title: only detect in a section of camera preview layer, iOS, Swift
body: <p>I am trying to get a detection zone in a live preview on my camera preview layer.</p>
  
  <p>Is it possible for this, say there is a live feed and you have face detect on and as you look around it will only put a box around the face in a certain area for example a rectangle in the centre of the screen. all other faces in the preview that are outside of the rectangle don't get detected?</p>
  
  <p>Im using Vision, iOS, Swift.</p>