%Fourier Transform z=fft2(H, 512,512); m=fftshift(z); q=abs(m);

figure(2) imagesc(q); colormap gray; axis image; axis off title('Focal Spots Of Grating Array'); ******* If we run the above code, the figure 2 gives an array of focal spots( i.e. the white spots). I need to select only one white spot ( which is to the right of central one) and then take the inverse Fourier transform of the selected focal (i.e. white) spot using matlab code. Can any one help me in this regard.

Thanking You!

[Image Analyst]

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Answer 1

[Thorsten]

Select the white spot

   znew = zeros(size(z));
   znew(244:270, 325:350) = z(244:270, 325:350);

Take the inverse FFT

   Hnew = real(ifft2(znew));
   imagesc(H)

This does what you asked for... Does you really want this result?