Let VecGeom turn large Boolean solids into MultiUnions

This lets VecGeom's BooleanFactory rewrite Boolean solids of at least 8 components into MultiUnions in the kVecGeom mode.

- In the ALICE geometry 170 composite shapes are rewritten, among them the MFT disk supports and the ITS end wheels.
- G4.vecgeomBooleanThreshold sets the threshold; 0 keeps the solids as converted.
- Other users of buildVecGeomGeometry keep the solids as converted.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: sawenzel

Common/SimConfig/include/SimConfig/G4Params.h

@@ -65,6 +65,8 @@ struct G4Params : public o2::conf::ConfigurableParamHelper<G4Params> {
                                         // shortens steps and so changes the random history
   bool vecgeomFlattenAssemblies = true; // dissolve TGeo assemblies into their content when converting
                                         // to VecGeom; the Geant4 touchable keeps the assembly levels
+  int vecgeomBooleanThreshold = 8;      // Boolean solids with at least this many components become
+                                        // VecGeom MultiUnions; 0 keeps them as converted
   int vecgeomCheckRays = 0;             // if > 0, step this many rays out of the interaction point with
                                         // TGeo and VecGeom and report the volumes they enter differently
   int vecgeomCheckLocation = 0;         // if > 0, locate this many random points with both and report

Detectors/Base/include/DetectorsBase/GeometryManager.h

@@ -139,8 +139,9 @@ class GeometryManager : public TObject
   /// locator and a safety estimator to every logical volume. Does the work once per process; later
   /// calls, whatever they ask for, return the geometry already built, so a caller that needs a
   /// particular assembly treatment must come first. \param flattenAssemblies dissolves TGeo
-  /// assemblies into their content.
-  static void buildVecGeomGeometry(bool flattenAssemblies);
+  /// assemblies into their content. \param booleanThreshold lets VecGeom turn Boolean solids with at
+  /// least this many components into MultiUnions; 0 keeps them as converted.
+  static void buildVecGeomGeometry(bool flattenAssemblies, int booleanThreshold = 0);
 #else
   static constexpr bool isVecGeomAvailable() { return false; }
 #endif

Detectors/Base/src/GeometryManager.cxx

@@ -587,17 +587,26 @@ void ensureVecGeomWorldBuilt()
 }
 } // namespace
 
-void GeometryManager::buildVecGeomGeometry(bool flattenAssemblies)
+void GeometryManager::buildVecGeomGeometry(bool flattenAssemblies, int booleanThreshold)
 {
   static std::once_flag onceFlag;
-  std::call_once(onceFlag, [flattenAssemblies]() {
+  std::call_once(onceFlag, [flattenAssemblies, booleanThreshold]() {
     if (!gGeoManager) {
       LOG(fatal) << "Cannot build VecGeom geometry: no TGeo geometry loaded (call GeometryManager::loadGeometry() first)";
     }
     // Translate geometry and material pointers, then build acceleration structures.
     tgeo2vecgeom::RootGeoManager::Instance().SetMaterialConversionHook([](TGeoMaterial const* m) { return (void*)m; });
     LOG(info) << "VecGeom conversion: flattenAssemblies=" << flattenAssemblies;
     tgeo2vecgeom::RootGeoManager::Instance().SetFlattenAssemblies(flattenAssemblies);
+#if __has_include(<VecGeom/management/BooleanFactory.h>)
+    // Applied when the conversion closes the geometry.
+    LOG(info) << "VecGeom conversion: booleanThreshold=" << booleanThreshold;
+    vecgeom::GeoManager::Instance().SetBooleanOptimizationThreshold(booleanThreshold > 0 ? booleanThreshold : 0);
+#else
+    if (booleanThreshold > 0) {
+      LOG(warning) << "VecGeom without BooleanFactory: Boolean solids are kept as converted";
+    }
+#endif
     tgeo2vecgeom::RootGeoManager::Instance().LoadRootGeometry();
 
     // Acceleration structures must be built before the navigators/locators reference them.

Detectors/gconfig/src/VecGeomNavigation.cxx

@@ -80,7 +80,7 @@ void installVecGeomNavigator()
 
   TStopwatch timer;
   timer.Start();
-  o2::base::GeometryManager::buildVecGeomGeometry(g4Params.vecgeomFlattenAssemblies);
+  o2::base::GeometryManager::buildVecGeomGeometry(g4Params.vecgeomFlattenAssemblies, g4Params.vecgeomBooleanThreshold);
   timer.Stop();
   LOG(info) << "VecGeom geometry built in " << timer.RealTime() << " s";