Generate small triangles randomly in the canvas. Each triangle has a unique id and a random color in the color palette

root > .x triangles.C

Then click on any triangle. A message showing the triangle number and its color will be printed.

Author: Rene Brun
This notebook tutorial was automatically generated with ROOTBOOK-izer from the macro found in the ROOT repository on Tuesday, November 29, 2022 at 11:16 AM.

Definition of a helper function:

In [1]:
%%cpp -d
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",

Arguments are defined.

In [2]:
Int_t ntriangles=50;
In [3]:
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];
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);
   Int_t ci = ncolors*r.Rndm();
   TColor *c = gROOT->GetColor(TColor::GetColorPalette(ci));

Draw all canvases

In [4]: