| 1 | README file for the Geant4 BlineTracer module
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| 2 |
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| 3 | Author : Laurent Desorgher (desorgher@phim.unibe.ch) - 04/10/2003
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| 4 |
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| 5 | -------------------------------------------------
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| 6 |
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| 7 | The BlineTracer module allows to trace and visualise magnetic field
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| 8 | lines in a Geant4 application where particle are tracked through the
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| 9 | magnetic field (in future, these functionalities may be integrated in
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| 10 | the Geant4 kernel).
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| 11 | To use the tracer, the user should copy the classes provided here in
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| 12 | his/her own application/example and create somewhere in his/her code
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| 13 | an instance of a G4BlineTracer object.
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| 14 | It can be anywhere (for example in the main code or in the user class
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| 15 | defining the magnetic field):
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| 16 |
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| 17 | #include "G4BlineTracer.hh"
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| 18 | G4BlineTracer* theBlineTool = new G4BlineTracer();
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| 19 |
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| 20 |
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| 21 | Design principles:
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| 22 |
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| 23 | The core of the tool is the method ComputeBlines() of G4BlineTracer class.
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| 24 | In this method a bline is computed by tracking a ChargedGeantino in the user
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| 25 | defined magnetic field and by using a Bline equation of motion (class
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| 26 | G4BlineEquation, motion along the field) instead of a Lorentz equation.
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| 27 |
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| 28 | During the execution of this method :
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| 29 |
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| 30 | -The user defined equations of motion associated to the different
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| 31 | global and local fields are replaced by instances of G4BlineEquation
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| 32 | associated to the same fields.
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| 33 | The G4BlineEquation class defines the differential equation of a
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| 34 | magnetic field line.
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| 35 |
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| 36 | -User defined ChordFinders are replaced by new ChordFinders
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| 37 | associated to the G4BlineEquation object.
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| 38 |
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| 39 | -The user primary generator action, run action, event action and
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| 40 | stepping action are replaced by instances of the classes
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| 41 | G4BlinePrimaryGeneratorAction, G4BlineTracer, G4BlineEventAction
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| 42 | and G4BlineSteppingAction respectively. Other actions are set to
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| 43 | a NULL pointer.
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| 44 |
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| 45 | After the execution of the method, the original user defined actions, equation
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| 46 | of motions and chord-finders are restored.
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| 47 |
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| 48 | In other words, the deafult run-action is temporarly replaced by a
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| 49 | G4BlineTracer run-action using the same Geometry, physics and magnetic fields
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| 50 | but with a different equation of motion and user actions.
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| 51 |
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| 52 | The GeneratePrimaries() method of the G4BlinePrimaryGeneratorAction class
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| 53 | call the GeneratePrimaries() method of the user defined PrimaryGeneratorAction
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| 54 | for defining the start position and start time of tracking.
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| 55 | Start position for user application and for Bline tracking are therefore
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| 56 | controlled by the same UI commands.
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| 57 | The type of particles to be tracked when tracing Blines is always set
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| 58 | to Charged-Geantino. This allows to switch off the effect of electromagnetic
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| 59 | and hadronics physics when tracing field lines.
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| 60 |
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| 61 |
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| 62 | The G4BlineEventAction class is responsible to store computed magnetic field
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| 63 | lines as a vector of Polylines and Polymarkers for later visualisation.
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| 64 | These vectors can be drawn and reset at any time.
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| 65 |
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| 66 | The stepping action does nothing in this implementation but it can be used
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| 67 | in future versions to limit field line tracing to physical volumes defined
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| 68 | by the user. For this purpose a G4BlineStackingAction could also be
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| 69 | implemented.
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| 70 |
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| 71 | User Manual:
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| 72 |
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| 73 | General description:
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| 74 |
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| 75 | The BlineTracer is controlled by the UI commands contained in the directory
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| 76 | /vis/blineTracer. By calling the command 'computeBline', several magnetic field
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| 77 | lines passing through user defined start positions are computed.
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| 78 | Start positions are generated by the user primary generator action.
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| 79 | By doing so, the definition of start positions is the same for usual particles
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| 80 | tracking and magnetic field line tracking.
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| 81 |
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| 82 | A magnetic field line is computed as a track of a charged geantino that moves
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| 83 | along the field line. The user can define the maximum length of a tracking step
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| 84 | (only valid for Bline tracing purposes) by the use of the 'setMaxStepLength'
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| 85 | command.
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| 86 | By using small enough maximum step length, smooth magnetic field lines are
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| 87 | obtained. By using the command 'stockLines' and 'stockPoints' the user
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| 88 | can decide to store the series of tracking step positions defining
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| 89 | a magnetic field line as a Polyline object and/or a PolyMarker object
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| 90 | (circles) respectively.
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| 91 | These objects are stored in vectors of PolyLines and PolyMarkers.
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| 92 | By using the command 'draw', these vectors are added to the scene
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| 93 | of the visualisation manager, provided that a scene handler and
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| 94 | visualisation driver have been properly created.
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| 95 | The scene is visualised by invoking the vis command '/vis/show'.
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| 96 | Polyline objects are visualised as line segments joining the different
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| 97 | step positions defining a line, while for a Polymarker object markers
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| 98 | (here circles), are drawn at each step positions.
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| 99 | By using the 'setColour' the user defines the visualisation colour
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| 100 | that will be associated to the next computed magnetic field lines.
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| 101 | By calling 'setPointSize' the user defines the size of visualisation markers
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| 102 | that will be associated to the next computed magnetic field lines.
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| 103 | The user can remove the vector of Polymarker and Polyline from the memory
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| 104 | by invoking 'resetMaterialToBeDrawn'.
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| 105 | When using small max step size and polymarkers for visualisation purposes, the
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| 106 | thickness of a smooth magnetic field line is obtained. It is controlled by the
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| 107 | Marker size parameter ('setPointSize')
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| 108 |
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| 109 | Command description:
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| 110 | /vis/blineTracer/computeBline nb_of_lines :
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| 111 | Parameters: integer nb_of_lines
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| 112 | Description: Compute nb_of_lines different magnetic field lines
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| 113 |
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| 114 | /vis/blineTracer/setMaxStepLength max_step_length :
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| 115 | Parameters: double max_step_length
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| 116 | Description: Set the maximum tracking step length for computing
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| 117 | magnetic field lines
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| 118 |
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| 119 | /vis/blineTracer/setColour red green blue
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| 120 | Parameters: double red, green, blue
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| 121 | Description: Define the colour for visualisation of the
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| 122 | next computed magnetic field lines. The color is
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| 123 | defined by a RGB code (red,green,blue) with all
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| 124 | parameters smaller than 1.
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| 125 |
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| 126 | /vis/blineTracer/stockLines aBool
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| 127 | Parameters: boolean aBool
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| 128 | Desscription: If true the next computed field lines are stored
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| 129 | as Polylines for further visualisation
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| 130 | /vis/blineTracer/stockLines aBool
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| 131 | Parameters: boolean aBool
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| 132 | Desscription: If true the next computed field lines are stored
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| 133 | as Polymarkers for further visualisation
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| 134 |
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| 135 | /vis/blineTracer/setPointSize point_size
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| 136 | Parameters: double point_size
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| 137 | DEscription: set the size of the visualisation markers
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| 138 | that will be associated with the next computed
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| 139 | magnetic field lines
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| 140 |
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| 141 | /vis/blineTracer/resetMaterialToBeDrawn
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| 142 | Parameters: none
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| 143 | Description: The vector of Polyline and Polymarker representing
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| 144 | magnetic field lines to be visualised are removed
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| 145 | from memory
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| 146 |
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| 147 | /tracking/storeTrajectory 1 :
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| 148 | If the storeTrajectory parameter is not set no field lines are
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| 149 | stored.
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| 150 |
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| 151 | Current limitations & known problems:
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| 152 |
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| 153 | The tool is working properly only for detectors parts where magnetic
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| 154 | field are defined. It is planned in the future to stop the tracking of
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| 155 | field lines in regions where no fields are existing.
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