/**
 * create_ideal_bandpass_filter -   create a 2-channel ideal band-pass filter
 *      with radius D, order n (assumes pre-aollocated size of dft_Filter specifies dimensions)
 *
 * @param dft_Filter    -   destinate filter
 * @param D             -   radius
 * @param W             -   band width
 */
void create_ideal_bandpass_filter(cv::Mat &dft_Filter, int D, int W)
{
    cv::Mat tmp = cv::Mat(dft_Filter.rows, dft_Filter.cols, CV_32F);

    Point centre = Point(dft_Filter.rows / 2, dft_Filter.cols / 2);
    double radius;

    for (int i = 0; i < dft_Filter.rows; i++)
    {
        for (int j = 0; j < dft_Filter.cols; j++)
        {
            // D(u, v)
            radius = (double) sqrt(pow((i - centre.x), 2.0) + pow((double) (j - centre.y), 2.0));

            if (radius >= D - W/2 && radius <= D + W/2)
                tmp.at<float>(i, j) = 1;
            else
                tmp.at<float>(i, j) = 0;
        }
    }

    cv::Mat toMerge[] = {tmp, tmp};
    merge(toMerge, 2, dft_Filter);
}