title: How to mask a UIImage so that white becomes transparent on iphone?
body: <p>I have a UIImage (generated by a user drawing) that has a white background. I'd like to make the white portions of this image transparent. I'd also like to save this to a PNG file. </p>
  
  <p>I've looked around on forums but can't figure out how to mask the image correctly. Has anyone tried this before?</p>
  
  <p>Thanks.</p>