Tips and Tricks¶

Code Snippets¶

Access Variables¶

Volume ID from Cell ID¶

Volume ID that obtained from Cell ID using dd4hep::VolumeManagerContext.identifier

auto volID = [&] (const std::vector<dd4hep::sim::Geant4Calorimeter::Hit*>& hits) {
      std::vector<double> result;
      for(const auto& h: hits) {
              auto volcontext = cellid_converter.findContext(h->cellID);
              result.push_back(volcontext->identifier);
      }
return result;
};

Volume ID that obtained from Cell ID using dd4hep::VolumeManagerContext.element and Readout/Segmentation

auto volID = [&] (const std::vector<dd4hep::sim::Geant4Calorimeter::Hit*>& hits) {
      std::vector<double> result;
      for(const auto& h: hits) {
              auto volcontext = cellid_converter.findContext(h->cellID);
              dd4hep::Readout r = cellid_converter.findReadout(volcontext->element);
              dd4hep::Segmentation seg = r.segmentation();
              result.push_back(seg.volumeID(h->cellID));
      }
return result;
};

FAQ¶

DD4hep¶

What is the difference between Volume ID and Cell ID?¶

Both are unique IDs, but in the case of volume ID, it is associated with a physical placement of a volume (in G4 or tgeo). The cell ID is used to further identify the subgeometry allocated to a volume through a segmentation. The volume ID is related to the cell ID through the readout’s id tag.

See dd4hep CellID Descriptors documentation

What is a segmentation?¶

A segmentation is a virtual geometry that is used to subdivide a volume. This avoids creating many small volumes to uniquely identify sensitive elements of a volume. An example would be a silicon detector with many channels at a fine pitch. Instead of create each pixel as a box of silicon, one silicon box is logically divide through the segmentation mechanism of DD4hep.

For a list of segmentations look at the headers in DDSegmentation.

What is a readout?¶

A readout is associated with a senstive detector in DD4hep and is used by setting the detector’s readout attribute. The attribute value is the name of a readout defined in the readouts.

<readouts>
  <readout name="ECalHits">
    <segmentation type="CartesianGridXY" grid_size_x="10.0*cm" grid_size_y="10.0*cm" />
    <id>system:5,layer:9,module:8,x:32:-16,y:-16</id>
  </readout>
</readouts>

How do I run the GEANT4 simulation?¶

todo

How can I visualize the GEANT4 simulation?¶

todo

How can I visualize the detector geometry?¶

todo