title: GPUImage background subtraction iOS
body: <p>I start capturing the camera with <code>startCameraCapture</code> and using <code>setFrameProcessingCompletionBlock</code> for take a <code>GPUImageOutput</code> (my background image).
  Now I want for each frame to subtract my background image from the other captured frames.</p>
  
  <p>How I can achieve this?
  Thanks</p>