// And compute indices:
              dfac = 3;
              for (i = 0; i < (nlat-1)/dfac; i++) {
                 for (j = 0; j < nl; j++) {
                    idx = 5*(i*nl+j);
                    ci[idx+0] = dfac*i*nl+(j)\%nl;
                    ci[idx+1] = dfac*i*nl+(j+1)\%nl;
                    ci[idx+2] = (dfac*i+1)*nl+(j+1)\%nl;
                    ci[idx+3] = (dfac*i+1)*nl+(j)\%nl;
                    ci[idx+4] = -1;  // end-of-polygon
                 }
              }
           }"
        ]
     }