CAD to Geant4 Converter Suite

A collection of tools for converting CAD files (STEP, STL) to GDML for use in Geant4 simulations.

Requirements

• Python 3.10 or above
• pyg4ometry >= 1.0.0
• vtk >= 9.0.0
• trimesh (for automatic mesh repair)
• pymeshlab (optional, for advanced mesh repair of stubborn cases)

Install dependencies:

pip install -r requirements.txt
pip install pymeshlab  # Optional but recommended

Overview

Converting CAD to GDML/Geant4 requires care; a few export best-practices greatly improve results (trim small parts, preserve named components and hierarchy, avoid intended overlaps).

What's new (concise):

• Automatic mesh check and repair on all tessellated volumes before GDML export
• CI added (pytest + Black + Flake8) to keep quality steady
• Geometry overlap checking with Geant4 via check_overlaps.py

Note: GDML output typically lacks detailed material assignments. Post-process the GDML with gdml-editor to add or correct materials: https://github.com/drflei/gdml-editor

Main Application: cad_g4_conv.py

Unified converter supporting three workflows. All conversions automatically check and repair tessellated meshes before saving to GDML.

Quick Start

# STEP native conversion
python cad_g4_conv.py --step-file assembly.STEP

# STL+STEP mesh conversion
python cad_g4_conv.py --step-file assembly.STEP --stl-dir STLs/

# Single STL conversion
python cad_g4_conv.py --stl-file mesh.stl

All conversions automatically check and repair tessellated meshes before export.

Automatic Mesh Repair

The converter includes a comprehensive 10-phase mesh repair system that automatically fixes common issues:

1. Basic cleanup: Removes duplicate/unreferenced vertices
2. Normal fixing: Ensures consistent face orientations
3. Degenerate removal: Removes zero-area faces and duplicates
4. Broken face repair: Detects and removes topologically broken faces
5. Hole filling: Iteratively attempts to close surface holes (up to 3 attempts)
6. Inversion fix: Corrects inverted face orientations
7. Final cleanup: Merges vertices after all repairs
8. Winding correction: Fixes inconsistent face winding
9. Fallback strategies: Uses convex hull or voxelization for severely broken meshes
10. PyMeshLab fallback (optional): For very stubborn meshes, uses MeshLab's industrial-strength repair algorithms

Most meshes are repaired automatically. The system tries progressively more aggressive techniques:

• Phases 1-8 handle 90%+ of issues using trimesh (lightweight, fast)
• Phase 9 applies convex hull for small holes or voxelization for complex geometry
• Phase 10 uses PyMeshLab's powerful algorithms for the most challenging cases (requires pip install pymeshlab)

Example repair output:

stl_solid_8: watertight_before=False watertight_after=True 
  notes=removed_92_broken_faces;subdivided;used_convex_hull(vol_diff=0.04);

Optional: PyMeshLab for stubborn meshes

pip install pymeshlab  # Optional but recommended for complex meshes

If installed, PyMeshLab provides a final fallback using MeshLab's industrial-strength repair algorithms.

Files in This Directory

Main Tools

• cad_g4_conv.py - Unified CAD to GDML converter
• run_vtkviewer.py - VTK-based geometry viewer for GDML/STL/STEP files
• check_overlaps.py - Geant4 geometry overlap checker

Documentation

• README.md - This file, main project documentation
• docs/QUICKREF.md - Quick reference and cheatsheet
• docs/DETAILED_USAGE.md - Complete usage guide
• docs/TESTING.md - Test suite documentation

Scripts

• demo_all_workflows.sh - Demonstrates all three workflows
• test_cad_g4_conv.sh - Automated testing script

Installation

From PyPI (Recommended)

pip install cad-g4-conv

From Source

git clone https://github.com/drflei/cad_g4_conv.git
cd cad_g4_conv
pip install -e .

Requirements

• Python 3.10+
• pyg4ometry >= 1.0.0
• VTK >= 9.0.0
• trimesh
• geant4_pybind (for overlap checking)

Usage

See docs/DETAILED_USAGE.md for complete documentation.

Three Workflows

Workflow	Command	Best For
STEP Native	--step-file X.STEP	Assemblies with hierarchy
STL+STEP	--step-file X.STEP --stl-dir STLs/	High-quality meshes
Single STL	--stl-file mesh.stl	Single meshes, prototyping

Examples

# 1. STEP native with hierarchy
python cad_g4_conv.py --step-file detector.STEP -o detector.gdml

# 2. STL+STEP with auto-sized world
python cad_g4_conv.py \
    --step-file assembly.STEP \
    --stl-dir parts/ \
    -o assembly.gdml

# 3. Single STL (simplest)
python cad_g4_conv.py --stl-file housing.stl -o housing.gdml

# 4. With overlap checking
python cad_g4_conv.py --step-file detector.STEP --check-overlaps

# 5. Flat mode (robust fallback)
python cad_g4_conv.py --step-file complex.STEP --flat

# 6. Center geometry at world origin
python cad_g4_conv.py --step-file detector.STEP --center-origin

# 7. Logging example
# Write detailed logs to 'convert.log'
python cad_g4_conv.py --stl-file mesh.stl --log-file convert.log --log-level DEBUG

# Notes:
# - All tessellated meshes are automatically checked and repaired before export
# - Repairs use trimesh (fix_normals, merge_vertices, remove degenerates, fill_holes, fix_inversion)
# - `--log-file` / `--log-level` control logging output for debugging and reproducibility

Running from Other Directories

You can run the tools from anywhere by specifying the full path:

# From any directory
python ~/cad_g4_conv/cad_g4_conv.py --stl-file /path/to/mesh.stl

Or add to PATH:

# Add to ~/.bashrc
export PATH="$HOME/cad_g4_conv:$PATH"

# Then use directly
cad_g4_conv.py --stl-file mesh.stl

Demo

Run the comprehensive demo to see all workflows in action:

cd ~/cad_g4_conv
./demo_all_workflows.sh

Testing

Run automated tests:

cd ~/cad_g4_conv
./test_cad_g4_conv.sh

Visualization

After conversion, visualize the GDML with the built-in VTK viewer:

python run_vtkviewer.py output.gdml

# Or use the viewer from gdml_editor package
python -m gdml_editor.run_vtkviewer output.gdml

The viewer supports:

• GDML files (Geant4 geometry)
• STL files (triangular meshes)
• STEP files (CAD assemblies)
• FLUKA input files (.inp)

See run_vtkviewer.py --help for all options.

Overlap Checking

Check for geometry overlaps using Geant4's built-in overlap checker:

python check_overlaps.py output.gdml

# With custom parameters
python check_overlaps.py output.gdml 2000 0.01 20.0

Arguments:

• gdml_file - Path to GDML geometry file
• resolution - Number of points to check (default: 1000)
• tolerance - Tolerance in mm (default: 0.001)
• world_size_m - World half-size in meters (default: 10.0)

Requirements:

pip install geant4_pybind

The tool loads the geometry into Geant4 and runs CheckOverlaps() on all volumes, reporting:

• Overlapping volume pairs
• Surface defects/holes in tessellated solids
• Detailed overlap locations and partners

Example output:

⚠️  FOUND 2 UNIQUE OVERLAPPING PAIR(S):

   1. sensor_1 === housing
   2. pcb_volume === sensor_2

Help

python cad_g4_conv.py --help

Features (short)

• Supports STEP-native conversion (hierarchy + CSG where possible), STL+STEP mesh-based conversion, and single-STL conversions.
• Automatic 10-phase mesh check and repair on all tessellated volumes before GDML export (trimesh + optional pymeshlab).
• Auto-sized and centered world volume, fuzzy name matching for STL→STEP associations, and overlap diagnostics.

Documentation

For detailed usage, see the docs/ folder:

• docs/QUICKREF.md - Quick reference and command cheatsheet
• docs/DETAILED_USAGE.md - Complete usage guide with examples
• docs/TESTING.md - Test suite and CI/CD documentation

License

GNU V3.0