Viewing the geometry

Last updated on 2026-07-10 | Edit this page

Estimated time: 20 minutes

Overview

Questions

  • How can we view the geometry?

Objectives

  • Understand how to export the geometry with dd_web_display.
  • Know of multiple ways in which to open ROOT TGeo geometries.

Introduction

Before we move to discussion of the detector plugins in the next part, let’s discuss visualization on your local system.

ROOT visualization

The geometry included in eic-shell can be viewed with the ROOT geometry browser. However, we first need to export it from the built-in DD4hep format to the ROOT TGeo format, with a small utility program dd_web_display. To do this, we will need to ensure we are in a directory where we have write access, such as the directory ~/eic/.

BASH 

$ cd ~/eic/
$ dd_web_display --export $DETECTOR_PATH/$DETECTOR_CONFIG.xml
Callout

Note: If you are not inside eic-shell, you may need to be connected to the internet as you run this command since a magnetic fieldmap will need to be downloaded.

The dd_web_display utility will create a ROOT file in the current directory that can be opened with the geometry viewer online at the ANL geometry viewer or the ROOT JSROOT viewer, or a local ROOT installation. VSCode also has a JSROOT extension which provides the same functionality as the browser.

Callout

Note: The version of JSROOT on the ANL geometry viewer is older than the ROOT JSROOT viewer version so some differences in the visualisation may be present.

The output file is by default named detector_geometry.root this can be changed if you want to look at several configurations using e.g.

BASH 

$ dd_web_display -o vertex_geometry.root --export $DETECTOR_PATH/epic_vertex_only.xml

For local ROOT installations, the following commands may be helpful:

CPP 

TGeoManager::Import("detector_geometry.root");
gGeoManager->GetTopVolume()->Draw("ogl")

The geometry viewer has to make decisions on what to draw in order to keep the number of facets small enough. This means that detectors with a large number of repeated components may not be drawn, or other detectors may not be drawn when those detectors are drawn. For this reason, we also have the subsystem-specific entry points xml files. For visualization of specific subsystems, these files are recommended.

Parameters describing how each component of the geometry should be vizualised are contained within the detector plugins and can be controlled through the xml description.

Challenge

Exercise: view a subsystem

  • Export a different detector configuration from the default epic.xml, and export this to ROOT TGeo format.
  • Open the exported ROOT file in the geometry viewer at the ANL geometry viewer or the ROOT JSROOT viewer.

For example dd_web_display -o vertex_geometry.root --export $DETECTOR_PATH/epic_vertex_only.xml produces vertex_geometry.root; loading that file in either viewer shows only the vertex barrel, which renders faster and is easier to inspect than the full epic.xml.

Geant4 visualization

Warning: might not work for various reasons. If you are used to the Geant4 geometry visualisation or want to visually inspect where a subset of your event sample this is still possible using npsim.

BASH 

npsim --runType qt --compactFile $DETECTOR_PATH/epic_vertex_only.xml --inputFiles root://dtn-eic.jlab.org//volatile/eic/EPIC/EVGEN/SIDIS/pythia6-eic/1.0.0/18x275/q2_0to1/pythia_ep_noradcor_18x275_q2_0.000000001_1.0_run9.ab.hepmc3.tree.root --macro macro/b0_vis.mac

This particular example uses pythia6 min-bias events stored on the xrootd server at jlab. The visualization of particle tracks and the detector is controlled by the macro/b0_vis.mac file.

Callout

Note: Geant4 does not try and limit its visualization of components so depending on your machine the full detector might struggle, please be patient. This performs much better outside of eic-shell but takes a long time to set up the environment.

Other visualisation you might want

  • ACTS Surfaces
  • Material Map
  • Event display

These are avaliable in various forms but not covered in this tutorial.

Key Points
  • The geometry, exported to the ROOT TGeo file, can be viewed with ROOT.