title: iPhone programmatically crop a square image to appear as circle
body: <p>I'm trying to create an image for a custom style UIButton using an image from the camera roll on iPhone. The button has a circular background and effectively appears as a circle. Now I need an image to go in the middle of the button that also appears round. </p>
  
  <p>How do I cut a square UIImage to appear round with transparency outside of the round area?</p>
  
  <p>If masking is involved, do I need to pre-render a mask or can I create one programmatically(ex: a circle)?</p>
  
  <p>Thank you!</p>