title: Difference between dFdxFine and dFdxCoarse
body: <p>From the OpenGL documentation:</p>
  
  <blockquote>
    <p>dFdxFine and dFdyFine calculate derivatives using local differencing based on on the value of p for the current fragment and its immediate neighbor(s).</p>
    
    <p>dFdxCoarse and dFdyCoarse calculate derivatives using local differencing based on the value of p for the current fragment's neighbors, and will possibly, but not necessarily, include the value for the current fragment. That is, over a given area, the implementation can compute derivatives in fewer unique locations than would be allowed for the corresponding dFdxFine and dFdyFine functions.</p>
  </blockquote>
  
  <p>Which is the difference between them? When should I care?</p>
  
  <p>I understand that both calculates the derivative of the value respect the window coordinates, but I don't understand the method used to computed them.</p>
  
  <p>I guess that they are both implemented in hardware, but could you post a dFdx pseudo-code implementation?</p>