# How to increase the resolution of a list of images

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BA on 10 Aug 2022
Commented: Walter Roberson on 10 Aug 2022
I did 'image variance' for a list of images but the region of interest shows black line (see picture 1) instead of white as in picture 2. What seems to be the problem? does it need to increase the resolution or high pass filtering?

Walter Roberson on 10 Aug 2022
You have a series of images that all happen to be bright in certain areas. Because they are all bright there, the variance between them is low. When you display the variance, low variance would show up dark.
Variance images highlight areas that differ a lot between images, not areas that are similar.
Walter Roberson on 10 Aug 2022
img1 = img; img1(1:150, 1:20) = img1(1:150, 1:20) + 1;
img2 = img; img2(1:50, 1:50) = double(img2(1:50, 1:50)) * rand(50,50);
img3 = img; img3(100:150,1:50) = img3(100:150,1:50) * 1.5;
combined = cat(3, img, img1, img2, img3);
varimg = var(double(combined), [], 3);
figure();
subplot(2,2,1); imshow(img); title('original');
subplot(2,2,2); imshow(img1); title('variation 1');
subplot(2,2,3); imshow(img2); title('variation 2');
subplot(2,2,4); imshow(img3); title('variation 3');
figure();
imshow(varimg); title('variance image');
variance image:
Black --> places where the images did not change much
White --> places where the image schanged a lot.
Gray --> places where the images changed a little
Your test2.jpg shows white where there was text, because the text changed between images. Something also changed near (0.8, 0.8) . There was also some small change in the bottom center.
If you were to mask out the text areas before taking the variance images, you would have more emphasis on the change in the bottom center.