| 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: geomOverlap.html,v 1.4 2006/06/09 13:58:38 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="geomG3toG4.html">
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| 17 | <IMG SRC="../../../../resources/html/IconsGIF/Previous.gif" ALT="Previous"></a>
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| 18 | <a href="geomDynamic.html">
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| 19 | <IMG SRC="../../../../resources/html/IconsGIF/Next.gif" ALT="Next"></a>
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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="4.1.11">
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| 34 | <h2>4.1.11 Detecting Overlapping Volumes</h2></a>
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| 35 |
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| 36 | <b>The problem of overlapping volumes</b>
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| 37 | <p>
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| 38 | Volumes are often positioned within other volumes with the intent that
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| 39 | one is fully contained within the other. If, however, a volume extends
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| 40 | beyond the boundaries of its mother volume, it is defined as overlapping.
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| 41 | It may also be intended that volumes are positioned within the same mother
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| 42 | volume such that they do not intersect one another. When such volumes do
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| 43 | intersect, they are also defined as overlapping.
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| 44 | </p>
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| 45 |
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| 46 | <p>
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| 47 | The problem of detecting overlaps between volumes is bounded by the
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| 48 | complexity of the solid model description. Hence it requires the same
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| 49 | mathematical sophistication which is needed to describe the most complex
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| 50 | solid topology, in general. However, a tunable accuracy can be obtained
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| 51 | by approximating the solids via first and/or second order surfaces and
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| 52 | checking their intersections.
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| 53 | </p>
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| 54 |
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| 55 | <P> </P>
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| 56 |
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| 57 | <b>Detecting overlaps: built-in kernel commands</b>
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| 58 |
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| 59 | <p>
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| 60 | In general, the most powerful clash detection algorithms are provided by
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| 61 | CAD systems, treating the intersection between the solids in their topological
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| 62 | form.
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| 63 | </p>
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| 64 |
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| 65 | <p>
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| 66 | Geant4 provides some built-in run-time commands to activate
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| 67 | verification tests for the user-defined geometry:
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| 68 | <PRE>
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| 69 | geometry/test/grid_test [recursion_flag]
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| 70 | --> to start verification of geometry for overlapping regions
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| 71 | based on standard lines grid setup. If the "recursion_flag" is
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| 72 | set to 'false' (the default), the check is limited to the first
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| 73 | depth level of the geometry tree; otherwise it visits recursively
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| 74 | the whole geometry tree. In the latter case, it may take a long
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| 75 | time, depending on the complexity of the geometry.
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| 76 | geometry/test/cylinder_test [recursion_flag]
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| 77 | --> shoots lines according to a cylindrical pattern. If the
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| 78 | "recursion_flag" is set to 'false' (the default), the check is
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| 79 | limited to the first depth level of the geometry tree; otherwise
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| 80 | it visits recursively the whole geometry tree. In the latter case,
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| 81 | it may take a long time, depending on the complexity of the geometry.
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| 82 | geometry/test/line_test [recursion_flag]
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| 83 | --> shoots a line according to a specified direction and position
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| 84 | defined by the user. If the "recursion_flag" is set to 'false'
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| 85 | (the default), the check is limited to the first depth level of the
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| 86 | geometry tree; otherwise it visits recursively the whole geometry
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| 87 | tree.
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| 88 | geometry/test/position
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| 89 | --> to specify position for the line_test.
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| 90 | geometry/test/direction
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| 91 | --> to specify direction for the line_test.
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| 92 | geometry/test/grid_cells
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| 93 | --> to define the resolution of the lines in the grid test as number
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| 94 | of cells, specifying them for each dimension, X, Y and Z.
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| 95 | The new settings will be applied to the grid_test command.
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| 96 | geometry/test/cylinder_geometry
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| 97 | --> to define the details of the cylinder geometry, by specifying:
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| 98 | nPhi - number of lines per Phi
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| 99 | nZ - number of Z points
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| 100 | nRho - number of Rho points
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| 101 | The new settings will be applied to the cylinder_test command.
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| 102 | geometry/test/cylinder_scaleZ
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| 103 | --> to define the resolution of the cylinder geometry, by specifying
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| 104 | the fraction scale for points along Z.
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| 105 | The new settings will be applied to the cylinder_test command.
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| 106 | geometry/test/cylinder_scaleRho
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| 107 | --> to define the resolution of the cylinder geometry, by specifying
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| 108 | the fraction scale for points along Rho.
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| 109 | The new settings will be applied to the cylinder_test command.
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| 110 | geometry/test/recursion_start
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| 111 | --> to set the initial level in the geometry tree for starting the
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| 112 | recursion (default value being zero, i.e. the world volume).
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| 113 | The new settings will then be applied to any recursive test.
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| 114 | geometry/test/recursion_depth
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| 115 | --> to set the depth in the geometry tree for recursion, so that
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| 116 | recursion will stop after having reached the specified depth (the
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| 117 | default being the full depth of the geometry tree).
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| 118 | The new settings will then be applied to any recursive test.
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| 119 | </PRE>
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| 120 | </P>
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| 121 |
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| 122 | To detect overlapping volumes, the built-in test uses the intersection
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| 123 | of solids with linear trajectories. For example, consider figure 4.1.4:</P>
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| 124 |
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| 125 | <center>
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| 126 | <table BORDER=1 CELLPADDING=8>
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| 127 | <tr><td>
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| 128 | <IMG SRC="geometry.src/geomtest.gif" ALT="|Geometry Test|" HEIGHT=162 WIDTH=402>
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| 129 | </td></tr>
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| 130 | <tr>
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| 131 | <td ALIGN=center>
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| 132 | Figure 4.1.4<BR>
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| 133 | Different cases of placed volumes overlapping each other.
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| 134 | </td></tr>
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| 135 | </table>
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| 136 | </center>
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| 137 |
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| 138 | <p>
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| 139 | Here we have a line intersecting some physical volume (large, black rectangle).
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| 140 | Belonging to the volume are four daughters: A, B, C, and D.
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| 141 | Indicated by the dots are the intersections of the line with the
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| 142 | mother volume and the four daughters.
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| 143 | </p>
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| 144 |
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| 145 | <p>
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| 146 | This example has two geometry errors. First, volume A sticks outside its
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| 147 | mother volume (this practice, sometimes used in GEANT3.21, is not allowed
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| 148 | in Geant4). This can be noticed because the intersection point (leftmost
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| 149 | magenta dot) lies outside the mother volume, as defined by the space
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| 150 | between the two black dots.
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| 151 | </p>
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| 152 |
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| 153 | <p>
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| 154 | The second error is that daughter volumes A and B overlap. This is noticeable
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| 155 | because one of the intersections with A (rightmost magenta dot) is
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| 156 | inside the volume B, as defined as the space between the red dots.
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| 157 | Alternatively, one of the intersections with B (leftmost red dot) is
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| 158 | inside the volume A, as defined as the space between the magenta dots.
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| 159 | </p>
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| 160 |
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| 161 | <p>
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| 162 | Each of these two types of errors is represented by a line segment,
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| 163 | which has a start point, an end point, and, a length.
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| 164 | Depending on the type of error, the points are most clearly recognized in
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| 165 | either the coordinate system of the volume, the global coordinate
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| 166 | system, or the coordinate system of the daughters involved.</P>
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| 167 |
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| 168 | <P>
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| 169 | Also notice that certain errors will be missed unless a line
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| 170 | is supplied in precisely the correct path. Unfortunately, it is hard to
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| 171 | predict which lines are best at uncovering potential geometry errors.
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| 172 | Instead, the geometry testing code uses a grid of lines, in the hope of
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| 173 | at least uncovering gross geometry errors.
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| 174 | More subtle errors could easily be missed.</P>
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| 175 |
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| 176 | <P>
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| 177 | Another difficult issue is roundoff error. For example, daughters C and D
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| 178 | lie precisely next to each other. It is possible, due to roundoff,
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| 179 | that one of the intersections points will lie just slightly inside the
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| 180 | space of the other. In addition, a volume that lies tightly up against
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| 181 | the outside of its mother may have an intersection point that just slightly
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| 182 | lies outside the mother.</P>
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| 183 |
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| 184 | <P>
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| 185 | To avoid spurious errors caused by roundoff, a rather generous tolerance
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| 186 | of 0.1 micron is used by default. This tolerance can be adjusted as
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| 187 | needed by the application through the run-time command:
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| 188 | <PRE>
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| 189 | geometry/test/tolerance <new-value>
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| 190 | </PRE>
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| 191 | </P>
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| 192 |
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| 193 | <P>
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| 194 | Finally, notice that no mention is made of the possible daughter volumes
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| 195 | of A, B, C, and D. To keep the code simple, only the immediate daughters
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| 196 | of a volume are checked at one pass. To test these "granddaughter" volumes,
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| 197 | the daughters A, B, C, and D each have to be tested themselves in turn.
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| 198 | To make this more automatic, an optional recursive algorithm is included;
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| 199 | it first tests a target volume, then it loops over all
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| 200 | daughter volumes and calls itself.</P>
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| 201 |
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| 202 | <P>Pay attention! For a complex geometry, checking the entire volume
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| 203 | hierarchy can be extremely time consuming.</P>
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| 204 |
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| 205 | <P> </P>
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| 206 |
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| 207 | <b>Detecting overlaps at construction</b>
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| 208 |
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| 209 | <P>
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| 210 | Since release 8.0, the Geant4 geometry modeler provides the ability to
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| 211 | detect overlaps of placed volumes (normal placements or parameterised)
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| 212 | at the time of construction. This check is optional and can be activated
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| 213 | when instantiating a placement (see <tt>G4PVPlacement</tt> constructor in
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| 214 | <a href="geomPhysical.html#4.1.4.1">Section 4.1.4.1</a>) or a parameterised
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| 215 | volume (see <tt>G4PVParameterised</tt> constructor in
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| 216 | <a href="geomPhysical.html#4.1.4.2">Section 4.1.4.2</a>).<br>
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| 217 | The positioning of that specific volume will be checked against all volumes
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| 218 | in the same hierarchy level and its mother volume.
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| 219 | Depending on the complexity of the geometry being checked, the check may
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| 220 | require considerable CPU time; it is therefore suggested to use it only
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| 221 | for debugging the geometry setup and to apply it only to the part of the
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| 222 | geometry setup which requires debugging.</P>
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| 223 | <P>
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| 224 | The classes <tt>G4PVPlacement</tt> and <tt>G4PVParameterised</tt> also
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| 225 | provide a method:
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| 226 | <pre>
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| 227 | G4bool CheckOverlaps(G4int res=1000)
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| 228 | </pre>
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| 229 | which will force the check for the specified volume. The check verifies
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| 230 | if each placed or parameterised instance is overlapping with other instances
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| 231 | or with its mother volume. A default resolution for the number of points to
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| 232 | be generated and verified is provided. The method returns <tt>true</tt> if
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| 233 | an overlap occurs.</P>
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| 234 |
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| 235 | <P> </P>
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| 236 |
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| 237 | <B>Using the visualization driver: DAVID</B>
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| 238 |
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| 239 | <p>
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| 240 | The Geant4 visualization offers a powerful debugging tool for detecting
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| 241 | potential intersections of physical volumes. The Geant4 DAVID visualization
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| 242 | tool <a href="#DAVID">[2]</a> can infact automatically detect the overlaps
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| 243 | between the volumes defined in Geant4 and converted to a graphical
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| 244 | representation for visualization purposes. The accuracy of the graphical
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| 245 | representation can be tuned onto the exact geometrical description. In the
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| 246 | debugging, physical-volume surfaces are automatically decomposed into 3D
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| 247 | polygons, and intersections of the generated polygons are investigated. If a
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| 248 | polygon intersects with another one, physical volumes which these polygons
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| 249 | belong to are visualized in color (red is the default). The figure 4.1.5 below
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| 250 | is a sample visualization of a detector geometry with intersecting physical
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| 251 | volumes highlighted:
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| 252 | </p>
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| 253 |
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| 254 | <center>
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| 255 | <table BORDER=1 CELLPADDING=8>
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| 256 | <tr><td>
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| 257 | <IMG SRC="geometry.src/DAVID_SAMPLE.gif" ALT="|DAVID Geometry Test|" HEIGHT=567 WIDTH=322></P>
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| 258 | </td></tr>
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| 259 | <tr>
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| 260 | <td ALIGN=center>
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| 261 | Figure 4.1.5<BR>
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| 262 | A geometry with overlapping volumes highlighted by DAVID.
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| 263 | </td></tr>
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| 264 | </table>
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| 265 | </center>
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| 266 |
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| 267 | <p>
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| 268 | At present physical volumes made of the following solids are able to
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| 269 | be debugged: <tt>G4Box</tt>, <tt>G4Cons</tt>, <tt>G4Para</tt>,
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| 270 | <tt>G4Sphere</tt>, <tt>G4Trd</tt>, <tt>G4Trap</tt>, <tt>G4Tubs</tt>.
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| 271 | (Existence of other solids is harmless.)
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| 272 | </p>
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| 273 |
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| 274 | <p>
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| 275 | Visual debugging of physical-volume surfaces is performed with the DAWNFILE
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| 276 | driver defined in the visualization category and with the two application
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| 277 | packages, i.e. Fukui Renderer "DAWN" and a visual intersection
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| 278 | debugger "DAVID". DAWN <a href="#DAWN">[1]</a> and
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| 279 | DAVID <a href="#DAVID">[2]</a> can be downloaded from the Web.
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| 280 | </p>
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| 281 |
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| 282 | <p>
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| 283 | How to compile Geant4 with the DAWNFILE driver incorporated is described
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| 284 | in <A HREF="../Visualization/visdrivers.html"> Section 8.3</A>.
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| 285 | </p>
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| 286 |
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| 287 | <p>
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| 288 | If the DAWNFILE driver, DAWN and DAVID are all working well in your
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| 289 | host machine, the visual intersection debugging of physical-volume surfaces
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| 290 | can be performed as follows:</P>
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| 291 |
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| 292 | <P>
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| 293 | Run your Geant4 executable, invoke the DAWNFILE driver, and execute
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| 294 | visualization commands to visualize your detector geometry:
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| 295 |
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| 296 | <PRE>
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| 297 | Idle> /vis/open DAWNFILE
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| 298 | .....(setting camera etc)...
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| 299 | Idle> /vis/drawVolume
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| 300 | Idle> /vis/viewer/update
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| 301 | </PRE></P>
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| 302 |
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| 303 | <P>
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| 304 | Then a file "g4.prim", which describes the detector geometry,
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| 305 | is generated in the current directory and DAVID is invoked to read it.
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| 306 | (The description of the format of the file g4.prim can be found from the
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| 307 | <a href="http://geant4.kek.jp/GEANT4/vis/DAWN/G4PRIM_FORMAT_24/">DAWN web
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| 308 | site documentation</a>.)</P>
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| 309 |
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| 310 | <P>
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| 311 | If DAVID detects intersection of physical-volume surfaces, it automatically
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| 312 | invokes DAWN to visualize the detector geometry with the intersected physical
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| 313 | volumes highlighted (See the above sample visualization).</P>
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| 314 |
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| 315 | <P>
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| 316 | If no intersection is detected, visualization is skipped and the following
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| 317 | message is displayed on the console:
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| 318 |
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| 319 | <PRE>
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| 320 | ------------------------------------------------------
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| 321 | !!! Number of intersected volumes : 0 !!!
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| 322 | !!! Congratulations ! \(^o^)/ !!!
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| 323 | ------------------------------------------------------
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| 324 | </PRE></P>
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| 325 |
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| 326 | <P>
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| 327 | If you always want to skip visualization, set an environmental variable
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| 328 | as follows beforehand:
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| 329 |
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| 330 | <PRE>
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| 331 | % setenv DAVID_NO_VIEW 1
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| 332 | </PRE></P>
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| 333 |
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| 334 | <P>
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| 335 | To control the precision associated to computation of intersections (default
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| 336 | precision is set to 9), it is possible to use the environmental variable for
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| 337 | the DAWNFILE graphics driver, as follows:
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| 338 |
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| 339 | <PRE>
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| 340 | % setenv G4DAWNFILE_PRECISION 10
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| 341 | </PRE></P>
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| 342 |
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| 343 | <P>
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| 344 | If necessary, re-visualize the detector geometry with intersected parts
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| 345 | highlighted. The data are saved in a file "g4david.prim" in the
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| 346 | current directory. This file can be re-visualized with DAWN as follows:
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| 347 |
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| 348 | <PRE>
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| 349 | % dawn g4david.prim
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| 350 | </PRE></P>
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| 351 |
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| 352 | <P>
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| 353 | It is also helpful to convert the generated file g4david.prim into a
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| 354 | VRML-formatted file and perform interactive visualization of it with your
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| 355 | WWW browser. The file conversion tool <tt>prim2wrml</tt> can be downloaded
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| 356 | from the <A HREF="http://geant4.kek.jp/GEANT4/vis/DAWN/About_prim2vrml1.html">DAWN
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| 357 | web site download pages</A>.</P>
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| 358 | </OL>
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| 359 |
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| 360 | <P>
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| 361 | For more details, see the <A HREF="http://geant4.kek.jp/GEANT4/vis/DAWN/About_DAVID.html">document
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| 362 | of DAVID</A> mentioned above.</P>
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| 363 |
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| 364 | <P> </P>
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| 365 |
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| 366 | <b>Using the geometry debugging tool OLAP</b>
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| 367 |
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| 368 | <p>
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| 369 | <b>OLAP</b> is a tool developed in the CMS experiment at CERN to help in
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| 370 | identifying ovelapping volumes in a detector geometry. It is placed in the
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| 371 | area for specific tools/examples, in
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| 372 | <tt>geant4/examples/extended/geometry</tt>. The technique consists in
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| 373 | shooting <tt>geantinos</tt> particles in one direction and the opposite one,
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| 374 | and verifying that the boundary crossings are the same.
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| 375 | <br>
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| 376 | The tool can be used for any Geant4 geometry, provided that the user
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| 377 | geometry to be debugged is available as a subclass of
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| 378 | <tt>G4VUserDetectorConstruction</tt> and is used to construct the
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| 379 | <tt>OlapDetConstr</tt> class of the tool. A dummy class
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| 380 | <tt>RandomDetector</tt> is provided for this purpose in the tool itself.<BR>
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| 381 | Run-time commands are provided by the tool to navigate in the geometry tree.
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| 382 | UNIX like navigation of the logical volume hierarchy is provided by the
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| 383 | <tt>/olap/cd</tt> command. The root of the logical volume tree can be accessed
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| 384 | by the character '/'. Any node in the volume tree can be accessed by a '/'
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| 385 | separated string of regular expressions. If '/' is at the beginning of the
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| 386 | string, the tree hierarchy is transversed from the root, otherwise from the
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| 387 | currently chosen logical volume. Further the command
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| 388 | <tt>/olap/goto [regexp]</tt> can be used to jump to the first logical volume
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| 389 | matching the expression <tt>[regexp]</tt>.
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| 390 | Every successful navigation command (<tt>/olap/cd</tt>, <tt>olap/goto</tt>)
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| 391 | results in the construction of a <tt>NewWorld</tt>, the mother volume being
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| 392 | the argument of the command and the daughter volumes being the direct
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| 393 | daughters of the mother volume.<BR>
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| 394 | <tt>/olap/pwd</tt> always shows where in the full geometrical hierarchy the
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| 395 | current <tt>NewWorld</tt> and mother volume are located.<BR>
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| 396 | For more detailed information, view the <tt>README</tt> file provided with
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| 397 | the tool.
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| 398 | </p>
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| 399 | <br>
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| 400 | <hr>
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| 401 | <p>
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| 402 | <table>
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| 403 | <tr><td valign=top><a name="DAWN">[1]</a>
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| 404 | <td><a href="http://geant4.kek.jp/GEANT4/vis/DAWN/About_DAWN.html">http://geant4.kek.jp/GEANT4/vis/DAWN/About_DAWN.html</a>
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| 405 | <tr><td valign=top><a name="DAVID">[2]</a>
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| 406 | <td><a href="http://geant4.kek.jp/GEANT4/vis/DAWN/About_DAVID.html">http://geant4.kek.jp/GEANT4/vis/DAWN/About_DAVID.html</a>
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| 407 | </table>
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| 408 | </p>
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| 409 |
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| 410 | <hr><a href="../../../../Authors/html/subjectsToAuthors.html">
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| 411 | <i>About the authors</a></i> </P>
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| 412 |
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| 413 | </body>
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| 414 | </html>
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