Set Geometric Representation Context

Overview

The specification of a project within IFC and every occurrence of a representation of a product (and specifically, its shape representation since this is currently the only type of representation defined with IFC) must have a context.
For a project, several contexts may be established which define global parameters that apply across the whole project. This includes the geometric representation context in conjunction with other values (see fp_set_project_context).
For a product, a geometric representation context is required that defines the context applying to the shape representations of that product. That is, it defines the type of the context in which the shape representation is defined, that can be used to describe the level of detailing for which the shape representation is valid and the numeric precision applicable to the geometric representation items defined in this context.
The geometric representation context establishes:

the world coordinate system
the coordinate space dimension
the precision used within the geometric representations
true north relative to the world coordinate system
different types and scales that can be used for representing stored shapes (symbols) where these have multiple representations (e.g. multi-view blocks)

Results

Geometric representation context is established for the identified context type.

Description	Entity/Pset/Functional Part	MAN	REC	OPT
Identify each context included. Presently there are no recommendations on how the context should be identified or on values (or types of values) that should be assigned to a context identifier	IfcGeometricRepresentationContext.ContextIdentifier
Identify the type of each context. Type values should be agreed for implementation and use and might typically describe the type of view or the stages of a project. Current recommendations for values that might be assigned include: graph view sketch view model view plan view reflected plan view section view elevation view	IfcGeometricRepresentationContext.ContextType
Set the coordinate space dimension This is the dimensionality in which geometric information within the project is to be defined. It must be an integer value of 2 or 3. Typically, for IFC information, this will take a value of 3 (for 3 dimensional representations). However, some contexts may have a value of 2 (for 2 dimensional representations).	IfcGeometricRepresentationContext.CoordinateSpaceDimension
Define the world coordinate system for the project context. This may be either a 2D coordinate system or a 3D coordinate system. Generally for IFC, a 3D coordinate system will be applied and this is assumed in the further guidance below. The world coordinate system is defined by its placement and axes as shown below.	IfcGeometricRepresentationContext.WorldCoordinateSystem
Define the location point of the world coordinate system. The location is given as a cartesian point with values for the 3 coordinates (X, Y, Z). Each coordinate is given as a length measure. Dimensionality of the cartesian point is derived from the overall dimensionality determined for the project (typically =3).	IfcAxis2Placement3D.Location IfcCartesianPoint.Coordinates::IfcLengthMeasure
Set the space dimensionality of the coordinate system.	IfcAxis2Placement3D.Dim
Define the direction of the Z axis. Use direction ratios to define its direction relative to the location of the coordinate system. Dimensionality of the direction is derived from the overall dimensionality determined for the project (typically =3). Note that although this attribute is shown as optional within the IFC model, use of this functional part overrides this with a mandatory requirement	IfcAxis2Placement3D.Axis IfcDirection.DirectionRatios
Define the direction of the local X axis. Defined as for the Z axis. Note that although this attribute is shown as optional within the IFC model, use of this functional part overrides this with a mandatory requirement	IfcAxis2Placement3D.RefDirection IfcDirection.DirectionRatios
Derive the normalized directions of the placement of the axes Note that these directions are determined by a function within the IFC model EXPRESS schema. However, if a different schema is being used, then thye normalized directions will need to be calculated using the algorithm shown in the EXPRESS function IfcBuildAxes.	IfcAxis2Placement3D.P
Define the precision of the representation. This is the default precision that should be applied to the project or the precision of a particular context. It must be asserted when used in the project context but may be omitted in a shape representation context in which case, the precision of the project context is used as a default.	IfcGeometricRepresentationContext.Precision
Set the direction of true north This is the direction of the true north relative to the world coordinate system established by the representation context. It must be asserted when used in the project context but may be omitted in a shape representation context in which case, the direction of true north of the project context is used as a default. Note that the direction of true north is established using direction ratios in a similar manner to establishing the direction of the coordinate system axes.	IfcGeometricRepresentationContext.TrueNorth IfcDirection.DirectionRatios

IFC Entities Required

IfcAxis2Placement2D
IfcAxis2Placement3D
IfcCartesianPoint
IfcDirection
IfcGeometricRepresentationContext
IfcGeometricRepresentationItem
IfcGeometricRepresentationSubContext
IfcPlacement
IfcPoint
IfcRepresentationContext

IFC Datatypes Required

IfcAxis2Placement
IfcDimensionCount
IfcGeometricProjectionEnum
IfcLabel
IfcLengthMeasure
IfcPositiveRatioMeasure
IfcRatioMeasure

IFC Functions Required

IfcBuild2Axes

IDM Functional Parts Required

-

EXPRESS-G

EXPRESS Schema

SCHEMA FP_SET_GEOMETRIC_REPRESENTATION_CONTEXT;

  TYPE IfcDimensionCount = INTEGER;
    WHERE
      WR1 : { 0 < SELF <= 3 };
  END_TYPE;

  TYPE IfcLabel = STRING;
  END_TYPE;

  TYPE IfcLengthMeasure = REAL;
  END_TYPE;

  TYPE IfcRatioMeasure = REAL;
  END_TYPE;

  TYPE IfcPositiveRatioMeasure = IfcRatioMeasure;
    WHERE
      WR1 : SELF > 0;
  END_TYPE;

  TYPE IfcGeometricProjectionEnum = ENUMERATION OF
    (GRAPH_VIEW,
     SKETCH_VIEW,
     MODEL_VIEW,
     PLAN_VIEW,
     REFLECTED_PLAN_VIEW,
     SECTION_VIEW,
     ELEVATION_VIEW,
     USERDEFINED,
     NOTDEFINED);
  END_TYPE;

  TYPE IfcAxis2Placement = SELECT
    (IfcAxis2Placement2D,
     IfcAxis2Placement3D);
  END_TYPE;

  ENTITY IfcAxis2Placement2D
    SUBTYPE OF(IfcPlacement);
      RefDirection : OPTIONAL IfcDirection;
    DERIVE
      p            : LIST [2:2] OF IfcDirection := IfcBuild2Axes(RefDirection);
    WHERE
      WR1 : (NOT (EXISTS (RefDirection))) OR (RefDirection.Dim = 2);
      WR2 : SELF\IfcPlacement.Location.Dim = 2;
  END_ENTITY;

  ENTITY IfcPlacement
    ABSTRACT SUPERTYPE OF (ONEOF(IfcAxis2Placement2D, IfcAxis2Placement3D))
    SUBTYPE OF(IfcGeometricRepresentationItem);
      Location : IfcCartesianPoint;
    DERIVE
      Dim      : IfcDimensionCount := Location.Dim;
  END_ENTITY;

  ENTITY IfcGeometricRepresentationItem
    ABSTRACT SUPERTYPE OF (ONEOF(IfcDirection, IfcPlacement, IfcPoint))
    SUBTYPE OF(IfcRepresentationItem);
  END_ENTITY;

  ENTITY IfcRepresentationItem
    ABSTRACT SUPERTYPE;
  END_ENTITY;

  ENTITY IfcDirection
    SUBTYPE OF(IfcGeometricRepresentationItem);
      DirectionRatios : LIST [2:3] OF REAL;
    DERIVE
      Dim             : IfcDimensionCount := HIINDEX(DirectionRatios);
  END_ENTITY;

  ENTITY IfcPoint
    ABSTRACT SUPERTYPE
    SUBTYPE OF(IfcGeometricRepresentationItem);
  END_ENTITY;

  ENTITY IfcCartesianPoint
    SUBTYPE OF(IfcPoint);
      Coordinates : LIST [1:3] OF IfcLengthMeasure;
    DERIVE
      Dim         : IfcDimensionCount := HIINDEX(Coordinates);
    WHERE
      WR1 : HIINDEX(Coordinates) >= 2;
  END_ENTITY;

  ENTITY IfcAxis2Placement3D
    SUBTYPE OF(IfcPlacement);
      Axis         : OPTIONAL IfcDirection;
      RefDirection : OPTIONAL IfcDirection;
    DERIVE
      p            : LIST [3:3] OF IfcDirection := IfcBuildAxes(Axis, RefDirection);
    WHERE
      WR1 : SELF\IfcPlacement.Location.Dim = 3;
      WR2 : (NOT (EXISTS (Axis))) OR (Axis.Dim = 3);
      WR3 : (NOT (EXISTS (RefDirection))) OR (RefDirection.Dim = 3);
      WR4 : (NOT (EXISTS (Axis))) OR (NOT (EXISTS (RefDirection))) OR (IfcCrossProduct(Axis,RefDirection).Magnitude > 0.0);
      WR5 : NOT ((EXISTS (Axis)) XOR (EXISTS (RefDirection)));
  END_ENTITY;

  ENTITY IfcRepresentationContext;
      ContextIdentifier : OPTIONAL IfcLabel;
      ContextType       : OPTIONAL IfcLabel;
  END_ENTITY;

  ENTITY IfcGeometricRepresentationContext
    SUBTYPE OF(IfcRepresentationContext);
      CoordinateSpaceDimension : IfcDimensionCount;
      Precision                : OPTIONAL REAL;
      WorldCoordinateSystem    : IfcAxis2Placement;
      TrueNorth                : OPTIONAL IfcDirection;
  END_ENTITY;

  ENTITY IfcGeometricRepresentationSubContext
    SUBTYPE OF(IfcGeometricRepresentationContext);
      ParentContext          : IfcGeometricRepresentationContext;
      TargetScale            : OPTIONAL IfcPositiveRatioMeasure;
      TargetView             : IfcGeometricProjectionEnum;
      UserDefinedTargetView  : OPTIONAL IfcLabel;
    DERIVE
      SELF\IfcGeometricRepresentationContext.WorldCoordinateSystem    : IfcAxis2Placement := ParentContext.WorldCoordinateSystem;
      SELF\IfcGeometricRepresentationContext.CoordinateSpaceDimension : IfcDimensionCount := ParentContext.CoordinateSpaceDimension;
      SELF\IfcGeometricRepresentationContext.TrueNorth                : IfcDirection := ParentContext.TrueNorth;
      SELF\IfcGeometricRepresentationContext.Precision                : REAL := ParentContext.Precision;
    WHERE
      WR31 : NOT('IFC2X2_FINAL.IFCGEOMETRICREPRESENTATIONSUBCONTEXT' IN TYPEOF(ParentContext));
      WR32 : (TargetView <> IfcGeometricProjectionEnum.USERDEFINED) OR 
             ((TargetView =  IfcGeometricProjectionEnum.USERDEFINED) AND 
             EXISTS(UserDefinedTargetView));
  END_ENTITY;

  FUNCTION IfcBuild2Axes
  (RefDirection : IfcDirection)
  	: LIST [2:2] OF IfcDirection;
    LOCAL
      D : IfcDirection := NVL(IfcNormalise(RefDirection),
          IfcRepresentationItem() || IfcGeometricRepresentationItem () || IfcDirection([1.0,0.0]));
    END_LOCAL;
      RETURN([D, IfcOrthogonalComplement(D)]);
  END_FUNCTION;

END_SCHEMA;

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