[OpenCV] gradient

std::pair<cv::Mat, cv::Mat> gradient(cv::Mat & img, float spaceX, float spaceY)
{

    cv::Mat gradY = gradientY(img, spaceY);
    cv::Mat gradX = gradientX(img, spaceX);
    std::pair<cv::Mat, cv::Mat> retValue(gradX, gradY);
    return retValue;
}

static cv::Mat gradientX(cv::Mat & mat, float spacing)
{
    cv::Mat grad = cv::Mat::zeros(mat.rows, mat.cols, CV_32F);

    /*  last row */
    int maxCols = mat.cols;
    int maxRows = mat.rows;

    /* get gradients in each border */
    /* first row */
    cv::Mat col = (-mat.col(0) + mat.col(1)) / (float)spacing;
    col.copyTo(grad(cv::Rect(0, 0, 1, maxRows)));

    col = (-mat.col(maxCols - 2) + mat.col(maxCols - 1)) / (float)spacing;
    col.copyTo(grad(cv::Rect(maxCols - 1, 0, 1, maxRows)));

    /* centered elements */
    cv::Mat centeredMat = mat(cv::Rect(0, 0, maxCols - 2, maxRows));
    cv::Mat offsetMat = mat(cv::Rect(2, 0, maxCols - 2, maxRows));
    cv::Mat resultCenteredMat = (-centeredMat + offsetMat) / (((float)spacing)*2.0);

    resultCenteredMat.copyTo(grad(cv::Rect(1, 0, maxCols - 2, maxRows)));
    return grad;
}

static cv::Mat gradientY(cv::Mat & mat, float spacing)
{
    cv::Mat grad = cv::Mat::zeros(mat.rows, mat.cols, CV_32F);

    /*  last row */
    const int maxWidth = mat.cols;
    const int maxHeight = mat.rows;

    /* get gradients in each border */
    /* first row */
    cv::Mat row = (-mat.row(0) + mat.row(1)) / (float)spacing;
    row.copyTo(grad(cv::Rect(0, 0, maxWidth, 1)));

    row = (-mat.row(maxHeight - 2) + mat.row(maxHeight - 1)) / (float)spacing;
    row.copyTo(grad(cv::Rect(0, maxHeight - 1, maxWidth, 1)));

    /* centered elements */
    cv::Mat centeredMat = mat(cv::Rect(0, 0, maxWidth, maxHeight - 2));
    cv::Mat offsetMat = mat(cv::Rect(0, 2, maxWidth, maxHeight - 2));
    cv::Mat resultCenteredMat = (-centeredMat + offsetMat) / (((float)spacing)*2.0);

    resultCenteredMat.copyTo(grad(cv::Rect(0, 1, maxWidth, maxHeight - 2)));
    return grad;
}

int main(void)
{
    cv::Mat imgMat = cv::imread("sample.jpg", CV_LOAD_IMAGE_COLOR);
    cv::Mat imgMat_bw = cv::Mat::zeros(imgMat.rows, imgMat.cols, CV_32FC(1));
    cv::cvtColor(imgMat, imgMat_bw, cv::COLOR_RGB2GRAY);
    std::pair<cv::Mat, cv::Mat> grad;
    grad = gradient(imgMat_bw, 1.0f, 1.0f);
    cv::Mat_<float> dX = grad.first;
    cv::Mat_<float> dY = grad.second;
    return 0;
}