Implementation of the TGeoTessellated navigation functionality #21095
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added 6 commits
February 1, 2026 19:08
Implementation of TGeoTessellated navigation functionality using triangle-ray-intersection similar to GEANT4 G4TessellatedSolid. Due to low simulation speeds for meshes of high triangle count, additional partitioning structures such as an Octree and the BVH (using the bvh library included in ROOT) can be used to reduce required simulation time. Here, the most benefitial partitioning structure seems to be mesh dependent. However, the usage of the BVH functionality is potentially not optimal and may require more intense testing. First comparisons from the CBM collaboration indicate that this implementation approach (including the Octree) is more time consuming than the VecGeom implementation for tessellated shapes (DOI: 10.1051/epjconf/202533701268). All usage modes (without partitioning structure and with) have been regularly tested against TGeoBBox navigation results by simulations of boxes with geant3/4_vmc TVirtualMCApplications and it is being actively used in simulations. Add TGeoVector3 include to TGeoTypedefs.h Add Tessellated helper classes REQUIRES REVERT: changed top_down_sah_builder.h to only include ROOT/span.hxx Changed TGeoTessellated to TGeoArbN code Adjusted ROOT classes to Tessellated changes Add DOI to TGeoTessellated.cxx Fix up constness of TGeoTessellated::ComputeNormal Remove TGeoFacet from LinkDef1.h Removes "Warning: Unused class rule: TGeoFacet" Remove overriding TPartitioningI::Print fix compile time warnings concerning TGDMLWrite size_t/int cast Add Reference for AABB-triangle overlap test Fix TGeoTessellated in tutorial scripts Fix: After resizing recompute triangle center Fixup spacing in TGeoTessellated::InspectShape TGeoTessellated add default implementations for minor TGeoShape functions Add DistancetoPrimitive, SavePrimitive and Sizeof3D default implementations. Adjust Octree tolerance to determine directions parallel to axis The Revelles Octree Traversal approach requires a tolerance value to determine if a direction is parallel to Octree axis. If a direction component is smaller than this value, this will be interpreted to be 0. If a particle is tracked close to a octant boundary, this can be problematic, as the wrong octant can be chosen to be traversed next. This is a minute effect, never encountered before, but seen by tessellatedNav.C
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This Pull request:
Introduce navigation functionality for TGeoTessellated using triangle-ray-intersection tests.
This is the navigation functionality as it is implemented for the historically grown, non-ROOT-implementation TGeoArbN.
To improve usability in VMC based simulations of meshes with high triangle count (>500), partitioning structures in the form of an Octree, as well as a BVH (using the bvh library included in root), are added. We found so far that the choice of partitioning structures seems to be mesh dependent (often BVH is best, however, for some particularly large meshes (~250k triangles) the Octree seems better). However, our BVH usage may be suboptimal.
As Dr. Sandro Wenzel opened a pull-request for his navigation implementation Add BVH-based navigation functions to TGeoTessellated #21045, we wanted to "offer" our implementation in parallel. Maybe something can be learned from it.
Tested using TGeoChecker CheckShape functionality (on a box mesh, the teddy.obj in tutorials/visualisation/geom/ contains triangle-intersections and hence is faulty), as well as by computing navigation information for 100 million random points for a box mesh and comparing results with TGeoBBox as quick test that no errors were introduced while translating our TGeoArbN to ROOT's TGeoTessellated.
Dr. Sandro Wenzel's 'rayTriangle' implementation was used to improve own implementation of Triangle ray intersection in TGeoTriangle.
Still required changes:
to compile for me, otherwise issues with std.span and modules