void KnotsViewer::set_scene(Bbox_3 &box)
{

    Mesh::Point     bbMin(box.xmin(), box.ymin(), box.zmin());
    Mesh::Point     bbMax(box.xmax(), box.ymax(), box.zmax());
    Mesh::Point     center= (bbMin + bbMax)*0.5;
    double radius=0.5*(bbMin - bbMax).norm();
    setSceneCenter(Vec(center[0], center[1], center[2]));
    setSceneRadius(radius);
    camera()->showEntireScene();
}