title: Using Objective-C, is there any way to compare two images and get a % difference value returned?
body: <p>Using Objective-C, is there any way to compare two images and get a % difference value returned? </p>
  
  <p>For example, one image will have and X in it. Another image will have an X in it. And a third will have a H in it. All the same font/color.</p>
  
  <p>Obviously the X and the X will have a % difference that is very small. The % between the X and the H would be very big.</p>
  
  <p>How would I write a function that can return me the % difference?</p>