title: Luma Key (create alpha mask from image) for iOS
body: <p>I'm building an app that allows people to upload an image of themselves against a white background and the app will create a silhouette of the person. </p>
  
  <p>I'm having a hard time keying out the background. I am using the <code>GPUImage</code> framework and the <code>GPUImageChromaKeyBlendFilter</code> works great for colors, but if you get to white/black it is really hard to key out one of those colors. If I set the key to white or black it keys both the same. </p>
  
  <p>Any advice?</p>