How do i assign OpenCASCADE representation to a IfcElement?

smr02

I'm trying to create a ifc element from a OpenCASCADE representation following the tutorial here:
https://docs.ifcopenshell.org/ifcopenshell-python/geometry_creation.html#opencascade-representations
I manage to create a IfcProductDefinitionShape but I don't understand how to assign this to a IfcElement
Here is my code:

import ifcopenshell
import ifcopenshell.geom
from OCC.Core.gp import gp_Pnt
from OCC.Core.BRepPrimAPI import BRepPrimAPI_MakeBox
from OCC.Core.BRepAlgoAPI import BRepAlgoAPI_Cut
from pprint import pprint

import ifcopenshell.api.root
import ifcopenshell.api.unit
import ifcopenshell.api.context
import ifcopenshell.api.project
import ifcopenshell.api.geometry

model = ifcopenshell.open('empty.ifc')

outer   = BRepPrimAPI_MakeBox(gp_Pnt(-5000., -180., -2000.), gp_Pnt(5000., 5180., 3000.)).Shape()
inner   = BRepPrimAPI_MakeBox(gp_Pnt(-4640.,  180.,     0.), gp_Pnt(4640., 4820., 3000.)).Shape()
window1 = BRepPrimAPI_MakeBox(gp_Pnt(-5000., -180.,   400.), gp_Pnt( 500., 1180., 2000.)).Shape()
window2 = BRepPrimAPI_MakeBox(gp_Pnt( 2070., -180.,   400.), gp_Pnt(3930.,  180., 2000.)).Shape()
building_shell = BRepAlgoAPI_Cut(
        BRepAlgoAPI_Cut(
            BRepAlgoAPI_Cut(outer, inner).Shape(),
            window1
            ).Shape(),
        window2
    ).Shape()

# model = ifcopenshell.file(schema="IFC4")
product_definition = ifcopenshell.geom.serialise("IFC4", building_shell, False)
product_definition = model.add(product_definition)

pprint(
    product_definition.get_info()
)

element = ifcopenshell.api.root.create_entity(model, ifc_class="IfcFurniture")

model.write('new.ifc')

{'Description': None,
 'Name': None,
 'Representations': (#240=IfcShapeRepresentation($,'Body','Brep',(#239)),),
 'id': 241,
 'type': 'IfcProductDefinitionShape'}

Andrej730

Hi, if it's IfcProductDefinitionShape, then you need to assign it to element.Representation.
See the documentation - https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcFurniture.htm

smr02

@Andrej730 i think that's what I'm doing with ifcopenshell.api.geometry.assign_representation, right?
But then I'm also producing a invalid IFC.
Here's my code:

import ifcopenshell
import ifcopenshell.geom
from OCC.Core.gp import gp_Pnt
from OCC.Core.BRepPrimAPI import BRepPrimAPI_MakeBox
from OCC.Core.BRepAlgoAPI import BRepAlgoAPI_Cut
from pprint import pprint

import ifcopenshell.api.root
import ifcopenshell.api.geometry
import ifcopenshell.api.context

# schema = "2x3"
schema = "4"


model = ifcopenshell.open(f'empty{schema}.ifc')
body = ifcopenshell.api.context.add_context(model,
    context_type="Model", context_identifier="Body", target_view="MODEL_VIEW")


outer   = BRepPrimAPI_MakeBox(gp_Pnt(-5000., -180., -2000.), gp_Pnt(5000., 5180., 3000.)).Shape()
inner   = BRepPrimAPI_MakeBox(gp_Pnt(-4640.,  180.,     0.), gp_Pnt(4640., 4820., 3000.)).Shape()
window1 = BRepPrimAPI_MakeBox(gp_Pnt(-5000., -180.,   400.), gp_Pnt( 500., 1180., 2000.)).Shape()
window2 = BRepPrimAPI_MakeBox(gp_Pnt( 2070., -180.,   400.), gp_Pnt(3930.,  180., 2000.)).Shape()
building_shell = BRepAlgoAPI_Cut(
        BRepAlgoAPI_Cut(
            BRepAlgoAPI_Cut(outer, inner).Shape(),
            window1
            ).Shape(),
        window2
    ).Shape()

# model = ifcopenshell.file(schema="IFC4")
product_definition = ifcopenshell.geom.serialise(f"IFC{schema}", building_shell, False)
product_definition = model.add(product_definition)


element = ifcopenshell.api.root.create_entity(model, ifc_class="IfcFurnishingElement" if schema == "2x3" else "IfcFurniture")

# Assign the product definition to the element
ifcopenshell.api.geometry.assign_representation(model, product=element, representation=product_definition)

pprint(element.get_info())

model.write('new.ifc')

{'Description': None,
 'GlobalId': '3MpIwn6cH2rALFgzeHaD4P',
 'Name': None,
 'ObjectPlacement': None,
 'ObjectType': None,
 'OwnerHistory': None,
 'PredefinedType': None,
 'Representation': #248=IfcProductDefinitionShape($,$,(#246)),
 'Tag': None,
 'id': 247,
 'type': 'IfcFurniture'}

and running the validation:

python -m ifcopenshell.validate new.ifc 
No stream support: No module named 'lark'
Validating new.ifc
For instance:
    #245=IfcShapeRepresentation($,'Body','Brep',(#244))
                                ^                      
With attribute:
    <attribute ContextOfItems: <entity IfcRepresentationContext>>
Not optional

For instance:
    #246=IfcProductDefinitionShape($,$,(#245))
With inverse:
    ShapeOfProduct : SET [1:?] OF IfcProduct FOR Representation
Value:
    ()
Not valid

For instance:
    #248=IfcProductDefinitionShape($,$,(#246))
                                       ^^^^^^ 
With attribute:
    <attribute Representations: <list [1:?] of <entity IfcRepresentation>>>
Value:
    #246=IfcProductDefinitionShape($,$,(#245))
Not valid

smr02

I've figured it out.
But the representation in Blender seems off. Is this a current limitation, or am i still doing something wrong?

import ifcopenshell
import ifcopenshell.geom
from OCC.Core.gp import gp_Pnt
from OCC.Core.BRepPrimAPI import BRepPrimAPI_MakeBox
from OCC.Core.BRepAlgoAPI import BRepAlgoAPI_Cut
from pprint import pprint

import ifcopenshell.api.root
import ifcopenshell.api.geometry
import ifcopenshell.api.context

schema = "4"
model = ifcopenshell.open(f'empty{schema}.ifc')

# Create or get the geometric representation context
body_context = ifcopenshell.api.context.add_context(model,
    context_type="Model", context_identifier="Body", target_view="MODEL_VIEW")

# Create OpenCASCADE geometry
outer = BRepPrimAPI_MakeBox(gp_Pnt(-5000., -180., -2000.), gp_Pnt(5000., 5180., 3000.)).Shape()
inner = BRepPrimAPI_MakeBox(gp_Pnt(-4640., 180., 0.), gp_Pnt(4640., 4820., 3000.)).Shape()
window1 = BRepPrimAPI_MakeBox(gp_Pnt(-5000., -180., 400.), gp_Pnt( 500., 1180., 2000.)).Shape()
window2 = BRepPrimAPI_MakeBox(gp_Pnt( 2070., -180., 400.), gp_Pnt(3930., 180., 2000.)).Shape()
building_shell = BRepAlgoAPI_Cut(
    BRepAlgoAPI_Cut(
        BRepAlgoAPI_Cut(outer, inner).Shape(),
        window1
    ).Shape(),
    window2
).Shape()

# Serialize the OpenCASCADE shape to IFC
product_definition = ifcopenshell.geom.serialise(f"IFC{schema}", building_shell, False)
product_definition = model.add(product_definition)

# Extract the IfcShapeRepresentation from the IfcProductDefinitionShape
shape_representation = product_definition.Representations[0]

# Set the context for the shape representation
shape_representation.ContextOfItems = body_context

# Create the element
element = ifcopenshell.api.root.create_entity(model, ifc_class="IfcFurniture")

# Option 1: Direct assignment (simpler approach)
element.Representation = product_definition

# Option 2: Using the API (alternative approach)
# ifcopenshell.api.geometry.assign_representation(model, product=element, representation=shape_representation)

pprint(element.get_info())
model.write('new.ifc')


from OCC.Display.SimpleGui import init_display
display, start_display, add_menu, add_function_to_menu = init_display()
display.DisplayShape(building_shell, update=True)


display.FitAll()
# Start viewer
start_display()

sjb007

Do you have "Backface Culling" turned on? That kind of looks like some of the normals are flipped, and backface culling is active.

smr02

@sjb007 no, that is turned off. Attached the IFC file

sjb007

Ah OK. Uneducated guess here, but it looks like a bug in the default Hybrid CGAL-OCC geometry library that gets used. The only one that works is plain OpenCASCADE. When opening the file use the Enable Advanced Mode, and set the Geometry Library option:

And you should get:

You should probably raise a bug against IfcOpenShell in Github as well. Considering your script is essentially the example in the documentation for geometry creation, it's curious that the other geometry libraries cannot handle it.

smr02

@sjb007 THANK YOU!
here the bug report: https://github.com/IfcOpenShell/IfcOpenShell/issues/7099