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