Create a mask for different values
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I want to subtract two images and show the results in another figure. For instance, if the subtraction matrix has 0 value indicate the elements with black, if it has negative value show them with red and if it has positive value show with blue. How can I do that?
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%sample data
firstImage = imread('cameraman.tif');
secondImage = imresize( rgb2gray( imread('flamingos.jpg') ), size(firstImage));
%assuming grayscale images
imgdiff = double(firstImage) - double(secondImage);
n0p = sign(imgdiff); %-1 for negative, 0 for equal, 1 for positive
%add 2 to n0p to get a 1-based index
cmap = [1 0 0; 0 0 0; 0 0 1]; %red, black, blue
diffImg = reshape(cmap(n0p+2,:), size(imgdiff,1), size(imgdiff,2), 3);
%show it off
subplot(3,1,1)
imshow(firstImage);
title('cameraman')
subplot(3,1,2)
imshow(secondImage);
title('flamingos')
subplot(3,1,3)
imshow(diffImg);
title('color difference image')
4 个评论
ali eskandari
2021-2-24
Walter Roberson
2021-2-24
Notice the
n0p = sign(imgdiff); %-1 for negative, 0 for equal, 1 for positive
As indicated in the comment, sign() is -1 when the value is negative, 0 when the value is 0, +1 when the value is positive -- which correspond to the three categories you asked for, as you asked for different colors for negative (red), equal (black) or positive (blue)
So you have an array of values that are all -1, 0, or +1, and you need to turn that into a colormap index. colormap indices start at +1, so the easiest way is to add 2 to the [-1, 0, +1] getting [+1, +2, +3] indices into the color map, which you then use to extract rows from the color map.
ali eskandari
2021-2-24
%sample data
firstImage = imread('cameraman.tif');
secondImage = imresize( rgb2gray( imread('flamingos.jpg') ), size(firstImage));
%assuming grayscale images
imgdiff = double(firstImage) - double(secondImage);
n0p = sign(imgdiff); %-1 for negative, 0 for equal, 1 for positive
%add 2 to n0p to get a 1-based index
cmap = [1 0 0; 0 0 0; 0 0 1]; %red, black, blue
diffImg = reshape(cmap(n0p+2,:), size(imgdiff,1), size(imgdiff,2), 3);
%show it off
subplot(3,1,1)
imshow(firstImage);
title('cameraman')
subplot(3,1,2)
imshow(secondImage);
title('flamingos')
subplot(3,1,3)
imshow(diffImg);
title('color difference image')
hold on
names = {'negative', 'equal', 'positive'};
for K = 1 : 3
L(K) = plot(nan,nan,'s', 'markeredgecolor', cmap(K,:), 'markerfacecolor', cmap(K,:));
end
hold off
legend(L, names, 'location', 'eastoutside')
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