MATLAB: Do I get unexpected results when using the NORMXCORR2 function

2dcorrelationimageImage Processing Toolboxnormxcorr2processingr12

Why do I get unexpected results when using the NORMXCORR2 function?
For example, I can create a pair of images of a black cross on a light background, and NORMXCORR2 will detect the shift between the two images in the first case (cross is 18 pixels wide). But when I widen the cross (to 38 pixels) NORMXCORR2 indicates that there has not been a shift
between the two images. It is clear that there is a shift from a visual examination of the images. Is there a bug?

Best Answer

The function NORMXCORR2 depends on the template being smaller than the image for the normalization to work properly. Here is a test function that shows how NORMXCORR2 can be used to recover an offset. Note that n must be larger than n_template.
%%%%%%%%%%%%%%%%%%%%%%%%%%%
function [corr_offset] = test_plus(n_template,n,coffset,roffset)
%
%  n               = the number of pixels per side of the image
%  n_template = the number of pixels per side in the template
%  coffset      = offset in columns
%  roffset       = offset in rows
if (n<n_template)
      error('n must be greater than n_template.')
end
% Make a template image containing a plus
template = .6*ones(n_template);
center = floor((n_template - 1)/2);
half_width =  floor(.1*n_template);
cmin = center - half_width;
cmax = center + half_width;
template(1:n_template,cmin:cmax) = .2;
template(cmin:cmax,1:n_template) = .2;
imshow(template)
template_size = size(template);
% Make an n-by-n image containing the template shifted by roffset rows
and coffset columns.
J = .6*ones(n);
rbegin = 1 + roffset;
rend = rbegin + template_size(1) - 1;
cbegin = 1 + coffset;
cend = cbegin + template_size(2) - 1;
J(rbegin:rend,cbegin:cend) = template;
figure;
imshow(J)
%cross-correlate the images
cc = normxcorr2(template,J);
%find the correlation peak coordinates
[max_cc, imax] = max(abs(cc(:)));
[ypeak, xpeak] = ind2sub(size(cc),imax(1));
corr_offset = [ (xpeak-template_size(2)) (ypeak-template_size(1)) ];
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