KAHIBARO
9.4. Volume Appearance
Table of Contents
Colors
The apparent color of each volume in the GATE geometry viewer is only a visual aid. It does not change the physics or materials, but it is extremely useful to understand and debug your setup.
When you create volumes, you can assign a color to each one. In OpenGATE this is done through the visualization attributes of the volume. Typically you will set a named color, such as "red", "blue", "green", "yellow", "cyan", or "magenta". Some backends also support more descriptive names. It is a good habit to use consistent color conventions, for example, blue for water phantoms, gray for CT based patient geometries, green for detector crystals, and dark gray for shielding.
You should choose colors that create a clear contrast between neighboring volumes. If two adjacent volumes share similar colors, it is much harder to see their exact boundary and to detect small position mistakes. If a volume is nested inside another one, a strong color contrast makes it easier to verify that the inner volume is placed correctly and not intersecting other components unintentionally.
Many visualization systems allow both solid and wireframe drawings. Color is applied in both cases, but wireframe colors mainly help to distinguish overlapping frames. When you display a complex detector, such as a PET ring or a SPECT camera head, using a coherent color scheme for repeated modules helps you to understand the tiling pattern and symmetry at a glance.
Use colors only as visual labels. The actual material and physics properties are defined elsewhere, so always verify materials and densities directly instead of relying on color alone.
Transparency
Transparency controls how much of the underlying or inner volumes you can see through a given object. It is especially important in medical physics simulations where a patient phantom, detector housing, collimator, and shields may all overlap in the field of view.
If a volume is completely opaque, you cannot see any volume that is inside or behind it from the current camera angle. This is often inconvenient when you want to check the placement of internal components such as detector crystals inside a housing, or sources inside a phantom. By adding transparency, you can keep the outer volume visible while still seeing the contents inside.
In practice you will set a transparency value between fully opaque and almost invisible. A moderate transparency value is usually best. Too little transparency hides inner parts, too much transparency can make the outer volume hard to recognize. For example, a lead shielding block might be given a dark color with medium transparency so that you can see both the shield outline and the detector inside. For patient CT based geometries, you might make soft tissue slightly transparent while keeping detector components more opaque for clarity.
Transparency is also helpful when you debug voxelized geometries. If your patient or phantom is represented as a large voxel volume, a transparent display allows you to check the orientation and to see where sources, beams, and detectors intersect the body.
Adjust transparency carefully when checking overlaps. High transparency can make overlapping volumes visually confusing, so combine transparency changes with rotation and zoom to interpret the scene reliably.
Visibility
Visibility simply controls whether a volume is drawn at all in the visualization window. Turning visibility off for a volume does not remove it from the simulation. The volume remains part of the geometry and still participates in particle transport and scoring. Only the graphical representation is hidden.
You will often use visibility to simplify the view. In a detailed scanner model you might hide mechanical supports, cables, or small screws. In a treatment simulation you might temporarily hide the treatment couch in order to see clearly the beam shaping devices and the patient. For voxelized patients or large phantoms, hiding outer containers or support structures can make the main volume much easier to inspect.
Another important use of visibility is to isolate specific components for debugging. For example, if you suspect that detector crystals are misplaced inside a module, you can hide all volumes except the module and the crystals. Similarly, you can hide everything except a collimator to inspect hole geometry and septa without interference from the crystal or housing.
Many visualization setups also support wireframe visibility as an alternative to fully filled shapes. In this case a visible volume shows only its edges, which is useful when you want to see multiple nested structures without relying too heavily on transparency.
Never assume that a hidden volume has been removed from the simulation. Always remember that visibility affects only what you see, not what the physics engine transports or scores.
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