title: How to draw a transparent stroke (or anyway clear part of an image) on the iPhone
body: <p>I have a small app that allows the user to draw on the screen with the finger.
  I have a <code>UIImageView</code> where the user draws, by creating a <code>CGContextRef</code> and the various <code>CG draw</code> functions. I primarily draw strokes/lines with the function <code>CGContextAddLineToPoint</code></p>
  
  <p>Now my issue is this: 
  The user can draw lines of various colors. I want to give him the ability to use a <code>"rubber"</code> tool to delete some part of the image drawn so far, with the finger. I initially did this by using a white color for the stroke (set with the <code>CGContextSetRGBStrokeColor</code> function) but it didn't work out...because I discovered later that the <code>UIImage</code> on the <code>UIImageView</code> actually had a transparent background, not white...so I would end up with a transparent image with white lines on it!</p>
  
  <p>Is there anyway to set a <code>"transparent"</code> stroke color or is there any other way to clear the content of the <code>CGContextRef</code> under the user's finger, when he moves it?
  Thanks</p>