goodDifference.swift

func differenceFrom(otherRGBA: UIImage.RGBA) -> Int {
   let aDiff = Int(max(alpha, otherRGBA.alpha) - min(alpha, otherRGBA.alpha))
   let rDiff = Int(max(red, otherRGBA.red) - min(red, otherRGBA.red))
   let gDiff = Int(max(green, otherRGBA.green) - min(green, otherRGBA.green))
   let bDiff = Int(max(blue, otherRGBA.green) - min(blue, otherRGBA.blue))
   return aDiff + rDiff + gDiff + bDiff
}