Install the software

The converter is a Python script, but it leans on OpenCascade — the CAD kernel that reads STEP files — through its Python bindings, pythonOCC. That is the one piece you have to provide yourself.

Warning

pythonOCC is not part of O2sim

It is a separate aliBuild package, and it is not pulled in when you build or load O2sim.If you have never built it, that is genuinely step one — no amount of loading O2sim willconjure it up.

So we build it first. This pulls in OpenCascade itself as a dependency, and takes a while the first time:

cd ~/alisw
aliBuild build pythonOCC --defaults o2 --no-system SWIG
 

The --no-system SWIG is worth keeping even when aliBuild tells you the system SWIG will do. The recipe asks for SWIG 4.2.1 and several distributions ship 4.2.0, which is close enough to be picked up and not close enough to build. Forcing aliBuild to build its own costs a few minutes once and saves a confusing failure later.

With that in place, everything happens in a single shell. We load pythonOCC together with O2sim, because the converter needs ROOT as well as OpenCascade — and the same environment then runs o2-sim afterwards, so there is no need to switch shells between converting and simulating:

alienv enter O2sim/latest,pythonOCC/latest

Two quick checks confirm the environment is sound. The first proves the CAD bindings import at all; the second runs the converter's own self-test, which builds its test cases in memory and needs no input file:

python3 -c "import OCC.Core.Bnd; print('OCC import OK')"
o2-cad-to-tgeo --self-test
 

OCC import OK
...
20/20 in-field media checks passed
 

The two commands you will use throughout are o2-cad-to-tgeo, which takes STEP to TGeo, and o2-tgeo-to-cad, which takes TGeo back to STEP. They are also installed under their older names, O2_CADtoTGeo.py and O2_TGeoToCAD.py, which work identically.

Note

If the import fails with “No module named ‘OCC’“

Some pythonOCC installations carry a modulefile that puts the OCC package directory itself onPYTHONPATH, rather than the site-packages directory containing it — so Python looks inside thepackage and never finds it. The cure is to drop the trailing /OCC from theprepend-path PYTHONPATH line in $PYTHONOCC_ROOT/etc/modulefiles/pythonOCC. A recipe fix is onits way to alidist.

Outside the ALICE stack

A conda environment with pythonocc-core also works. There, run the script from the source tree:

conda create -n occ -c conda-forge python=3.10 pythonocc-core -y
conda activate occ
python3 $O2_SRC/Detectors/CADSupport/tools/O2_CADtoTGeo.py --help
 