OpenLoom

This project is under heavy development and not yet ready for production use.

A constrained Delaunay triangulation library for 2D and 3D mesh generation with OpenCASCADE CAD support, written in C++20.

Status

Component	Status
2D constrained Delaunay mesher	Working
3D surface mesher	Under development
3D volume mesher	Not yet started

Background

OpenLoom is designed with extensibility as a core principle. The architecture makes it straightforward to add new meshing algorithms, refinement strategies, and import/export formats without touching the existing pipeline. Key extension points are defined as interfaces (IMesher, ICorner, IEdge, ISurface), and the context pattern cleanly separates geometry, topology, and mesh data.

The current meshing and refinement implementation is based on Shewchuk's algorithm — a quality Delaunay refinement method that guarantees well-shaped elements by iteratively inserting Steiner points to eliminate poorly-conditioned triangles and tetrahedra. This is used in both the 2D and 3D meshers.

Features

• 2D constrained Delaunay triangulation (CDT)
• OpenCASCADE integration for importing STEP/IGES CAD geometry
• VTK export (.vtu) for visualization in ParaView
• Quality-driven mesh refinement (Shewchuk-style)
• Mesh verification and debug utilities

Dependencies

Library	Purpose	Required
Eigen3	Linear algebra	Yes
VTK	Mesh export	Yes
spdlog	Logging	Yes
fmt	String formatting	Yes
GoogleTest	Testing	Yes
OpenCASCADE	CAD geometry import	Optional
OpenMP	Parallel mesh verification	Optional

Building

Install dependencies (Fedora/RHEL)

sudo dnf install clang clang-tools-extra cmake
sudo dnf install vtk-devel eigen3-devel spdlog-devel fmt-devel gtest-devel

Configure and build

cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug
cmake --build build -j$(nproc)

Optional build flags

# Enable OpenMP for parallel mesh verification
cmake -S . -B build -DOPENLOOM_USE_OPENMP=ON

# Point to a custom OpenCASCADE installation
cmake -S . -B build -DOpenCASCADE_DIR=$HOME/local/opencascade/lib/cmake/opencascade

Run tests

ctest --test-dir build/tests --output-on-failure
# or directly:
./build/tests/runTests

# Run a specific test
./build/tests/runTests --gtest_filter="TestClass.TestName"

Examples

After building, executables are in build/src/Examples2D/ and build/src/Examples3D/. Output .vtu files can be opened with ParaView.

2D examples

Executable	Description
SimpleDelaunay2D	Basic 2D Delaunay triangulation
RectangleWithHole2D	Rectangle mesh with a hole constraint
RectangleWithCrack	Mesh with crack/slit constraints
SquareWithCircularHole	Square domain with a circular cutout
SquareWithInternalCircles	Multiple internal circle constraints
MeshStepFile2D	Mesh a 2D profile loaded from a STEP file

3D examples

Executable	Description
CreateBox	Basic 3D box mesh
BoxWithHole	3D box with a through-hole
ShewchukBox	Quality-refined 3D box mesh
ShewchukBoxWithHole	Quality refinement with hole constraints
ConstrainedBoxExample	3D constrained mesh example

# Run an example and view the result
./build/src/Examples2D/RectangleWithHole2D
paraview output.vtu

Architecture

src/
├── Common/         # Types, BoundingBox, Exceptions
├── Geometry/       # CAD geometry entities (ICorner, IEdge, ISurface interfaces)
├── Topology/       # Topological relationships (3D)
├── Topology2D/     # Topological relationships (2D)
├── Meshing/
│   ├── Core/       # Delaunay algorithms (2D, 3D, Bowyer-Watson, constraint recovery)
│   └── Data/       # MeshData2D/3D, Node, TriangleElement, TetrahedralElement
├── Readers/        # OpenCASCADE STEP/IGES import
└── Export/         # VtkExporter (VTU format)

Key design patterns:

• Strategy: IMesher interface with pluggable algorithm implementations
• Context: MeshingContext2D/3D manages geometry, topology, and mesh lifecycle
• Controlled mutation: MeshData exposes mutation only through MeshMutator

Logging

Runtime logging level is controlled via environment variable:

SPDLOG_LEVEL=debug ./build/src/Examples2D/RectangleWithHole2D
# levels: trace, debug, info, warn, error

Building OpenCASCADE (optional)

If you need STEP file support and your distribution doesn't package OpenCASCADE:

cd ~/software/OCCT-7_8_1/build
cmake .. \
    -DCMAKE_BUILD_TYPE=Release \
    -DCMAKE_INSTALL_PREFIX=$HOME/local/opencascade \
    -DBUILD_LIBRARY_TYPE=Shared \
    -DBUILD_MODULE_DataExchange=ON \
    -DBUILD_MODULE_ApplicationFramework=OFF \
    -DBUILD_MODULE_Draw=OFF \
    -DBUILD_MODULE_Visualization=OFF \
    -DUSE_TK=OFF
make -j$(nproc)
make install

Then configure OpenLoom with:

cmake -S . -B build -DOpenCASCADE_DIR=$HOME/local/opencascade/lib/cmake/opencascade

Documentation

• doc/Terminology.md - CAD and mesh terminology glossary
• doc/Meshing_Core_Overview.md - Architecture overview
• doc/Error_Handling.md - Error handling guide

Contributing

Contributions are welcome. Please open an issue before starting significant work so we can discuss the approach. Bug reports and feedback are equally appreciated.

Acknowledgements

The meshing and refinement algorithms are based on the work of J.R. Shewchuk:

J.R. Shewchuk, "Delaunay Refinement Algorithms for Triangular Mesh Generation," Computational Geometry: Theory and Applications, 22(1-3):21-74, 2002.

About

OpenLoom is developed by Sandbricius Consulting AB. We offer consulting services for tailored mesh generation solutions — contact us if you need custom adaptations, integration support, or domain-specific extensions.

License

MIT — see LICENSE for the full text. This software is provided as-is, without warranty of any kind.