#include "TROOT.h"
#include "TStyle.h"
#include "TCanvas.h"
#include "TView.h"
#include "TPolyLine3D.h"
#include "TLatex.h"

void cir_solid_angle(){
  gROOT->SetStyle("Plain");
  gStyle->SetTextFont(132);
  TCanvas *c1 = new TCanvas("c1","c1",400,400);
  c1->SetTheta(20.);
  c1->SetPhi(-141.);
  TView *view = TView::CreateView(1);
  Double_t view_size = 0.8;
  view->SetRange(-view_size*0.3,-view_size*1.0,-view_size*1.0,
		  view_size*1.7, view_size*1.0, view_size*1.0);

  const Int_t nPolyLine3D = 20;
  TPolyLine3D *guide[nPolyLine3D];
  for (Int_t i = 0; i<nPolyLine3D; i++){
    guide[i]=new TPolyLine3D();
  }

  Double_t pi = acos(-1.);
  Int_t    n = 30;
  Double_t theta0 = pi/6.0;
  Double_t x0 =  2.*tan(theta0);

  for (Int_t i=0;i<=n;i++) {
    Double_t r = 0.6;
    Double_t theta = -i*theta0/n+pi;
    guide[4]->SetNextPoint(2.0+r*cos(theta),r*sin(theta),0);
  }

  for (Int_t i=0;i<=n;i++) {
    Double_t r = x0;
    Double_t theta = i*2.*pi/n;
    guide[5]->SetNextPoint(0,r*cos(theta),r*sin(theta));
  }

  guide[6]->SetNextPoint(0,x0,0);
  guide[6]->SetNextPoint(2,0,0);
  guide[6]->SetNextPoint(0,0,0);

  Double_t virlen = 0.1;
  guide[7]->SetNextPoint(virlen,0,0);
  guide[7]->SetNextPoint(virlen,0,virlen);
  guide[7]->SetNextPoint(0,0,virlen);

  for (Int_t i = 0; i<nPolyLine3D; i++){
    guide[i]->Draw();
  }

  Double_t xx[3],xxt[3];
  TLatex Tl;
  Tl.SetTextSize(0.06);
  xx[0]=2.04; xx[1]=0.0; xx[2]=-0.15; view->WCtoNDC(xx,xxt);
  Tl.DrawLatex(xxt[0],xxt[1],"O");
  xx[0]=0.00; xx[1]=0.0; xx[2]=-0.15; view->WCtoNDC(xx,xxt);
  Tl.DrawLatex(xxt[0],xxt[1],"C");
  xx[0]=1.4; xx[1]=0.25; xx[2]=0.00; view->WCtoNDC(xx,xxt);
  Tl.DrawLatex(xxt[0],xxt[1],"#it{#alpha}");
}