How to do the image detection with opencv?

double Detector::getColorDiff(const cv::Mat& current_image, const cv::Mat& target_image) {
    auto current_image_color_means = mean(current_image);
    auto target_image_color_means = mean(target_image);

    double diff = 0;
    for (int i = 0; i < 3; i++) {
        diff += abs(current_image_color_means.val[i] - target_image_color_means.val[i]);
    }
    return (diff * 100) / (255 * 3);
}

If 0 <= diff <= 20, we’ll consider it matchs. the smaller the similar. 

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