首先看一下Rect对象的定义：
typedef Rect_ Rect;
再看Rect_的定义：
/*! The 2D up-right rectangle class The class represents a 2D rectangle with coordinates of the specified data type. Normally, cv::Rect ~ cv::Rect_ is used.*/templateclass Rect_{public: typedef _Tp value_type; //! various constructors Rect_(); Rect_(_Tp _x, _Tp _y, _Tp _width, _Tp _height); Rect_(const Rect_& r); Rect_(const CvRect& r); Rect_(const Point_<_Tp>& org, const Size_<_Tp>& sz); Rect_(const Point_<_Tp>& pt1, const Point_<_Tp>& pt2); Rect_& operator = ( const Rect_& r ); //! the top-left corner Point_<_Tp> tl() const; //! the bottom-right corner Point_<_Tp> br() const; //! size (width, height) of the rectangle Size_<_Tp> size() const; //! area (width*height) of the rectangle _Tp area() const; //! conversion to another data type template operator Rect_<_Tp2>() const; //! conversion to the old-style CvRect operator CvRect() const; //! checks whether the rectangle contains the point bool contains(const Point_<_Tp>& pt) const; _Tp x, y, width, height; //< the top-left corner, as well as width and height of the rectangle};
从上面的定义至少可以发现两点：一，类Rect_的类模板中的数据类型_Tp在Rect_<int>中被指定为整型；二，从Rect_的构造函数可以看出，其形参列表一共有6种形式：
Rect_()，形参列表为空，即定义一个空窗口（默认值为：x=y=width=height=0）；Rect_(_Tp _x, _Tp _y, _Tp _width, _Tp _height)，定义一个左上角点坐标为(_x, _y)的_width*_height矩形窗口；Rect_(const Rect_& r)，使用其他的Rect_对象初始化；Rect_(const CvRect& r)，使用CvRect对象初始化；Rect_(const Point_<_Tp>& org, const Size_<_Tp>& sz)，分别将位置坐标(_x, _y)和窗口大小(_width, _height)用Point_和Size_对象初始化；Rect_(const Point_<_Tp>& pt1, const Point_<_Tp>& pt2)，分别将坐标位置(_x, _y)和窗口大小(_width, _height)用Point_和Point_对象初始化。
在OpenCV库中，图像像素坐标与所在行列数的对应关系为：
x -> col, y -> row, width -> cols, height -> rows
下面给出一段代码，基本可以把Rect的常见用法涵盖：
Mat image = imread("C:\\Users\\Leo\\Desktop\\lena.jpg");Rect rect1(256, 256, 128, 128);Rect rect2(224, 224, 128, 128);Mat roi1;image(rect1).copyTo(roi1); // copy the region rect1 from the image to roi1imshow("1", roi1);waitKey(0);Mat roi2;image(rect2).copyTo(roi2); // copy the region rect2 from the image to roi2imshow("2", roi2);waitKey(0);cv::Rect rect3 = rect1&rect2; // intersection of the two setsMat roi3;image(rect3).copyTo(roi3);imshow("3", roi3);waitKey(0);Rect rect4 = rect1|rect2; // union of the two sets (the minimum bounding rectangle)Mat roi4;image(rect4).copyTo(roi4);imshow("4", roi4);waitKey(0);Rect rect5(10, 10, 128, 128);roi1.copyTo(image(rect5)); // copy the region rect1 to the designated region in the imageimshow("5", image);waitKey(0); 结果为：
相应代码，可以在Github账户中下载：。