void mirror()
{
// Produce a ray-traced image of a Davis-Cotton mirror component
   gSystem->Load("libGeom.so");
   gGeoManager = new TGeoManager("mirror","Davis-Cotton mirror");
   TGeoMaterial *mat = new TGeoMaterial("void", 0,0,0);
   TGeoMedium *med = new TGeoMedium("void", 1, mat);

   TGeoPgon *pgon = new TGeoPgon("pg",0.,360.,6,2); 
   pgon->DefineSection(0,0,0,20);
   pgon->DefineSection(1, 30,0,20);

   TGeoSphere *sph = new TGeoSphere("sph", 40,45);
   TGeoTranslation *tr = new TGeoTranslation(0,0,45);
   tr->SetName("tr");
   tr->RegisterYourself();
   
   TGeoCompositeShape *cs = new TGeoCompositeShape("mir", "pg*sph:tr");

   TGeoVolume *top = gGeoManager->MakeBox("TOP", med, 30,30,20);
   gGeoManager->SetTopVolume(top);
   
   TGeoVolume *mirror = new TGeoVolume("MIR",cs);
   mirror->SetLineColor(kRed);

   top->AddNode(mirror,1);

   gGeoManager->CloseGeometry();
   
   top->Raytrace();
   // point the mouse to the pad where the mirror is drawn, then
   // press j/k/u/i/h/l to zoom/unzoom/move arround the object
}