G4ITNavigator2.hh

Go to the documentation of this file.

//
// ********************************************************************
// * License and Disclaimer                                           *
// *                                                                  *
// * The  Geant4 software  is  copyright of the Copyright Holders  of *
// * the Geant4 Collaboration.  It is provided  under  the terms  and *
// * conditions of the Geant4 Software License,  included in the file *
// * LICENSE and available at  http://cern.ch/geant4/license .  These *
// * include a list of copyright holders.                             *
// *                                                                  *
// * Neither the authors of this software system, nor their employing *
// * institutes,nor the agencies providing financial support for this *
// * work  make  any representation or  warranty, express or implied, *
// * regarding  this  software system or assume any liability for its *
// * use.  Please see the license in the file  LICENSE  and URL above *
// * for the full disclaimer and the limitation of liability.         *
// *                                                                  *
// * This  code  implementation is the result of  the  scientific and *
// * technical work of the GEANT4 collaboration.                      *
// * By using,  copying,  modifying or  distributing the software (or *
// * any work based  on the software)  you  agree  to acknowledge its *
// * use  in  resulting  scientific  publications,  and indicate your *
// * acceptance of all terms of the Geant4 Software license.          *
// ********************************************************************
//
//
// Original author: Paul Kent, July 95/96
//
/// \brief { Class description:
///
/// G4ITNavigator is a duplicate version of G4Navigator started from Geant4.9.5
/// initially written by Paul Kent and colleagues.
/// The only difference resides in the way the information is saved and managed
///
/// A class for use by the tracking management, able to obtain/calculate
/// dynamic tracking time information such as the distance to the next volume,
/// or to find the physical volume containing a given point in the world
/// reference system. The navigator maintains a transformation history and
/// other information to optimise the tracking time performance.}
//
// Contact : Mathieu Karamitros (kara (AT) cenbg . in2p3 . fr)
//
// WARNING : This class is released as a prototype.
// It might strongly evolve or even disappear in the next releases.
//
// We would be very happy hearing from you, send us your feedback! :)
//
// History:
// - Created.                                  Paul Kent,     Jul 95/96
// - Zero step protections                     J.A. / G.C.,   Nov  2004
// - Added check mode                          G. Cosmo,      Mar  2004
// - Made Navigator Abstract                   G. Cosmo,      Nov  2003
// - G4ITNavigator created                     M.K.,          Nov  2012
// *********************************************************************
 
#ifndef G4ITNAVIGATOR_HH
#define G4ITNAVIGATOR_HH
 
#include "geomdefs.hh"
 
#include "G4ThreeVector.hh"
#include "G4AffineTransform.hh"
#include "G4RotationMatrix.hh"
 
#include "G4LogicalVolume.hh"             // Used in inline methods
#include "G4GRSVolume.hh"                 //    "         "
#include "G4GRSSolid.hh"                  //    "         "
#include "G4TouchableHandle.hh"           //    "         "
#include "G4TouchableHistoryHandle.hh"
 
#include "G4NavigationHistory.hh"
#include "G4NormalNavigation.hh"
#include "G4VoxelNavigation.hh"
#include "G4ParameterisedNavigation.hh"
#include "G4ReplicaNavigation.hh"
#include "G4RegularNavigation.hh"
 
#include <iostream>
#include "G4TrackState.hh"
#include <memory>
 
class G4VPhysicalVolume;
 
struct G4ITNavigatorState_Lock2
{
  virtual ~G4ITNavigatorState_Lock2()
  {
    ;
  }
protected:
  G4ITNavigatorState_Lock2()
  {
    ;
  }
  G4ITNavigatorState_Lock2(const G4ITNavigatorState_Lock2&)
  {
    ;
  }
};
 
class G4ITNavigator2
{
public:
  static const G4int fMaxNav = 8; // rename to kMaxNoNav ??
 
public:
  // with description
 
  friend std::ostream& operator <<(std::ostream &os, const G4ITNavigator2 &n);
 
  G4ITNavigator2();
  // Constructor - initialisers and setup.
 
  virtual ~G4ITNavigator2();
  // Destructor. No actions.
 
  // !>
  G4ITNavigatorState_Lock2* GetNavigatorState();
  void SetNavigatorState(G4ITNavigatorState_Lock2*);
  void NewNavigatorState();
  void NewNavigatorState(const G4TouchableHistory &h);
  void ResetNavigatorState();
  G4VPhysicalVolume* NewNavigatorStateAndLocate(const G4ThreeVector &p,
                                                const G4ThreeVector &direction);
  void CheckNavigatorState() const;
 
  inline std::shared_ptr<G4ITNavigatorState_Lock2> GetSnapshotOfState();
  inline void ResetFromSnapshot(std::shared_ptr<G4ITNavigatorState_Lock2>);
  // <!
 
  virtual G4double ComputeStep(const G4ThreeVector &pGlobalPoint,
                               const G4ThreeVector &pDirection,
                               const G4double pCurrentProposedStepLength,
                                     G4double  &pNewSafety);
  // Calculate the distance to the next boundary intersected
  // along the specified NORMALISED vector direction and
  // from the specified point in the global coordinate
  // system. LocateGlobalPointAndSetup or LocateGlobalPointWithinVolume
  // must have been called with the same global point prior to this call.
  // The isotropic distance to the nearest boundary is also
  // calculated (usually an underestimate). The current
  // proposed Step length is used to avoid intersection
  // calculations: if it can be determined that the nearest
  // boundary is >pCurrentProposedStepLength away, kInfinity
  // is returned together with the computed isotropic safety
  // distance. Geometry must be closed.
 
  G4double CheckNextStep(const G4ThreeVector &pGlobalPoint,
                         const G4ThreeVector &pDirection,
                         const G4double pCurrentProposedStepLength,
                               G4double &pNewSafety);
  // Same as above, but do not disturb the state of the Navigator.
 
  virtual G4VPhysicalVolume* ResetHierarchyAndLocate(const G4ThreeVector &point,
                                                     const G4ThreeVector &direction,
                                                     const G4TouchableHistory &h);
 
  // Resets the geometrical hierarchy and search for the volumes deepest
  // in the hierarchy containing the point in the global coordinate space.
  // The direction is used to check if a volume is entered.
  // The search begin is the geometrical hierarchy at the location of the
  // last located point, or the endpoint of the previous Step if
  // SetGeometricallyLimitedStep() has been called immediately before.
  //
  // Important Note: In order to call this the geometry MUST be closed.
 
  virtual
  G4VPhysicalVolume* LocateGlobalPointAndSetup(const G4ThreeVector& point,
                                             const G4ThreeVector* direction=0,
                                             const G4bool pRelativeSearch=true,
                                             const G4bool ignoreDirection=true);
    // Search the geometrical hierarchy for the volumes deepest in the hierarchy
    // containing the point in the global coordinate space. Two main cases are:
    //  i) If pRelativeSearch=false it makes use of no previous/state
    //     information. Returns the physical volume containing the point, 
    //     with all previous mothers correctly set up.
    // ii) If pRelativeSearch is set to true, the search begin is the
    //     geometrical hierarchy at the location of the last located point,
    //     or the endpoint of the previous Step if SetGeometricallyLimitedStep()
    //     has been called immediately before.
    // The direction is used (to check if a volume is entered) if either
    //   - the argument ignoreDirection is false, or
    //   - the Navigator has determined that it is on an edge shared by two or
    //     more volumes.  (This is state information.)
    // 
    // Important Note: In order to call this the geometry MUST be closed.
 
  //----------------------------------------------------------------------------
  EInside InsideCurrentVolume(const G4ThreeVector& globalPoint) const;
 
  void GetRandomInCurrentVolume(G4ThreeVector& rndmPoint) const;
  //----------------------------------------------------------------------------
 
  virtual
  void LocateGlobalPointWithinVolume(const G4ThreeVector& position);
    // Notify the Navigator that a track has moved to the new Global point
    // 'position', that is known to be within the current safety.
    // No check is performed to ensure that it is within  the volume. 
    // This method can be called instead of LocateGlobalPointAndSetup ONLY if
    // the caller is certain that the new global point (position) is inside the
    // same volume as the previous position.  Usually this can be guaranteed
    // only if the point is within safety.
 
  inline void LocateGlobalPointAndUpdateTouchableHandle(
                const G4ThreeVector&       position,
                const G4ThreeVector&       direction,
                      G4TouchableHandle&   oldTouchableToUpdate,
                const G4bool               RelativeSearch = true);
    // First, search the geometrical hierarchy like the above method
    // LocateGlobalPointAndSetup(). Then use the volume found and its
    // navigation history to update the touchable.
 
  inline void LocateGlobalPointAndUpdateTouchable(
                const G4ThreeVector&       position,
                const G4ThreeVector&       direction,
                      G4VTouchable*        touchableToUpdate,
                const G4bool               RelativeSearch = true);
    // First, search the geometrical hierarchy like the above method
    // LocateGlobalPointAndSetup(). Then use the volume found and its
    // navigation history to update the touchable.
 
  inline void LocateGlobalPointAndUpdateTouchable(
                const G4ThreeVector&       position,
                      G4VTouchable*        touchableToUpdate,
                const G4bool               RelativeSearch = true);
    // Same as the method above but missing direction.
 
  inline void SetGeometricallyLimitedStep();
    // Inform the navigator that the previous Step calculated
    // by the geometry was taken in its entirety.
 
  virtual G4double ComputeSafety(const G4ThreeVector &globalpoint,
                                 const G4double pProposedMaxLength = DBL_MAX,
                                 const G4bool keepState = true);
    // Calculate the isotropic distance to the nearest boundary from the
    // specified point in the global coordinate system. 
    // The globalpoint utilised must be within the current volume.
    // The value returned is usually an underestimate.  
    // The proposed maximum length is used to avoid volume safety
    //  calculations.  The geometry must be closed.
    // To ensure minimum side effects from the call, keepState
    //  must be true.
  
   virtual G4bool RecheckDistanceToCurrentBoundary(
                               const G4ThreeVector &pGlobalPoint,
                               const G4ThreeVector &pDirection,
                               const G4double  CurrentProposedStepLength,
                                     G4double *prDistance,
                                     G4double *prNewSafety=0) const;
    // Trial method for checking potential displacement for MS
    // Check new Globalpoint, to see whether it is in current volume
    // (mother) and not in potential entering daughter.
    // If in mother, check distance to boundary along pDirection.
    // If in entering daughter, check distance back to boundary. 
    // NOTE:
    // Can be called only after ComputeStep is called - before ReLocation
    // Deals only with current volume (and potentially entered)
 
  inline G4VPhysicalVolume* GetWorldVolume() const;
    // Return the current  world (`topmost') volume.
 
  inline void SetWorldVolume(G4VPhysicalVolume* pWorld);
    // Set the world (`topmost') volume. This must be positioned at
    // origin (0,0,0) and unrotated.
 
  inline G4GRSVolume* CreateGRSVolume() const;
  inline G4GRSSolid* CreateGRSSolid() const;
  inline G4TouchableHistory* CreateTouchableHistory() const;
  inline G4TouchableHistory* CreateTouchableHistory(const G4NavigationHistory*) const;
    // `Touchable' creation methods: caller has deletion responsibility.
 
  virtual G4TouchableHistoryHandle CreateTouchableHistoryHandle() const;
    // Returns a reference counted handle to a touchable history.
 
  virtual G4ThreeVector GetLocalExitNormal(G4bool* valid);
  virtual G4ThreeVector GetLocalExitNormalAndCheck(const G4ThreeVector& point,
                                                         G4bool* valid);
  virtual G4ThreeVector GetGlobalExitNormal(const G4ThreeVector& point,
                                                  G4bool* valid);
    // Return Exit Surface Normal and validity too.
    // Can only be called if the Navigator's last Step has crossed a
    // volume geometrical boundary.
    // It returns the Normal to the surface pointing out of the volume that
    // was left behind and/or into the volume that was entered.
    // Convention:
    //   The *local* normal is in the coordinate system of the *final* volume.
    // Restriction:
    //   Normals are not available for replica volumes (returns valid= false)
    // These methods takes full care about how to calculate this normal,
    // but if the surfaces are not convex it will return valid=false.
 
  inline G4int GetVerboseLevel() const;
  inline void  SetVerboseLevel(G4int level);
    // Get/Set Verbose(ness) level.
    // [if level>0 && G4VERBOSE, printout can occur]
 
  inline G4bool IsActive() const;
    // Verify if the navigator is active.
  inline void  Activate(G4bool flag);
    // Activate/inactivate the navigator.
 
  inline G4bool EnteredDaughterVolume() const;
    // The purpose of this function is to inform the caller if the track is
    // entering a daughter volume while exiting from the current volume.
    // This method returns 
    // - True only in case 1) above, that is when the Step has caused
    //   the track to arrive at a boundary of a daughter.
    // - False in cases 2), 3) and 4), i.e. in all other cases.
    // This function is not guaranteed to work if SetGeometricallyLimitedStep()
    // was not called when it should have been called.
  inline G4bool ExitedMotherVolume() const;
  // Verify if the step has exited the mother volume.
 
  inline void   CheckMode(G4bool mode);
    // Run navigation in "check-mode", therefore using additional
    // verifications and more strict correctness conditions.
    // Is effective only with G4VERBOSE set.
  inline G4bool IsCheckModeActive() const;
  inline void   SetPushVerbosity(G4bool mode);
    // Set/unset verbosity for pushed tracks (default is true).
 
  void PrintState() const;
    // Print the internal state of the Navigator (for debugging).
    // The level of detail is according to the verbosity.
 
  inline const G4AffineTransform& GetGlobalToLocalTransform() const;
  inline const G4AffineTransform  GetLocalToGlobalTransform() const;
    // Obtain the transformations Global/Local (and inverse).
    // Clients of these methods must copy the data if they need to keep it.
 
  G4AffineTransform GetMotherToDaughterTransform(G4VPhysicalVolume* dVolume, 
                                                 G4int dReplicaNo,
                                                 EVolume dVolumeType );
    // Obtain mother to daughter transformation
 
  inline void ResetStackAndState();
    // Reset stack and minimum or navigator state machine necessary for reset
    // as needed by LocalGlobalPointAndSetup.
    // [Does not perform clears, resizes, or reset fLastLocatedPointLocal]
 
  inline G4int SeverityOfZeroStepping( G4int* noZeroSteps ) const;
  // Report on severity of error and number of zero steps,
  // in case Navigator is stuck and is returning zero steps.
  // Values: 1 (small problem),  5 (correcting),
  //         9 (ready to abandon), 10 (abandoned)
 
  /*
   void SetSavedState();
   // ( fValidExitNormal, fExitNormal, fExiting, fEntering,
   //   fBlockedPhysicalVolume, fBlockedReplicaNo, fLastStepWasZero);
   void RestoreSavedState();
   // Copy aspects of the state, to enable a non-state changing
   //  call to ComputeStep
   */
  inline G4ThreeVector GetCurrentLocalCoordinate() const;
    // Return the local coordinate of the point in the reference system
    // of its containing volume that was found by LocalGlobalPointAndSetup.
    // The local coordinate of the last located track.
 
  inline G4ThreeVector NetTranslation() const;
  inline G4RotationMatrix NetRotation() const;
  // Compute+return the local->global translation/rotation of current volume.
 
  inline void EnableBestSafety( G4bool value= false );
  // Enable best-possible evaluation of isotropic safety
 
protected:// with description
 
  inline G4ThreeVector ComputeLocalPoint(const G4ThreeVector& rGlobPoint) const;
  // Return position vector in local coordinate system, given a position
  // vector in world coordinate system.
 
  inline G4ThreeVector ComputeLocalAxis(const G4ThreeVector& pVec) const;
  // Return the local direction of the specified vector in the reference
  // system of the volume that was found by LocalGlobalPointAndSetup.
  // The Local Coordinates of point in world coordinate system.
 
  virtual void ResetState();
  // Utility method to reset the navigator state machine.
 
  inline EVolume VolumeType(const G4VPhysicalVolume *pVol) const;
  // Characterise `type' of volume - normal/replicated/parameterised.
 
  inline EVolume CharacteriseDaughters(const G4LogicalVolume *pLog) const;
  // Characterise daughter of logical volume.
 
  inline G4int GetDaughtersRegularStructureId(const G4LogicalVolume *pLog) const;
  // Get regular structure ID of first daughter
 
  virtual void SetupHierarchy();
  // Renavigate & reset hierarchy described by current history
  // o Reset volumes
  // o Recompute transforms and/or solids of replicated/parameterised
  //   volumes.
 
private:
 
  G4ITNavigator2(const G4ITNavigator2&);
  G4ITNavigator2& operator=(const G4ITNavigator2&);
  // Private copy-constructor and assignment operator.
 
  void ComputeStepLog(const G4ThreeVector& pGlobalpoint,
  G4double moveLenSq) const;
  // Log and checks for steps larger than the tolerance
 
protected:// without description
 
  G4double kCarTolerance;
  // Geometrical tolerance for surface thickness of shapes.
 
  G4int fVerbose;
  // Verbose(ness) level  [if > 0, printout can occur].
 
private:
 
  G4bool fActive;
  // States if the navigator is activated or not.
 
  G4int fActionThreshold_NoZeroSteps;
  // After this many failed/zero steps, act (push etc)
  G4int fAbandonThreshold_NoZeroSteps;
  // After this many failed/zero steps, abandon track
 
protected:
public:
  struct G4SaveNavigatorState;
 
  //
  // BEGIN State information
  //
  struct G4NavigatorState : public G4ITNavigatorState_Lock2
  {
    G4NavigatorState();
    G4NavigatorState(const G4NavigatorState&);
    virtual ~G4NavigatorState()
    { ;}
 
    G4NavigatorState& operator=(const G4NavigatorState& );
    G4NavigatorState& operator=(const G4SaveNavigatorState& );
 
    void ResetState();
    void ResetStack();
    void ResetStackAndState();
 
    G4NavigationHistory fHistory;
    // Transformation and history of the current path
    // through the geometrical hierarchy.
 
    G4bool fEnteredDaughter;
    // A memory of whether in this Step a daughter volume is entered
    // (set in Compute & Locate).
    //  After Compute: it expects to enter a daughter
    //  After Locate:  it has entered a daughter
 
    G4bool fExitedMother;
    // A similar memory whether the Step exited current "mother" volume
    // completely, not entering daughter.
 
    G4bool fWasLimitedByGeometry;
    // Set true if last Step was limited by geometry.
 
    G4ThreeVector fStepEndPoint;
    // Endpoint of last ComputeStep
    // can be used for optimisation (e.g. when computing safety).
    G4ThreeVector fLastStepEndPointLocal;
    // Position of the end-point of the last call to ComputeStep
    // in last Local coordinates.
 
    G4bool fPushed;
    // Push flags  [if true, means a stuck particle has been pushed].
 
  private:
    friend class G4ITNavigator2;
    // The friend class would allow G4Navigator to access the private members
    // of G4NavigatorState but not the classes inheriting from G4Navigator
 
    friend struct G4SaveNavigatorState;
    friend std::ostream& operator << (std::ostream &os,const G4ITNavigator2 &n);
 
    G4bool fLastTriedStepComputation;
    // Whether ComputeStep was called since the last call to a Locate method
    // Uses: - distinguish parts of state which differ before/after calls
    //         to ComputeStep or one of the Locate methods;
    //       - avoid two consecutive calls to compute-step (illegal).
 
    G4bool fEntering,fExiting;
    // Entering/Exiting volumes blocking/setup
    // o If exiting
    //      volume ptr & replica number (set & used by Locate..())
    //      used for blocking on redescent of geometry
    // o If entering
    //      volume ptr & replica number (set by ComputeStep(),used by
    //      Locate..()) of volume for `automatic' entry
 
    G4VPhysicalVolume *fBlockedPhysicalVolume;
    G4int fBlockedReplicaNo;
 
    G4ThreeVector fLastLocatedPointLocal;
    // Position of the last located point relative to its containing volume.
    G4bool fLocatedOutsideWorld;
    // Whether the last call to Locate methods left the world
 
    G4bool fValidExitNormal;// Set true if have leaving volume normal
    G4ThreeVector fExitNormal;// Leaving volume normal, in the
    // volume containing the exited
    // volume's coordinate system
    G4ThreeVector fGrandMotherExitNormal;// Leaving volume normal, in its
    // own coordinate system
    G4bool fChangedGrandMotherRefFrame;// Whether frame is changed
 
    G4ThreeVector fExitNormalGlobalFrame;// Leaving volume normal, in the
    // global coordinate system
    G4bool fCalculatedExitNormal;// Has it been computed since
    // the last call to ComputeStep
    // Covers both Global and GrandMother
 
    // Count zero steps - as one or two can occur due to changing momentum at
    //                    a boundary or at an edge common between volumes
    //                  - several are likely a problem in the geometry
    //                    description or in the navigation
    //
    G4bool fLastStepWasZero;
    // Whether the last ComputeStep moved Zero. Used to check for edges.
 
    G4bool fLocatedOnEdge;
    // Whether the Navigator has detected an edge
    G4int fNumberZeroSteps;
    // Number of preceding moves that were Zero. Reset to 0 after finite step
 
    G4ThreeVector fPreviousSftOrigin;
    G4double fPreviousSafety;
    // Memory of last safety origin & value. Used in ComputeStep to ensure
    // that origin of current Step is in the same volume as the point of the
    // last relocation
 
    //
    // END State information
    //
  };
 
  G4NavigatorState* fpNavigatorState;
 
  // Save key state information (NOT the navigation history stack)
  //
  struct G4SaveNavigatorState
  {
    G4SaveNavigatorState();
    G4SaveNavigatorState(G4NavigatorState*);
    virtual ~G4SaveNavigatorState()
    { ;}
    G4ThreeVector sExitNormal;
    G4bool sValidExitNormal;
    G4bool sEntering, sExiting;
    G4VPhysicalVolume* spBlockedPhysicalVolume;
    G4int sBlockedReplicaNo;
    G4int sLastStepWasZero;
 
    // !>
//    G4bool sLocatedOnEdge;
//    G4bool sPushed;
//    G4int sNumberZeroSteps;
    // <!
 
    //  Potentially relevant
    //
    G4bool sLocatedOutsideWorld;
    G4ThreeVector sLastLocatedPointLocal;
    G4bool sEnteredDaughter, sExitedMother;
    G4bool sWasLimitedByGeometry;
 
    G4ThreeVector sPreviousSftOrigin;
    G4double sPreviousSafety;
  };
 
  // G4SaveNavigatorState* fpSaveState;
 
  // Tracking Invariants
  //
  G4VPhysicalVolume *fTopPhysical;
  // A link to the topmost physical volume in the detector.
  // Must be positioned at the origin and unrotated.
 
  // Utility information
  //
  G4bool fCheck;
  // Check-mode flag  [if true, more strict checks are performed].
 
  G4bool fWarnPush;
  // Push flag  [for verbose].
 
  // Helpers/Utility classes
  //
  G4NormalNavigation fnormalNav;
  G4VoxelNavigation fvoxelNav;
  G4ParameterisedNavigation fparamNav;
  G4ReplicaNavigation freplicaNav;
  G4RegularNavigation fregularNav;
  G4VoxelSafety *fpVoxelSafety;
};
 
RegisterTrackState(G4ITNavigator2, G4NavigatorState)
 
#define CheckNavigatorStateIsValid() \
if(fpNavigatorState == 0) \
{ \
    G4ExceptionDescription exceptionDescription; \
    exceptionDescription << "The navigator state is NULL. "; \
    exceptionDescription << "Either NewNavigatorStateAndLocate was not called "; \
    exceptionDescription << "or the provided navigator state was already NULL."; \
    G4Exception((G4String("G4Navigator")+G4String(__FUNCTION__)).c_str(),\
                "NavigatorStateNotValid",FatalException,exceptionDescription); \
}
 
#include "G4ITNavigator2.icc"
 
#endif
 
// NOTES:
//
// The following methods provide detailed information when a Step has
// arrived at a geometrical boundary.  They distinguish between the different
// causes that can result in the track leaving its current volume.
//
// Four cases are possible:
//
// 1) The particle has reached a boundary of a daughter of the current volume:
//     (this could cause the relocation to enter the daughter itself
//     or a potential granddaughter or further descendant)
//     
// 2) The particle has reached a boundary of the current
//     volume, exiting into a mother (regardless the level
//     at which it is located in the tree):
//
// 3) The particle has reached a boundary of the current
//     volume, exiting into a volume which is not in its
//     parental hierarchy:
//
// 4) The particle is not on a boundary between volumes:
//     the function returns an exception, and the caller is
//     reccomended to compare the G4touchables associated
//     to the preStepPoint and postStepPoint to handle this case.
//
//   G4bool        EnteredDaughterVolume()
//   G4bool        IsExitNormalValid()
//   G4ThreeVector GetLocalExitNormal()
//
// The expected usefulness of these methods is to allow the caller to
// determine how to compute the surface normal at the volume boundary. The two
// possibilities are to obtain the normal from:
//
//   i) the solid associated with the volume of the initial point of the Step.
//      This is valid for cases 2 and 3.  
//      (Note that the initial point is generally the PreStepPoint of a Step).
//   or
// 
//  ii) the solid of the final point, ie of the volume after the relocation.
//      This is valid for case 1.
//      (Note that the final point is generally the PreStepPoint of a Step).
//
// This way the caller can always get a valid normal, pointing outside
// the solid for which it is computed, that can be used at his own
// discretion.