# Ifc Syntax Geometry Representations > Use when you need to attach geometry to an IFC product, read a model's shape data, or debug why a representation is rejected : the product-to-shape chain, IfcShapeRepresentation, the RepresentationIdentifier and RepresentationType value tables, the Model representation context and its sub-contexts, topology representations, named shape aspects, and mapped (instanced) geometry. Prevents a shape representation with no placement, a non-shape-model entity inside IfcProductDefinitionShape, a missing or mismatched RepresentationType, a sub-context with literal instead of derived attributes, and a self-referential mapped item. Covers IfcProduct.Representation, IfcProductRepresentation, IfcProductDefinitionShape, IfcShapeRepresentation, IfcTopologyRepresentation, IfcGeometricRepresentationContext, IfcGeometricRepresentationSubContext, IfcGeometricProjectionEnum, IfcShapeAspect, IfcRepresentationMap, IfcMappedItem, IfcCartesianTransformationOperator, IfcRepresentationItem, and IFC2x3 / IFC4 / IFC4.3 version difference - Skill: `impertio-studio/ifc-syntax-geometry-representations` (Agent Skill, multi-file: 4 files) - Install (CLI): `npx skillmds@latest add impertio-studio/ifc-syntax-geometry-representations` - Raw SKILL.md: https://api.skillmd.com/api/skills/impertio-studio/ifc-syntax-geometry-representations/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: AI & ML - License: MIT - Author: Impertio-Studio (https://skillmd.com/u/impertio-studio) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/impertio-studio/ifc-syntax-geometry-representations --- # 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 - IfcProduct : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcProduct.htm - IfcProductDefinitionShape : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcProductDefinitionShape.htm - IfcShapeRepresentation : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcShapeRepresentation.htm - IfcGeometricRepresentationContext : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcGeometricRepresentationContext.htm - IfcGeometricRepresentationSubContext : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcGeometricRepresentationSubContext.htm - IfcRepresentationMap : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcRepresentationMap.htm - IfcMappedItem : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMappedItem.htm ### 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.