Why do I get unexpected results when using the NORMXCORR2 function?

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?

Answer 1

[MathWorks Support Team]

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)) ];