exterior-shell

Extract lightweight exterior shells from BIM models (IFC) for GIS and visualization workflows.

exterior-shell reads an IFC file, classifies elements as exterior/interior using a rule-based engine, and produces:

• Stripped IFC (.ifc) with interior elements removed, preserving a structurally valid IFC file
• 2D Footprint (.geojson) with base elevation, height, and area attributes for GIS extrusion

Optional AI-assisted classification handles ambiguous elements (walls, columns, slabs) that rule-based logic alone can't resolve.

Why

GIS teams don't ask for BIM models. They receive them. Architecture practices export IFC files and hand them off, and the GIS analyst is left with 500MB of pipes, HVAC, and furniture when they need just the building envelope for a web map or spatial analysis.

exterior-shell does one thing: clean exterior shell, lightweight output, under 30 seconds. The output is a structurally valid IFC file that any BIM or GIS tool can read, plus an optional GeoJSON footprint ready for ArcGIS Pro, QGIS, or web maps.

Install

pip install exterior-shell

Or from source:

git clone https://github.com/Asem-D/exterior-shell.git
cd exterior-shell
pip install -e .

Requirements

• Python 3.10+
• ifcopenshell, shapely, numpy, click

Usage

Extract exterior shell (default)

Produces a stripped IFC file with interior elements removed:

exterior-shell extract building.ifc
# Output: building_stripped.ifc + building.report.md

Extract with 2D footprint

exterior-shell extract building.ifc --footprint
# Output: building_stripped.ifc + building_footprint.geojson + building.report.md

The footprint GeoJSON includes base_elevation, height, min_elevation, max_elevation, and area properties. Load it in ArcGIS Pro or QGIS and extrude by the height attribute, or use it directly in web maps (MapLibre, CesiumJS).

Stripped IFC only (no GIS output)

exterior-shell extract building.ifc --no-stripped-ifc --footprint
# Output: building_footprint.geojson only

Other options

exterior-shell extract building.ifc --ai                # AI-assisted classification
exterior-shell extract building.ifc --crs EPSG:3857     # Footprint in Web Mercator
exterior-shell extract building.ifc --keep-interior      # Include interior-facing faces
exterior-shell extract building.ifc --no-report          # Skip report generation
exterior-shell extract building.ifc --json-stats         # Machine-readable output
exterior-shell info building.ifc                         # Inspect IFC file

How It Works

IFC File
  │
  ├─ Parse ─── ifcopenshell extracts elements, geometry, spatial hierarchy
  │
  ├─ Classify ─ Rule-based engine assigns EXTERIOR / INTERIOR / AMBIGUOUS
  │   │
  │   └─ [--ai] Vision model reclassifies ambiguous elements from rendered views
  │
  ├─ Assemble ─ Merge exterior faces, remove hidden interior-facing geometry
  │
  └─ Export
      ├─ Stripped IFC (.ifc) ─ structurally valid IFC with interiors removed
      ├─ Footprint (.geojson) ─ 2D outline with base_elevation and height
      └─ Report (.md) ─ extraction summary with element counts

Classification Rules

Performance

Tested on real-world IFC models:

Development

pip install -e ".[dev]"
pytest tests/ -v

Project Structure

exterior_shell/
├── cli.py              # Click CLI entry point
├── core/
│   ├── parser.py       # IFC parsing with ifcopenshell
│   ├── classifier.py   # Rule-based classification engine
│   ├── assembler.py    # Geometry assembly + face deduplication
│   └── models.py       # Data classes (Element, Classification, Shell)
├── ai/
│   └── ...             # Vision model classification (Phase 2)
├── export/
│   ├── stripped_ifc.py # Stripped IFC export (remove interior elements)
│   └── footprint.py    # 2D footprint GeoJSON with elevation attributes
└── utils/
    └── ...             # Geometry helpers, I/O utilities

Prior Art

This problem has been approached from different angles:

• IfcEnvelopeExtractor (TU Delft): Academic-grade tool outputting CityJSON, STEP, and OBJ with full LoD coverage (LOD0 through LOD5). Built on the Biljecki et al. LoD framework. Different output ecosystem from exterior-shell. If you're building 3D city models with CityJSON, that's the tool. GitHub
• IfcConvert (--exterior-only): Open-source, extracts exterior shell as mesh. No structurally valid IFC output, no GIS attributes.
• Esri ExteriorShell: Built into ArcGIS Pro. Automatic sublayer extraction when loading IFC/RVT. Often misses roofs, ground floors, and includes interior geometry.

exterior-shell targets a narrower niche: GIS practitioners who need a clean stripped IFC plus a GeoJSON footprint with elevation attributes, with zero heavy GIS dependencies.

Roadmap

• v1.2 (current) - Stripped IFC + 2D footprint output, rule-based extraction
• v1.3 - AI-assisted classification for ambiguous elements (multi-view rendering + vision API)
• v1.4 - Provenance metadata: link output features to IFC GlobalIds, record extraction parameters, validate spatial consistency with source shell
• v2.0 - Revit direct integration (.rvt), 3D Tiles export, LOD generation

License

MIT

Acknowledgments

Built with ifcopenshell and Shapely.

Inspired by the daily pain of GIS teams who receive BIM models they didn't ask for and need just the envelope.

Trademarks

ArcGIS is a registered trademark of Esri. This project is not affiliated with or endorsed by Esri.