title: tilt shift image processing effect
body: <p>I'm working on certain image processing project.</p>
  
  <p>I need to implement tilt shift effect like Instagram one. I have already implemented basic effects like sepia, grayscale, various color effects and rotation using android.graphics package (Bitmap, Canvas, Paint).</p>
  
  <p>I have found some code for blurring whole the image, but I do not know how to define circle that will be focused and the complement area that will be blurred.</p>
  
  <p>How can I implement it using android.graphics or some other third party framework?
  Is there some literature about the subject?</p>
  
  <p>Thanks.</p>