Mat img= imread（image）；
Rect rect(50,20, 200, 50);
Mat ROI = img(rect);
imshow("ROI_WIN",ROI);

#include <opencv2/highgui/highgui.hpp>
#include <opencv2/core/core.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <iostream>

using namespace cv;
using namespace std;

cv::Mat img;
bool select_flag = false;
cv::Rect m_select; 
cv::Point origin;
int ROI_count;

void onMouseRectPicking(int event, int x, int y, int, void*)
{
 if (select_flag)
 {
 m_select.x = MIN(origin.x, x);//不一定要等鼠標(biāo)彈起才計算矩形框，而應(yīng)該在鼠標(biāo)按下開始到彈起這段時間實時計算所選矩形框
 m_select.y = MIN(origin.y, y);
 m_select.width = abs(x - origin.x);//算矩形寬度和高度
 m_select.height = abs(y - origin.y);
 m_select &= cv::Rect(0, 0, img.cols, img.rows);//保證所選矩形框在視頻顯示區(qū)域之內(nèi)
 }
 if (event == CV_EVENT_LBUTTONDOWN)
 {
 select_flag = true;  //鼠標(biāo)按下的標(biāo)志賦真值
 origin = cv::Point(x, y); //保存下來單擊捕捉到的點(diǎn)
 m_select = cv::Rect(x, y, 0, 0); //這里一定要初始化，寬和高為(0,0)是因為在opencv中Rect矩形框類內(nèi)的點(diǎn)是包含左上角那個點(diǎn)的，但是不含右下角那個點(diǎn) 
 }
 else if (event == CV_EVENT_LBUTTONUP)
 {
 select_flag = false;
 ROI_count++;
 }
}

int main(int argc, char* argv[] )
{

 img=imread("/home/drew/picture_cut/build/scenary.jpg");
 bool stop = false;

 cv::namedWindow("capframe", CV_WINDOW_AUTOSIZE);
 cv::setMouseCallback("capframe", onMouseRectPicking, 0);

 char pic_name[40];
 ROI_count=0;

 while(!stop)
 {
 img=imread("/home/drew/picture_cut/build/scenary.jpg");
 cv::rectangle(img, m_select, cv::Scalar(255,0,0), 2, 8, 0); // 畫矩形框
 cv::imshow("capframe",img);



 if((m_select.x!=0)&&(m_select.y!=0)&&(m_select.width!=0)&&(m_select.height!=0))
 {
  sprintf(pic_name,"ROI_%d.jpg",ROI_count);
  Mat ROI = img(m_select);
  imshow("ROI_WIN",ROI); 
  imwrite(pic_name,ROI);  
 }


 char key = cv::waitKey(30);
 if(key == 27) 
  stop = true; 
 }
 waitKey(0);
 return 0;
}