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@uraqtlee You can do like this,

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

using namespace cv;
using namespace std;

int main()
{

VideoCapture cam(0); //change index accordingly
if(!cam.isOpened())
{
    cout<<"error camera failed to open ! "<< endl;
    return -1;
}   

while(1)
{
    Mat img, img_roi, mask;
    bool success = cam.read(img);

    if(!success)
    {
        cout<<"error cannot read image ! "<<endl;
        return -1;
    }

    img.copyTo(mask);

    int width = img.cols;
    int height = img.rows;

    Rect roi(width/2 - 100, height/2 - 100, 200, 200); // set roi for rectangle according to your choice

    rectangle(img, roi, Scalar(255,0,0), 2, 8, 0); // draw rectangle using above roi
    img_roi = mask(roi); 

    imshow("img", img); // showing orignal image with rectangle
    imshow("img_roi", img_roi); // showing image in the rectangle

    //now process img_roi image for further text detection processing.

    if (waitKey(3) == 27)
        break;

}
}