triangles.C

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/// \file
/// \ingroup tutorial_graphics
/// \notebook
/// Generate small triangles randomly in the canvas.
/// Each triangle has a unique id and a random color in the color palette
///
/// ~~~{.cpp}
/// root > .x triangles.C
/// ~~~
///
/// Then click on any triangle. A message showing the triangle number
/// and its color will be printed.
///
/// \macro_image
/// \macro_code
///
/// \author Rene Brun

void triangles(Int_t ntriangles=50) {
   TCanvas *c1 = new TCanvas("c1","triangles",10,10,700,700);
   TRandom r;
   Double_t dx = 0.2; Double_t dy = 0.2;
   Int_t ncolors = gStyle->GetNumberOfColors();
   Double_t x[4],y[4];
   TColor *c;
   Int_t ci;
   for (Int_t i=0;i<ntriangles;i++) {
      x[0] = r.Uniform(.05,.95); y[0] = r.Uniform(.05,.95);
      x[1] = x[0] + dx*r.Rndm(); y[1] = y[0] + dy*r.Rndm();
      x[2] = x[1] - dx*r.Rndm(); y[2] = y[1] - dy*r.Rndm();
      x[3] = x[0];               y[3] = y[0];
      TPolyLine *pl = new TPolyLine(4,x,y);
      pl->SetUniqueID(i);
      ci = ncolors*r.Rndm();
      c  = gROOT->GetColor(TColor::GetColorPalette(ci));
      c->SetAlpha(r.Rndm());
      pl->SetFillColor(ci);
      pl->Draw("f");
   }
   c1->AddExec("ex","TriangleClicked()");
}

void TriangleClicked() {
   //this action function is called whenever you move the mouse
   //it just prints the id of the picked triangle
   //you can add graphics actions instead
   int event = gPad->GetEvent();
   if (event != 11) return; //may be comment this line
   TObject *select = gPad->GetSelected();
   if (!select) return;
   if (select->InheritsFrom(TPolyLine::Class())) {
      TPolyLine *pl = (TPolyLine*)select;
      printf("You have clicked triangle %d, color=%d\n",
              pl->GetUniqueID(),pl->GetFillColor());
   }
}