Bool_t HandleInput()
{
    TTimer timer("gSystem->ProcessEvents();", 50, kFALSE);
    while (1)
    {
        //
        // While reading the input process gui events asynchronously
        //
        timer.TurnOn();
        TString input = Getline("Type 'q' to exit, <return> to go on: ");
        timer.TurnOff();

        if (input=="q\n")
            return kFALSE;

        if (input=="\n")
            return kTRUE;
    };

    return kFALSE;
}

void PropagateZ(TVector3 &p, TVector3 &w, double z, TArrow &arr)
{
    arr.SetX1(p.X());
    arr.SetY1(p.Z());

    p += (z-p.Z())/w.Z()*w;

    arr.SetX2(p.X());
    arr.SetY2(p.Z());
}

void PropagateDZ(TVector3 &p, TVector3 &w, double dz, TArrow &arr)
{
    arr.SetX1(p.X());
    arr.SetY1(p.Z());

    p += dz/w.Z()*w;

    arr.SetX2(p.X());
    arr.SetY2(p.Z());
}

void transition()
{
    TH1C h("", "", 100, -2, 2);
    h.SetMinimum(-1.5);
    h.SetMaximum(1.5);
    h.SetStats(kFALSE);
    h.DrawCopy();

    TLine line;
    line.DrawLine(-2, 0, 2, 0);

    TVector3 norm(0, 0, -1);
    norm *= 1./norm.Mag();

    TArrow arr;
    arr.SetArrowSize(0.01);
    arr.DrawArrow(0, 0, 0+norm.X(), 0+norm.Z());

    TArrow arr_in(1, 1, 0, 0);
    arr_in.SetArrowSize(0.01);
    arr_in.SetLineColor(kBlue);
    arr_in.Draw();

    TArrow arr_out(0, 0, 1, 1);
    arr_out.SetArrowSize(0.01);
    arr_out.SetLineColor(kRed);
    arr_out.Draw();

    while (1)
    {
        double x, y;
        gRandom->Circle(x, y, 1);

        TVector3 pos(x, 0, y);
        TVector3 dir = -pos;

        arr_in.SetX1(-dir.X()*1.5);
        arr_in.SetY1(-dir.Z()*1.5);

        double n1 = dir.Z()>0 ? 1.5 : 1;   // where the ray comes from
        double n2 = dir.Z()>0 ? 1   : 1.5; // where the ray goes to

        cout << sin(acos(dir*norm)) << endl;

        int rc = MOptics::ApplyTransition(dir, norm, n1, n2);

        cout << rc << endl;

        if (rc>=3) // Refraction
        {
            // Snell's law: n2*sin(theta2) = n1*sin(theta1) -- Is thge result correct?
            cout << n2/n1*sin(acos(dir*norm)) << endl;
        }

        arr_out.SetX2(dir.X()*1.5);
        arr_out.SetY2(dir.Z()*1.5);

        gPad->Modified();
        gPad->Update();

        if (!HandleInput())
            break;
    }
}