Ersilia,
Assuming your image is a 2D matrix, you could take "the rectangular main diagonal" from it by using the following code.
% M is your image % v is the "rectangular main diagonal"
[a b] = size(M);
nPix = max(a,b);
xi = linspace(1,b,nPix);
yi = linspace(1,a,nPix);
v = interp2(M,xi,yi);
Hope this helps!
