/**
  * Returns wheter the given sphere is in the frustum.
  *
  * @param center
  *                 The center of the sphere
  * @param radius
  *                 The radius of the sphere
  * @return Wheter the sphere is in the frustum
  */
public boolean sphereInFrustum(Vector3 center, float radius) {
  for (int i = 0; i < 6; i++)
   if ((planes[i].normal.x * center.x + planes[i].normal.y * center.y + planes[i].normal.z
         * center.z) < (-radius - planes[i].d))
    return false;
  return true;
}