title: How do I clip or change alpha of an image (pixels) in Quartz?
body: <p>I'm working on making an iPhone App where there are two ImageViews and when you touch the top one, wherever you tapped, the bottom one shows instead.</p>
  
  <p>Basically what I want to do is cut an ellipse/roundedrect out of an image. To do this I was thinking on either clipping the image, or changing the alpha pixels in the rect to zero. I am new to Quartz 2D Programming so I am not sure how to do this.</p>
  
  <p>Assuming I have:</p>
  
  <pre><code>UIImageView *topImage;
  UIImageView *bottomImage;
  </code></pre>
  
  <p>How do I delete a CGRect/Ellipse/RoundedRect from these images.
  This is kind of like those lottery tickets that you have to scratch off to reveal if you won.</p>