Extracting Non-Triangulated Surface Polygons from IFC Models #6458

ChSchlaeger · 2025-03-28T11:36:08Z

ChSchlaeger
Mar 28, 2025

Hi everyone,

I'm looking for some guidance on extracting primitive geometry directly from IFC models. Currently, I'm utilizing the create_shape function, which provides me a TriangulationElement. From there, I extract the faces and vertices using get_vertices and get_faces.

import ifcopenshell.geom
import ifcopenshell.util.shape

ifc_model = ifcopenshell.open('model.ifc')
element = ifc_model.by_type('IfcWall')[0]

settings = ifcopenshell.geom.settings()
shape = ifcopenshell.geom.create_shape(settings, element)

points = ifcopenshell.util.shape.get_vertices(shape.geometry)
"""
[[ 0.   -0.24  0.  ]
 [ 0.   -0.24  2.5 ]
 [ 4.29 -0.24  2.5 ]
 [ 4.29 -0.24  0.  ]
 [ 4.29  0.    2.5 ]
 [ 4.29  0.    0.  ]
 [ 0.24  0.    2.5 ]
 [ 0.24  0.    0.  ]]
"""

faces = ifcopenshell.util.shape.get_faces(shape.geometry)
"""
[[1 0 3]
 [2 1 3]
 [2 3 5]
 [4 2 5]
 [4 5 7]
 [6 4 7]
 [6 7 0]
 [1 6 0]
 [0 7 3]
 [3 7 5]
 [2 6 1]
 [4 6 2]]
"""

However, my objective is to perform operations on complete surfaces rather than working with a triangulated mesh. Specifically, for a simple wall o 8000 bject I would want to extract the six rectangular faces without breaking them down into triangles.

As far as I understand, the triangulation isn't stored in the IFC file itself but is rather generated within IfcOpenShell?
Is there any way to suppress this triangulation so that I can obtain non-triangulated surface polygons directly?

I am aware of some possibilities using OpenCascade to extract geometry as BRep, but I am struggeling with the install process, and therefore it seems a bit overkill for my current use case. Or is this the route I have to take?

Thanks in advance for your help!

Answered by aothms

Mar 28, 2025

I am aware of some possibilities using OpenCascade to extract geometry as BRep, but I am struggeling with the install process, and therefore it seems a bit overkill for my current use case. Or is this the route I have to take?

Well, that depends on the question: do you care about the possibility of having a curved wall with openings? Because in that case you do really need the full occt brep model that will tell you you have a Face with a cylindrical surface with exterior and interior bounding wires.

If you do not care / do not expect that, you can have ifcopenshell reverse engineer the polygons (with holes) back from the triangulation.

>>> import ifcopenshell.geom
>>> settings = ifcope…

View full answer

aothms · 2025-03-28T12:46:28Z

aothms
Mar 28, 2025
Maintainer

I am aware of some possibilities using OpenCascade to extract geometry as BRep, but I am struggeling with the install process, and therefore it seems a bit overkill for my current use case. Or is this the route I have to take?

Well, that depends on the question: do you care about the possibility of having a curved wall with openings? Because in that case you do really need the full occt brep model that will tell you you have a Face with a cylindrical surface with exterior and interior bounding wires.

If you do not care / do not expect that, you can have ifcopenshell reverse engineer the polygons (with holes) back from the triangulation.

>>> import ifcopenshell.geom
>>> settings = ifcopenshell.geom.settings()
>>> ifc_model = ifcopenshell.open('Duplex_A_20110907_optimized.ifc')
>>> element = sorted(ifc_model.by_type("IfcWall"), key=lambda w: len(w.HasOpenings))[-1]           
>>> settings = ifcopenshell.geom.settings()
>>> settings.set('triangulation-type', ifcopenshell.ifcopenshell_wrapper.POLYHEDRON_WITH_HOLES)
>>> shape = ifcopenshell.geom.create_shape(settings, element)
>>> shape.geometry.faces
(((3, 2, 1, 0),), ((7, 6, 5, 4, 0, 1, 9, 8), (13, 12, 10, 11), (20, 18, 19, 21), (14, 15, 17, 16), (24, 22, 23, 25)), ((2, 3, 27, 26), (41, 40, 43, 42), (45, 44, 47, 46), (30, 29, 28, 31), (33, 32, 35, 34), (37, 36, 39, 38)), ((27, 3, 0, 4),), ((1, 2, 26, 9),), ((9, 26, 27, 4, 5, 8),), ((7, 8, 42, 43),), ((6, 7, 43, 40),), ((5, 6, 40, 41),), ((11, 10, 29, 30),), ((13, 11, 30, 31),), ((28, 12, 13, 31),), ((10, 12, 28, 29),), ((15, 14, 33, 34),), ((17, 15, 34, 35),), ((32, 16, 17, 35),), ((14, 16, 32, 33),), ((19, 18, 45, 46),), ((21, 19, 46, 47),), ((44, 20, 21, 47),), ((18, 20, 44, 45),), ((22, 37, 38, 23),), ((23, 38, 39, 25),), ((24, 25, 39, 36),), ((22, 24, 36, 37),), ((8, 5, 41, 42),))

1 reply

ChSchlaeger Mar 31, 2025
Author

Thank you, Thomas! That’s exactly what I was looking for.

Openings are quite relevant for me, but I don’t expect curved walls in my models—and I’m fine with accepting this limitation in such edge cases. I already tried your suggestion, and it works perfectly. Really appreciate the quick help!

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Extracting Non-Triangulated Surface Polygons from IFC Models #6458

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Extracting Non-Triangulated Surface Polygons from IFC Models #6458

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