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一、知识点
1、void setMouseCallback(const String & winname, MouseCallback onMouse, void * userdata = 0);
  (1)、为指定窗口设置鼠标事件回调函数。
  (2)、参数说明:
      winname: 窗口名称。
      onMouse: 自定义的鼠标事件回调函数，函数类型见2。
      userdata: 传递给鼠标事件回调函数的可选参数。
  (3)、一旦鼠标事件发生，就会自动调用绑定的onMouse函数。
      
2、typedef void (*MouseCallback)(int event, int x, int y, int flags, void * userdata);
  (1)、鼠标事件回调函数的类型。
  (2)、参数说明:
      event: MouseEventTypes宏值。
             enum MouseEventTypes {
                   EVENT_MOUSEMOVE      = 0, //!< indicates that the mouse pointer has moved over the window.
                   EVENT_LBUTTONDOWN    = 1, //!< indicates that the left mouse button is pressed.
                   EVENT_RBUTTONDOWN    = 2, //!< indicates that the right mouse button is pressed.
                   EVENT_MBUTTONDOWN    = 3, //!< indicates that the middle mouse button is pressed.
                   EVENT_LBUTTONUP      = 4, //!< indicates that left mouse button is released.
                   EVENT_RBUTTONUP      = 5, //!< indicates that right mouse button is released.
                   EVENT_MBUTTONUP      = 6, //!< indicates that middle mouse button is released.
                   EVENT_LBUTTONDBLCLK  = 7, //!< indicates that left mouse button is double clicked.
                   EVENT_RBUTTONDBLCLK  = 8, //!< indicates that right mouse button is double clicked.
                   EVENT_MBUTTONDBLCLK  = 9, //!< indicates that middle mouse button is double clicked.
                   EVENT_MOUSEWHEEL     = 10,//!< positive and negative values mean forward and backward scrolling, respectively.
                   EVENT_MOUSEHWHEEL    = 11 //!< positive and negative values mean right and left scrolling, respectively.
             };
      x: 鼠标事件的x坐标。
      y: 鼠标事件的y坐标。
      flags: MouseEventFlags宏值。
             enum MouseEventFlags {
                   EVENT_FLAG_LBUTTON   = 1, //!< indicates that the left mouse button is down.
                   EVENT_FLAG_RBUTTON   = 2, //!< indicates that the right mouse button is down.
                   EVENT_FLAG_MBUTTON   = 4, //!< indicates that the middle mouse button is down.
                   EVENT_FLAG_CTRLKEY   = 8, //!< indicates that CTRL Key is pressed.
                   EVENT_FLAG_SHIFTKEY  = 16,//!< indicates that SHIFT Key is pressed.
                   EVENT_FLAG_ALTKEY    = 32 //!< indicates that ALT Key is pressed.
             };
      userdata: setMouseCallback()传递的可选参数。
      
3、Mat Mat::operator()( const Rect & roi ) const
  (1)、从图像中提取感兴趣区域(ROI)。
  (2)、提取的方式不是复制，而是通过创建指向原图数据的新矩阵指针，对新矩阵的操作会直接影响原图像。
      
      
      
二、示例代码

#include <iostream>
#include <opencv2/opencv.hpp>cv::Point sp(-1, -1);
cv::Point ep(-1, -1);
cv::Mat tempSrc;//鼠标事件处理函数
void onMouseDraw(int event, int x, int y, int flags, void * userdata)
{cv::Mat image = *((cv::Mat*)(userdata));if (event == cv::EVENT_LBUTTONDOWN){sp.x = x;sp.y = y;}else if (event == cv::EVENT_MOUSEMOVE){if (sp.x > 0 && sp.y > 0){ep.x = x;ep.y = y;int dx = ep.x - sp.x;int dy = ep.y - sp.y;if (dx > 0 && dy > 0){//清空图像tempSrc.copyTo(image);cv::Rect box(sp.x, sp.y, dx, dy);cv::rectangle(image, box, cv::Scalar(0, 0, 255), 2, 8, 0);cv::imshow("鼠标绘制ROI", image);cv::imshow("ROI区域", image(box));}}}else if (event == cv::EVENT_LBUTTONUP){//标志着鼠标左键已松开，不会影响鼠标移动sp.x = -1;sp.y = -1;}
}int main()
{//本地读取原图cv::Mat src = cv::imread("../images/9.png");if (src.empty()){std::cout << "load src image error..." << std::endl;return -1;}//全局备份一张原图，达到清空ROI目的tempSrc = src.clone();//创建一个窗口, 名为"鼠标绘制ROI"cv::namedWindow("鼠标绘制ROI", cv::WINDOW_AUTOSIZE);//设置鼠标事件回调函数cv::setMouseCallback("鼠标绘制ROI", onMouseDraw, &src);cv::imshow("鼠标绘制ROI", src);cv::waitKey(0);return 0;
}

  演示结果: