Question

I am working on some fourier transform code in matlab, and have come across the following:

xx = meshgrid(1:N);
% Center on DC
xx = xx - dcN;
% normalize dynamic range from -1 to 1
xx = xx./max(abs(xx(:)));
% form y coordinate from negative transpose of x coordinate (maintains symmetry about DC)
yy = -xx';
% compute the related radius of the x/y coordinates centered on DC
rr = sqrt(xx.^2 + yy.^2);

How can I generalize this for non-square matrices? This code is assuming my matrix is square, so dcN is the center of the square matrix (in other words, with 11×11, dcN = 6).

The math doesnt work out for that yy variable when the transpose is taken for a non-square matrix.

I have tried to figure out if I can make a meshgrid going from “top to bottom” instead of left to right – but I havent been able to figure taht out either.

Thanks

Answer 1

From your question I guess that you want to find rr, i.e. the distance of any element in the matrix from the center.

If you want this for a M-by-N array, you’d do the following

%# note that using meshgrid instead of ndgrid will swap xx and yy
[xx,yy] = ndgrid(-(M-1)/2:(M-1)/2,-(N-1)/2:(N-1)/2);

%# normalize to the max of xx,yy
nrm = max((M-1)/2,(N-1)/2);
xx = xx./nrm; 
yy = yy./nrm;

rr = sqrt(xx.^2+yy.^2)