title: CIAreaHistogram + CIHistogramDisplayFilter to get Luminance ONLY Histogram
body: <p>I believe this could be relative to Core Image in iOS as well as in Mac OS.</p>
  
  <p>I am able to get a RGB Histogram to show up using CIAreaHistogram + CIHistogramDisplayFilter in Core Image.  Is there a way to get just LUMINANCE instead of RGB separately?</p>