code: private void normalize(final Element vertex) {
          double nx = vertex.getDouble("nx");
          double ny = vertex.getDouble("ny");
          double nz = vertex.getDouble("nz");
          double n = Math.sqrt(nx * nx + ny * ny + nz * nz);
          if (n < EPSILON) {
              vertex.setDouble("nx", 0);
              vertex.setDouble("ny", 0);
              vertex.setDouble("nz", 0);
          }
          vertex.setDouble("nx", nx / n);
          vertex.setDouble("ny", ny / n);
          vertex.setDouble("nz", nz / n);
      }
docs: Normalizes the normal vector of a vertex.
  @param vertex Vertex to normalize.