OpenCV setroi

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Hello to whomever is reading this. I'm new to opencv and I've only recently installed opencv.

I would like to ask as to how do I go on about in setting the ROI of an image. An example is I have an image of an electric meter and I would just like to get the ROI of the values part of the electric meter. Not the entire electric meter. I would love to post some image as example but I'm lacking in karma as I'm a new user. Sorry about that

I've used this code over here

#include "opencv2/core/core.hpp"
#include "opencv2/highgui/highgui.hpp"
using namespace cv;

int main() 
{
    namedWindow("Example1");
    Mat img = imread("D:/sample.jpg");
    if ( img.empty() ) { /* bark!!*/ }
    Mat roi = img( Rect(170,150,250,100));
    imshow("Example1",roi);
    waitKey(0);
    return 0; // yea, c++, no cleanup nessecary!
}

I've not set the proper rect for the ROI but for some reason I'm always getting an error even before I can test it out. The error has something to do with the + &ThisException 0x0000004532f7f6d0 {ExceptionCode=3765269347 ExceptionFlags=1 ExceptionRecord=0x0000000000000000 {ExceptionCode=...} ...} EHExceptionRecord *

When I use a different image, it isn't an issue.

Bear in mind I'm new to opencv so if the question does sound pretty noobish, sorry.