title: Crop a circle area (ROI) of an image and put it onto a white mask
body: <p>I want to crop a circle area of an image (ROI) and put it onto a white mask with same dimensions and positions.</p>
  
  <p><strong>Build a white mask:</strong></p>
  
  <pre><code>h, w = img.shape[:2]
  mask = np.ones((h,w,3), np.uint8)*255
  </code></pre>
  
  <p><strong>The circle area (ROI) in the image (--> this is wrong, because it is only a circle and I want an area with all pixels in the circle of this and copy it in the white mask):</strong></p>
  
  <pre><code>roi = cv2.circle(img, (656,517), 505, (0,0,0), -1)
  </code></pre>
  
  <p>How I can copy all the pixels in the circle of the image onto a mask with same dimensions and the same position of the circle?</p>
  
  <p>The image has the dimensions: 1280x1024.</p>