IfcSurfaceCurve

BACnet MVD

#### IfcSurfaceCurve

Natural language names

Change log

Item | SPF | XML | Change | Description
--- | --- | --- | --- | ---
IFC4 Addendum 2
IfcSurfaceCurve |  |  | ADDED |

##### Semantic definitions at the entity

Entity definition

An IfcSurfaceCurve is a 3-dimensional curve that has additional representations provided by one or two pcurves.

> NOTE Definition according to ISO/CD 10303-42:1992
> A surface curve is a type of curve, that is a curve on a surface. The curve is represented as a curve (curve_3d) in three-dimensional space and possibly as a curve, corresponding to a pcurve, in the two-dimensional parametric space of a surface. The ability of this curve to reference a list of 1 or 2 pcurves enables this entity to define either a curve on a single surface, or an intersection curve which has two distinct surface associations. A `seam' on a closed surface can also be represented by this entity; in this case each associated_geometry will be a pcurve lying on the same surface. Each pcurve shall be parametrised to have the same sense as curve_3d. The surface curve takes its parametrisation directly from either curve_3d or pcurve as indicated by the attribute master representation.

> NOTE Entity adapted from surface_curve defined in ISO 10303-42.

> HISTORY New entity in IFC4 Add2.

Informal Propositions:

1. Where Curve3D and one or two pcurves exist they shall represent the same mathematical point set. (i.e., they shall coincide geometrically but may differ in parametrisation.)
2. The Curve3D and any associated pcurves shall agree with respect to their senses.

Attribute definitions

# | Attribute | Type | Cardinality | Description | B
--- | --- | --- | --- | --- | ---
1 | Curve3D | IfcCurve |  | The curve which is the three-dimensional representation of the surface curve. |  | X
2 | AssociatedGeometry | IfcPcurve | L[1:2] | A list of one or two pcurves which define the surface or surfaces associated with the surface curve. Two elements in this list indicate that the curve has two surface associations which need not be two distinct surfaces. Being a pcurve, it also associates a basis curve in the parameter space of this surface as an alternative representation of the surface curve. |  | X

3

MasterRepresentation

IfcPreferredSurfaceCurveRepresentation

The defines the curve used to determine the unique parametrisation of the IfcSurfaceCurve.
The master_representation takes one of the values Curve3D, PCurve_S1 or PCurve_S2 to indicate a preference for the 3D curve, or the first or second pcurve, in the associated geometry list, respectively. Multiple representations provide the ability to communicate data in more than one form, even though the data is expected to be geometrically identical.
NOTE The master representation attribute acknowledges the impracticality of ensuring that multiple forms are indeed identical and allows the indication of a preferred form. This would probably be determined by the creator of the data. All characteristics, such as parametrisation, domain, and results of evaluation, for an entity having multiple representations, are derived from the master representation. Any use of the other representations is a compromise for practical considerations.

X

| BasisSurface :=IfcGetBasisSurface(SELF) | IfcSurface | S[1:2] | The surface, or surfaces on which the IfcSurfaceCurve lies. This is determined from the AssociatedGeometry list. |  | X

Formal Propositions

Rule | Description
--- | ---
CurveIs3D | The Curve2D shall be defined in three-dimensional space.
CurveIsNotPcurve | The Curve3D shall not be a pcurve.

##### Inherited definitions from supertypes

Entity inheritance
Attribute inheritance

# | Attribute | Type | Cardinality | Description | B
--- | --- | --- | --- | --- | ---
IfcRepresentationItem

LayerAssignment

IfcPresentationLayerAssignment
@AssignedItems

S[0:1]

Assignment of the representation item to a single or multiple layer(s). The LayerAssignments can override a LayerAssignments of the IfcRepresentation it is used within the list of Items.

> IFC2x3 CHANGE The inverse attribute LayerAssignments has been added.

> IFC4 CHANGE The inverse attribute LayerAssignment has been restricted to max 1. Upward compatibility for file based exchange is guaranteed.

X

StyledByItem

IfcStyledItem
@Item

S[0:1]

Reference to the IfcStyledItem that provides presentation information to the representation, e.g. a curve style, including colour and thickness to a geometric curve.

> IFC2x3 CHANGE The inverse attribute StyledByItem has been added.

X

X

IfcGeometricRepresentationItem
IfcCurve
| Dim :=IfcCurveDim(SELF) | IfcDimensionCount |  | The space dimensionality of this abstract class, defined differently for all subtypes, i.e. for IfcLine, IfcConic and IfcBoundedCurve. | X | X
IfcSurfaceCurve
1 | Curve3D | IfcCurve |  | The curve which is the three-dimensional representation of the surface curve. |  | X
2 | AssociatedGeometry | IfcPcurve | L[1:2] | A list of one or two pcurves which define the surface or surfaces associated with the surface curve. Two elements in this list indicate that the curve has two surface associations which need not be two distinct surfaces. Being a pcurve, it also associates a basis curve in the parameter space of this surface as an alternative representation of the surface curve. |  | X

3

MasterRepresentation

IfcPreferredSurfaceCurveRepresentation

The defines the curve used to determine the unique parametrisation of the IfcSurfaceCurve.
The master_representation takes one of the values Curve3D, PCurve_S1 or PCurve_S2 to indicate a preference for the 3D curve, or the first or second pcurve, in the associated geometry list, respectively. Multiple representations provide the ability to communicate data in more than one form, even though the data is expected to be geometrically identical.
NOTE The master representation attribute acknowledges the impracticality of ensuring that multiple forms are indeed identical and allows the indication of a preferred form. This would probably be determined by the creator of the data. All characteristics, such as parametrisation, domain, and results of evaluation, for an entity having multiple representations, are derived from the master representation. Any use of the other representations is a compromise for practical considerations.

X

| BasisSurface :=IfcGetBasisSurface(SELF) | IfcSurface | S[1:2] | The surface, or surfaces on which the IfcSurfaceCurve lies. This is determined from the AssociatedGeometry list. |  | X

##### Formal representations

XML Specification
`<xs:element name="IfcSurfaceCurve" type="ifc:IfcSurfaceCurve" substitutionGroup="ifc:IfcCurve" nillable="true"/>
<xs:complexType name="IfcSurfaceCurve">
<xs:complexContent>
<xs:extension base="ifc:IfcCurve">
<xs:sequence>
<xs:element name="Curve3D" type="ifc:IfcCurve" nillable="true"/>
<xs:element name="AssociatedGeometry">
<xs:complexType>
<xs:sequence>
<xs:element ref="ifc:IfcPcurve" maxOccurs="2"/>
</xs:sequence>
<xs:attribute ref="ifc:itemType" fixed="ifc:IfcPcurve"/>
<xs:attribute ref="ifc:cType" fixed="list"/>
<xs:attribute ref="ifc:arraySize" use="optional"/>
</xs:complexType>
</xs:element>
</xs:sequence>
<xs:attribute name="MasterRepresentation" type="ifc:IfcPreferredSurfaceCurveRepresentation" use="optional"/>
</xs:extension>
</xs:complexContent>
</xs:complexType>
`
EXPRESS Specification
`ENTITY IfcSurfaceCurve
SUPERTYPE OF(ONEOF(IfcIntersectionCurve, IfcSeamCurve))
SUBTYPE OF (IfcCurve);
Curve3D : IfcCurve;
AssociatedGeometry : LIST [1:2] OF IfcPcurve;
MasterRepresentation : IfcPreferredSurfaceCurveRepresentation;
DERIVE
BasisSurface : SET [1:2] OF IfcSurface := IfcGetBasisSurface(SELF);
WHERE
CurveIs3D : Curve3D.Dim = 3;
CurveIsNotPcurve : NOT ('IFCGEOMETRYRESOURCE.IfcPcurve' IN TYPEOF(Curve3D));
END_ENTITY;

EXPRESS-G diagram

`

References: IfcCurveOnSurface

Link to this page