KAHIBARO
9.1. Visualization in Geant4
Table of Contents
Visualization drivers
Geant4 includes a flexible visualization system that can work with several different backends, called visualization drivers. A driver is a concrete implementation of a generic visualization interface and is responsible for actually drawing your geometry, trajectories, and hits on the screen or into a file.
Internally, Geant4 provides a central visualization manager that receives drawing requests from the simulation and forwards them to one or more active drivers. As a user, you normally do not interact directly with the manager in a beginner application. Instead, you select and configure drivers using macro commands in a terminal or in the Geant4 interactive session.
A typical Geant4 installation can support interactive, file-based, and batch-oriented visualization drivers. Examples include OpenGL for real-time interactive viewing, RayTracer for simple ray-traced images, and various drivers that produce output for external viewers. For absolute beginners, OpenGL is by far the most convenient starting point, because it gives immediate visual feedback while you interact with your simulation.
To use any visualization driver, you must first have compiled Geant4 with visualization support enabled in CMake. After that, you usually start visualization with a macro command of the form /vis/open <driver-name>. This tells the visualization manager to create a new viewer window using the specified driver.
The choice of driver affects only how things are displayed, not the physics results. You can switch drivers between runs or even have multiple viewers open at the same time. For a beginner course, it is sufficient to know that the visualization driver is a pluggable component that you select at run time and that OpenGL is the default choice in many examples.
Important: The visualization driver does not change the physics. It only changes how geometry, tracks, and hits are displayed. Always choose your physics list and geometry independently of your visualization settings.
OpenGL
OpenGL is the most widely used interactive visualization driver in Geant4. It draws your detector geometry directly in a window and allows you to rotate, zoom, and inspect volumes in real time. For beginners, this is usually the first driver you enable to check whether your geometry is correctly defined and placed.
There are several OpenGL-based drivers in Geant4, for example OGL with different backends or flavors. They all follow the same idea: they open a window and show the current scene, which can include the world volume, all placed volumes, and later, particle trajectories and hits once the simulation has run. When you run a standard example that has been built with visualization enabled, you can usually start an OpenGL viewer with a macro line similar to /vis/open OGL. After that, you can use additional visualization macro commands, covered in later sections, to draw the geometry and adjust the view.
In many builds, OpenGL is combined with Qt to provide a graphical user interface with menus, buttons, and an integrated command line. In that case, the OpenGL viewer is embedded inside the Qt window, but conceptually it is still the same OpenGL driver, drawing the same geometry. Regardless of the exact flavor, the OpenGL driver is intended for interactive work, not for producing high quality publication figures. It is ideal for quickly verifying that volumes do not overlap, that placements are correct, and that tracks behave as expected.
Rule: Use OpenGL visualization to check your geometry and trajectory behavior during development. Do not rely on it as a substitute for proper physics validation or as final scientific output.
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