opencv c++访问某一区域

int main(){
    cv::Mat m1(5,5, CV_8UC1);
    for(int i=0;i<m1.rows;++i)
        for(int j=0;j<m1.cols;++j)
            m1.at<uchar>(i,j)=i*2+j*2+1;  // uchar与8U匹配。
    std::cout<<m1<<std::endl;
    std::cout<<std::endl;

    cv::Mat roi1=m1(cv::Rect(cv::Point(2,1),cv::Point(3,2)));  // Rect_(const Point_<_Tp>& pt1, const Point_<_Tp>& pt2);
    std::cout<<roi1<<std::endl;
    std::cout<<std::endl;

    cv::Mat roi2=m1(cv::Rect(2,1,1,1));  // Rect_(_Tp _x, _Tp _y, _Tp _width, _Tp _height);
    std::cout<<roi2<<std::endl;
    std::cout<<std::endl;

    cv::Mat roi3=m1(cv::Rect(cv::Point(2,1),cv::Size(1,1)));  // Rect_(const Point_<_Tp>& pt1, const Point_<_Tp>& pt2)
    std::cout<<roi3<<std::endl;
    std::cout<<std::endl;
    return 0;
}