title: How to detect edge of the document using GPUImage
body: <p>I am creating an app that detect the edge of document using the <code>Open-CV framework</code> but result is not great.</p>
  
  <p>Than i came to this project <code>GPUImage</code>. I am don't know anything about this project can anyone suggest me how to detect edge using <code>GPUImage</code>. Code or tutorial will be really helpful.</p>