Geant4 Geometry and Transport Working Group Meeting

Monday 12 Jan 2026, 14:30 → 17:00 Europe/Zurich

1/1-025 (CERN)

14:30 → 15:20 Part 1 - VecGeom: Plans for 2026 - open issues, identifying priorities and 'top-level' deliverables

POW VecGeom 2026: restructure VecGeom to deliver higher GPU navigation performance, consistent behavior across backends, and a maintainable, portable code base (CUDA/HIP). The detailed plan is drafted here

• Simplify and standardize the navigation interface (clear inputs/outputs; consistent boundary behavior; consistency of VNavigator/GPU interfaces).

• Improve GPU navigation performance with a more locality-aware accelerator (beyond today’s BVH)

• Replace recursion-heavy algorithms (esp. Boolean navigation) with iterative, stack-friendly implementations

• Remove legacy vector/SIMD remnants in kernels to simplify code and improve scalar performance

• Add a portability layer for CUDA/HIP and refactor device code toward POD + explicit dispatch (no device virtuals)

Mid-term items:

• Mixed-precision implementation (global API in double; per-solid precision policies)
• Documentation: user guides + algorithm notes to match the restructured core

 

Long-term goal: Unified solids common library providing solid-level and global navigation algorithms for Geant4, ROOT, reconstruction programs.

 

15:20 → 15:30 Short break

15:30 → 16:50 Part 2. Other Topics: Status and Plans for 2026

15:30 Status and Outlook

Speaker: John Apostolakis (CERN)

15:40 Goals for 2026

Recall topics mentioned in 2025:

Navigation / modelling
- Vecgeom surface-based modeller
Performance / interfaces
- Improvement of implementation of parallel voxel creation
- Overlap checking using Multi-Threading
Field:
- Robustness improvement of QSS3
- High order symplectic stepper (needs person)

Impact / Coupling with other developments (in Geometry & Transport and beyond)

See Deliverables (2024 & 2025) and potential deliverables (future)

16:30 Reports, Open Topics and Plans - roundtable

16:50 → 17:00 AOB: Other topics