title: How do I implement a soft eraser stroke in a CGBitmapContext
body: <p>I am trying to erase an image with my touch in iOS.  By setting the blend mode to kCGBlendModeClear I am able to do this - but only with hard edges.  I could draw my stroke with varying line widths and alphas, but it appears that CGContextSetAlpha does not have an effect with kCGBlendModeClear.  </p>
  
  <p>How do I go about this?</p>