void align();

//______________________________________________________________________________
void parallel_world(Bool_t usepw = kTRUE, Bool_t useovlp = kTRUE)
{
   // web geometry display does not support "parallel world" feature
   gROOT->SetWebDisplay("off");

   TGeoManager *geom = new TGeoManager("parallel_world", "Showcase for prioritized physical paths");
   TGeoMaterial *matV = new TGeoMaterial("Vac", 0, 0, 0);
   TGeoMedium *medV = new TGeoMedium("MEDVAC", 1, matV);
   TGeoMaterial *matAl = new TGeoMaterial("Al", 26.98, 13, 2.7);
   TGeoMedium *medAl = new TGeoMedium("MEDAL", 2, matAl);
   TGeoMaterial *matSi = new TGeoMaterial("Si", 28.085, 14, 2.329);
   TGeoMedium *medSi = new TGeoMedium("MEDSI", 3, matSi);
   TGeoVolume *top = gGeoManager->MakeBox("TOP", medV, 100, 400, 1000);
   gGeoManager->SetTopVolume(top);

   // Shape for the support block
   TGeoBBox *sblock = new TGeoBBox("sblock", 20, 10, 2);
   // The volume for the support
   TGeoVolume *support = new TGeoVolume("block", sblock, medAl);
   support->SetLineColor(kGreen);

   // Shape for the sensor to be prioritized in case of overlap
   TGeoBBox *ssensor = new TGeoBBox("sensor", 19, 9, 0.2);
   // The volume for the sensor
   TGeoVolume *sensor = new TGeoVolume("sensor", ssensor, medSi);
   sensor->SetLineColor(kRed);

   // Chip assembly of support+sensor
   TGeoVolumeAssembly *chip = new TGeoVolumeAssembly("chip");
   chip->AddNode(support, 1);
   chip->AddNode(sensor, 1, new TGeoTranslation(0, 0, -2.1));

   // A ladder that normally sags
   TGeoBBox *sladder = new TGeoBBox("sladder", 20, 300, 5);
   // The volume for the ladder
   TGeoVolume *ladder = new TGeoVolume("ladder", sladder, medAl);
   ladder->SetLineColor(kBlue);

   // Add nodes
   top->AddNode(ladder, 1);
   for (Int_t i = 0; i < 10; i++)
      top->AddNode(chip, i + 1, new TGeoTranslation(0, -225. + 50. * i, 10));

   gGeoManager->CloseGeometry();
   TGeoParallelWorld *pw = nullptr;
   if (usepw)
      pw = gGeoManager->CreateParallelWorld("priority_sensors");
   // Align chips
   align();
   if (usepw) {
      if (useovlp)
         pw->AddOverlap(ladder);
      pw->CloseGeometry();
      gGeoManager->SetUseParallelWorldNav(kTRUE);
   }
   TString cname;
   cname = usepw ? "cpw" : "cnopw";
   TCanvas *c = (TCanvas *)gROOT->GetListOfCanvases()->FindObject(cname);
   if (c)
      c->cd();
   else
      c = new TCanvas(cname, "", 800, 600);
   top->Draw();
   //   top->RandomRays(0,0,0,0,sensor->GetName());
   // Track random "particles" coming from the block side and draw only the tracklets
   // actually crossing one of the sensors. Note that some of the tracks coming
   // from the outer side may see the full sensor, while the others only part of it.
   TStopwatch timer;
   timer.Start();
   top->RandomRays(100000, 0, 0, -30, sensor->GetName());
   timer.Stop();
   timer.Print();
   TView3D *view = (TView3D *)gPad->GetView();
   if (view) {
      view->SetParallel();
      view->Side();
   }
   if (usepw)
      pw->PrintDetectedOverlaps();
}

//______________________________________________________________________________
void align()
{
   // Aligning 2 sensors so they will overlap with the support. One sensor is positioned
   // normally while the other using the shared matrix
   TGeoPhysicalNode *node;
   TGeoParallelWorld *pw = gGeoManager->GetParallelWorld();
   Double_t sag;
   for (Int_t i = 0; i < 10; i++) {
      node = gGeoManager->MakePhysicalNode(TString::Format("/TOP_1/chip_%d", i + 1));
      sag = 8. - 0.494 * (i - 4.5) * (i - 4.5);
      TGeoTranslation *tr = new TGeoTranslation(0., -225. + 50. * i, 10 - sag);
      node->Align(tr);
      if (pw)
         pw->AddNode(TString::Format("/TOP_1/chip_%d", i + 1));
   }
}