1 | <html> |
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2 | <head> |
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3 | <title>ADG: Geometry</title> |
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4 | </head> |
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5 | |
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6 | <!-- Changed by: Gabriele Cosmo, 18-Apr-2005 --> |
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7 | <!-- $Id: geomTouch.html,v 1.2 2006/06/09 13:55:26 gcosmo Exp $ --> |
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8 | <!-- $Name: $ --> |
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9 | <body> |
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10 | <table WIDTH="100%"><TR> |
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11 | <td> |
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12 | <a href="../../../../Overview/html/index.html"> |
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13 | <IMG SRC="../../../../resources/html/IconsGIF/Overview.gif" ALT="Overview"></a> |
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14 | <a href="geometry.html"> |
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15 | <IMG SRC="../../../../resources/html/IconsGIF/Contents.gif" ALT="Contents"></a> |
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16 | <a href="geomPhysical.html"> |
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17 | <IMG SRC="../../../../resources/html/IconsGIF/Previous.gif" ALT="Previous"></a> |
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18 | <a href="geomAssembly.html"> |
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20 | </td> |
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21 | <td ALIGN="Right"> |
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22 | <font SIZE="-1" COLOR="#238E23"> |
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23 | <b>Geant4 User's Guide</b> |
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24 | <br> |
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25 | <b>For Application Developers</b> |
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26 | <br> |
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27 | <b>Geometry</b> |
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28 | </font> |
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29 | </td> |
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30 | </tr></table> |
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31 | <br><br> |
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32 | |
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33 | <a name="4.1.5"> |
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34 | <h2>4.1.5 Touchables: Uniquely Identifying a Volume</h2></a> |
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35 | |
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36 | <b>Introduction to Touchables</b> |
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37 | <p> |
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38 | A <i>touchable</i> for a volume serves the purpose of providing a unique identification for |
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39 | a detector element. This can be useful for description of the geometry |
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40 | alternative to the one used by the Geant4 tracking system, such as a Sensitive |
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41 | Detectors based read-out geometry, or a parameterised geometry for fast |
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42 | Monte Carlo. In order to create a <i>touchable volume</i>, several techniques can be |
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43 | implemented: for example, in Geant4 touchables are implemented as solids |
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44 | associated to a transformation-matrix in the global reference system, or as a |
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45 | hierarchy of physical volumes up to the root of the geometrical tree. |
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46 | </p> |
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47 | |
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48 | <p> |
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49 | A touchable is a geometrical entity (volume or solid) which has a unique |
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50 | placement in a detector description. It is represented by an abstract base class |
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51 | which can be implemented in a variety of ways. Each way must provide the |
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52 | capabilities of obtaining the transformation and solid that is described by |
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53 | the touchable. |
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54 | </p> |
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55 | |
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56 | <br> |
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57 | |
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58 | <b>What can a Touchable do ?</b> |
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59 | <p> |
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60 | All <tt>G4VTouchable</tt> implementations must respond to the two following |
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61 | "requests", where in all cases, by <tt>depth</tt> it is meant the number of |
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62 | levels <i>up</i> in the tree to be considered (the default and current one |
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63 | is <tt>0</tt>): |
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64 | <P> |
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65 | <ol> |
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66 | <li><tt>GetTranslation(depth)</tt></li> |
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67 | <li><tt>GetRotation(depth)</tt></li> |
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68 | </ol> |
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69 | that return the components of the volume's transformation. |
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70 | <P> |
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71 | Additional capabilities are available from implementations with more |
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72 | information. These have a default implementation that causes an exception. |
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73 | <P> |
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74 | Several capabilities are available from touchables with physical volumes: |
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75 | <P> |
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76 | <ol start=3> |
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77 | <li><tt>GetSolid(depth)</tt> gives the solid associated to the touchable. |
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78 | <li><tt>GetVolume(depth)</tt> gives the physical volume. |
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79 | <li><tt>GetReplicaNumber(depth)</tt> or <tt>GetCopyNumber(depth)</tt> which |
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80 | return the copy number of the physical volume (replicated or not). |
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81 | </ol> |
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82 | <P> |
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83 | Touchables that store volume hierarchy (history) have the whole stack of |
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84 | parent volumes available. Thus it is possible to add a little more state |
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85 | in order to extend its functionality. We add a "pointer" to a level and a |
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86 | member function to move the level in this stack. Then |
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87 | calling the above member functions for another level the information for |
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88 | that level can be retrieved. |
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89 | <P> |
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90 | The top of the history tree is, by convention, the world volume. |
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91 | <P> |
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92 | <ol start=6> |
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93 | <li><tt>GetHistoryDepth()</tt> gives the depth of the history tree. |
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94 | <li><tt>MoveUpHistory(num)</tt> moves the current pointer inside the |
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95 | touchable to point <tt>num</tt> levels up the history tree. Thus, e.g., calling |
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96 | it with <tt>num=1</tt> will cause the internal pointer to move to the mother |
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97 | of the current volume.<br> |
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98 | WARNING: this function changes the state of the touchable and can cause |
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99 | errors in tracking if applied to Pre/Post step touchables. |
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100 | </ol> |
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101 | These methods are valid only for the <i>touchable-history</i> type, as |
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102 | specified also below. |
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103 | <P> |
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104 | An update method, with different arguments is available, so |
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105 | that the information in a touchable can be updated: |
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106 | <P> |
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107 | <ol start=8> |
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108 | <li><tt>UpdateYourself(vol, history)</tt> takes a physical volume pointer |
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109 | and can additionally take a <tt>NavigationHistory</tt> pointer. |
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110 | </ol> |
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111 | <P> |
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112 | |
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113 | <b>Touchable history holds stack of geometry data</b> |
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114 | <p> |
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115 | As shown in Sections <a href="geomLogical.html#4.1.3">4.1.3</a> and |
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116 | <a href="geomPhysical.html#4.1.4">4.1.4</a>, a logical volume represents |
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117 | unpositioned detector elements, and a physical volume can represent multiple |
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118 | detector elements. On the other hand, touchables provide a unique |
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119 | identification for a detector element. In particular, the Geant4 |
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120 | transportation process and the tracking system exploit touchables as |
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121 | implemented in <tt>G4TouchableHistory</tt>. The touchable history is the |
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122 | minimal set of information required to specify the full genealogy of a |
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123 | given physical volume (up to the root of the geometrical tree). These |
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124 | touchable volumes are made available to the user at every step of the Geant4 |
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125 | tracking in <tt>G4VUserSteppingAction</tt>. |
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126 | </p> |
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127 | |
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128 | <p> |
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129 | To create/access a <tt>G4TouchableHistory</tt> the user must message |
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130 | <tt>G4Navigator</tt> which provides the method <tt>CreateTouchableHistoryHandle()</tt>: |
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131 | <PRE> |
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132 | G4TouchableHistoryHandle CreateTouchableHistoryHandle() const; |
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133 | </PRE> |
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134 | this will return a handle to the touchable. |
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135 | <p> |
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136 | The methods that differentiate the touchable-history from other touchables |
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137 | (since they have meaning only for this type...), are: |
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138 | <pre> |
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139 | G4int GetHistoryDepth() const; |
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140 | G4int MoveUpHistory( G4int num_levels = 1 ); |
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141 | </pre> |
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142 | </p> |
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143 | |
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144 | <p> |
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145 | The first method is used to find out how many levels deep in the |
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146 | geometry tree the current volume is. The second method asks the |
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147 | touchable to eliminate its deepest level. |
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148 | </p> |
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149 | |
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150 | <p> |
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151 | As mentioned above, <tt>MoveUpHistory(num)</tt> significantly modifies the |
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152 | state of a touchable. |
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153 | </p> |
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154 | |
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155 | <hr><a href="../../../../Authors/html/subjectsToAuthors.html"> |
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156 | <i>About the authors</a></i> </P> |
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157 | |
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158 | </body> |
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159 | </html> |
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