title: Drawing convexHull in OpenCV Python
body: <p>So I am trying to draw the convexHull from a contour in python, however when i print the image it is not changing.</p>
  
  <pre><code>roi=mask[y:y+h,x:x+w]
  roi = cv2.fastNlMeansDenoisingColored(roi,None,15,15,7,21)
  hull = cv2.convexHull(cnt)
  cv2.drawContours(roi,[hull],0,(147,0,255),2)
  cv2.imshow(str(i),roi)
  blank_image[y:y+h,x:x+w] = roi
  </code></pre>
  
  <p>However, the images that show are the exact same if I did not include the code. I looked online, but cannot seem to find the answer.
  Here is a sample Image:<a href="https://i.stack.imgur.com/CfXQ9.png" rel="noreferrer"><img src="https://i.stack.imgur.com/CfXQ9.png" alt="Sample Image"></a></p>