G4OCCT

G4OCCT

[](LICENSE)

Geant4 interface to Open CASCADE Technology (OCCT) geometry definitions.

G4OCCT provides a compatibility layer between Geant4 geometry descriptions and the Open CASCADE Technology (OCCT) geometry framework. The goal is to enable physics simulations to be driven by CAD geometry imported from STEP (and other CAD exchange) files, while retaining full compatibility with Geant4's navigation, scoring, and visualisation subsystems.

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Motivation

Geant4 is the de-facto standard toolkit for high-energy physics detector simulations. Its geometry is traditionally hand-coded using constructive solid geometry (CSG) primitives or tessellated meshes. Engineering designs, however, are almost universally stored in CAD tools and exchanged in formats such as STEP. G4OCCT bridges this gap by:

• Providing accurate detector simulations from the engineering design directly, eliminating geometry discrepancies between the CAD model and the simulation.
• Reducing maintenance burden — geometry changes in the CAD tool propagate automatically to the simulation.
• Enabling richer geometry — OCCT BRep (boundary representation) shapes can capture design intent (fillets, chamfers, swept surfaces) that CSG primitives cannot.

For more detail see the Project Goals documentation page.

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Architecture

G4OCCT uses a thin-wrapper approach: OCCT shapes are embedded inside Geant4 constructs so that the Geant4 navigator, scoring, and visualisation infrastructure remain unchanged.

G4OCCT class	Inherits from	Embeds
G4OCCTSolid	G4VSolid	TopoDS_Shape
G4OCCTLogicalVolume	G4LogicalVolume	TopoDS_Shape (optional)
G4OCCTPlacement	G4PVPlacement	TopLoc_Location

Navigation queries (Inside, DistanceToIn/Out, SurfaceNormal, …) are delegated to OCCT BRep algorithms. See the Geometry Mapping and Solid Navigation Design documentation pages for details.

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Requirements

Dependency	Minimum version
CMake	3.16
C++	20
Geant4	11.3
OpenCASCADE (OCCT)	7.8

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Building

cmake -S . -B build \
  -DCMAKE_BUILD_TYPE=Release \
  -DBUILD_TESTING=ON \
  -DBUILD_BENCHMARKS=ON \
  -DCMAKE_INSTALL_PREFIX=/path/to/install
 
cmake --build build -- -j$(nproc)
ctest --test-dir build --output-on-failure -j$(nproc)
cmake --install build

Both BUILD_TESTING and BUILD_BENCHMARKS default to OFF; enable them during development to run the CTest suite and the geantino navigator benchmarks.

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Downstream Usage

After installation, integrate G4OCCT into a CMake project with:

find_package(G4OCCT REQUIRED)
target_link_libraries(myApp PRIVATE G4OCCT::G4OCCT)

The installed G4OCCTConfig.cmake propagates the Geant4 and OCCT dependencies automatically.

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Repository Layout

G4OCCT/
├── CMakeLists.txt          # Top-level build; installs G4OCCTConfig.cmake
├── cmake/
│   └── G4OCCTConfig.cmake.in
├── include/G4OCCT/
│   ├── G4OCCTSolid.hh      # G4VSolid wrapping TopoDS_Shape
│   ├── G4OCCTLogicalVolume.hh
│   └── G4OCCTPlacement.hh  # G4PVPlacement + TopLoc_Location
├── src/
│   ├── G4OCCTSolid.cc
│   ├── G4OCCTLogicalVolume.cc
│   ├── G4OCCTPlacement.cc
│   ├── tests/              # CTest-integrated unit tests
│   └── benchmarks/         # Geantino navigator benchmarks
└── docs/
    ├── goals.md            # Project goals and design philosophy
    ├── geometry_mapping.md # Geant4 ↔ OCCT class correspondence
    ├── solid_navigation.md # G4VSolid ↔ OCCT algorithm mapping
    └── material_bridging.md

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Documentation

Full documentation is available at **https://eic.github.io/G4OCCT/**, including:

• Project Overview Slides — 20-slide deck covering motivation, architecture, performance, and roadmap.
• Project Goals — Vision, motivation, design philosophy, and roadmap.
• Geometry Mapping — Correspondence between Geant4 and OCCT class hierarchies.
• Solid Navigation Design — Per-function mapping of G4VSolid queries to OCCT algorithms.
• Material Bridging — Strategies for mapping STEP/OCCT material names to G4Material.
• API Reference — Doxygen-generated API documentation.

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Contributing

Contributor conventions (coding style, CI setup, documentation requirements, and design principles) are described in AGENTS.md.

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License

This project is licensed under the [GNU Lesser General Public License v2.1 or later](LICENSE) (SPDX identifier: LGPL-2.1-or-later).