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