G4TwistBoxSide.cc

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// $Id: G4TwistBoxSide.cc 72937 2013-08-14 13:20:38Z gcosmo $
//
// 
// --------------------------------------------------------------------
// GEANT 4 class source file
//
//
// G4TwistBoxSide.cc
//
// Author:
//
//   18/03/2005 - O.Link (Oliver.Link@cern.ch)
//
// --------------------------------------------------------------------

#include <cmath>

#include "G4TwistBoxSide.hh"
#include "G4PhysicalConstants.hh"
#include "G4JTPolynomialSolver.hh"

//=====================================================================
//* constructors ------------------------------------------------------

G4TwistBoxSide::G4TwistBoxSide(const G4String     &name,
                           G4double      PhiTwist,    // twist angle
                           G4double      pDz,         // half z lenght
                           G4double      pTheta,      // direction between end planes
                           G4double      pPhi,        // defined by polar and azimutal angles.
                           G4double      pDy1,        // half y length at -pDz
                           G4double      pDx1,        // half x length at -pDz,-pDy
                           G4double      pDx2,        // half x length at -pDz,+pDy
                           G4double      pDy2,        // half y length at +pDz
                           G4double      pDx3,        // half x length at +pDz,-pDy
                           G4double      pDx4,        // half x length at +pDz,+pDy
                           G4double      pAlph,       // tilt angle at +pDz
                           G4double      AngleSide    // parity
                                               ) : G4VTwistSurface(name)
{  
  
                 
  fAxis[0]    = kYAxis; // in local coordinate system
  fAxis[1]    = kZAxis;
  fAxisMin[0] = -kInfinity ;  // Y Axis boundary
  fAxisMax[0] = kInfinity ;   //   depends on z !!
  fAxisMin[1] = -pDz ;      // Z Axis boundary
  fAxisMax[1] = pDz ;
  
  fDx1  = pDx1 ;
  fDx2  = pDx2 ;  // box
  fDx3  = pDx3 ;
  fDx4  = pDx4 ;  // box

  // this is an overhead. But the parameter naming scheme fits to the other surfaces.
 
  if ( ! (fDx1 == fDx2 && fDx3 == fDx4 ) ) {
    std::ostringstream message;
    message << "TwistedTrapBoxSide is not used as a the side of a box: "
            << GetName() << G4endl
            << "        Not a box !";
    G4Exception("G4TwistBoxSide::G4TwistBoxSide()", "GeomSolids0002",
                FatalException, message);
  }
 
  fDy1   = pDy1 ;
  fDy2   = pDy2 ;

  fDz   = pDz ;

  fAlph = pAlph  ;
  fTAlph = std::tan(fAlph) ;

  fTheta = pTheta ;
  fPhi   = pPhi ;

 // precalculate frequently used parameters
  fDx4plus2  = fDx4 + fDx2 ;
  fDx4minus2 = fDx4 - fDx2 ;
  fDx3plus1  = fDx3 + fDx1 ; 
  fDx3minus1 = fDx3 - fDx1 ;
  fDy2plus1  = fDy2 + fDy1 ;
  fDy2minus1 = fDy2 - fDy1 ;

  fa1md1 = 2*fDx2 - 2*fDx1  ; 
  fa2md2 = 2*fDx4 - 2*fDx3 ;


  fPhiTwist = PhiTwist ;     // dphi
  fAngleSide = AngleSide ;  // 0,90,180,270 deg

  fdeltaX = 2 * fDz * std::tan(fTheta) * std::cos(fPhi)  ;  // dx in surface equation
  fdeltaY = 2 * fDz * std::tan(fTheta) * std::sin(fPhi)  ;  // dy in surface equation
  
  fRot.rotateZ( AngleSide ) ; 
  
  fTrans.set(0, 0, 0);  // No Translation
  fIsValidNorm = false;
  
  SetCorners() ;
  SetBoundaries() ;

}


//=====================================================================
//* Fake default constructor ------------------------------------------

G4TwistBoxSide::G4TwistBoxSide( __void__& a )
  : G4VTwistSurface(a), fTheta(0.), fPhi(0.), fDy1(0.), fDx1(0.), fDx2(0.), 
    fDy2(0.), fDx3(0.), fDx4(0.), fDz(0.), fAlph(0.), fTAlph(0.), fPhiTwist(0.), 
    fAngleSide(0.), fdeltaX(0.), fdeltaY(0.), fDx4plus2(0.), fDx4minus2(0.), 
    fDx3plus1(0.), fDx3minus1(0.), fDy2plus1(0.), fDy2minus1(0.), fa1md1(0.), 
    fa2md2(0.)
{
}


//=====================================================================
//* destructor --------------------------------------------------------

G4TwistBoxSide::~G4TwistBoxSide()
{
}

//=====================================================================
//* GetNormal ---------------------------------------------------------

G4ThreeVector G4TwistBoxSide::GetNormal(const G4ThreeVector &tmpxx, 
                                                G4bool isGlobal) 
{
   // GetNormal returns a normal vector at a surface (or very close
   // to surface) point at tmpxx.
   // If isGlobal=true, it returns the normal in global coordinate.
   //

   G4ThreeVector xx;
   if (isGlobal) {
      xx = ComputeLocalPoint(tmpxx);
      if ((xx - fCurrentNormal.p).mag() < 0.5 * kCarTolerance) {
         return ComputeGlobalDirection(fCurrentNormal.normal);
      }
   } else {
      xx = tmpxx;
      if (xx == fCurrentNormal.p) {
         return fCurrentNormal.normal;
      }
   }

   G4double phi ;
   G4double u ;

   GetPhiUAtX(xx,phi,u) ;   // phi,u for point xx close to surface 

   G4ThreeVector normal =  NormAng(phi,u) ;  // the normal vector at phi,u

#ifdef G4TWISTDEBUG
   G4cout  << "normal vector = " << normal << G4endl ;
   G4cout << "phi = " << phi << " , u = " << u << G4endl ;
#endif

   //    normal = normal/normal.mag() ;

   if (isGlobal) {
      fCurrentNormal.normal = ComputeGlobalDirection(normal.unit());
   } else {
      fCurrentNormal.normal = normal.unit();
   }
   return fCurrentNormal.normal;
}

//=====================================================================
//* DistanceToSurface -------------------------------------------------

G4int G4TwistBoxSide::DistanceToSurface(const G4ThreeVector &gp,
                                        const G4ThreeVector &gv,
                                              G4ThreeVector  gxx[],
                                              G4double       distance[],
                                              G4int          areacode[],
                                              G4bool         isvalid[],
                                              EValidate      validate)
{

  static const G4double pihalf = pi/2 ;
  const G4double ctol = 0.5 * kCarTolerance;

  G4bool IsParallel = false ;
  G4bool IsConverged =  false ;

  G4int nxx = 0 ;  // number of physical solutions

  fCurStatWithV.ResetfDone(validate, &gp, &gv);

  if (fCurStatWithV.IsDone()) {
    G4int i;
    for (i=0; i<fCurStatWithV.GetNXX(); i++) {
      gxx[i] = fCurStatWithV.GetXX(i);
      distance[i] = fCurStatWithV.GetDistance(i);
      areacode[i] = fCurStatWithV.GetAreacode(i);
      isvalid[i]  = fCurStatWithV.IsValid(i);
    }
    return fCurStatWithV.GetNXX();
  } else {
   
   // initialize
    G4int i;
    for (i=0; i<G4VSURFACENXX ; i++) {
      distance[i] = kInfinity;
      areacode[i] = sOutside;
      isvalid[i]  = false;
      gxx[i].set(kInfinity, kInfinity, kInfinity);
    }
  }

  G4ThreeVector p = ComputeLocalPoint(gp);
  G4ThreeVector v = ComputeLocalDirection(gv);
  
#ifdef G4TWISTDEBUG
  G4cout << "Local point p = " << p << G4endl ;
  G4cout << "Local direction v = " << v << G4endl ; 
#endif

  G4double phi=0.,u=0.,q=0.;  // parameters

  // temporary variables

  G4double      tmpdist = kInfinity ;
  G4ThreeVector tmpxx;
  G4int         tmpareacode = sOutside ;
  G4bool        tmpisvalid  = false ;

  std::vector<Intersection> xbuf ;
  Intersection xbuftmp ;
  
  // prepare some variables for the intersection finder

  G4double L = 2*fDz ;

  G4double phixz = fPhiTwist * ( p.x() * v.z() - p.z() * v.x() ) ;
  G4double phiyz = fPhiTwist * ( p.y() * v.z() - p.z() * v.y() ) ;

  // special case vz = 0

  if ( v.z() == 0. ) {         

    if ( (std::fabs(p.z()) <= L) && fPhiTwist ) {  // intersection possible in z
      
      phi = p.z() * fPhiTwist / L ;  // phi is determined by the z-position 

      q = (2.* fPhiTwist*((v.x() - fTAlph*v.y())*std::cos(phi)
                             + (fTAlph*v.x() + v.y())*std::sin(phi)));
      
      if (q) {
        u = (2*(-(fdeltaY*phi*v.x()) + fPhiTwist*p.y()*v.x()
                + fdeltaX*phi*v.y() - fPhiTwist*p.x()*v.y())
             + (fDx4plus2*fPhiTwist + 2*fDx4minus2*phi)
             * (v.y()*std::cos(phi) - v.x()*std::sin(phi))) / q;
      }

      xbuftmp.phi = phi ;
      xbuftmp.u = u ;
      xbuftmp.areacode = sOutside ;
      xbuftmp.distance = kInfinity ;
      xbuftmp.isvalid = false ;

      xbuf.push_back(xbuftmp) ;  // store it to xbuf

    }

    else {                        // no intersection possible

      distance[0] = kInfinity;
      gxx[0].set(kInfinity,kInfinity,kInfinity);
      isvalid[0] = false ;
      areacode[0] = sOutside ;
      fCurStatWithV.SetCurrentStatus(0, gxx[0], distance[0],
                                     areacode[0], isvalid[0],
                                     0, validate, &gp, &gv);
      
      return 0;


    }  // end std::fabs(p.z() <= L 

  } // end v.z() == 0
  

  // general solution for non-zero vz

  else {

    G4double c[8],srd[7],si[7] ;  

    c[7] = -14400*(-2*phixz + 2*fTAlph*phiyz + fDx4plus2*fPhiTwist*v.z()) ;
    c[6] = 28800*(phiyz + 2*fDz*v.x() - (fdeltaX + fDx4minus2)*v.z() + fTAlph*(phixz - 2*fDz*v.y() + fdeltaY*v.z())) ;
    c[5] = -1200*(10*phixz - 48*fDz*v.y() + 24*fdeltaY*v.z() + fDx4plus2*fPhiTwist*v.z() - 2*fTAlph*(5*phiyz + 24*fDz*v.x() - 12*fdeltaX*v.z())) ;
    c[4] = -2400*(phiyz + 10*fDz*v.x() - 5*fdeltaX*v.z() + fDx4minus2*v.z() + fTAlph*(phixz - 10*fDz*v.y() + 5*fdeltaY*v.z())) ;
    c[3] = 24*(2*phixz - 200*fDz*v.y() + 100*fdeltaY*v.z() - fDx4plus2*fPhiTwist*v.z() - 2*fTAlph*(phiyz + 100*fDz*v.x() - 50*fdeltaX*v.z())) ;
    c[2] = -16*(7*fTAlph* phixz + 7*phiyz - 6*fDz*v.x() + 6*fDz*fTAlph*v.y() + 3*(fdeltaX + fDx4minus2 - fdeltaY*fTAlph)*v.z()) ;
    c[1] = 4*(9*phixz - 9*fTAlph*phiyz - 56*fDz*fTAlph*v.x() - 56*fDz*v.y() + 28*(fdeltaY + fdeltaX*fTAlph)*v.z()) ;
    c[0] = 36*(2* fDz*(v.x() - fTAlph*v.y()) - fdeltaX*v.z() + fdeltaY*fTAlph*v.z()) ;


#ifdef G4TWISTDEBUG
    G4cout << "coef = " << c[0] << " " 
           <<  c[1] << " "  
           <<  c[2] << " "  
           <<  c[3] << " "  
           <<  c[4] << " "  
           <<  c[5] << " "  
           <<  c[6] << " "  
           <<  c[7] << G4endl ;
#endif    

    G4JTPolynomialSolver trapEq ;
    G4int num = trapEq.FindRoots(c,7,srd,si);
  

    for (G4int i = 0 ; i<num ; i++ ) {  // loop over all mathematical solutions
      if ( (si[i]==0.0) && fPhiTwist ) {  // only real solutions
#ifdef G4TWISTDEBUG
        G4cout << "Solution " << i << " : " << srd[i] << G4endl ;
#endif
        phi = std::fmod(srd[i] , pihalf)  ;

        u   = (2*phiyz + 4*fDz*phi*v.y() - 2*fdeltaY*phi*v.z()
             - fDx4plus2*fPhiTwist*v.z()*std::sin(phi)
             - 2*fDx4minus2*phi*v.z()*std::sin(phi))
             /(2*fPhiTwist*v.z()*std::cos(phi)
               + 2*fPhiTwist*fTAlph*v.z()*std::sin(phi)) ;

        xbuftmp.phi = phi ;
        xbuftmp.u = u ;
        xbuftmp.areacode = sOutside ;
        xbuftmp.distance = kInfinity ;
        xbuftmp.isvalid = false ;
        
        xbuf.push_back(xbuftmp) ;  // store it to xbuf
      
#ifdef G4TWISTDEBUG
        G4cout << "solution " << i << " = " << phi << " , " << u  << G4endl ;
#endif

      }  // end if real solution
    }  // end loop i
    
  }    // end general case


  nxx = xbuf.size() ;  // save the number of  solutions

  G4ThreeVector xxonsurface  ;       // point on surface
  G4ThreeVector surfacenormal  ;     // normal vector  
  G4double deltaX  ;                 // distance between intersection point and point on surface
  G4double theta  ;                  // angle between track and surfacenormal
  G4double factor ;                  // a scaling factor
  G4int maxint = 30 ;                // number of iterations


  for ( size_t k = 0 ; k<xbuf.size() ; k++ ) {

#ifdef G4TWISTDEBUG
    G4cout << "Solution " << k << " : " 
           << "reconstructed phiR = " << xbuf[k].phi
           << ", uR = " << xbuf[k].u << G4endl ; 
#endif
    
    phi = xbuf[k].phi ;  // get the stored values for phi and u
    u = xbuf[k].u ;

    IsConverged = false ;   // no convergence at the beginning
    
    for ( G4int i = 1 ; i<maxint ; i++ ) {
      
      xxonsurface = SurfacePoint(phi,u) ;
      surfacenormal = NormAng(phi,u) ;
      tmpdist = DistanceToPlaneWithV(p, v, xxonsurface, surfacenormal, tmpxx); 
      deltaX = ( tmpxx - xxonsurface ).mag() ; 
      theta = std::fabs(std::acos(v*surfacenormal) - pihalf) ;
      if ( theta < 0.001 ) { 
        factor = 50 ;
        IsParallel = true ;
      }
      else {
        factor = 1 ;
      }

#ifdef G4TWISTDEBUG
      G4cout << "Step i = " << i << ", distance = " << tmpdist << ", " << deltaX << G4endl ;
      G4cout << "X = " << tmpxx << G4endl ;
#endif
      
      GetPhiUAtX(tmpxx, phi, u) ; // the new point xx is accepted and phi/u replaced
      
#ifdef G4TWISTDEBUG
      G4cout << "approximated phi = " << phi << ", u = " << u << G4endl ; 
#endif
      
      if ( deltaX <= factor*ctol ) { IsConverged = true ; break ; }
      
    }  // end iterative loop (i)
    

    // new code  21.09.05 O.Link
    if ( std::fabs(tmpdist)<ctol ) tmpdist = 0 ; 

#ifdef G4TWISTDEBUG
    G4cout << "refined solution "  << phi << " , " << u  <<  G4endl ;
    G4cout << "distance = " << tmpdist << G4endl ;
    G4cout << "local X = " << tmpxx << G4endl ;
#endif
    
    tmpisvalid = false ;  // init 

    if ( IsConverged ) {
      
      if (validate == kValidateWithTol) {
        tmpareacode = GetAreaCode(tmpxx);
        if (!IsOutside(tmpareacode)) {
          if (tmpdist >= 0) tmpisvalid = true;
        }
      } else if (validate == kValidateWithoutTol) {
        tmpareacode = GetAreaCode(tmpxx, false);
        if (IsInside(tmpareacode)) {
          if (tmpdist >= 0) tmpisvalid = true;
        }
      } else { // kDontValidate
        G4Exception("G4TwistBoxSide::DistanceToSurface()",
                    "GeomSolids0001", FatalException,
                    "Feature NOT implemented !");
      }

    } 
    else {
      tmpdist = kInfinity;     // no convergence after 10 steps 
      tmpisvalid = false ;     // solution is not vaild
    }  


    // store the found values 
    xbuf[k].xx = tmpxx ;
    xbuf[k].distance = tmpdist ;
    xbuf[k].areacode = tmpareacode ;
    xbuf[k].isvalid = tmpisvalid ;


  }  // end loop over physical solutions (variable k)


  std::sort(xbuf.begin() , xbuf.end(), DistanceSort ) ;  // sorting

#ifdef G4TWISTDEBUG
  G4cout << G4endl << "list xbuf after sorting : " << G4endl ;
  G4cout << G4endl << G4endl ;
#endif


  // erase identical intersection (within kCarTolerance) 
  xbuf.erase( std::unique(xbuf.begin(), xbuf.end() , EqualIntersection ) , xbuf.end() ) ;


  // add guesses

  G4int nxxtmp = xbuf.size() ;

  if ( nxxtmp<2 || IsParallel  ) {

    // positive end
#ifdef G4TWISTDEBUG
    G4cout << "add guess at +z/2 .. " << G4endl ;
#endif

    phi = fPhiTwist/2 ;
    u   = 0 ;

    
     
    xbuftmp.phi = phi ;
    xbuftmp.u = u ;
    xbuftmp.areacode = sOutside ;
    xbuftmp.distance = kInfinity ;
    xbuftmp.isvalid = false ;
    
    xbuf.push_back(xbuftmp) ;  // store it to xbuf


#ifdef G4TWISTDEBUG
    G4cout << "add guess at -z/2 .. " << G4endl ;
#endif

    phi = -fPhiTwist/2 ;
    u   = 0 ;

    xbuftmp.phi = phi ;
    xbuftmp.u = u ;
    xbuftmp.areacode = sOutside ;
    xbuftmp.distance = kInfinity ;
    xbuftmp.isvalid = false ;
    
    xbuf.push_back(xbuftmp) ;  // store it to xbuf

    for ( size_t k = nxxtmp ; k<xbuf.size() ; k++ ) {

#ifdef G4TWISTDEBUG
      G4cout << "Solution " << k << " : " 
             << "reconstructed phiR = " << xbuf[k].phi
             << ", uR = " << xbuf[k].u << G4endl ; 
#endif
      
      phi = xbuf[k].phi ;  // get the stored values for phi and u
      u   = xbuf[k].u ;

      IsConverged = false ;   // no convergence at the beginning
      
      for ( G4int i = 1 ; i<maxint ; i++ ) {
        
        xxonsurface = SurfacePoint(phi,u) ;
        surfacenormal = NormAng(phi,u) ;
        tmpdist = DistanceToPlaneWithV(p, v, xxonsurface, surfacenormal, tmpxx); 
        deltaX = ( tmpxx - xxonsurface ).mag() ; 
        theta = std::fabs(std::acos(v*surfacenormal) - pihalf) ;
        if ( theta < 0.001 ) { 
          factor = 50 ;    
        }
        else {
          factor = 1 ;
        }
        
#ifdef G4TWISTDEBUG
        G4cout << "Step i = " << i << ", distance = " << tmpdist << ", " << deltaX << G4endl ;
        G4cout << "X = " << tmpxx << G4endl ;
#endif

        GetPhiUAtX(tmpxx, phi, u) ; // the new point xx is accepted and phi/u replaced
      
#ifdef G4TWISTDEBUG
        G4cout << "approximated phi = " << phi << ", u = " << u << G4endl ; 
#endif
      
        if ( deltaX <= factor*ctol ) { IsConverged = true ; break ; }
      
      }  // end iterative loop (i)
    

    // new code  21.09.05 O.Link
    if ( std::fabs(tmpdist)<ctol ) tmpdist = 0 ; 

#ifdef G4TWISTDEBUG
      G4cout << "refined solution "  << phi << " , " << u  <<  G4endl ;
      G4cout << "distance = " << tmpdist << G4endl ;
      G4cout << "local X = " << tmpxx << G4endl ;
#endif

      tmpisvalid = false ;  // init 

      if ( IsConverged ) {

        if (validate == kValidateWithTol) {
          tmpareacode = GetAreaCode(tmpxx);
          if (!IsOutside(tmpareacode)) {
            if (tmpdist >= 0) tmpisvalid = true;
          }
        } else if (validate == kValidateWithoutTol) {
          tmpareacode = GetAreaCode(tmpxx, false);
          if (IsInside(tmpareacode)) {
            if (tmpdist >= 0) tmpisvalid = true;
          }
        } else { // kDontValidate
          G4Exception("G4TwistedBoxSide::DistanceToSurface()",
                      "GeomSolids0001", FatalException,
                      "Feature NOT implemented !");
        }
        
      } 
      else {
        tmpdist = kInfinity;     // no convergence after 10 steps 
        tmpisvalid = false ;     // solution is not vaild
      }  
        
        
      // store the found values 
      xbuf[k].xx = tmpxx ;
      xbuf[k].distance = tmpdist ;
      xbuf[k].areacode = tmpareacode ;
      xbuf[k].isvalid = tmpisvalid ;


    }  // end loop over physical solutions 


  }  // end less than 2 solutions


  // sort again
  std::sort(xbuf.begin() , xbuf.end(), DistanceSort ) ;  // sorting

  // erase identical intersection (within kCarTolerance) 
  xbuf.erase( std::unique(xbuf.begin(), xbuf.end() , EqualIntersection ) , xbuf.end() ) ;

#ifdef G4TWISTDEBUG
  G4cout << G4endl << "list xbuf after sorting : " << G4endl ;
  G4cout << G4endl << G4endl ;
#endif

  nxx = xbuf.size() ;   // determine number of solutions again.

  for ( size_t i = 0 ; i<xbuf.size() ; i++ ) {
    
    distance[i] = xbuf[i].distance;
    gxx[i]      = ComputeGlobalPoint(xbuf[i].xx);
    areacode[i] = xbuf[i].areacode ;
    isvalid[i]  = xbuf[i].isvalid ;
    
    fCurStatWithV.SetCurrentStatus(i, gxx[i], distance[i], areacode[i],
                                     isvalid[i], nxx, validate, &gp, &gv);

#ifdef G4TWISTDEBUG
    G4cout << "element Nr. " << i 
           << ", local Intersection = " << xbuf[i].xx 
           << ", distance = " << xbuf[i].distance 
           << ", u = " << xbuf[i].u 
           << ", phi = " << xbuf[i].phi 
           << ", isvalid = " << xbuf[i].isvalid 
           << G4endl ;
#endif

  }  // end for( i ) loop

    
#ifdef G4TWISTDEBUG
  G4cout << "G4TwistBoxSide finished " << G4endl ;
  G4cout << nxx << " possible physical solutions found" << G4endl ;
  for ( G4int k= 0 ; k< nxx ; k++ ) {
    G4cout << "global intersection Point found: " << gxx[k] << G4endl ;
    G4cout << "distance = " << distance[k] << G4endl ;
    G4cout << "isvalid = " << isvalid[k] << G4endl ;
  }
#endif

  return nxx ;
    
}


//=====================================================================
//* DistanceToSurface -------------------------------------------------

G4int G4TwistBoxSide::DistanceToSurface(const G4ThreeVector &gp,
                                                G4ThreeVector  gxx[],
                                                G4double       distance[],
                                                G4int          areacode[])
{  
  // to do

  const G4double ctol = 0.5 * kCarTolerance;

  fCurStat.ResetfDone(kDontValidate, &gp);

   if (fCurStat.IsDone()) {
      G4int i;
      for (i=0; i<fCurStat.GetNXX(); i++) {
         gxx[i] = fCurStat.GetXX(i);
         distance[i] = fCurStat.GetDistance(i);
         areacode[i] = fCurStat.GetAreacode(i);
      }
      return fCurStat.GetNXX();
   } else {
      // initialize
      G4int i;
      for (i=0; i<G4VSURFACENXX; i++) {
         distance[i] = kInfinity;
         areacode[i] = sOutside;
         gxx[i].set(kInfinity, kInfinity, kInfinity);
      }
   }
   
   G4ThreeVector p = ComputeLocalPoint(gp);
   G4ThreeVector xx;  // intersection point
   G4ThreeVector xxonsurface ; // interpolated intersection point 

   // the surfacenormal at that surface point
   G4double phiR = 0  ; // 
   G4double uR = 0 ;

   G4ThreeVector surfacenormal ; 
   G4double deltaX ;
   
   G4int maxint = 20 ;

   for ( G4int i = 1 ; i<maxint ; i++ ) {

     xxonsurface = SurfacePoint(phiR,uR) ;
     surfacenormal = NormAng(phiR,uR) ;
     distance[0] = DistanceToPlane(p, xxonsurface, surfacenormal, xx); // new XX
     deltaX = ( xx - xxonsurface ).mag() ; 

#ifdef G4TWISTDEBUG
     G4cout << "i = " << i << ", distance = " << distance[0] << ", " << deltaX << G4endl ;
     G4cout << "X = " << xx << G4endl ;
#endif

     // the new point xx is accepted and phi/psi replaced
     GetPhiUAtX(xx, phiR, uR) ;
     
     if ( deltaX <= ctol ) { break ; }

   }

   // check validity of solution ( valid phi,psi ) 

   G4double halfphi = 0.5*fPhiTwist ;
   G4double uMax = GetBoundaryMax(phiR) ;

   if (  phiR > halfphi ) phiR =  halfphi ;
   if ( phiR < -halfphi ) phiR = -halfphi ;
   if ( uR > uMax ) uR = uMax ;
   if ( uR < -uMax ) uR = -uMax ;

   xxonsurface = SurfacePoint(phiR,uR) ;
   distance[0] = (  p - xx ).mag() ;
   if ( distance[0] <= ctol ) { distance[0] = 0 ; } 

   // end of validity 

#ifdef G4TWISTDEBUG
   G4cout << "refined solution "  << phiR << " , " << uR << " , " <<  G4endl ;
   G4cout << "distance = " << distance[0] << G4endl ;
   G4cout << "X = " << xx << G4endl ;
#endif

   G4bool isvalid = true;
   gxx[0]      = ComputeGlobalPoint(xx);
   
#ifdef G4TWISTDEBUG
   G4cout << "intersection Point found: " << gxx[0] << G4endl ;
   G4cout << "distance = " << distance[0] << G4endl ;
#endif

   fCurStat.SetCurrentStatus(0, gxx[0], distance[0], areacode[0],
                            isvalid, 1, kDontValidate, &gp);
   return 1;
   

}


//=====================================================================
//* GetAreaCode -------------------------------------------------------

G4int G4TwistBoxSide::GetAreaCode(const G4ThreeVector &xx, 
                                          G4bool withTol)
{
   // We must use the function in local coordinate system.
   // See the description of DistanceToSurface(p,v).
   
   const G4double ctol = 0.5 * kCarTolerance;

   G4double phi ;
   G4double yprime ;
   GetPhiUAtX(xx, phi,yprime ) ;

   G4double fYAxisMax =  GetBoundaryMax(phi) ;   // Boundaries are symmetric
   G4double fYAxisMin =  - fYAxisMax ;

#ifdef G4TWISTDEBUG
   G4cout << "GetAreaCode: phi = " << phi << G4endl ;
   G4cout << "GetAreaCode: yprime = " << yprime << G4endl ;
   G4cout << "Intervall is " << fYAxisMin << " to " << fYAxisMax << G4endl ;
#endif

   G4int areacode = sInside;
   
   if (fAxis[0] == kYAxis && fAxis[1] == kZAxis) {

      G4int zaxis = 1;
      
      if (withTol) {

        G4bool isoutside   = false;
        
        // test boundary of yaxis

         if (yprime < fYAxisMin + ctol) {
            areacode |= (sAxis0 & (sAxisY | sAxisMin)) | sBoundary; 
            if (yprime <= fYAxisMin - ctol) isoutside = true;

         } else if (yprime > fYAxisMax - ctol) {
            areacode |= (sAxis0 & (sAxisY | sAxisMax)) | sBoundary;
            if (yprime >= fYAxisMax + ctol)  isoutside = true;
         }

         // test boundary of z-axis

         if (xx.z() < fAxisMin[zaxis] + ctol) {
            areacode |= (sAxis1 & (sAxisZ | sAxisMin)); 

            if   (areacode & sBoundary) areacode |= sCorner;  // xx is on the corner.
            else                        areacode |= sBoundary;
            if (xx.z() <= fAxisMin[zaxis] - ctol) isoutside = true;

         } else if (xx.z() > fAxisMax[zaxis] - ctol) {
            areacode |= (sAxis1 & (sAxisZ | sAxisMax));

            if   (areacode & sBoundary) areacode |= sCorner;  // xx is on the corner.
            else                        areacode |= sBoundary; 
            if (xx.z() >= fAxisMax[zaxis] + ctol) isoutside = true;
         }

         // if isoutside = true, clear inside bit.             
         // if not on boundary, add axis information.             
         
         if (isoutside) {
            G4int tmpareacode = areacode & (~sInside);
            areacode = tmpareacode;
         } else if ((areacode & sBoundary) != sBoundary) {
            areacode |= (sAxis0 & sAxisY) | (sAxis1 & sAxisZ);
         }           
         
      } else {

         // boundary of y-axis

         if (yprime < fYAxisMin ) {
            areacode |= (sAxis0 & (sAxisY | sAxisMin)) | sBoundary;
         } else if (yprime > fYAxisMax) {
            areacode |= (sAxis0 & (sAxisY | sAxisMax)) | sBoundary;
         }
         
         // boundary of z-axis

         if (xx.z() < fAxisMin[zaxis]) {
            areacode |= (sAxis1 & (sAxisZ | sAxisMin));
            if   (areacode & sBoundary) areacode |= sCorner;  // xx is on the corner.
            else                        areacode |= sBoundary; 
           
         } else if (xx.z() > fAxisMax[zaxis]) {
            areacode |= (sAxis1 & (sAxisZ | sAxisMax)) ;
            if   (areacode & sBoundary) areacode |= sCorner;  // xx is on the corner.
            else                        areacode |= sBoundary; 
         }

         if ((areacode & sBoundary) != sBoundary) {
            areacode |= (sAxis0 & sAxisY) | (sAxis1 & sAxisZ);
         }           
      }
      return areacode;
   } else {
      G4Exception("G4TwistBoxSide::GetAreaCode()",
                  "GeomSolids0001", FatalException,
                  "Feature NOT implemented !");
   }
   return areacode;
}

//=====================================================================
//* SetCorners() ------------------------------------------------------

void G4TwistBoxSide::SetCorners()
{

  // Set Corner points in local coodinate.   

  if (fAxis[0] == kYAxis && fAxis[1] == kZAxis) {
    
    G4double x, y, z;

    // corner of Axis0min and Axis1min

    x = -fdeltaX/2. + (fDx2 - fDy1*fTAlph)*std::cos(fPhiTwist/2.) - fDy1*std::sin(fPhiTwist/2.) ;
    y = -fdeltaY/2. - fDy1*std::cos(fPhiTwist/2.) + (-fDx2 + fDy1*fTAlph)*std::sin(fPhiTwist/2.) ;
    z = -fDz ;

    SetCorner(sC0Min1Min, x, y, z);

    // corner of Axis0max and Axis1min

    x = -fdeltaX/2. + (fDx2 + fDy1*fTAlph)*std::cos(fPhiTwist/2.) + fDy1*std::sin(fPhiTwist/2.) ;
    y = -fdeltaY/2. + fDy1*std::cos(fPhiTwist/2.) - (fDx2 + fDy1*fTAlph)*std::sin(fPhiTwist/2.) ;
    z = -fDz ;

    SetCorner(sC0Max1Min, x, y, z);
      
    // corner of Axis0max and Axis1max

    x = fdeltaX/2. + (fDx4 + fDy2*fTAlph)*std::cos(fPhiTwist/2.) - fDy2*std::sin(fPhiTwist/2.) ;
    y = fdeltaY/2. + fDy2*std::cos(fPhiTwist/2.) + (fDx4 + fDy2*fTAlph)*std::sin(fPhiTwist/2.) ;
    z = fDz ;

    SetCorner(sC0Max1Max, x, y, z);
      
    // corner of Axis0min and Axis1max

    x = fdeltaX/2. + (fDx4 - fDy2*fTAlph)*std::cos(fPhiTwist/2.) + fDy2*std::sin(fPhiTwist/2.) ;
    y = fdeltaY/2. - fDy2*std::cos(fPhiTwist/2.) + (fDx4 - fDy2*fTAlph)*std::sin(fPhiTwist/2.) ;
    z = fDz ;

    SetCorner(sC0Min1Max, x, y, z);

  } else {

    G4Exception("G4TwistBoxSide::SetCorners()",
                "GeomSolids0001", FatalException,
                "Method NOT implemented !");
  }
}

//=====================================================================
//* SetBoundaries() ---------------------------------------------------

void G4TwistBoxSide::SetBoundaries()
{
   // Set direction-unit vector of boundary-lines in local coodinate. 
   //   

  G4ThreeVector direction;
   
  if (fAxis[0] == kYAxis && fAxis[1] == kZAxis) {
      
    // sAxis0 & sAxisMin
    direction = GetCorner(sC0Min1Max) - GetCorner(sC0Min1Min);
    direction = direction.unit();
    SetBoundary(sAxis0 & (sAxisY | sAxisMin), direction, 
                GetCorner(sC0Min1Min), sAxisZ) ;
      
      // sAxis0 & sAxisMax
    direction = GetCorner(sC0Max1Max) - GetCorner(sC0Max1Min);
    direction = direction.unit();
    SetBoundary(sAxis0 & (sAxisY | sAxisMax), direction, 
                GetCorner(sC0Max1Min), sAxisZ);
    
    // sAxis1 & sAxisMin
    direction = GetCorner(sC0Max1Min) - GetCorner(sC0Min1Min);
    direction = direction.unit();
    SetBoundary(sAxis1 & (sAxisZ | sAxisMin), direction, 
                GetCorner(sC0Min1Min), sAxisY);
    
    // sAxis1 & sAxisMax
    direction = GetCorner(sC0Max1Max) - GetCorner(sC0Min1Max);
    direction = direction.unit();
    SetBoundary(sAxis1 & (sAxisZ | sAxisMax), direction, 
                GetCorner(sC0Min1Max), sAxisY);
    
  } else {
    
  G4Exception("G4TwistBoxSide::SetCorners()",
              "GeomSolids0001", FatalException,
              "Feature NOT implemented !");
  }
}


void G4TwistBoxSide::GetPhiUAtX( G4ThreeVector p, G4double &phi, G4double &u) 
{
  // find closest point XX on surface for a given point p
  // X0 is a point on the surface,  d is the direction ( both for a fixed z = pz)
  
  // phi is given by the z coordinate of p

  phi = p.z()/(2*fDz)*fPhiTwist ;

  u =  -(fTAlph*(fDx4plus2*fPhiTwist + 2*fDx4minus2*phi) + 2*(fdeltaY*phi + fdeltaX*fTAlph*phi - fPhiTwist*(fTAlph*p.x() + p.y()))* std::cos(phi) + 2*(-(fdeltaX*phi) + fdeltaY*fTAlph*phi + fPhiTwist*(p.x() - fTAlph*p.y()))*std::sin(phi))/(2.*(fPhiTwist + fPhiTwist*fTAlph*fTAlph)) ;


}


G4ThreeVector G4TwistBoxSide::ProjectPoint(const G4ThreeVector &p, 
                                                    G4bool isglobal) 
{
  // Get Rho at p.z() on Hyperbolic Surface.
  G4ThreeVector tmpp;
  if (isglobal) {
     tmpp = fRot.inverse()*p - fTrans;
  } else {
     tmpp = p;
  }

  G4double phi ;
  G4double u ;

  GetPhiUAtX( tmpp, phi, u ) ;  // calculate (phi, u) for a point p close the surface
  
  G4ThreeVector xx = SurfacePoint(phi,u) ;  // transform back to cartesian coordinates

  if (isglobal) {
     return (fRot * xx + fTrans);
  } else {
     return xx;
  }
}

void G4TwistBoxSide::GetFacets( G4int k, G4int n,  G4double xyz[][3], G4int faces[][4], G4int iside ) 
{

  G4double phi ;
  G4double b ;   

  G4double z, u ;     // the two parameters for the surface equation
  G4ThreeVector p ;  // a point on the surface, given by (z,u)

  G4int nnode ;
  G4int nface ;

  // calculate the (n-1)*(k-1) vertices

  G4int i,j ;

  for ( i = 0 ; i<n ; i++ ) {

    z = -fDz+i*(2.*fDz)/(n-1) ;
    phi = z*fPhiTwist/(2*fDz) ;
    b = GetValueB(phi) ;

    for ( j = 0 ; j<k ; j++ ) {

      nnode = GetNode(i,j,k,n,iside) ;
      u = -b/2 +j*b/(k-1) ;
      p = SurfacePoint(phi,u,true) ;  // surface point in global coordinate system

      xyz[nnode][0] = p.x() ;
      xyz[nnode][1] = p.y() ;
      xyz[nnode][2] = p.z() ;

      if ( i<n-1 && j<k-1 ) {   // conterclock wise filling
        
        nface = GetFace(i,j,k,n,iside) ;
        faces[nface][0] = GetEdgeVisibility(i,j,k,n,0,-1) * (GetNode(i  ,j  ,k,n,iside)+1) ;  // fortran numbering
        faces[nface][1] = GetEdgeVisibility(i,j,k,n,1,-1) * (GetNode(i  ,j+1,k,n,iside)+1) ;
        faces[nface][2] = GetEdgeVisibility(i,j,k,n,2,-1) * (GetNode(i+1,j+1,k,n,iside)+1) ;
        faces[nface][3] = GetEdgeVisibility(i,j,k,n,3,-1) * (GetNode(i+1,j  ,k,n,iside)+1) ;

      }
    }
  }
}