KAHIBARO
9.3. Changing the View
Table of Contents
Rotation
Once the geometry is visible in a viewer, you often need to rotate the scene to understand the detector layout. In Geant4 interactive sessions with visualization, rotation is controlled either by macro commands or, when using Qt or similar GUI viewers, by the mouse.
The core idea is that the visualization system defines a camera that looks at your geometry. Rotation changes the camera orientation around the scene while keeping the geometry fixed in space.
In command line macros, there are two main ways to control rotation. The first uses the /vis/viewer/set/viewpointThetaPhi command. This sets the camera direction using spherical angles $\theta$ and $\phi$ in degrees, measured with respect to the world coordinate system. For example, you can write
/vis/viewer/set/viewpointThetaPhi 90 0 to look along the $+X$ direction, or /vis/viewer/set/viewpointThetaPhi 90 90 to look along $+Y$. The second approach is to use rotation matrices with /vis/viewer/set/rotation. This command expects a rotation matrix that you usually specify in an external macro or script after you compute it, so it is less common for beginners.
In Qt or OpenGL Qt viewers, you can usually click and drag with the mouse to rotate the view interactively. This does not change your geometry, only the camera orientation. When you later run macros again, the view can be reset to a known direction using /vis/viewer/set/viewpointThetaPhi. It is a good practice to include at least one explicit viewpoint command in your macros so that anyone running your example sees a consistent orientation.
Always remember that visualization rotation affects only the camera, not the detector geometry. To rotate a detector component physically, you must use the correct rotation in your geometry construction, not a visualization command.
Zoom
Zooming changes how close the camera appears to be to your geometry without altering the geometry itself. In most Geant4 viewers, zoom is controlled in two ways, either through macro commands or GUI interactions.
The basic command is /vis/viewer/zoom. This command takes a scaling factor. A factor greater than 1 zooms in, and a factor between 0 and 1 zooms out. For example, /vis/viewer/zoom 1.2 makes the objects appear larger, while /vis/viewer/zoom 0.8 makes them appear smaller. The zoom operation is multiplicative, so several zoom commands in sequence compound the effect. If you get lost because you zoomed too far, you can reset the view with /vis/viewer/reset which restores the default zoom and orientation.
In many GUI viewers, the mouse wheel or equivalent gesture controls zoom. The viewer may also offer menu items such as “Zoom in,” “Zoom out,” or “Fit all” which adjust the camera distance automatically so that the entire geometry fits in the window. The “fit all” type of function is particularly useful after you modify the geometry or after you have zoomed in to inspect a small region and want to return to an overview.
You should be careful not to confuse zoom with changing the geometry scale. Zoom only affects how the scene is displayed. The geometry dimensions and physics calculations remain exactly the same regardless of zoom level. When preparing macro files that others will use, consider including a zoom command that provides a reasonable initial view, for example enough zoom to see both the world volume and the main detector components clearly.
Zoom never changes real detector size. It only changes how large objects appear on the screen. Physics quantities and distances in the simulation are unaffected by any visualization zoom.
Viewpoints
Viewpoints are predefined or user defined camera orientations and positions that make it easy to look at the geometry from meaningful directions. Instead of repeatedly rotating interactively, you can select a viewpoint that corresponds to a front, side, or top view of your detector.
Geant4 provides the /vis/viewer/set/style command to control how the scene is drawn, but the actual viewpoint direction is usually defined by /vis/viewer/set/viewpointThetaPhi or related commands. For example, you can define a “beam view” that looks along the beam axis by setting the camera direction to align with the primary particle direction. If your beam travels along $+Z$, then /vis/viewer/set/viewpointThetaPhi 0 0 gives a view along the $+Z$ axis, so you can see what the beam “sees” as it enters the detector.
You can also look at the detector from orthogonal engineering views. A typical set of useful viewpoints includes a top view, a side view, and a front view. For a detector whose main axis is $Z$, you could use:
/vis/viewer/set/viewpointThetaPhi 90 0 for an $+X$ view, /vis/viewer/set/viewpointThetaPhi 90 90 for a $+Y$ view, and /vis/viewer/set/viewpointThetaPhi 0 0 for $+Z$. Some GUI viewers offer buttons or menu entries that correspond to these canonical views.
In addition to view direction, a viewpoint also includes the camera position relative to the target point and a field of view. Advanced commands like /vis/viewer/set/target can change which point in space the camera is looking at, and /vis/viewer/set/autoRefresh controls when the view is updated. For beginners, it is usually enough to set the viewpoint angles and then use zoom to obtain a good framing.
It is helpful to store your favorite viewpoints in separate macro files. For example, you might have a file views.mac that contains several /vis/viewer/set/viewpointThetaPhi and /vis/viewer/zoom commands, each preceded by a comment that describes the intended view. You can then include this macro in interactive sessions or run it automatically at startup, which makes it easier to return to consistent views when you change geometry or run new simulations.
Define consistent viewpoints for your project and store them in macros. This ensures that visual comparisons of different geometries or configurations are meaningful because they are always viewed from the same directions and distances.
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