Register an Image Using Normalized Cross-Correlation

This example shows how to determine the translation needed to align two images by using normalized cross-correlation.

Read Images

Read and display two images. For this example, the images are different sizes.

onion = imread("onion.png");
imshow(onion)

peppers = imread("peppers.png");
imshow(peppers)

Choose Subregions of Each Image

Choose subregions of each image so that the peak of the cross-correlation matrix will be easy to identify. It is important to choose regions that are similar. The image sub_onion will be the template, and must be smaller than the image sub_peppers. You can get these subregions by using these commands.

rect_onion = [111 33 65 58];
rect_peppers = [163 47 143 151];
sub_onion = imcrop(onion,rect_onion);
sub_peppers = imcrop(peppers,rect_peppers);

If you want to select subregions interactively, then you can use these commands.

[sub_onion,rect_onion] = imcrop(onion); % Choose the pepper below the onion
[sub_peppers,rect_peppers] = imcrop(peppers); % Choose the whole onion

Display the subregions.

imshow(sub_onion)

imshow(sub_peppers)

Calculate Normalized Cross-Correlation and Find Coordinates of Peak

Calculate the normalized cross-correlation and display it as a surface plot. The peak of the cross-correlation matrix occurs where the subimages are best correlated. normxcorr2 only works on grayscale images, so pass it the first channel of each subimage.

c = normxcorr2(sub_onion(:,:,1),sub_peppers(:,:,1));
surf(c) 
shading flat

Find the coordinates of the peak.

[max_c,imax] = max(abs(c(:)));
[ypeak,xpeak] = ind2sub(size(c),imax(1));

Find the Total Offset Between the Images

The total offset or translation between images depends on the location of the peak in the cross-correlation matrix, and on the size and position of the subimages.

% Offset found by correlation
corr_offset = [(xpeak-size(sub_onion,2)) 
               (ypeak-size(sub_onion,1))];

% Relative offset of position of subimages
rect_offset = [(rect_peppers(1)-rect_onion(1)) 
               (rect_peppers(2)-rect_onion(2))];

% Total offset
offset = corr_offset + rect_offset;
xoffset = offset(1);
yoffset = offset(2);

Register Images and Verify Results

Figure out where onion falls inside of peppers.

xbegin = round(xoffset + 1);
xend   = round(xoffset + size(onion,2));
ybegin = round(yoffset + 1);
yend   = round(yoffset + size(onion,1));

% Extract region from peppers and compare to onion
extracted_onion = peppers(ybegin:yend,xbegin:xend,:);
if isequal(onion,extracted_onion) 
   disp("onion.png was extracted from peppers.png")
end

onion.png was extracted from peppers.png

Pad the onion image to overlay on peppers, using the offset determined above.

recovered_onion = uint8(zeros(size(peppers)));
recovered_onion(ybegin:yend,xbegin:xend,:) = onion;
imshow(recovered_onion)

Confirm the registration by displaying one plane of the peppers image with the recovered_onion image using alpha blending.

imshowpair(peppers(:,:,1),recovered_onion,"blend")