Geant4 Visualization Tutorial using the OpenGL Event Display

• Introduction
• OpenGL Modes
• A Node on JAIDA/JAS
• Running Geant4 with OpenGL
• Recap of Geant4 Commands Used in this Tutorial
• Further Resources

Introduction

This tutorial can be used on its own, but gives the most comprehensive introduction to Geant4 visualization when used as part of the following full set of documents:

• Geant4 Installation Guides
• Introduction to Geant4 Visualization (ppt, pdf)
• Geant4 Visualization Tutorial using the HepRApp HepRep Browser
• Geant4 Visualization Tutorial using the DAWN Event Display
• Geant4 Visualization Tutorial using the OpenGL Event Display
• Geant4 Visualization Commands (ppt, pdf)
• Geant4 Advanced Visualization (ppt, pdf)

While some Geant4 visualization drivers are always included by default in your Geant4 build (because they require no external libraries), OpenGL is not part of the default set. You have to specifically ask to include OpenGL.

If you followed the Geant4 Installation Guides referenced above, you used the Geant4 Configuration script and said "yes" to including OpenGL.

If you are following some other set of build instructions, refer to them for how to include OpenGL. This may involve explicitly setting environment variables.

OpenGL Modes

A Note on JAIDA/JAS

• The problem is that the OpenGL libraries pre-load the library libXt.so which makes the Java virtual machine crash when it tries to open its first Window - no fix was available as of this writing.
• For now, you can run Geant4 with either JAIDA/JAS or OpenGL, but you cannot run with both at the same time.

Running Geant4 with OpenGL

The Installation Guides listed above already showed you how to start Geant4 with OpenGL included. They then showed you how to use a few simple commands to do the most minimal visualization of some detector geometry and some event data. This guide will show you some additional commands.

• Remember to do the appropriate setup commands before you start Geant4 from a new windows (see the installation guides for details).
• Start a fresh Geant4 session.
• Open the visualization driver:

     /vis/open OGL
  

  An empty graphics windows should appear
• Issue the first command to draw something:

    /vis/drawVolume
  

  The graphics window should redraw to show the detector geometry:
  
• Next, change the view angle by providing a theta and phi "camera" angle (taking care about the correct mixed case in the command):

     /vis/viewer/set/viewpointThetaPhi 70 20
  

  
  
• Next, zoom in by a factor of 2:

     /vis/viewer/zoom 2
  

  
  
• To reset to the initial default view:

     /vis/viewer/reset
  

• Type "help" to see a list of other available commands:

  Command directory path : /
   Sub-directories :
   1) /control/   UI control commands.
   2) /units/   Available units.
   3) /geometry/   Geometry control commands.
   4) /tracking/   TrackingManager and SteppingManager control commands.
   5) /event/   EventManager control commands.
   6) /run/   Run control commands.
   7) /random/   Random number status control commands.
   8) /particle/   Particle control commands.
   9) /process/   Process Table control commands.
   10) /vis/   Visualization commands.
   11) /mydet/   A01 detector setup control commands.
   12) /hits/   Sensitive detectors and Hits
   13) /gun/   Particle Gun control commands.
   Commands :
  
  Type the number ( 0:end, -n:n level back ) :
  

  
  Navigate the help system by typing in the numbers that correspond to the various commands. Item 10 gives the visualization commands. And then item 6 gives the viewer commands:

  Guidance :
  Operations on Geant4 viewers.
  
   Sub-directories :
   1) /vis/viewer/set/   Set view parameters of current viewer.
   Commands :
   2) clear * /vis/viewer/clear []
   3) create * /vis/viewer/create  [] [] []
   4) dolly * /vis/viewer/dolly [] []
   5) dollyTo * /vis/viewer/dollyTo [] []
   6) flush * /vis/viewer/flush []
   7) lightsThetaPhi * /vis/viewer/lightsThetaPhi  [] [] [deg|rad]
   8) lightsVector * /vis/viewer/lightsVector  [] [] []
   9) list * /vis/viewer/list [] []
   10) pan * /vis/viewer/pan [] [] []
   11) panTo * /vis/viewer/panTo [] [] []
   12) refresh * /vis/viewer/refresh []
   13) remove * /vis/viewer/remove 
   14) reset * /vis/viewer/reset []
   15) select * /vis/viewer/select 
   16) update * /vis/viewer/update []
   17) show * Synonym for "/vis/viewer/update" - see that command for guidance.
   18) viewpointThetaPhi * /vis/viewer/viewpointThetaPhi  [] [] [deg|r
  ad]
   19) viewpointVector * /vis/viewer/viewpointVector  [] [] []
   20) zoom * /vis/viewer/zoom []
   21) zoomTo * /vis/viewer/zoomTo []
  

• Try another view:

     /vis/viewer/set/viewpointThetaPhi 40 40
  

  
  
• Then change the center using the pan command and zoom in a factor of 4:

     /vis/viewer/panTo -5 -1
     /vis/viewer/zoom 4.
  

  
  
• So far, we have just looked at detector geometry. Now lets add some particles.
  Declare that trajectories and hits should be added to the scene:

     /vis/scene/add/trajectories
     /vis/scene/add/hits
  

• Run the beam:

     /run/beamOn 1
  

  
  
• The output appears as soon as the /run/beamOn is complete since a /vis/viewer/flush is included automatically at the end of each event.
• One last command for this quick tutorial. You will have noticed in the previous image that the detector was drawn as a wireframe. Change this now by issuing the following command:

     /vis/viewer/set/style surface
     /run/beamOn 1
  

  The detector will then appear as a solid (though it is somewhat transparent, allowing you to still seee the tracks passing within).
  
• You can change this back with:

     /vis/viewer/set/style wireframe
     /run/beamOn 1
  

  Notice that the style command does not affect the hits. They are always drawn as surfaces because of code in the relevant c++ classes (which override any style commands issued at run time):

     src/A01EmCalorimeterHit.cc:  attribs.SetForceSolid(true);
     src/A01HadCalorimeterHit.cc: attribs.SetForceSolid(true);
     src/A01HodoscopeHit.cc:      attribs.SetForceSolid(true);
  

• OpenGL supports hidden line removal. You can control whether this removal is done and whether trajectories and hits are affected by this feature.

  By default, hidden line removal is disabled:
  

• To turn on hidden line removal

    /vis/viewer/set/hiddenEdge 1
  

  This hides edges of geometry, but lets trajectories through:
  
• To hide trajectories and hits as well

    /vis/viewer/set/hiddenMarker 1
  

  
• Geant4 visualization for OpenGL has many more options that you can best learn by playing around. If you get confused about the current view, just remember that you can reset with:

     /vis/viewer/reset
  

• This tutorial has only scratched the surface of Geant4's OpenGL capabilities. As mentioned above, Geant4 has other modes in which you can run OpenGL to control whether or not to store information into an intermediate memory area ("Immediate Mode" versus "Display List Mode") and to optionally wrap the OpenGL window in a Qt or Motif user interface. This tutorial was meant only to give you a brief, hands on introduction to this powerful tool and to encourage you to further explore its abilities.
• For futher information on OpenGL and Mesa, see:
  http://www.opengl.org/
  http://www.mesa3d.org

Recap of Geant4 Commands Used in this Tutorial

/vis/open OGL
/vis/drawVolume
/vis/viewer/set/viewpointThetaPhi 70 20
/vis/viewer/zoom 2
/vis/viewer/reset
/vis/viewer/set/viewpointThetaPhi 40 40
/vis/viewer/panTo -5 -1
/vis/viewer/zoom 4.
/vis/scene/add/trajectories
/vis/scene/add/hits
/run/beamOn 1
/vis/viewer/set/style surface
/run/beamOn 1
/vis/viewer/set/style wireframe
/run/beamOn 1

Further Resources

• Geant4 Installation Guides
• Introduction to Geant4 Visualization (ppt, pdf)
• Geant4 Visualization Tutorial using the HepRApp HepRep Browser
• Geant4 Visualization Tutorial using the DAWN Event Display
• Geant4 Visualization Tutorial using the OpenGL Event Display
• Geant4 Visualization Commands (ppt, pdf)
• Geant4 Advanced Visualization (ppt, pdf)

Other Resources:

• On-line documentation on Geant4visualisation:
  http://geant4.web.cern.ch/geant4/UserDocumentation/UsersGuides/ForApplicationDeveloper/html/Visualization
• List of visualization commands:
  http://geant4.web.cern.ch/geant4/UserDocumentation/UsersGuides/ForApplicationDeveloper/html/Control/UIcommands/_vis_.html
• Questions, comments about Geant4 Visualization -Contact us through the Geant4 Visualization online forum:
  http://geant4-hn.slac.stanford.edu:5090/HyperNews/public/get/visualization.html