title: How to black out everything outside a circle in Open CV
body: <p>I am currently trying to black out everything outside a circle.
  I am drawing the circle using the following lines of code:</p>
  
  <pre><code>cv::Point center(cvRound(circles[i][0]), cvRound(circles[i][1])); // CVRound converts floating numbers to integer
  int radius = cvRound(circles[i][2]);                              // Radius is the third parameter [i][0] = x [i][1]= y [i][2] = radius
  circle( image, center, 3, cv::Scalar(0,255,0), -1, 8, 0 );        // Drawing little circle to Image Center , next Line of Code draws the real circle
  circle( image, center, radius, cv::Scalar(0,0,255), 3, 8, 0 );    // Circle(img, center, radius, color, thickness=1, lineType=8, shift=0)
  </code></pre>
  
  <p>What is the best approach of painting everything of the circly black, if I have a radius and the center of my circle?
  Does OpenCV provide an easy mechanism of doing this or should I iterate through all the pixels of my image and depending on the position color them black or not?</p>