title: Select a bounding box on an image and annotate
body: <p>I'm working on an project where I would like to take a bounding box already drawn on a subject and select it (via mouse click) so I can have something like a text dialogue box hover above the image, so I can then type in a label.  I'm already using OpenCV to detect the object and draw an initial bounding box on it using the Haar Cascade classifier, but so far I can't find the right combination of OpenCV directives to be able to select that bounding box and then annotate it.  The relevant code is below.</p>
  
  <pre><code>faces = faceCascade.detectMultiScale(
      gray,
      scaleFactor=1.1,
      minNeighbors=5,
      minSize=(30, 30),
  )
  
  # Draw a rectangle around the faces
  for (x, y, w, h) in faces:
      cv2.rectangle(frame, (x, y), (x+w, y+h), (0, 255, 0), 2)
  </code></pre>
  
  <p>Would appreciate some good pointers.  Thanks.</p>