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0001 //
0002 // ********************************************************************
0003 // * License and Disclaimer                                           *
0004 // *                                                                  *
0005 // * The  Geant4 software  is  copyright of the Copyright Holders  of *
0006 // * the Geant4 Collaboration.  It is provided  under  the terms  and *
0007 // * conditions of the Geant4 Software License,  included in the file *
0008 // * LICENSE and available at  http://cern.ch/geant4/license .  These *
0009 // * include a list of copyright holders.                             *
0010 // *                                                                  *
0011 // * Neither the authors of this software system, nor their employing *
0012 // * institutes,nor the agencies providing financial support for this *
0013 // * work  make  any representation or  warranty, express or implied, *
0014 // * regarding  this  software system or assume any liability for its *
0015 // * use.  Please see the license in the file  LICENSE  and URL above *
0016 // * for the full disclaimer and the limitation of liability.         *
0017 // *                                                                  *
0018 // * This  code  implementation is the result of  the  scientific and *
0019 // * technical work of the GEANT4 collaboration.                      *
0020 // * By using,  copying,  modifying or  distributing the software (or *
0021 // * any work based  on the software)  you  agree  to acknowledge its *
0022 // * use  in  resulting  scientific  publications,  and indicate your *
0023 // * acceptance of all terms of the Geant4 Software license.          *
0024 // ********************************************************************
0025 //
0026 //
0027 // Original author: Paul Kent, July 95/96
0028 //
0029 /// \brief { Class description:
0030 ///
0031 /// G4ITNavigator1 is a duplicate version of G4Navigator started from Geant4.9.5
0032 /// initially written by Paul Kent and colleagues.
0033 /// The only difference resides in the way the information is saved and managed
0034 ///
0035 /// A class for use by the tracking management, able to obtain/calculate
0036 /// dynamic tracking time information such as the distance to the next volume,
0037 /// or to find the physical volume containing a given point in the world
0038 /// reference system. The navigator maintains a transformation history and
0039 /// other information to optimise the tracking time performance.}
0040 //
0041 // Contact : Mathieu Karamitros (kara (AT) cenbg . in2p3 . fr)
0042 //
0043 // WARNING : This class is released as a prototype.
0044 // It might strongly evolve or even disapear in the next releases.
0045 //
0046 // We would be very happy hearing from you, send us your feedback! :)
0047 //
0048 // History:
0049 // - Created.                                  Paul Kent,     Jul 95/96
0050 // - Zero step protections                     J.A. / G.C.,   Nov  2004
0051 // - Added check mode                          G. Cosmo,      Mar  2004
0052 // - Made Navigator Abstract                   G. Cosmo,      Nov  2003
0053 // - G4ITNavigator1 created                     M.K.,          Nov  2012
0054 // *********************************************************************
0055 
0056 #ifndef G4ITNAVIGATOR_HH
0057 #define G4ITNAVIGATOR_HH
0058 
0059 #include "geomdefs.hh"
0060 
0061 #include "G4ThreeVector.hh"
0062 #include "G4AffineTransform.hh"
0063 #include "G4RotationMatrix.hh"
0064 
0065 #include "G4LogicalVolume.hh"             // Used in inline methods
0066 #include "G4TouchableHandle.hh"           //    "         "
0067 
0068 #include "G4NavigationHistory.hh"
0069 #include "G4NormalNavigation.hh"
0070 #include "G4VoxelNavigation.hh"
0071 #include "G4ParameterisedNavigation.hh"
0072 #include "G4ReplicaNavigation.hh"
0073 #include "G4RegularNavigation.hh"
0074 
0075 #include <iostream>
0076 
0077 class G4VPhysicalVolume;
0078 
0079 
0080 struct G4ITNavigatorState_Lock1
0081 {
0082     virtual ~G4ITNavigatorState_Lock1()= default;
0083 protected:
0084     G4ITNavigatorState_Lock1(){}
0085 };
0086 
0087 class G4ITNavigator1
0088 {
0089 public:
0090     static const G4int fMaxNav = 8;   // rename to kMaxNoNav ??
0091 
0092   public:  // with description
0093 
0094   friend std::ostream& operator << (std::ostream &os, const G4ITNavigator1 &n);
0095 
0096   G4ITNavigator1();
0097     // Constructor - initialisers and setup.
0098 
0099   virtual ~G4ITNavigator1();
0100     // Destructor. No actions.
0101 
0102   G4ITNavigator1(const G4ITNavigator1&) = delete;
0103   G4ITNavigator1& operator=(const G4ITNavigator1&) = delete;
0104 
0105   // !>
0106     G4ITNavigatorState_Lock1* GetNavigatorState();
0107     void SetNavigatorState(G4ITNavigatorState_Lock1*);
0108     void NewNavigatorState();
0109   // <!
0110 
0111   virtual G4double ComputeStep(const G4ThreeVector &pGlobalPoint,
0112                                const G4ThreeVector &pDirection,
0113                                const G4double pCurrentProposedStepLength,
0114                                      G4double  &pNewSafety);
0115     // Calculate the distance to the next boundary intersected
0116     // along the specified NORMALISED vector direction and
0117     // from the specified point in the global coordinate
0118     // system. LocateGlobalPointAndSetup or LocateGlobalPointWithinVolume 
0119     // must have been called with the same global point prior to this call.
0120     // The isotropic distance to the nearest boundary is also
0121     // calculated (usually an underestimate). The current
0122     // proposed Step length is used to avoid intersection
0123     // calculations: if it can be determined that the nearest
0124     // boundary is >pCurrentProposedStepLength away, kInfinity
0125     // is returned together with the computed isotropic safety
0126     // distance. Geometry must be closed.
0127 
0128   G4double CheckNextStep(const G4ThreeVector &pGlobalPoint,
0129                          const G4ThreeVector &pDirection,
0130                          const G4double pCurrentProposedStepLength,
0131                                G4double &pNewSafety); 
0132     // Same as above, but do not disturb the state of the Navigator.
0133 
0134   virtual
0135   G4VPhysicalVolume* ResetHierarchyAndLocate(const G4ThreeVector &point,
0136                                              const G4ThreeVector &direction,
0137                                              const G4TouchableHistory &h);
0138 
0139     // Resets the geometrical hierarchy and search for the volumes deepest
0140     // in the hierarchy containing the point in the global coordinate space.
0141     // The direction is used to check if a volume is entered.
0142     // The search begin is the geometrical hierarchy at the location of the
0143     // last located point, or the endpoint of the previous Step if
0144     // SetGeometricallyLimitedStep() has been called immediately before.
0145     // 
0146     // Important Note: In order to call this the geometry MUST be closed.
0147 
0148   virtual
0149   G4VPhysicalVolume* LocateGlobalPointAndSetup(const G4ThreeVector& point,
0150                                              const G4ThreeVector* direction=nullptr,
0151                                              const G4bool pRelativeSearch=true,
0152                                              const G4bool ignoreDirection=true);
0153     // Search the geometrical hierarchy for the volumes deepest in the hierarchy
0154     // containing the point in the global coordinate space. Two main cases are:
0155     //  i) If pRelativeSearch=false it makes use of no previous/state
0156     //     information. Returns the physical volume containing the point, 
0157     //     with all previous mothers correctly set up.
0158     // ii) If pRelativeSearch is set to true, the search begin is the
0159     //     geometrical hierarchy at the location of the last located point,
0160     //     or the endpoint of the previous Step if SetGeometricallyLimitedStep()
0161     //     has been called immediately before.
0162     // The direction is used (to check if a volume is entered) if either
0163     //   - the argument ignoreDirection is false, or
0164     //   - the Navigator has determined that it is on an edge shared by two or
0165     //     more volumes.  (This is state information.)
0166     // 
0167     // Important Note: In order to call this the geometry MUST be closed.
0168 
0169   virtual
0170   void LocateGlobalPointWithinVolume(const G4ThreeVector& position);
0171     // Notify the Navigator that a track has moved to the new Global point
0172     // 'position', that is known to be within the current safety.
0173     // No check is performed to ensure that it is within  the volume. 
0174     // This method can be called instead of LocateGlobalPointAndSetup ONLY if
0175     // the caller is certain that the new global point (position) is inside the
0176     // same volume as the previous position.  Usually this can be guaranteed
0177     // only if the point is within safety.
0178 
0179   inline void LocateGlobalPointAndUpdateTouchableHandle(
0180                 const G4ThreeVector&       position,
0181                 const G4ThreeVector&       direction,
0182                       G4TouchableHandle&   oldTouchableToUpdate,
0183                 const G4bool               RelativeSearch = true);
0184     // First, search the geometrical hierarchy like the above method
0185     // LocateGlobalPointAndSetup(). Then use the volume found and its
0186     // navigation history to update the touchable.
0187 
0188   inline void LocateGlobalPointAndUpdateTouchable(
0189                 const G4ThreeVector&       position,
0190                 const G4ThreeVector&       direction,
0191                       G4VTouchable*        touchableToUpdate,
0192                 const G4bool               RelativeSearch = true);
0193     // First, search the geometrical hierarchy like the above method
0194     // LocateGlobalPointAndSetup(). Then use the volume found and its
0195     // navigation history to update the touchable.
0196 
0197   inline void LocateGlobalPointAndUpdateTouchable(
0198                 const G4ThreeVector&       position,
0199                       G4VTouchable*        touchableToUpdate,
0200                 const G4bool               RelativeSearch = true);
0201     // Same as the method above but missing direction.
0202 
0203   inline void SetGeometricallyLimitedStep();
0204     // Inform the navigator that the previous Step calculated
0205     // by the geometry was taken in its entirety.
0206 
0207   virtual G4double ComputeSafety(const G4ThreeVector &globalpoint,
0208                                  const G4double pProposedMaxLength = DBL_MAX,
0209                                  const G4bool keepState = true);
0210     // Calculate the isotropic distance to the nearest boundary from the
0211     // specified point in the global coordinate system. 
0212     // The globalpoint utilised must be within the current volume.
0213     // The value returned is usually an underestimate.  
0214     // The proposed maximum length is used to avoid volume safety
0215     //  calculations.  The geometry must be closed.
0216     // To ensure minimum side effects from the call, keepState
0217     //  must be true.
0218   
0219   inline G4VPhysicalVolume* GetWorldVolume() const;
0220     // Return the current  world (`topmost') volume.
0221 
0222   inline void SetWorldVolume(G4VPhysicalVolume* pWorld);
0223     // Set the world (`topmost') volume. This must be positioned at
0224     // origin (0,0,0) and unrotated.
0225 
0226   inline G4TouchableHistory* CreateTouchableHistory() const;
0227   inline G4TouchableHistory* CreateTouchableHistory(const G4NavigationHistory*) const;
0228     // `Touchable' creation methods: caller has deletion responsibility.
0229 
0230   virtual G4TouchableHandle CreateTouchableHistoryHandle() const;
0231     // Returns a reference counted handle to a touchable history.
0232 
0233   virtual G4ThreeVector GetLocalExitNormal(G4bool* valid);
0234   virtual G4ThreeVector GetLocalExitNormalAndCheck(const G4ThreeVector& point,
0235                                                          G4bool* valid);
0236   virtual G4ThreeVector GetGlobalExitNormal(const G4ThreeVector& point,
0237                                                   G4bool* valid);
0238     // Return Exit Surface Normal and validity too.
0239     // Can only be called if the Navigator's last Step has crossed a
0240     // volume geometrical boundary.
0241     // It returns the Normal to the surface pointing out of the volume that
0242     // was left behind and/or into the volume that was entered.
0243     // Convention:
0244     //   The *local* normal is in the coordinate system of the *final* volume.
0245     // Restriction:
0246     //   Normals are not available for replica volumes (returns valid= false)
0247     // These methods takes full care about how to calculate this normal,
0248     // but if the surfaces are not convex it will return valid=false.
0249 
0250   inline G4int GetVerboseLevel() const;
0251   inline void  SetVerboseLevel(G4int level);
0252     // Get/Set Verbose(ness) level.
0253     // [if level>0 && G4VERBOSE, printout can occur]
0254 
0255   inline G4bool IsActive() const;
0256     // Verify if the navigator is active.
0257   inline void  Activate(G4bool flag);
0258     // Activate/inactivate the navigator.
0259 
0260   inline G4bool EnteredDaughterVolume() const;
0261     // The purpose of this function is to inform the caller if the track is
0262     // entering a daughter volume while exiting from the current volume.
0263     // This method returns 
0264     // - True only in case 1) above, that is when the Step has caused
0265     //   the track to arrive at a boundary of a daughter.
0266     // - False in cases 2), 3) and 4), i.e. in all other cases.
0267     // This function is not guaranteed to work if SetGeometricallyLimitedStep()
0268     // was not called when it should have been called.
0269   inline G4bool ExitedMotherVolume() const;
0270     // Verify if the step has exited the mother volume.
0271 
0272   inline void   CheckMode(G4bool mode);
0273     // Run navigation in "check-mode", therefore using additional
0274     // verifications and more strict correctness conditions.
0275     // Is effective only with G4VERBOSE set.
0276   inline G4bool IsCheckModeActive() const;
0277   inline void   SetPushVerbosity(G4bool mode);
0278     // Set/unset verbosity for pushed tracks (default is true).
0279 
0280   void PrintState() const;
0281     // Print the internal state of the Navigator (for debugging).
0282     // The level of detail is according to the verbosity.
0283 
0284   inline const G4AffineTransform& GetGlobalToLocalTransform() const;
0285   inline const G4AffineTransform  GetLocalToGlobalTransform() const;
0286     // Obtain the transformations Global/Local (and inverse).
0287     // Clients of these methods must copy the data if they need to keep it.
0288 
0289   G4AffineTransform GetMotherToDaughterTransform(G4VPhysicalVolume* dVolume, 
0290                                                  G4int dReplicaNo,
0291                                                  EVolume dVolumeType );
0292     // Obtain mother to daughter transformation
0293 
0294   inline void ResetStackAndState();
0295     // Reset stack and minimum or navigator state machine necessary for reset
0296     // as needed by LocalGlobalPointAndSetup.
0297     // [Does not perform clears, resizes, or reset fLastLocatedPointLocal]
0298 
0299   inline G4int SeverityOfZeroStepping( G4int* noZeroSteps ) const; 
0300     // Report on severity of error and number of zero steps,
0301     // in case Navigator is stuck and is returning zero steps.
0302     // Values: 1 (small problem),  5 (correcting), 
0303     //         9 (ready to abandon), 10 (abandoned)
0304 
0305   void SetSavedState(); 
0306     // ( fValidExitNormal, fExitNormal, fExiting, fEntering, 
0307     //   fBlockedPhysicalVolume, fBlockedReplicaNo, fLastStepWasZero); 
0308   void RestoreSavedState(); 
0309     // Copy aspects of the state, to enable a non-state changing
0310     //  call to ComputeStep
0311 
0312   inline G4ThreeVector GetCurrentLocalCoordinate() const;
0313     // Return the local coordinate of the point in the reference system
0314     // of its containing volume that was found by LocalGlobalPointAndSetup.
0315     // The local coordinate of the last located track.
0316 
0317   inline G4ThreeVector NetTranslation() const;
0318   inline G4RotationMatrix NetRotation() const;
0319     // Compute+return the local->global translation/rotation of current volume.
0320 
0321   inline void EnableBestSafety( G4bool value= false );
0322     // Enable best-possible evaluation of isotropic safety
0323 
0324   virtual void ResetState();
0325     // Utility method to reset the navigator state machine.
0326 
0327  protected:  // with description
0328 
0329   inline G4ThreeVector ComputeLocalPoint(const G4ThreeVector& rGlobPoint) const;
0330     // Return position vector in local coordinate system, given a position
0331     // vector in world coordinate system.
0332 
0333   inline G4ThreeVector ComputeLocalAxis(const G4ThreeVector& pVec) const;
0334     // Return the local direction of the specified vector in the reference
0335     // system of the volume that was found by LocalGlobalPointAndSetup.
0336     // The Local Coordinates of point in world coordinate system.
0337 
0338   inline EVolume VolumeType(const G4VPhysicalVolume *pVol) const;
0339     // Characterise `type' of volume - normal/replicated/parameterised.
0340 
0341   inline EVolume CharacteriseDaughters(const G4LogicalVolume *pLog) const;
0342     // Characterise daughter of logical volume.
0343 
0344   inline G4int GetDaughtersRegularStructureId(const G4LogicalVolume *pLog) const;
0345     // Get regular structure ID of first daughter
0346 
0347   virtual void SetupHierarchy();
0348     // Renavigate & reset hierarchy described by current history
0349     // o Reset volumes
0350     // o Recompute transforms and/or solids of replicated/parameterised
0351     //   volumes.
0352 
0353  private:
0354 
0355   void ComputeStepLog(const G4ThreeVector& pGlobalpoint,
0356                             G4double moveLenSq) const;
0357     // Log and checks for steps larger than the tolerance
0358 
0359  protected:  // without description
0360 
0361   G4double kCarTolerance;
0362     // Geometrical tolerance for surface thickness of shapes.
0363 
0364   //
0365   // BEGIN State information
0366   //
0367 
0368   G4NavigationHistory fHistory;
0369     // Transformation and history of the current path
0370     // through the geometrical hierarchy.
0371 
0372   G4bool fEnteredDaughter;
0373     // A memory of whether in this Step a daughter volume is entered 
0374     // (set in Compute & Locate).
0375     //  After Compute: it expects to enter a daughter
0376     //  After Locate:  it has entered a daughter
0377 
0378   G4bool fExitedMother;
0379     // A similar memory whether the Step exited current "mother" volume
0380     // completely, not entering daughter.
0381 
0382   G4bool fWasLimitedByGeometry{false};
0383     // Set true if last Step was limited by geometry.
0384 
0385   G4ThreeVector fStepEndPoint;
0386     // Endpoint of last ComputeStep 
0387     // can be used for optimisation (e.g. when computing safety).
0388   G4ThreeVector fLastStepEndPointLocal; 
0389     // Position of the end-point of the last call to ComputeStep 
0390     // in last Local coordinates.
0391 
0392   G4int  fVerbose{0};
0393     // Verbose(ness) level  [if > 0, printout can occur].
0394 
0395  private:
0396 
0397   G4bool fActive;
0398     // States if the navigator is activated or not.
0399 
0400   G4bool fLastTriedStepComputation; 
0401     // Whether ComputeStep was called since the last call to a Locate method
0402     // Uses: - distinguish parts of state which differ before/after calls
0403     //         to ComputeStep or one of the Locate methods;
0404     //       - avoid two consecutive calls to compute-step (illegal).
0405 
0406   G4bool fEntering,fExiting;
0407     // Entering/Exiting volumes blocking/setup
0408     // o If exiting
0409     //      volume ptr & replica number (set & used by Locate..())
0410     //      used for blocking on redescent of geometry
0411     // o If entering
0412     //      volume ptr & replica number (set by ComputeStep(),used by
0413     //      Locate..()) of volume for `automatic' entry
0414 
0415   G4VPhysicalVolume *fBlockedPhysicalVolume;
0416   G4int fBlockedReplicaNo;
0417 
0418   G4ThreeVector fLastLocatedPointLocal;
0419     // Position of the last located point relative to its containing volume.
0420   G4bool fLocatedOutsideWorld;
0421     // Whether the last call to Locate methods left the world
0422 
0423   G4bool fValidExitNormal;    // Set true if have leaving volume normal
0424   G4ThreeVector fExitNormal;  // Leaving volume normal, in the
0425                               // volume containing the exited
0426                               // volume's coordinate system
0427   G4ThreeVector fGrandMotherExitNormal;  // Leaving volume normal, in its 
0428                                          // own coordinate system
0429   G4bool  fChangedGrandMotherRefFrame;   // Whether frame is changed
0430 
0431   G4ThreeVector fExitNormalGlobalFrame;  // Leaving volume normal, in the
0432                                          // global coordinate system
0433   G4bool  fCalculatedExitNormal;  // Has it been computed since
0434                                   // the last call to ComputeStep
0435                                   // Covers both Global and GrandMother
0436    
0437   // Count zero steps - as one or two can occur due to changing momentum at
0438   //                    a boundary or at an edge common between volumes
0439   //                  - several are likely a problem in the geometry
0440   //                    description or in the navigation
0441   //
0442   G4bool fLastStepWasZero;
0443     // Whether the last ComputeStep moved Zero. Used to check for edges.
0444 
0445   G4bool fLocatedOnEdge;       
0446     // Whether the Navigator has detected an edge
0447   G4int fNumberZeroSteps;
0448     // Number of preceding moves that were Zero. Reset to 0 after finite step
0449   G4int fActionThreshold_NoZeroSteps;  
0450     // After this many failed/zero steps, act (push etc) 
0451   G4int fAbandonThreshold_NoZeroSteps; 
0452     // After this many failed/zero steps, abandon track
0453 
0454   G4ThreeVector  fPreviousSftOrigin;
0455   G4double       fPreviousSafety; 
0456     // Memory of last safety origin & value. Used in ComputeStep to ensure
0457     // that origin of current Step is in the same volume as the point of the
0458     // last relocation
0459 
0460   //
0461   // END State information
0462   //
0463 
0464   // Save key state information (NOT the navigation history stack)
0465   //
0466   struct G4SaveNavigatorState : public G4ITNavigatorState_Lock1
0467   { 
0468      G4SaveNavigatorState();
0469      ~G4SaveNavigatorState() override= default;
0470      G4ThreeVector sExitNormal;  
0471      G4bool sValidExitNormal;    
0472      G4bool sEntering, sExiting;
0473      G4VPhysicalVolume* spBlockedPhysicalVolume;
0474      G4int sBlockedReplicaNo;  
0475      G4int sLastStepWasZero; 
0476 
0477      // !>
0478      G4bool sLocatedOnEdge;
0479      G4bool sWasLimitedByGeometry;
0480      G4bool sPushed;
0481      G4int  sNumberZeroSteps;
0482      // <!
0483 
0484      //  Potentially relevant
0485      //
0486      G4bool sLocatedOutsideWorld;
0487      G4ThreeVector sLastLocatedPointLocal; 
0488      G4bool sEnteredDaughter, sExitedMother;
0489      G4ThreeVector  sPreviousSftOrigin;
0490      G4double       sPreviousSafety; 
0491   } ;
0492 
0493   G4SaveNavigatorState* fpSaveState;
0494 
0495 
0496   // Tracking Invariants
0497   //
0498   G4VPhysicalVolume  *fTopPhysical{nullptr};
0499     // A link to the topmost physical volume in the detector.
0500     // Must be positioned at the origin and unrotated.
0501 
0502   // Utility information
0503   //
0504   G4bool fCheck{false};
0505     // Check-mode flag  [if true, more strict checks are performed].
0506   G4bool fPushed{false}, fWarnPush{true};
0507     // Push flags  [if true, means a stuck particle has been pushed].
0508 
0509   // Helpers/Utility classes
0510   //
0511   G4NormalNavigation  fnormalNav;
0512   G4VoxelNavigation fvoxelNav;
0513   G4ParameterisedNavigation fparamNav;
0514   G4ReplicaNavigation freplicaNav;
0515   G4RegularNavigation fregularNav;
0516   G4VoxelSafety       *fpVoxelSafety;
0517 };
0518 
0519 #include "G4ITNavigator1.icc"
0520 
0521 #endif
0522 
0523 
0524 // NOTES:
0525 //
0526 // The following methods provide detailed information when a Step has
0527 // arrived at a geometrical boundary.  They distinguish between the different
0528 // causes that can result in the track leaving its current volume.
0529 //
0530 // Four cases are possible:
0531 //
0532 // 1) The particle has reached a boundary of a daughter of the current volume:
0533 //     (this could cause the relocation to enter the daughter itself
0534 //     or a potential granddaughter or further descendant)
0535 //     
0536 // 2) The particle has reached a boundary of the current
0537 //     volume, exiting into a mother (regardless the level
0538 //     at which it is located in the tree):
0539 //
0540 // 3) The particle has reached a boundary of the current
0541 //     volume, exiting into a volume which is not in its
0542 //     parental hierarchy:
0543 //
0544 // 4) The particle is not on a boundary between volumes:
0545 //     the function returns an exception, and the caller is
0546 //     reccomended to compare the G4touchables associated
0547 //     to the preStepPoint and postStepPoint to handle this case.
0548 //
0549 //   G4bool        EnteredDaughterVolume()
0550 //   G4bool        IsExitNormalValid()
0551 //   G4ThreeVector GetLocalExitNormal()
0552 //
0553 // The expected usefulness of these methods is to allow the caller to
0554 // determine how to compute the surface normal at the volume boundary. The two
0555 // possibilities are to obtain the normal from:
0556 //
0557 //   i) the solid associated with the volume of the initial point of the Step.
0558 //      This is valid for cases 2 and 3.  
0559 //      (Note that the initial point is generally the PreStepPoint of a Step).
0560 //   or
0561 // 
0562 //  ii) the solid of the final point, ie of the volume after the relocation.
0563 //      This is valid for case 1.
0564 //      (Note that the final point is generally the PreStepPoint of a Step).
0565 //
0566 // This way the caller can always get a valid normal, pointing outside
0567 // the solid for which it is computed, that can be used at his own
0568 // discretion.