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Skillscrosstech-impl-freecad-ifc-bridge

crosstech-impl-freecad-ifc-bridge

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Updated 10-15 22:17

About this skill

crosstech-impl-freecad-ifc-bridge

Quick Reference

FreeCAD IFC Modes

Mode FreeCAD Version IFC Engine Data Loss Round-Trip
NativeIFC Locked 1.0+ IfcOpenShell 0.8.x NONE Lossless
NativeIFC Unlocked 1.0+ IfcOpenShell 0.8.x NONE (IFC part) Lossless
Legacy Import/Export < 1.0 / 1.0+ fallback IfcOpenShell YES Lossy

BIM Workbench Tools (FreeCAD 1.0+)

Tool Purpose
IFC Elements Manager Control which elements export to IFC
IFC Properties Manager Manage property set attachments
IFC Quantities Manager Handle explicit quantity exports
IFC Explorer Pre-import structural analysis
IFC Diff Visual comparison of two IFC files

Critical Warnings

ALWAYS use NativeIFC mode for IFC workflows. Legacy import converts IFC to FreeCAD Part objects, losing parametric IFC data permanently.

NEVER save an IFC file opened in NativeIFC locked mode as .FCStd — the IFC file IS the document. Saving as .FCStd breaks the NativeIFC link.

NEVER mix NativeIFC locked mode with non-IFC geometry. Use unlocked mode when combining IFC and non-IFC elements in one document.

ALWAYS verify IfcOpenShell version compatibility with FreeCAD. Mismatched versions cause silent import failures. Check with import ifcopenshell; print(ifcopenshell.version).

NEVER use the legacy importIFC module for new projects. Use the BIM Workbench NativeIFC workflow instead.

ALWAYS load shapes on demand for large IFC files (100MB+). NativeIFC supports selective geometry loading to manage memory.

Technology Boundary

Side A: FreeCAD (Part objects, BIM Workbench)

Version: FreeCAD 1.0+ (BIM Workbench replaces legacy Arch Workbench)

Internal representation:

  • Part::TopoShape — OpenCASCADE BREP geometry (the native geometry kernel)
  • Arch::Wall, Arch::Structure, Arch::Window — parametric BIM objects
  • BIM Workbench objects: walls, columns, beams, slabs, doors, windows, pipes, stairs, roofs, panels, equipment
  • Properties stored as FreeCAD object properties (obj.Label, obj.IfcType, custom properties)
  • Document format: .FCStd (ZIP containing XML + BREP files)

Scripting API:

  • FreeCAD module — document and object management
  • Arch module — BIM object creation (walls, structures, windows)
  • Part module — geometry operations (shapes, booleans, extrusions)
  • Draft module — 2D drawing tools

Side B: IFC (IfcOpenShell data model)

Version: IFC4 (ISO 16739-1:2018), IFC4X3 partial support via IfcOpenShell

Internal representation:

  • ifcopenshell.file — in-memory IFC model
  • entity_instance — individual IFC entities with typed attributes
  • Geometry: CSG, swept solids, BREP, extrusions, tessellation
  • Properties: IfcPropertySet attached via IfcRelDefinesByProperties
  • Spatial hierarchy: IfcProject > IfcSite > IfcBuilding > IfcBuildingStorey

Data model:

  • Entities inherit from IfcRoot (GlobalId, Name, Description, OwnerHistory)
  • Relationships are objectified (IfcRelAggregates, IfcRelContainedInSpatialStructure)
  • Types shared via IfcRelDefinesByType
  • Materials assigned via IfcRelAssociatesMaterial

The Bridge: NativeIFC / Traditional Import-Export

NativeIFC (PREFERRED): The IFC file IS the FreeCAD document. No conversion happens. IfcOpenShell reads and writes IFC data directly. FreeCAD renders shapes on demand using IfcOpenShell's OpenCASCADE geometry converter.

Traditional Import: IfcOpenShell parses IFC geometry and converts it to FreeCAD Part::TopoShape objects. This is a one-way conversion that loses IFC semantic data unless explicitly mapped to FreeCAD properties.

Traditional Export: FreeCAD BIM objects are serialized to IFC entities using IfcOpenShell. BIM type mappings (Arch.Wall -> IfcWall) are applied automatically. Non-BIM objects are excluded unless explicitly converted.

Data flow:

NativeIFC:     IFC file <--direct--> FreeCAD viewport (via IfcOpenShell)
Traditional:   IFC file --> IfcOpenShell --> Part::TopoShape --> FreeCAD
               FreeCAD --> Arch/BIM objects --> IfcOpenShell --> IFC file

Critical Rules

  1. ALWAYS prefer NativeIFC mode over legacy import/export for IFC workflows.
  2. ALWAYS check IfcOpenShell availability before IFC operations: import ifcopenshell.
  3. ALWAYS use locked NativeIFC mode when the IFC file is the single source of truth.
  4. ALWAYS use unlocked NativeIFC mode when mixing IFC with non-IFC elements.
  5. ALWAYS load shapes selectively for large models — NativeIFC supports on-demand loading.
  6. ALWAYS verify schema version with ifcopenshell.open(path).schema before processing.
  7. NEVER assume FreeCAD preserves IFC parametric geometry in legacy mode — it converts to BREP.
  8. NEVER edit IFC files outside FreeCAD while they are open in NativeIFC locked mode.
  9. NEVER rely on FreeCAD-specific parametric constraints surviving IFC export — Sketcher constraints and Part constraints are NOT part of IFC.
  10. NEVER use importIFC.export() for NativeIFC workflows — save the document directly.

Decision Tree

START: You need to work with IFC in FreeCAD
|
+-- Q1: Is FreeCAD 1.0+ available?
|   +-- YES --> Use BIM Workbench with NativeIFC
|   +-- NO  --> Use legacy Arch import/export (expect data loss)
|
+-- Q2: What is the workflow?
|   +-- View/inspect IFC --> NativeIFC locked mode (read-only is fine)
|   +-- Edit IFC and save back --> NativeIFC locked mode
|   +-- Combine IFC with non-IFC geometry --> NativeIFC unlocked mode
|   +-- Create BIM model from scratch --> BIM Workbench, export to IFC
|   +-- One-time geometry extraction --> Legacy import is acceptable
|
+-- Q3: Is the IFC file large (>100MB)?
|   +-- YES --> Use NativeIFC with selective shape loading
|   +-- NO  --> Full shape loading is acceptable
|
+-- Q4: Is round-trip fidelity required?
|   +-- YES --> NativeIFC locked mode ONLY
|   +-- NO  --> Legacy export is acceptable for one-way conversion
|
+-- Q5: Do you need scripting access to IFC data?
    +-- YES, to IFC entities --> Use ifcopenshell directly on the file
    +-- YES, to FreeCAD objects --> Use FreeCAD.ActiveDocument.Objects
    +-- YES, to both --> NativeIFC mode (objects ARE IFC entities)

Essential Patterns

Pattern 1: Open IFC in NativeIFC Locked Mode

import FreeCAD

# Open IFC file directly — NativeIFC treats it as the document
doc = FreeCAD.openDocument("/path/to/model.ifc")

# Iterate IFC objects
for obj in doc.Objects:
    if hasattr(obj, "IfcType"):
        print(f"{obj.Label}: {obj.IfcType}")

# Modify an element
wall = doc.getObjectsByLabel("Exterior Wall 01")[0]
wall.Label = "Renamed Wall"

# Save — writes directly to the IFC file, minimal diff
doc.save()

Pattern 2: Access IFC Data via IfcOpenShell (NativeIFC)

import FreeCAD
import ifcopenshell
import ifcopenshell.util.element

# Open the IFC file with IfcOpenShell for semantic queries
ifc_file = ifcopenshell.open("/path/to/model.ifc")

# Query all walls with their properties
for wall in ifc_file.by_type("IfcWall"):
    psets = ifcopenshell.util.element.get_psets(wall)
    name = wall.Name or "Unnamed"
    is_external = psets.get("Pset_WallCommon", {}).get("IsExternal", None)
    print(f"{name}: external={is_external}")

# Check schema version
print(f"Schema: {ifc_file.schema}")  # "IFC2X3", "IFC4", "IFC4X3"

Pattern 3: Create BIM Objects and Export to IFC

import FreeCAD
import Arch
import Draft

# Create a new document
doc = FreeCAD.newDocument("BIM_Project")

# Create BIM objects using the Arch module
wall = Arch.makeWall(length=5000, width=200, height=3000)
wall.IfcType = "IfcWall"
wall.Label = "Exterior Wall 01"

# Create a slab
slab = Arch.makeStructure(length=6000, width=5000, height=200)
slab.IfcType = "IfcSlab"
slab.Label = "Ground Floor Slab"

# Recompute to update geometry
doc.recompute()

# Export to IFC using the legacy exporter
import importIFC
importIFC.export([wall, slab], "/path/to/output.ifc")

Pattern 4: NativeIFC Unlocked Mode (Mixed Content)

import FreeCAD
import Arch

# Create document with mixed content
doc = FreeCAD.newDocument("Mixed_Project")

# Add non-IFC geometry (parametric FreeCAD part)
import Part
box = doc.addObject("Part::Box", "SiteContext")
box.Length = 10000
box.Width = 10000
box.Height = 100

# Add IFC project (unlocked mode — IFC and non-IFC coexist)
# The IFC project attaches to a separate .ifc file
# Non-IFC objects (like the box) remain in the .FCStd file

doc.recompute()

Pattern 5: Selective Shape Loading for Large Models

import FreeCAD

# Open large IFC file — NativeIFC loads metadata only
doc = FreeCAD.openDocument("/path/to/large_model.ifc")

# Shapes are loaded on demand per object
# Access an object to trigger shape loading
obj = doc.getObjectsByLabel("Foundation Wall")[0]
shape = obj.Shape  # Triggers geometry computation via IfcOpenShell

# For batch processing without visualization, avoid loading shapes
for obj in doc.Objects:
    if hasattr(obj, "IfcType"):
        # Access metadata only — no shape loading
        print(f"{obj.Label}: {obj.IfcType}")

Common Operations

Import/Export Operations

Operation NativeIFC (Preferred) Legacy
Open IFC FreeCAD.openDocument("f.ifc") File > Import > IFC
Save IFC doc.save() (writes to .ifc) importIFC.export(objs, "f.ifc")
Access properties Via IfcOpenShell on same file Custom properties on FreeCAD object
Round-trip edit Direct — changes write to IFC Lossy — re-export required
Large file handling Selective shape loading Full load into memory

FreeCAD BIM Type Mapping

FreeCAD BIM Object IFC Entity Notes
Arch.makeWall() IfcWall Maps directly
Arch.makeStructure() IfcColumn / IfcBeam / IfcSlab Based on IfcType property
Arch.makeWindow() IfcWindow / IfcDoor Based on IfcType property
Arch.makeStairs() IfcStair Maps directly
Arch.makeRoof() IfcRoof Maps directly
Arch.makePipe() IfcPipeSegment Maps directly
Arch.makeSite() IfcSite Spatial element
Arch.makeBuilding() IfcBuilding Spatial element
Arch.makeFloor() IfcBuildingStorey Spatial element

Data Preservation Matrix

Data Type NativeIFC Legacy Import Legacy Export
IFC entity class FULL FULL FULL
GlobalId FULL Stored as property Generated new
Property sets FULL Partial (custom props) Partial
Quantities FULL Partial Partial
Relationships FULL Simplified Simplified
Parametric geometry FULL Converted to BREP From BREP
Material layers FULL Simplified Simplified
Spatial hierarchy FULL Collection hierarchy Regenerated
FreeCAD constraints N/A N/A LOST
Sketcher data N/A N/A LOST

Scripting Quick Reference

# Check FreeCAD version
import FreeCAD
print(FreeCAD.Version())  # ['1', '0', '0', ...]

# Check IfcOpenShell version
import ifcopenshell
print(ifcopenshell.version)  # "0.8.0" or similar

# Check if NativeIFC is available
try:
    import nativeifc
    NATIVEIFC_AVAILABLE = True
except ImportError:
    NATIVEIFC_AVAILABLE = False

# Get all IFC objects in current document
doc = FreeCAD.ActiveDocument
ifc_objects = [o for o in doc.Objects if hasattr(o, "IfcType")]

# Access underlying IfcOpenShell entity (NativeIFC)
import ifcopenshell
ifc_file = ifcopenshell.open(doc.FileName)
entity = ifc_file.by_id(42)  # Access by expressID

Reference Links

Official Sources