void gltf3()
{
   gStyle->SetCanvasPreferGL(1);
   TCanvas *cnv = new TCanvas("gltf3", "TF3: Klein bottle", 200, 10, 600, 600);

   TPaveLabel *title =
      new TPaveLabel(0.04, 0.86, 0.96, 0.98, "\"gl\" option for TF3. Select plot and press 's' to change the color.");
   title->SetFillColor(32);
   title->Draw();

   TPad *tf3Pad = new TPad("box", "box", 0.04, 0.04, 0.96, 0.8);
   tf3Pad->Draw();

   TFormula *f1 = new TFormula("f1", "x*x + y*y + z*z + 2*y - 1");
   TFormula *f2 = new TFormula("f2", "x*x + y*y + z*z - 2*y - 1");

   // Klein bottle with cut top&bottom parts
   // The Klein bottle is a closed non-orientable surface that has no
   // inside or outside.

   TF3 *tf3 = new TF3("Klein Bottle", "f1*(f2*f2-8*z*z) + 16*x*z*f2", -3.5, 3.5, -3.5, 3.5, -2.5, 2.5);

   tf3->SetFillColor(kRed);
   tf3Pad->cd();
   tf3->Draw("gl");
}