MATLAB-SSIM calculating for images

Question

I am searching a aulgorithm to find the similarity between two images, and I found SSIM, and even the codes like:

function [mssim, ssim_map]  = SSIM(img1, img2, K, window, L)
if (size(img1) ~= size(img2))
   ssim_index = -Inf;
   ssim_map = -Inf;
   return;
end

[M N] = size(img1);

if (nargin == 2)
   if ((M < 11) || (N < 11))
       ssim_index = -Inf;
       ssim_map = -Inf;
      return
   end
   window = fspecial('gaussian', 11, 1.5);  %
   K(1) = 0.01;                                   % default settings
   K(2) = 0.03;                                   %
   L = 255;                                  %
end

C1 = (K(1)*L)^2;
C2 = (K(2)*L)^2;
window = window/sum(sum(window));
img1 = double(img1);
img2 = double(img2);

mu1   = filter2(window, img1, 'valid');
mu2   = filter2(window, img2, 'valid');
mu1_sq = mu1.*mu1;
mu2_sq = mu2.*mu2;
mu1_mu2 = mu1.*mu2;
sigma1_sq = filter2(window, img1.*img1, 'valid') - mu1_sq;
sigma2_sq = filter2(window, img2.*img2, 'valid') - mu2_sq;
sigma12 = filter2(window, img1.*img2, 'valid') - mu1_mu2;

if (C1 > 0 && C2 > 0)
   ssim_map = ((2*mu1_mu2 + C1).*(2*sigma12 + C2))./((mu1_sq + mu2_sq + C1).*(sigma1_sq + sigma2_sq + C2));
else
   numerator1 = 2*mu1_mu2 + C1;
   numerator2 = 2*sigma12 + C2;
    denominator1 = mu1_sq + mu2_sq + C1;
   denominator2 = sigma1_sq + sigma2_sq + C2;
   ssim_map = ones(size(mu1));
   index = (denominator1.*denominator2 > 0);
   ssim_map(index) = (numerator1(index).*numerator2(index))./(denominator1(index).*denominator2(index));
   index = (denominator1 ~= 0) & (denominator2 == 0);
   ssim_map(index) = numerator1(index)./denominator1(index);
end

mssim = mean2(ssim_map);
return

img1=imread (image1);
img2=imread (image2);
[mssim ssim_map] = SSIM(img1, img2);

I can get some values from this source code, but may I know whether this method is posible for rotation situations, such as one picture I rotate to a certain degree and does this method will detect actually the rotated image and original image have the same shape?

Thanks very much, please help me!

score 2 · Accepted Answer · answered Sep 15 '13 at 08:07

2

SSIM is not rotation invariant. That is, if ImgA is a rotated version of ImgB the SSIM( ImgA, ImgB ) is not likely to be high.
So, if you want to detect the relative rotation angle between ImgA and ImgB you would have to rotate ImgA through all possible angles and compare the rotated version to ImgB.
This is not a very efficient method and you might find other methods that are more efficient for detecting rotation.

If I recall correctly, you are dealing mostly with binary masks of closed curves. I believe a better choise for rotation detection in your case would be to use rotation invariane version of shape-context descriptors combined in some robust rigid transformation estimation method (like Ransac).

answered Sep 15 '13 at 08:07

Shai

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Thanks very much, I will do research about that, by the way, for SSIM, does it can compare the two images with same shape of pattern but the patterns are in different sizes (one very small, one very large, but the same are the same), can SSIM return the correct similarity value of that situation? Thanks a lot – user2753594 Sep 15 '13 at 12:39
@user2753594 AFAIK ssim is designed to compare noisy/blured versions of image, it is not designed to overcome geometrical transformation such as translation scale and/or rotation. You might want to use a different similarity measure for these types of deformations. – Shai Sep 15 '13 at 13:00

MATLAB-SSIM calculating for images

1 Answers1