.IFC File Format – Placement and representation

Object placement

IfcObjectPlacement is the abstract supertype of the ways an object coordinate system is given: absolute or relative by an axis placement, by reference to a design grid, or by distance along a curve. Whatever the kind, the result is a 2D or 3D axis placement. The placement must be given when a product has a shape representation, and one placement instance may be shared by several products without that sharing meaning anything.

Conventions for relative placement, from IfcLocalPlacement:

• IfcSite is placed absolutely in the world coordinate system of the project’s representation context.
• IfcFacility (building, bridge, road and so on) is placed relative to the site, and IfcFacilityPart relative to its facility.
• IfcGrid and IfcAnnotation are placed relative to their container (site, building or storey), and IfcAlignment relative to the project’s world coordinate system.
• IfcPort is placed relative to the element it belongs to.
• An IfcElement is placed relative to its spatial container (the one named by IfcRelContainedInSpatialStructure), or to the element it is tied to: the host of an opening or projection, the opening a door or window fills, the element a covering covers, or the whole it is aggregated or nested into.

Applications must prevent cycles of relative placements themselves.

Axis placements, points and directions

All placements define right-handed Cartesian coordinate systems and do not allow mirroring; mirroring and scaling are done with Cartesian transformation operators (see mapped representations). IfcAxis2Placement is a select that accepts either a 2D or a 3D axis placement.

The derived attributes are computed, not stored; how a derived attribute appears in a file is ISO 10303-21 syntax (see the STEP page).

Representation contexts

The project’s RepresentationContexts declare the coordinate space every shape is in. One IfcGeometricRepresentationContext for the 3D model is mandatory; a second for 2D plan views is optional, and needed when there are scale-dependent plan views. There is at most one context for 2D and one for 3D. Finer, scale- or view-dependent contexts are sub-contexts of one of them. The project concept template names them Model_3D, for all 3D geometry, and Plan_2D, for 2D annotations.

IfcGeometricRepresentationSubContext shares its parent’s world coordinate system, dimension, precision and true north: it redeclares those four inherited attributes (positions 3 to 6) as derived from the parent. Its own attributes:

The sub-context’s ContextIdentifier takes the same values as the representation identifiers below. Only the main context may have a coordinate operation (rule NoCoordOperation).

IfcGeometricProjectionEnum

Shape representations

A product’s Representation is an IfcProductRepresentation, for shapes the subtype IfcProductDefinitionShape, which may hold only shape or topology representations (rule OnlyShapeModel). It lists one or more IfcRepresentations, each a set of items in one context. Its subtypes are IfcShapeModel (with IfcShapeRepresentation and IfcTopologyRepresentation) and IfcStyleModel.

IfcProductDefinitionShape and IfcShapeRepresentation add no attributes. Rules on IfcShapeRepresentation: an identifier and a type should be given; no topological item may be used directly except IfcVertexPoint, IfcEdgeCurve and IfcFaceSurface; and the items must suit the type (CorrectItemsForType). Most building elements have a 'Body' representation, typically 'Tessellation' or 'Brep'.

Representation identifiers

Representation types

Mapped representations

A shape shared by many products (a block, cell or macro) is defined once as an IfcRepresentationMap, usually listed in a type’s RepresentationMaps, and inserted with an IfcMappedItem in a representation of type 'MappedRepresentation'. Several maps on one type make a multi-view block.

The 2D operators IfcCartesianTransformationOperator2D and IfcCartesianTransformationOperator2DnonUniform complete the family.

Georeferencing

A main geometric context can be tied to a map coordinate reference system by an IfcCoordinateOperation whose source is the context; the context finds it through its inverse attribute HasCoordinateOperation. All main contexts of a project must use the same operation and target system. The subtypes of IfcCoordinateOperation are IfcMapConversion (with IfcMapConversionScaled) and IfcRigidOperation; those of IfcCoordinateReferenceSystem are IfcProjectedCRS and IfcGeographicCRS.

When a map conversion is present, TrueNorth is for information only and must not be applied on top of the conversion; where they disagree, the map conversion wins.

Geometric representation items

Shapes are made of IfcRepresentationItems. Its direct subtypes are IfcGeometricRepresentationItem, IfcMappedItem, IfcStyledItem and IfcTopologicalRepresentationItem. The direct subtypes of IfcGeometricRepresentationItem in IFC 4.3: IfcAnnotationFillArea, IfcBooleanResult, IfcBoundingBox, IfcCartesianPointList, IfcCartesianTransformationOperator, IfcCsgPrimitive3D, IfcCurve, IfcDirection, IfcFaceBasedSurfaceModel (deprecated), IfcFillAreaStyleHatching, IfcFillAreaStyleTiles, IfcGeometricSet, IfcHalfSpaceSolid, IfcLightSource, IfcPlacement, IfcPlanarExtent, IfcPoint, IfcSectionedSpine, IfcSegment, IfcShellBasedSurfaceModel, IfcSolidModel, IfcSurface, IfcTessellatedItem, IfcTextLiteral and IfcVector. The schemas page lists every entity of the geometry, geometric model, topology and profile schemas.

Worked example

A wall 5 m long, 0.2 m thick and 3 m high, extruded from a rectangle, in a project measured in metres. The wall’s placement is relative to its storey’s placement #21; the identifiers and numbers are made up for the example.

#100=IFCGEOMETRICREPRESENTATIONCONTEXT($,'Model',3,1.E-05,#101,$);
#101=IFCAXIS2PLACEMENT3D(#102,$,$);
#102=IFCCARTESIANPOINT((0.,0.,0.));
#30=IFCWALL('1kTvXnbbzCWw8lcMd1dR4o',$,'Wall 1',$,$,#31,#40,$,.SOLIDWALL.);
#31=IFCLOCALPLACEMENT(#21,#32);
#32=IFCAXIS2PLACEMENT3D(#102,$,$);
#40=IFCPRODUCTDEFINITIONSHAPE($,$,(#41));
#41=IFCSHAPEREPRESENTATION(#100,'Body','SweptSolid',(#42));
#42=IFCEXTRUDEDAREASOLID(#43,#44,#45,3.);
#43=IFCRECTANGLEPROFILEDEF(.AREA.,$,$,5.,0.2);
#44=IFCAXIS2PLACEMENT3D(#46,$,$);
#45=IFCDIRECTION((0.,0.,1.));
#46=IFCCARTESIANPOINT((2.5,0.1,0.));

The attributes used for the solid and profile: IfcSweptAreaSolid has SweptArea (IfcProfileDef) and Position (IfcAxis2Placement3D, opt.); IfcExtrudedAreaSolid adds ExtrudedDirection (IfcDirection) and Depth (IfcPositiveLengthMeasure). IfcProfileDef has ProfileType (IfcProfileTypeEnum) and ProfileName (IfcLabel, opt.); IfcParameterizedProfileDef adds Position (IfcAxis2Placement2D, opt.); IfcRectangleProfileDef adds XDim and YDim (IfcPositiveLengthMeasure). IfcProfileTypeEnum has two values: AREA, a surface that sweeps into a solid with volume, and CURVE, a closed curve (open only for IfcArbitraryOpenProfileDef) that sweeps into a surface, used for thin sheets such as ducts. Every parameterised profile is defined with the centre of its bounding box at the origin of its position. The documentation advises leaving the profile’s own Position unset when the containing solid can place it, as here.

Not confirmed

• The members of the selects IfcCoordinateReferenceSystemSelect and IfcGridPlacementDirectionSelect beyond what the documentation describes in prose.
• The attributes of the curve, surface, solid, tessellation and topology entities; they are listed by name only.

Sources

• IFC4.3.x-development – buildingSMART’s IFC 4.3 source repository, master branch: schemas/IFC.xml (attribute order, types, optionality, derived attributes, generalisations and enumeration values), the entity pages IfcObjectPlacement, IfcLocalPlacement, IfcLinearPlacement, IfcGridPlacement, IfcPlacement, IfcAxis2Placement2D, IfcAxis2Placement3D, IfcAxis2PlacementLinear, IfcCartesianPoint, IfcDirection, IfcRepresentationContext, IfcGeometricRepresentationContext, IfcGeometricRepresentationSubContext, IfcProductRepresentation, IfcProductDefinitionShape, IfcRepresentation, IfcShapeRepresentation, IfcRepresentationMap, IfcMappedItem, IfcCoordinateOperation, IfcMapConversion, IfcMapConversionScaled, IfcCoordinateReferenceSystem, IfcProjectedCRS, IfcExtrudedAreaSolid, IfcParameterizedProfileDef, IfcRectangleProfileDef, IfcProduct, IfcProject, the type pages IfcAxis2Placement, IfcGeometricProjectionEnum, IfcProfileTypeEnum, and the concept templates Project Representation Context, Product Placement and Product Geometric Representation