title: Matlab - take gradient of a matrix at any arbitrary point
body: <p>I am trying to implement Runge-Kutta's 4th order integration method in matlab on a 2-d matrix (x, y).  The matrix contains a height value (float) at each point.  The idea is place a particle in the matrix and watch it's path as it gets 'pushed' around by the directional vectors u and v.</p>
  
  <p>I have implemented Euler's integration method already using simply the 'gradient' function built in matlab.  However, for RK4 integration, I need to take 4 slopes at variable points, not the predetermined slope given by the gradient.</p>
  
  <p>I think the Matlab gradient function itself doesn't have these kind of parameters (from what I understand from the Matlab docs).  What kind of approach should I take for this?  </p>
  
  <p>If I were to write my own gradient function that computes the slope at any arbitrary point, I am not quite sure how to write it, since it is not a straight-foward function.  Are there any suggestions for this approach?</p>
  
  <p>Thanks.   </p>