void test_roi()
{
    IplImage *img = cvLoadImage("opencv-logo.png");

    int x = 100;
    int y = 100;
    int width = 200;
    int height = 200;
    int add_val = 200;
    cvSetImageROI(img, cvRect(x, y, width, height)); 
    cvAddS(img, cvScalar(add_val), img);
    cvResetImageROI(img);

    cvNamedWindow("Roi_Add", 1);
    cvShowImage("Roi_add", img);

    cvWaitKey();
}