text: 
  #include <opencv2/opencv.hpp>
  
  
  int main()
  {
    cv::Mat im0;
    im0 = cv::Mat(cv::Size(640, 480), CV_8UC1, cv::Scalar(255));
  
    im0.at<uchar>(200, 100) = 0;    // (y, x)
  
    cv::imwrite("z132.png", im0);
  }
  
  