Hello,
   some may experience a problem with OpenGL visualization with displaying the detector geometry.
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Problem Description:
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On SLC5 (Scientific Linux CERN 5) with Geant4 using OGL (OpenGL) drivers (both X and Qt variants) the detector geometry is not displayed, while tracks are.

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Details:
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The OpenGL library that is used is of version 1.2 : 
ls -l /usr/lib64/libGL*
/usr/lib64/libGL.so -> libGL.so.1
/usr/lib64/libGL.so.1 -> libGL.so.1.2
/usr/lib64/libGL.so.1.2
The system has an installation of Mesa 
rpm -qa | grep mesa
mesa-libGL-6.5.1-7.8.el5.x86_64
The Mesa package provides the OpenGL library:
rpm -ql mesa-libGL-6.5.1-7.8.el5.x86_6

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Solution:
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Updating to a newer version of Mesa solves the problem. I fixed the problem installing on my system Mesa version 7.6.1
This version provides a version of GL library 1.5.
After compilation of mesa (follow provided instruction) it is enough to modify LD_LIBRARY_PATH:
export LD_LIBRARY_PATH=<path-where-mesa7.6.1-is-installed>/lib:$LD_LIBRARY_PATH
(or setenv in tcsh)

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Notes:
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- This is tested in SLC5, but other Linux distributions may be as well affected. SLC5 is based on RedHad Enterprise Linux 5
- Installing Mesa 7.6.1 does not require re-compilation of Geant4. Indeed it is enough to change the run-time variable LD_LIBRARY_PATH