title: Detecting what percent of an drawn image has been "erased"
body: <p>Let's say I have a solid, irregularly shaped (but enclosed) shape on screen in iOS (one colour). I then want to "erase" portions of that shape by dragging my finger around like you would in a typical kids colouring app, erasing with a fixed brush size where I touch the screen.</p>
  
  <p>I could easily accomplish all this with something like an image mask and touch detection however, as a requirement, I also need to determine the rough percentage of the shape that remains.</p>
  
  <p>For example I need to know when 50% of the random enclosed shape has been "erased".</p>
  
  <p>What's the best way of approaching this problem? Are there any existing iOS compatible libraries that can handle it? I'm thinking that I would need to keep track of a ton of polygons and calculate all the overlaps but it seems like there must be a solution to this problem.</p>
  
  <p>EDIT: I have done research into this problem however tracking all the polygons manually and calculating all their positions and area overlaps seems overly complicated. I was simply wondering if anyone else has run into a similar issue and found a better solution. </p>