title: Determine if an image is of "blank" paper
body: <p>We are making a tool that will allow a chain of photos to be taken. Mixed in with real photos will be photos of blank sheets of paper.  I want to separate the series of photos by identifying the images of blank pages.</p>
  
  <p>I'm trying to find a way to identify the blank sheet. Either by counting colors, or some other method. Maybe filesize?</p>
  
  <p>I've got GraphicsMagick, so maybe there's something useful there, and the code will be in PHP, but could be in anything if it works well.</p>