读取图片，并通过鼠标交互，在图片上选择一块矩形区域，然后把这个矩形区域的子图像显示出来

// ROI.cpp : 定义控制台应用程序的入口点。
//

#include "stdafx.h"
#include "cv.h"
#include "cvaux.h"
#include "highgui.h"

IplImage* srcImg;
CvRect ROI_rect;
bool flag = false;//true表示鼠标按下状态
CvScalar color = CV_RGB(255, 0, 0);
void on_mouse(int event, int x, int y, int flags, void* param)
{
switch (event)
{
case CV_EVENT_LBUTTONDOWN:
ROI_rect.x = x;
ROI_rect.y = y;
cvDestroyWindow("wnd2");
flag = true;
break;
case CV_EVENT_MOUSEMOVE:
if (flag){
ROI_rect.width = abs(ROI_rect.x - x);
ROI_rect.height = abs(ROI_rect.y - y);
IplImage* temp = cvCloneImage(srcImg);
cvRectangle(temp, cvPoint(ROI_rect.x, ROI_rect.y), cvPoint(x, y), color);
cvShowImage("wnd1", temp);
cvReleaseImage(&temp);
}
break;
case CV_EVENT_LBUTTONUP:
if (ROI_rect.x != x && ROI_rect.y != y){
int xx = MIN(ROI_rect.x, x);
int yy = MIN(ROI_rect.y, y);
cvSetImageROI(srcImg, cvRect(xx, yy, ROI_rect.width, ROI_rect.height));
cvNamedWindow("wnd2", 1);
cvShowImage("wnd2", srcImg);
cvResetImageROI(srcImg);
}

flag = false;
break;
default:
break;
}
}
int _tmain(int argc, _TCHAR* argv[])
{
srcImg = cvLoadImage("src.bmp", -1);
cvNamedWindow("wnd1",1);
cvShowImage("wnd1", srcImg);
cvSetMouseCallback("wnd1", on_mouse,0);
cvWaitKey(0);
cvReleaseImage(&srcImg);
cvDestroyWindow("wnd1");
return 0;
}