title: Android canvas: draw transparent circle on image
body: <p>I am creating a pixel-hunting game. So my activity shows an ImageView. And I want to create a hint "show me where is the object". For this I need to blur whole image except a circle around a point where the object is located. Instead of blur I can show a just semitransparent black background.
  There is there is no problem to draw a semitransparent rectangle on the Canvas.
  But I don't know how to crop a transparent circle from it. 
  The result should look like like this:
  <img src="https://i.stack.imgur.com/tcpdJ.jpg" alt="enter image description here"></p>
  
  <p>Please help me to achieve same result on Android SDK.</p>