1 Automatic generation of CityGML LoD3 building models from IFC models M.Sc. Geomatics P5 presentation by Sjors Donkers Supervisors: Hugo Ledoux / Junqiao (John) Zhao Graduation professor: Jantien Stoter
Sjors Donkers - Automatic generation of CityGML LoD3 building models from IFC models
May 24, 2015
CityGML is a standardized data format used to store the semantic information and geometries of buildings and other object classes of 3D city models. The Level of Detail of current state of the art city models (LoD2) is not sufficient for accurate environmental simulations like noise, the solar potential of windows and other types of analyses. An LoD3 building model represents the full architectural exterior of a building with balconies, windows and so forth. The generation of these models needs to be automated as it is otherwise infeasible due to the required high amount of manual labour. In the architectural world, detailed building models are created in IFC format. This thesis shows that it is possible to automatically generate valid and semantically rich CityGML LoD3 building models directly from IFC models. Also an initial investigation is done on the possibilities for the conversion of IFC models to CityGML LoD4.
For the conversion the semantic and geometric validity requirements are determined for CityGML. A methodology for the conversion is developed and a prototype implementation is made to prove the effectiveness of the conversion. The conversion consists of three parts: 1) The extraction and mapping of IFC semantics to CityGML semantics; 2) A geometric generalization which extracts the exterior shell using a transformation based on Boolean and morphological operations; 3) Semantic and geometric refinements which optimize the model for analyses. The developed prototype is able to successfully convert IFC models to CityGML LoD3. All the resulting models were geometrically validated according to the ISO19107 standard, and semantics were checked manually. Few improper semantics occur in the output due to missing semantics in IFC. For example, there are no semantics for balconies or dormers in IFC. Recommendations are given to improve the alignment between the two formats. For IFC additional semantics are recommended whereas it is important for CityGML to specify how certain aspects are to be modelled.
The research presented in this thesis can be used as the foundation for future work on the interoperability between Architecture and Geomatics. The software package is open source and freely available at https://github.com/tudelft-gist/ifc2citygml.
For the conversion the semantic and geometric validity requirements are determined for CityGML. A methodology for the conversion is developed and a prototype implementation is made to prove the effectiveness of the conversion. The conversion consists of three parts: 1) The extraction and mapping of IFC semantics to CityGML semantics; 2) A geometric generalization which extracts the exterior shell using a transformation based on Boolean and morphological operations; 3) Semantic and geometric refinements which optimize the model for analyses. The developed prototype is able to successfully convert IFC models to CityGML LoD3. All the resulting models were geometrically validated according to the ISO19107 standard, and semantics were checked manually. Few improper semantics occur in the output due to missing semantics in IFC. For example, there are no semantics for balconies or dormers in IFC. Recommendations are given to improve the alignment between the two formats. For IFC additional semantics are recommended whereas it is important for CityGML to specify how certain aspects are to be modelled.
The research presented in this thesis can be used as the foundation for future work on the interoperability between Architecture and Geomatics. The software package is open source and freely available at https://github.com/tudelft-gist/ifc2citygml.
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1
Automatic generation of CityGML LoD3 building models from IFC modelsM.Sc. Geomatics P5 presentation by Sjors Donkers
Supervisors: Hugo Ledoux / Junqiao (John) ZhaoGraduation professor: Jantien Stoter
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Geomatics
3
Architecture
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Where the fields overlap
Architecture
Geomatics Large scale(mm)
Small scale(km)
WorldCountryCityRegionBuildin
g
ComponentRoomSectio
nFloorBuilding
CityGML IFC
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What is high detail for CityGML?
LoD4LoD3LoD2LoD0LoD1
• LoD ≤ 2 can be automatically generated• LoD ≥ 3 requires manual labour
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Research question
Is it possible to generate valid and
semantically rich CityGML building models at
LoD3 from IFC models,
and can this method be extended to LoD4?
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Structure
• What is IFC / CityGML and when is it valid?
• Methodology for the conversion
• Experimental results
• Possibilities for LoD4
• Conclusions, recommendations & future work
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Conversion from IFC to CityGML
IFCCityGML
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IFC example
•Many objects
•Complex semantic network
•For content creators
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CityGML LoD3 example
•Few objects
•Simple hierarchical semantics
•For users / analyses
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Differences
IFC CityGML
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Validity criteria for CityGML LoD3
•Semantics:• Normal vector constraints on surface types
•Geometry:CityGML:• A building has only an exterior shellISO19107:• The shell must be 2-manifold
Vertices should not be close to other geometries
You should be able to walk on a FloorSurface,but not on a CeilingSurface
A
B
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Degenerate geometry
Floating-point arithmetic: 0.1 + 0.2 ≠ 0.3
Degenerate when:Float-orientation ≠ Exact-orientationDistance d = 0 (float or exact)
Exact arithmetic:
0.1 + 0.2 = 0.3= 0.30000000000004
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Existing converters
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Structure
• What is IFC / CityGML and when is it valid?
• Methodology for the conversion
• Experimental results
• Possibilities for LoD4
• Conclusions, recommendations & future work
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1 Semantics mapping2 Geometric transformation
Methodology for the Conversion
3 Refinements
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CityGML has semantics for and between:
• Solids / Objects• Trivial attributes
• Faces• Boundary surfaces
• Curves• Requires the terrain which is out of scope
1 Semantic mapping
(But not limited to only boundary surfaces!)
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Project
Site
Building
Building
Building
Storey Storey Space
1 Semantic mapping
Geometry
IFC example structure
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1 Semantic mapping
CityGML• Ceiling• Door• Floor• Roof• Wall• Window• Ground• Closure
IFC• Beam• Member• Stairs• Door• Roof• Wall• Slab• Window• …
??
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1. Type
2. Predefined type
3. Decomposed by
4. Surface normal
• FLOOR• ROOF• LANDING• BASESLAB• USERDEFINED• NOTDEFINED
1 Semantic mapping
Building
Roof
Slab
Slab
Constraints:
The object has to be contained in a building
Must be a Space or a subtype of BuildingElement
ROOF
A combination of:
• IfcSlab• IfcMember• IfcRoof
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2 Geometric transformation
Methodology for the Conversion
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1. All IFC geometries are connected using the Boolean union operation
2. Interior geometries are removed
2 Geometric transformation
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Real buildings are not watertight !
?
2 Geometric transformation
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Concepts for extracting the exterior shell• Generated• Implemented• Evaluated
Morphological closingUsing an oriented cubical structuring element
2 Geometric transformation
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Morphological closing = dilation followed by erosion
Dilation:
Erosion:
2 Morphological closing
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1. Geometries are dilated thereby closing the gaps
2. Interior geometries are removed
3. The exterior shell is eroded back to it original size
2 Morphological closing
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2 BuildingInstallations
• BuildingInstallations are separate objects
• Objects must not overlap each other
1. BuildingInstallations are unioned
2. The building solid is subtracted
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Methodology for the Conversion
3 Refinements
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3 Refinements - geometric
• Coordinates are rounded in preparation of writing
• The geometry is regularized
• Solid geometry is made 2-manifold
• Degeneracies are removed
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• Faces without semantics are created during closing
• Semantics are asigned based on the normal and the neighbours
Conversion complete!
3 Refinements - semantic
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• Nef polyhedra are used for Boolean operations
• Nef polyhedra in CGAL do not support semantic faces
• Semantics are reattached after the geometric transformation
Implementation
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Structure
• What is IFC / CityGML and when is it valid?
• Methodology for the conversion
• Experimental results
• Possibilities for LoD4
• Conclusions, recommendations & future work
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Experimental resultsExperimental results – no roof
IFC CityGML
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Experimental results
IFC CityGML
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Validity and quality
• Geometric validity checked using 3D Validator: All models are valid!
• Some near degeneracies remain
• Artefacts occur when closing is applied
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Validity and quality
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Validity and quality
• Geometric validity checked using 3D Validator: All models are valid!
• Some near degeneracies remain
• Artefacts occur when closing is applied
• Mismatched semantic
• Lacking semantics in IFC
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Conversion evaluation
• Computation time ~5-15 minutes• With outliers from 6 seconds to 95 minutes• Creation of Nef polyhedra and Boolean operations are slow
• Morphological closing roughly double the computation time
• Generated CityGML files are smaller than input IFC files• Detriangulation leads to a file size reduction of ~66%• ~50% of the file space is dedicated to BuildingInstallations
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Structure
• What is IFC / CityGML and when is it valid?
• Methodology for the conversion
• Experimental results
• Possibilities for LoD4
• Conclusions, recommendations & future work
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Generation of LoD4 rooms
• Rooms cannot be detected from the geometry
• IfcSpaces are (almost) equivalent to Rooms in CityGML
• In the implementation:Geometry from IfcSpacesSemantics base on surface normal
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LoD4 experimental results
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Possibilities for LoD4 conversion
• In IFC objects can be linked to spaces
• The same semantic mapping can be used• But needs to be extended with:
Furniture and other LoD4 specific objects
Connectivity relations between openings
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Structure
• What is IFC / CityGML and when is it valid?
• Methodology for the conversion
• Experimental results
• Possibilities for LoD4
• Conclusions, recommendations & future work
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Conclusions
• A new source for CityGML LoD3 building models• Small additions to the IFC will align the two standards even more
• Generating LoD4 building models is only a small step away
• The methodology enables the creation of• up-to-date• high detail models that • adhere to the standards of CityGML and ISO19107, thereby • increasing the availability of high detail models• and the interoperability between Geomatics and Architecture and • reducing the costs for the creation of high detail city models
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Conclusions
Other uses of this research:
• Semantic mapping for use in a reverse conversion or UBM
• Geometric transformations for the simplification of any CAD model
• Refinement methods to optimize the geometry for analyses
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Recommendations
Recommendations for IFC:
• IfcSpace for the exterior of the building• Add semantics for balconies, dormers, external IfcSpaces
Recommendations for CityGML:
• Refine the definitions of how to model CityGML• For the geometry of BuildingParts & -Installations • For the semantics of doors and windows
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Future work
• Mapping of new IFC4 classes and trivial attributes like the address
• Extraction of the terrain intersection curve
• A higher level of interoperability between IFC and CityGML• Alignment of the standards• Generation of LoD2 and LoD4 building models• Generation of other city objects (tunnels, bridges)
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Something extra
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Automatic generation of CityGML LoD3 building models from IFC modelsM.Sc. Geomatics P5 presentation by Sjors Donkers
Supervisors: Hugo Ledoux / Junqiao (John) ZhaoGraduation professor: Jantien Stoter
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