SuperAPI AI Directory
Partner中文AI newsFavorites
Skillsifc-syntax-geometry-representations

ifc-syntax-geometry-representations

Office & productivity

View repository

Updated 10-16 05:48

About this skill

IFC Syntax : Geometry Representations

IFC separates what an object is (semantics : IfcWall, IfcBeam, IfcDoor) from what it looks like (geometry). The bridge is the representation framework : a layered chain every shape-bearing product walks, plus the context entities that fix the coordinate frame. This skill owns the framework and the wiring. The individual geometry primitives (extrusions, Breps, tessellation) live in their own dedicated skills.

Applies to IFC2x3, IFC4, and IFC4.3. The core framework entities are version-stable ; the value tables grew across versions. See Version Differences.

Quick Reference

The product-to-shape chain

Every shape-bearing product resolves its geometry down a fixed chain :

IfcProduct.Representation
  -> IfcProductDefinitionShape          (a kind of IfcProductRepresentation)
       .Representations LIST OF
         -> IfcShapeRepresentation       (or IfcTopologyRepresentation)
              .Items SET OF
                -> IfcRepresentationItem (the actual geometry primitives)

ALWAYS walk this exact chain when reading or writing geometry. The semantics (IfcWall) and the geometry (IfcExtrudedAreaSolid) meet only here.

IfcProduct : placement plus representation

IfcProduct is the abstract supertype of every object with a geometric or spatial context. Two attributes matter here :

  • ObjectPlacement : OPTIONAL IfcObjectPlacement. The object coordinate system.
  • Representation : OPTIONAL IfcProductRepresentation. For a shape-bearing product this is always its subtype IfcProductDefinitionShape.

The WHERE rule PlacementForShapeRepresentation : if Representation is an IfcProductDefinitionShape, then ObjectPlacement MUST also be given. Geometry items are defined in a local coordinate system ; the placement positions that system in the world.

ALWAYS supply an ObjectPlacement whenever a shape representation is present. NEVER leave ObjectPlacement as $ on a product that carries geometry : the file is schema-invalid. See ifc-syntax-geometry-placement.

IfcProductDefinitionShape

The container for all shape information of one product. Subtype of IfcProductRepresentation ; inherits Name, Description, and Representations (LIST [1:?] OF IfcRepresentation).

  • WHERE rule OnlyShapeModel : Representations MUST contain ONLY IfcShapeRepresentation or IfcTopologyRepresentation instances (the two subtypes of IfcShapeModel). Nothing else is permitted.
  • Inverse ShapeOfProduct : SET [1:?] OF IfcProduct (cardinality 1:N from IFC4 : one shape definition may be shared by several products).
  • Inverse HasShapeAspects : SET [0:?] OF IfcShapeAspect.

IfcShapeRepresentation : the workhorse

One geometric representation of a product in one context. Supertype chain IfcRepresentation -> IfcShapeModel -> IfcShapeRepresentation. Four attributes (all inherited from IfcRepresentation) :

  • ContextOfItems : IfcRepresentationContext (required ; in practice an IfcGeometricRepresentationContext or one of its sub-contexts).
  • RepresentationIdentifier : OPTIONAL IfcLabel. The kind of representation.
  • RepresentationType : OPTIONAL IfcLabel. The geometry/topology type.
  • Items : SET [1:?] OF IfcRepresentationItem. The geometric primitives.

Five WHERE rules :

  1. CorrectContext : ContextOfItems MUST be an IfcGeometricRepresentationContext.
  2. CorrectItemsForType : Items MUST comply with RepresentationType.
  3. HasRepresentationIdentifier : RepresentationIdentifier MUST be present.
  4. HasRepresentationType : RepresentationType MUST be present.
  5. NoTopologicalItem : no topological item in Items, except IfcVertexPoint, IfcEdgeCurve, IfcFaceSurface.

Rules 3 and 4 make RepresentationIdentifier and RepresentationType effectively mandatory even though they are schema-level OPTIONAL. ALWAYS populate both.

RepresentationIdentifier : the kind (most common values)

Body (3D solid/surface/wireframe), Axis (2D/3D axis line), FootPrint (2D ground projection), Box (bounding box), Profile (3D planar profile outline), Surface (analytical surface), Reference (3D non-Body geometry), CoG (centre of gravity point), Clearance (clearance volume), Annotation (2D/3D annotation). IFC4.3 adds Body-FallBack and Lighting. The full verified table is in references/methods.md. The usual identifier for a building element is Body.

RepresentationType : the geometry type (most common values)

SweptSolid (extruded/revolved area solids), AdvancedSweptSolid (tapered sweeps along a directrix), Brep (faceted boundary representations), AdvancedBrep (b-spline surface Breps), CSG (Boolean results), Clipping (Boolean differences with half-spaces), Tessellation (tessellated face sets), MappedRepresentation (mapped item references), Curve2D / Curve3D (axis curves), BoundingBox, SurfaceModel. The full verified table is in references/methods.md.

CorrectItemsForType couples this label to Items : SweptSolid may hold ONLY swept area solids, Tessellation ONLY tessellated face sets. The label and the geometry kind MUST agree.

Representation contexts

Geometric items are meaningless without a context : the frame fixing the dimension, precision, world origin, and true north.

IfcGeometricRepresentationContext is the root frame. Key attributes : CoordinateSpaceDimension (2 or 3), Precision (OPTIONAL IfcReal, typically 1E-5), WorldCoordinateSystem (IfcAxis2Placement), TrueNorth (OPTIONAL IfcDirection). Inherited : ContextIdentifier, ContextType ('Model', 'Plan', 'NotDefined'). A project with 3D geometry MUST declare one context with ContextType = 'Model'.

IfcGeometricRepresentationSubContext is a subtype that targets a specific view. Explicit attributes : ParentContext (required), TargetScale (OPTIONAL), TargetView (IfcGeometricProjectionEnum, required), UserDefinedTargetView (OPTIONAL). Its WorldCoordinateSystem, CoordinateSpaceDimension, TrueNorth, and Precision are DERIVED from the parent.

In a STEP file the four derived attributes appear as the asterisk * placeholder, NEVER as repeated literal values. WHERE rules : NoCoordOperation (a sub-context carries no coordinate operation), ParentNoSub (the parent is a plain context, one level deep only), UserTargetProvided (UserDefinedTargetView required when TargetView = USERDEFINED).

IfcGeometricProjectionEnum has nine values : MODEL_VIEW, PLAN_VIEW, GRAPH_VIEW, ELEVATION_VIEW, SECTION_VIEW, REFLECTED_PLAN_VIEW, SKETCH_VIEW, USERDEFINED, NOTDEFINED. A Body representation uses a sub-context with TargetView = MODEL_VIEW ; an Axis uses GRAPH_VIEW ; a FootPrint uses PLAN_VIEW.

IfcTopologyRepresentation

The sibling of IfcShapeRepresentation (both subtype IfcShapeModel). It carries topological structure (vertices, edges, paths, faces, shells), potentially without coordinate geometry. Same four attributes. Permitted RepresentationType values : Vertex, Edge, Path, Face, Shell, Undefined. Rare in normal building exchange ; used mainly in analysis and structural-connectivity models.

IfcShapeAspect

Names an addressable sub-feature of a product's geometry (one material layer of a wall, one panel of a curtain wall). Attributes : ShapeRepresentations (LIST [1:?] OF IfcShapeModel), Name, Description, ProductDefinitional (IfcLogical), PartOfProductDefinitionShape (the owning IfcProductDefinitionShape or IfcRepresentationMap).

Mapped representations : geometry reuse

One geometric definition reused under many transformations, the IFC equivalent of a CAD block. Two entities collaborate :

  • IfcRepresentationMap : the base definition. Attributes : MappingOrigin (IfcAxis2Placement), MappedRepresentation (IfcRepresentation). WHERE rule ApplicableMappedRepr : only an IfcShapeRepresentation or IfcTopologyRepresentation is permitted as MappedRepresentation.
  • IfcMappedItem : the inserted instance. It IS an IfcRepresentationItem, so it sits directly in an Items set. Attributes : MappingSource (IfcRepresentationMap), MappingTarget (IfcCartesianTransformationOperator).

An IfcShapeRepresentation whose Items are IfcMappedItem instances uses RepresentationType = 'MappedRepresentation'. Typed product families (IfcDoorType, IfcFurnitureType) carry geometry once on the type and instance it per occurrence, shrinking file size for repetitive elements.

Version differences (compact)

Item IFC2x3 IFC4 IFC4.3
Core framework entities present present present
ShapeOfProduct cardinality 1:1 1:? (shared shapes) 1:?
RepresentationIdentifier / Type tables smaller larger largest
Body-FallBack, Lighting identifiers absent absent present
PointCloud representation type present present DEPRECATED, use Point
HasCoordinateOperation inverse (georeferencing) absent present present

Decision Trees

Does my product need a representation, a placement, or both?

Is the product physically located and shape-bearing?
|
+-- It carries geometry
|   --> give it an IfcProductDefinitionShape AND an IfcObjectPlacement.
|       Both are required together (PlacementForShapeRepresentation rule).
|
+-- It is located but has no own geometry (e.g. a spatial zone)
|   --> give it an IfcObjectPlacement, leave Representation as $.
|
+-- It is an abstract grouping product
    --> leave both as $.

Which RepresentationIdentifier?

What does this representation depict?
|
+-- The 3D solid / surface body of the element     --> Body
+-- A 2D or 3D axis line (wall axis, beam axis)     --> Axis
+-- A 2D ground-level outline                       --> FootPrint
+-- A simplified bounding box                       --> Box
+-- A 3D planar profile outline (door, window)      --> Profile
+-- An analytical surface                           --> Surface
+-- 3D geometry not part of Body (opening ref)      --> Reference
+-- A centre-of-gravity point                       --> CoG
+-- A clearance volume                              --> Clearance
+-- A 2D or 3D annotation                           --> Annotation

Which context and TargetView?

Declare ONE IfcGeometricRepresentationContext, ContextType = 'Model', dimension 3.
Then one IfcGeometricRepresentationSubContext per representation kind:
|
+-- Body      --> sub-context, TargetView = MODEL_VIEW
+-- Axis      --> sub-context, TargetView = GRAPH_VIEW
+-- FootPrint --> sub-context, TargetView = PLAN_VIEW
+-- Box       --> sub-context, TargetView = MODEL_VIEW
+-- Annotation--> sub-context, TargetView = PLAN_VIEW / SECTION_VIEW / ELEVATION_VIEW

Shape representation or topology representation?

Does the data carry coordinate geometry (points, curves, solids)?
|
+-- YES --> IfcShapeRepresentation. The normal case for building geometry.
|
+-- NO, it is pure topological structure (connectivity, no coordinates)
    --> IfcTopologyRepresentation. Rare ; analysis / structural models.

Should I reuse geometry across many elements?

Is the same geometry repeated for many occurrences (doors, furniture, fixtures)?
|
+-- YES --> define it once in an IfcRepresentationMap, instance each occurrence
|           with an IfcMappedItem ; RepresentationType = 'MappedRepresentation'.
|
+-- NO  --> give each element its own IfcShapeRepresentation with concrete items.

Patterns

Pattern : Wire a Body representation into the Model context

ALWAYS build the chain bottom-up : create the geometry items, wrap them in an IfcShapeRepresentation whose ContextOfItems is the Body sub-context, RepresentationIdentifier is 'Body', and RepresentationType matches the geometry ('SweptSolid' for an extrusion). Put that representation in an IfcProductDefinitionShape.Representations list, point the product's Representation at it, and give the product an ObjectPlacement. NEVER omit the placement : the PlacementForShapeRepresentation rule rejects a shape representation with no placement. See references/examples.md for the full worked STEP file.

Pattern : Declare the Model context and its sub-contexts

ALWAYS declare exactly one IfcGeometricRepresentationContext with ContextType = 'Model' and CoordinateSpaceDimension = 3 for a 3D model. Add one IfcGeometricRepresentationSubContext per representation kind, each pairing a RepresentationIdentifier with a TargetView. ALWAYS write the four derived sub-context attributes as the * placeholder in the STEP file. NEVER copy the parent's WorldCoordinateSystem, dimension, precision, or true north as literal values into a sub-context. NEVER nest a sub-context under another sub-context : the ParentNoSub rule allows exactly one level.

Pattern : Carry multiple representations on one product

ALWAYS place every representation of a product (Body, Axis, FootPrint, Box) as a separate IfcShapeRepresentation in the single IfcProductDefinitionShape.Representations list, each tied to its own identifier-and-view sub-context. They are complementary views. NEVER treat multiple representations as conflicting duplicates : a wall legitimately has a Body solid and an Axis line at the same time.

Pattern : Instance geometry with a mapped representation

ALWAYS define repeated geometry once as an IfcRepresentationMap whose MappedRepresentation is an IfcShapeRepresentation, and instance it per occurrence with an IfcMappedItem whose MappingSource is that map and whose MappingTarget is an IfcCartesianTransformationOperator. ALWAYS set the consuming representation's RepresentationType to 'MappedRepresentation' when its Items are IfcMappedItem instances. NEVER let an IfcMappedItem map a representation that transitively contains the item itself : that infinite recursion is forbidden.

Pattern : Name a sub-feature with IfcShapeAspect

ALWAYS use IfcShapeAspect to give a name to a distinct geometric part (a material layer, a curtain-wall panel, a named connection zone). Set ShapeRepresentations to the shape models forming the aspect and PartOfProductDefinitionShape to the owning IfcProductDefinitionShape or IfcRepresentationMap.

Pattern : Honour the version differences

ALWAYS treat the core framework entities as version-stable across IFC2x3, IFC4, and IFC4.3. ALWAYS allow one IfcProductDefinitionShape to be shared by several products in IFC4 and IFC4.3 (ShapeOfProduct is 1:N) ; in IFC2x3 the relation is 1:1. ALWAYS draw RepresentationIdentifier and RepresentationType values from the table of the target version : Body-FallBack and Lighting are IFC4.3 identifiers, and PointCloud is deprecated in IFC4.3 in favour of Point.

Reference Links

  • references/methods.md : the complete verified entity definitions, the full RepresentationIdentifier and RepresentationType value tables, the context entities with their derived attributes and WHERE rules, the IfcCartesianTransformationOperator family, and the per-version difference matrix.
  • references/examples.md : worked STEP scenarios : a Body representation wired into the Model context, a product carrying both Body and Axis, a mapped representation instancing a door, and a named shape aspect.
  • references/anti-patterns.md : the failure modes this skill prevents, each with the WHERE rule it breaks and the correct form.

Official sources

Related skills

  • ifc-syntax-geometry-placement : IfcObjectPlacement, the placement tree, and resolving local geometry to world coordinates.
  • ifc-syntax-geometry-swept-solid : IfcExtrudedAreaSolid and the swept-solid geometry that fills a SweptSolid representation.
  • ifc-impl-geometry-extraction : reading geometry back out of a model.