You can refer this code I written for cropping image, re-write as your requirement
#include <iostream>
#include "opencv2/opencv.hpp"
#include <stdio.h>
using namespace std;
using namespace cv;
Mat src,img,ROI;
Rect cropRect(0,0,0,0);
Point P1(0,0);
Point P2(0,0);
const char* winName="Crop Image";
bool clicked=false;
int i=0;
char imgName[15];
void checkBoundary(){
//check croping rectangle exceed image boundary
if(cropRect.width>img.cols-cropRect.x)
cropRect.width=img.cols-cropRect.x;
if(cropRect.height>img.rows-cropRect.y)
cropRect.height=img.rows-cropRect.y;
if(cropRect.x<0)
cropRect.x=0;
if(cropRect.y<0)
cropRect.height=0;
}
void showImage(){
img=src.clone();
checkBoundary();
if(cropRect.width>0&&cropRect.height>0){
ROI=src(cropRect);
imshow("cropped",ROI);
}
rectangle(img, cropRect, Scalar(0,255,0), 1, 8, 0 );
imshow(winName,img);
}
void onMouse( int event, int x, int y, int f, void* ){
switch(event){
case CV_EVENT_LBUTTONDOWN :
clicked=true;
P1.x=x;
P1.y=y;
P2.x=x;
P2.y=y;
break;
case CV_EVENT_LBUTTONUP :
P2.x=x;
P2.y=y;
clicked=false;
break;
case CV_EVENT_MOUSEMOVE :
if(clicked){
P2.x=x;
P2.y=y;
}
break;
default : break;
}
if(clicked){
if(P1.x>P2.x){ cropRect.x=P2.x;
cropRect.width=P1.x-P2.x; }
else { cropRect.x=P1.x;
cropRect.width=P2.x-P1.x; }
if(P1.y>P2.y){ cropRect.y=P2.y;
cropRect.height=P1.y-P2.y; }
else { cropRect.y=P1.y;
cropRect.height=P2.y-P1.y; }
}
showImage();
}
int main()
{
cout<<"Click and drag for Selection"<<endl<<endl;
cout<<"------> Press 's' to save"<<endl<<endl;
cout<<"------> Press '8' to move up"<<endl;
cout<<"------> Press '2' to move down"<<endl;
cout<<"------> Press '6' to move right"<<endl;
cout<<"------> Press '4' to move left"<<endl<<endl;
cout<<"------> Press 'w' increas top"<<endl;
cout<<"------> Press 'x' increas bottom"<<endl;
cout<<"------> Press 'd' increas right"<<endl;
cout<<"------> Press 'a' increas left"<<endl<<endl;
cout<<"------> Press 't' decrease top"<<endl;
cout<<"------> Press 'b' decrease bottom"<<endl;
cout<<"------> Press 'h' decrease right"<<endl;
cout<<"------> Press 'f' decrease left"<<endl<<endl;
cout<<"------> Press 'r' to reset"<<endl;
cout<<"------> Press 'Esc' to quit"<<endl<<endl;
src=imread("src.png",1);
namedWindow(winName,WINDOW_NORMAL);
setMouseCallback(winName,onMouse,NULL );
imshow(winName,src);
while(1){
char c=waitKey();
if(c=='s'&&ROI.data){
sprintf(imgName,"%d.jpg",i++);
imwrite(imgName,ROI);
cout<<" Saved "<<imgName<<endl;
}
if(c=='6') cropRect.x++;
if(c=='4') cropRect.x--;
if(c=='8') cropRect.y--;
if(c=='2') cropRect.y++;
if(c=='w') { cropRect.y--; cropRect.height++;}
if(c=='d') cropRect.width++;
if(c=='x') cropRect.height++;
if(c=='a') { cropRect.x--; cropRect.width++;}
if(c=='t') { cropRect.y++; cropRect.height--;}
if(c=='h') cropRect.width--;
if(c=='b') cropRect.height--;
if(c=='f') { cropRect.x++; cropRect.width--;}
if(c==27) break;
if(c=='r') {cropRect.x=0;cropRect.y=0;cropRect.width=0;cropRect.height=0;}
showImage();
}
return 0;
}