Mat_<uchar> im2=img1; im2(i,j)=255;

    Mat img1(200,500,CV_8U,cv::Scalar(0));
    for(int k=0;k<3000;k++)
    {
        int i=rand() % img1.rows;
        int j=rand() % img1.cols;
        //img1.at<uchar>(i,j)=255;
        Mat_<uchar> im2=img1;
        im2(i,j)=255;

    }