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Make a copy at each iteration of your image and draw on that copy.

cv::Mat orig, result;
orig=imread("image.jpg");
while(1) {
    orig.copyTo(result); //make a new copy of the original image
    Rect detection=processimage(orig); //here you put the processing that returns some result.
    cv::rectangle(result,detection,cv::Scalar(0,0,0)); //draw the result to the copy of the image
    cv::imshow("Result",result); // display it
    cv::waitKey();
}