title: formulae / algorithm to select centre cells in a grid?
body: <p>If I have a grid say consisting of 15 x 15 cells (but variable), what formulae or algorithm  would I use to randomly select 20 cells clustered around the centre cell?</p>
  
  <p>I guess I would ideally like to be able to set the centre point, cluster radius, density etc...</p>
  
  <p>any pointers would be really appreciated! cheers.</p>