Discrete Fast Fourier Transform in MATLAB

I'm having difficulties to implement and answer for below question. If someone could help/guide me that would be much appreciated.

· multiply the image by (-1)^(x+y); · compute the DFT using "fft2"; · take the complex conjugate of the transform; · compute the inverse DFT; · multiply the real part of the result by (-1)^(x+y); · write the resulting matrix to an image file. · compare the result image by using "subplot" function.

I have managed to find answers for some, but not sure how to proceed or whether below is correct?

close all clear all X=imread('.\img02.jpg','jpg'); figure; imshow(X);axis equal;axis off; [m,n]=size(X); for i = 1:m; for j = 1:n; f1(i,j) =double(X(i,j)) * (-1)^(i+j); end; end; F=fft2(double(f1)); S=abs(F); S2=log(1+abs(S)); figure; imshow(S3); axis equal;axis off;

[Andy L]

(-1)^(x+y);

...are x & y meant to be the coordinates, or the values at those coordinates? Also you say

imshow(S3)

but i haven't seen S3 defined anywhere in your code. Is the code complete or is this a typo? It is good practice to display matlab code properly (use a new line and double space indent). This makes it easier to read in general.

Finally with images it would be useful to have it attached.