/// used in display_beamprofile() : not working !
void getEllipseParameters(const float * x_data, const float * y_data, const unsigned int N, float& x_width, float& y_width, float& angle) {
// In order to fit a good ellipse on the scattered plot :
// 1) The TH2 is copied into a TGraph, to fit it with y(x) = ax => to retrieve the angle
// 2) Rotation of the Graph to get the RMS in X and Y
// 3) Creation of the final ellipse, with the good widths and angle
TCanvas * ca0 = new TCanvas;
ca0->Divide(2,1);
ca0->cd(1);
TGraph * draft = new TGraph(N,x_data,y_data);
draft->Draw("AP");
draft->Fit("pol1","Q");
TF1 * pol1 = draft->GetFunction("pol1");
pol1->Draw("same");
// gets the angle [rad]
angle = asin(1.) - atan(pol1->GetParameter(1));
double x_datarot[N], y_datarot[N];
for (unsigned int i=0; i<N; i++) {
x_datarot[i]= x_data[i]*cos(angle) - y_data[i]*sin(angle);
y_datarot[i]= x_data[i]*sin(angle) + y_data[i]*cos(angle);
}
ca0->cd(2);
TGraph * draft2 = new TGraph(N,x_datarot,y_datarot);
draft2->Draw("AP");
x_width = draft2->GetRMS(1);
y_width = draft2->GetRMS(2);
angle = 180-90*angle/asin(1.);
// draft->Draw("AP");
ca0->cd(1);
TEllipse * ell = new TEllipse(draft->GetMean(1),draft2->GetMean(2),x_width*3,y_width*3);
ell->SetTheta(angle);
ell->Draw("same");
//cout << "x = " << x_width << "\t y = " << y_width << "\t angle = " << angle << endl;
// delete draft2;
// delete draft;
// delete ca0;
return;
}
