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Assuming both image1 and image2 are the same size:

float correlation(cv::Mat &image1, cv::Mat &image2) 
{
    cv::Mat corr;
    cv::matchTemplate(image1, image2, corr, cv::TM_CCORR_NORMED);
    return corr.at<float>(0,0);  // corr only has one pixel
}