int ir,ig,ib,index;
   double r,g,b;        /* r,b,g colour [0,1] */
   int n=6;             /* Number of samples */

   for (ib=0;ib<n;ib++) {
      for (ig=0;ig<n;ig++) {
         for (ir=0;ir<n;ir++) {
            index = ib*n*n + ig*n + ir;
            r = ir / (double)(n-1);
            g = ig / (double)(n-1);
            b = ib / (double)(n-1);
         }
      }
   }