http://blog.csdn.net/chenli2010/article/details/7899773

#include <cv.h>
#include <highgui.h>

#include<iostream>  

#pragma comment(lib, "cv.lib")
#pragma comment(lib, "cxcore.lib")
#pragma comment(lib, "highgui.lib")

CvPoint pt1 = cvPoint(0,0);  
CvPoint pt2 = cvPoint(0,0);  
bool is_selecting = false
 
;  

// callback function  
void cvMouseCallback(int mouseEvent,int x,int y,int flags,void* param)  
{  
	switch(mouseEvent)  
	{  
	case CV_EVENT_LBUTTONDOWN:  
		pt1 = cvPoint(x,y);  
		pt2 = cvPoint(x,y);  
		is_selecting = true;  
		break;  
	case CV_EVENT_MOUSEMOVE:  
		if(is_selecting)  
			pt2 = cvPoint(x,y);  
		break
 
;  
	case CV_EVENT_LBUTTONUP:  
		pt2 = cvPoint(x,y);  
		is_selecting = false;  
		break;  
	}  
	return;  
}  

int main(int argc,char* argv[])  
{  
	printf("用滑鼠左鍵在影象視窗中畫框，以得到目標區域的大致位置.\
		   \n然後用a,s,d,w移動所選區域，用1，2,3,5放大或縮小之\
		   \n由於opencv中鍵盤響應函式不能響應鍵盤中的方向鍵、shift鍵和ctrl鍵\
		   \n所以用tab來切換狀態（放大和縮小）。\
		   \n選擇結束後用enter鍵儲存。Esc鍵退出。"
 
);

	char img_path[80] = ".\\test.png";  
	char save_path[80] = ".\\save.jpg";  
	char* window = "img";  

	IplImage* img = cvLoadImage(img_path);    
	IplImage* img_show = cvCloneImage(img);  

	cvNamedWindow(window,CV_WINDOW_AUTOSIZE);  
	cvSetMouseCallback(window,cvMouseCallback);  

	bool shift_on = false;  

	char text[80];  
	CvFont font;  
	cvInitFont(&font,CV_FONT_HERSHEY_PLAIN,1.0,1.0);  
	while(true)  
	{  
		cvCopy(img,img_show);  
		cvRectangle(img_show,pt1,pt2,cvScalar(255,255,255));  
		sprintf(text,"roi = cvRect(%d,%d,%d,%d)",pt1.x,pt1.y,std::abs(pt2.x-pt1.x),std::abs(pt2.y-pt1.y));  
		cvPutText(img_show,text,cvPoint(10,20),&font,cvScalar(0,0,255));  

		cvShowImage(window,img_show);  
		char key = cvWaitKey(10);  
		switch(key)  
		{  
		case '\t':  
			shift_on = !shift_on; break;  

		case 'a':  
			pt1.x--; pt2.x--; break;  
		case 's':  
			pt1.y++; pt2.y++; break;  
		case 'd':  
			pt1.x++; pt2.x++; break;  
		case 'w':  
			pt1.y--; pt2.y--; break;  

		case '1':  
			if(shift_on) pt1.x--;   
			else pt2.x--;  
			break;  
		case '2':  
			if(shift_on) pt2.y++;   
			else pt1.y++;  
			break;  
		case '3':  
			if(shift_on) pt2.x++;  
			else pt1.x++;  
			break;  
		case '5':  
			if(shift_on) pt1.y--;  
			else pt2.y--;  
			break;  

		case '\r':  
			cvSetImageROI(img, cvRect(pt1.x,pt1.y,std::abs(pt2.x-pt1.x), std::abs(pt2.y-pt1.y)));  
			cvSaveImage(save_path, img);  
			cvResetImageROI(img);  
			break;  
		};  

		if(key==27) break;  
	}  

	cvReleaseImage(&img);  
	cvReleaseImage(&img_show);  
	return 0;  
}  

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