Scalar getMSSIM(const Mat& i1, const Mat& i2)
{
    const double C1 = 6.5025, C2 = 58.5225;
    int d = CV_32F;

    Mat I1, I2;
    i1.convertTo(I1, d);
    i2.convertTo(I2, d);

    Mat I2_2 = I2.mul(I2);
    Mat I1_2 = I1.mul(I1);
    Mat I1_I2 = I1.mul(I2);


    Mat mu1, mu2;
    GaussianBlur(I1, mu1, Size(11, 11), 1.5);
    GaussianBlur(I2, mu2, Size(11, 11), 1.5);

    Mat mu1_2 = mu1.mul(mu1);
    Mat mu2_2 = mu2.mul(mu2);
    Mat mu1_mu2 = mu1.mul(mu2);

    Mat sigma1_2, sigma2_2, sigma12;

    GaussianBlur(I1_2, sigma1_2, Size(11, 11), 1.5);
    sigma1_2 -= mu1_2;

    GaussianBlur(I2_2, sigma2_2, Size(11, 11), 1.5);
    sigma2_2 -= mu2_2;

    GaussianBlur(I1_I2, sigma12, Size(11, 11), 1.5);
    sigma12 -= mu1_mu2;

    Mat t1, t2, t3;

    t1 = 2 * mu1_mu2 + C1;
    t2 = 2 * sigma12 + C2;
    t3 = t1.mul(t2);

    t1 = mu1_2 + mu2_2 + C1;
    t2 = sigma1_2 + sigma2_2 + C2;
    t1 = t1.mul(t2);

    Mat ssim_map;
    divide(t3, t1, ssim_map);

    Scalar mssim = mean(ssim_map);
    return mssim;
}