Convert your first model

Rather than start on your own detector, it is worth converting something small and known-good first, so that anything odd later is clearly your model and not your installation. A toy excavator arm is committed to the repository for exactly this purpose:

$O2_ROOT/share/CADSupport/examples/ExcavatorArm.step   # 13 leaf solids, ~500 kB
 

It converts in seconds and is varied enough to be interesting: the hydraulic rams and pivot pins are plain cylinders, the boom and stick are machined bodies full of concave features, and the bucket has a torus in it. Run the converter over it, asking for all three representations at once — we come back to what those are in the next section:

mkdir -p cad_out/excavator
o2-cad-to-tgeo \
    $O2_ROOT/share/CADSupport/examples/ExcavatorArm.step \
    --output-folder cad_out/excavator \
    -o geom.C \
    --step-unit auto \
    --csg auto --exact-surfaces auto --mesh --mesh-prec 0.05
 

That takes about thirteen seconds. Along the way the converter prints three lines worth reading on every run, because each one catches a different common mistake:

Detected STEP length unit: mm (scale to cm = 0.1)
Placement check: 13 leaf placement(s), all at distinct world transforms.
Emitting 13/13 logical volumes as exact O2BVHSurfaceSolid
 

The unit line bites hardest. TGeo works in centimetres and most CAD systems export millimetres, so a silent unit error gives you a detector ten times too big and a simulation that still looks almost plausible. --step-unit auto reads the declaration in the file; pass --step-unit mm explicitly when the file declares something you do not believe. The placement line then tells you whether two leaves landed on the same world transform, which almost always means a duplicated part in the CAD model rather than a real coincidence.

Finally the converter prints what it decided for each part, ending in a one-line summary:

=== REPRESENTATION CASCADE (per leaf solid) ===
  volume                carried by  evidence
  BasePin               csg         TGeoTube(rmin=0, rmax=1, dz=5) [tier1-tube], dV_sym=0 cm^3
  Base                  surface     declined CSG: 7 axis clusters: beyond the recogniser's scope ...
  BoomCylinderOuter     csg         TGeoTube(0.6,1,7.991) u TGeoTube(0.7,1.5,1.5), dV_sym=0 cm^3
  ...
  tiers: CSG 7, exact surfaces 6, tessellated 0  (of 13 leaf solids)
 

Seven parts came out as ordinary ROOT shapes, six as exact surface solids, and none had to fall back to an approximate mesh. The dV_sym=0 is the reassuring part: it is the symmetric-difference volume between what was emitted and the original CAD solid, so zero means the conversion is exact rather than merely close.

Look at what you made

Numbers in a terminal are no substitute for seeing the thing. The macro can build the geometry and write it out as an ordinary ROOT file:

cd cad_out/excavator
root -l -b -q -e '.L geom.C' -e 'build_and_export("geom.root");'
 

A shaded render of the converted excavator arm: bucket, stick, boom and hydraulic rams, seen from above and to the side.

The converted model, drawn by casting one ray per pixel through the TGeo navigator — so this is the geometry as the transport sees it, not a separate preview mesh.

The simplest interactive way to inspect the result is ROOT's own web display, which renders the geometry with JSROOT in your browser and lets you rotate it, hide volumes and click through the tree:

root --web geom.root

If you are on a remote machine where opening a browser is awkward, export the geometry as a JSROOT document instead and open that file locally. It is a self-contained 32 kB for this model, and can be dragged straight onto root.cern/js:

root -l -b -q -e 'TGeoManager::Import("geom.root");' \
             -e 'TBufferJSON::ExportToFile("excavator.json.gz", gGeoManager);'
 

Spend a minute here. Turning the model around is the fastest way to notice that a subassembly is missing, that something sits at the wrong scale, or that the part you care about was quietly filtered out.