[904] | 1 | <!-- ******************************************************** --> |
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| 2 | <!-- --> |
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| 3 | <!-- [History] --> |
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| 4 | <!-- Converted to DocBook: Katsuya Amako, Aug-2006 --> |
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| 5 | <!-- Changed by: Gabriele Cosmo, 18-Apr-2005 --> |
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| 6 | <!-- --> |
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| 7 | <!-- ******************************************************** --> |
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| 8 | |
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| 9 | |
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| 10 | <!-- ******************* Section (Level#2) ****************** --> |
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| 11 | <sect2 id="sect.Geom.Touch"> |
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| 12 | <title> |
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| 13 | Touchables: Uniquely Identifying a Volume |
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| 14 | </title> |
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| 15 | |
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| 16 | |
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| 17 | <!-- ******************* Section (Level#3) ****************** --> |
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| 18 | <sect3 id="sect.Geom.Touch.Intro"> |
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| 19 | <title> |
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| 20 | Introduction to Touchables |
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| 21 | </title> |
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| 22 | |
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| 23 | <para> |
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| 24 | A <emphasis>touchable</emphasis> for a volume serves the purpose of providing |
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| 25 | a unique identification for a detector element. This can be useful |
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| 26 | for description of the geometry alternative to the one used by the |
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| 27 | Geant4 tracking system, such as a Sensitive Detectors based |
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| 28 | read-out geometry, or a parameterised geometry for fast Monte |
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| 29 | Carlo. In order to create a <emphasis>touchable volume</emphasis>, several |
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| 30 | techniques can be implemented: for example, in Geant4 touchables |
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| 31 | are implemented as solids associated to a transformation-matrix in |
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| 32 | the global reference system, or as a hierarchy of physical volumes |
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| 33 | up to the root of the geometrical tree. |
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| 34 | </para> |
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| 35 | |
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| 36 | <para> |
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| 37 | A touchable is a geometrical entity (volume or solid) which has |
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| 38 | a unique placement in a detector description. It is represented by |
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| 39 | an abstract base class which can be implemented in a variety of |
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| 40 | ways. Each way must provide the capabilities of obtaining the |
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| 41 | transformation and solid that is described by the touchable. |
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| 42 | </para> |
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| 43 | |
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| 44 | </sect3> |
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| 45 | |
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| 46 | <!-- ******************* Section (Level#3) ****************** --> |
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| 47 | <sect3 id="sect.Geom.Touch.WhatCan"> |
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| 48 | <title> |
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| 49 | What can a Touchable do? |
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| 50 | </title> |
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| 51 | |
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| 52 | <para> |
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| 53 | All <literal>G4VTouchable</literal> implementations must respond to the |
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| 54 | two following "requests", where in all cases, by <literal>depth</literal> it |
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| 55 | is meant the number of levels <emphasis>up</emphasis> in the tree to be |
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| 56 | considered (the default and current one is <literal>0</literal>): |
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| 57 | |
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| 58 | <orderedlist spacing="compact"> |
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| 59 | <listitem><para> |
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| 60 | <literal>GetTranslation(depth)</literal> |
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| 61 | </para></listitem> |
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| 62 | <listitem><para> |
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| 63 | <literal>GetRotation(depth)</literal> |
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| 64 | </para></listitem> |
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| 65 | </orderedlist> |
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| 66 | |
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| 67 | that return the components of the volume's transformation. |
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| 68 | </para> |
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| 69 | |
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| 70 | <para> |
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| 71 | Additional capabilities are available from implementations with |
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| 72 | more information. These have a default implementation that causes |
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| 73 | an exception. |
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| 74 | </para> |
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| 75 | |
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| 76 | <para> |
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| 77 | Several capabilities are available from touchables with physical |
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| 78 | volumes: |
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| 79 | |
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| 80 | <orderedlist continuation="continues" > |
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| 81 | <listitem><para> |
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| 82 | <literal>GetSolid(depth)</literal> gives the solid associated to the |
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| 83 | touchable. |
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| 84 | </para></listitem> |
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| 85 | <listitem><para> |
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| 86 | <literal>GetVolume(depth)</literal> gives the physical volume. |
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| 87 | </para></listitem> |
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| 88 | <listitem><para> |
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| 89 | <literal>GetReplicaNumber(depth)</literal> or |
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| 90 | <literal>GetCopyNumber(depth)</literal> which return the copy number of the |
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| 91 | physical volume (replicated or not). |
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| 92 | </para></listitem> |
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| 93 | </orderedlist> |
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| 94 | </para> |
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| 95 | |
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| 96 | <para> |
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| 97 | Touchables that store volume hierarchy (history) have the whole |
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| 98 | stack of parent volumes available. Thus it is possible to add a |
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| 99 | little more state in order to extend its functionality. We add a |
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| 100 | "pointer" to a level and a member function to move the level in |
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| 101 | this stack. Then calling the above member functions for another |
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| 102 | level the information for that level can be retrieved. |
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| 103 | </para> |
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| 104 | |
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| 105 | <para> |
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| 106 | The top of the history tree is, by convention, the world |
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| 107 | volume. |
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| 108 | |
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| 109 | <orderedlist continuation="continues" > |
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| 110 | <listitem><para> |
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| 111 | <literal>GetHistoryDepth()</literal> gives the depth of the history |
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| 112 | tree. |
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| 113 | </para></listitem> |
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| 114 | <listitem><para> |
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| 115 | <para> |
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| 116 | <literal>MoveUpHistory(num)</literal> moves the current pointer inside |
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| 117 | the touchable to point <literal>num</literal> levels up the history tree. |
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| 118 | Thus, e.g., calling it with <literal>num=1</literal> will cause the internal |
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| 119 | pointer to move to the mother of the current volume. |
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| 120 | </para> |
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| 121 | <para> |
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| 122 | WARNING: this function changes the state of the touchable and can |
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| 123 | cause errors in tracking if applied to Pre/Post step |
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| 124 | touchables. |
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| 125 | </para> |
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| 126 | </para></listitem> |
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| 127 | </orderedlist> |
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| 128 | </para> |
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| 129 | |
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| 130 | <para> |
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| 131 | These methods are valid only for the <emphasis>touchable-history</emphasis> type, |
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| 132 | as specified also below. |
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| 133 | </para> |
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| 134 | |
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| 135 | <para> |
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| 136 | An update method, with different arguments is available, so that |
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| 137 | the information in a touchable can be updated: |
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| 138 | |
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| 139 | <orderedlist continuation="continues" > |
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| 140 | <listitem><para> |
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| 141 | <literal>UpdateYourself(vol, history)</literal> takes a physical volume |
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| 142 | pointer and can additionally take a <literal>NavigationHistory</literal> |
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| 143 | pointer. |
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| 144 | </para></listitem> |
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| 145 | </orderedlist> |
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| 146 | </para> |
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| 147 | |
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| 148 | </sect3> |
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| 149 | |
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| 150 | <!-- ******************* Section (Level#3) ****************** --> |
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| 151 | <sect3 id="sect.Geom.Touch.History"> |
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| 152 | <title> |
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| 153 | Touchable history holds stack of geometry data |
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| 154 | </title> |
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| 155 | |
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| 156 | <para> |
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| 157 | As shown in Sections |
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| 158 | <xref linkend="sect.Geom.LogVol" /> and <xref linkend="sect.Geom.PhysVol" />, |
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| 159 | a logical volume represents unpositioned detector elements, and a physical |
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| 160 | volume can represent multiple detector elements. On the other hand, |
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| 161 | touchables provide a unique identification for a detector element. |
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| 162 | In particular, the Geant4 transportation process and the tracking |
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| 163 | system exploit touchables as implemented in |
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| 164 | <literal>G4TouchableHistory</literal>. The touchable history is the minimal |
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| 165 | set of information required to specify the full genealogy of a |
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| 166 | given physical volume (up to the root of the geometrical tree). |
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| 167 | These touchable volumes are made available to the user at every |
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| 168 | step of the Geant4 tracking in <literal>G4VUserSteppingAction</literal>. |
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| 169 | </para> |
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| 170 | |
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| 171 | <para> |
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| 172 | To create/access a <literal>G4TouchableHistory</literal> the user must |
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| 173 | message <literal>G4Navigator</literal> which provides the method |
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| 174 | <literal>CreateTouchableHistoryHandle()</literal>: |
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| 175 | |
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| 176 | <informalexample> |
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| 177 | <programlisting> |
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| 178 | G4TouchableHistoryHandle CreateTouchableHistoryHandle() const; |
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| 179 | </programlisting> |
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| 180 | </informalexample> |
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| 181 | |
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| 182 | this will return a handle to the touchable. |
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| 183 | </para> |
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| 184 | |
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| 185 | <para> |
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| 186 | The methods that differentiate the touchable-history from other |
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| 187 | touchables (since they have meaning only for this type...), |
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| 188 | are: |
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| 189 | |
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| 190 | <informalexample> |
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| 191 | <programlisting> |
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| 192 | G4int GetHistoryDepth() const; |
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| 193 | G4int MoveUpHistory( G4int num_levels = 1 ); |
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| 194 | </programlisting> |
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| 195 | </informalexample> |
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| 196 | </para> |
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| 197 | |
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| 198 | <para> |
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| 199 | The first method is used to find out how many levels deep in the |
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| 200 | geometry tree the current volume is. The second method asks the |
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| 201 | touchable to eliminate its deepest level. |
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| 202 | </para> |
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| 203 | |
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| 204 | <para> |
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| 205 | As mentioned above, <literal>MoveUpHistory(num)</literal> significantly |
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| 206 | modifies the state of a touchable. |
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| 207 | </para> |
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| 208 | |
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| 209 | |
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| 210 | </sect3> |
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| 211 | </sect2> |
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