8.9.3.75 IfcTrimmedCurve

8.9.3.75.1 Semantic definition

An IfcTrimmedCurve is a bounded curve that is trimmed at both ends. The trimming points may be provided by a Cartesian point or by a parameter value, based on the parameterization of the BasisCurve. The SenseAgreement attribute indicates whether the direction of the IfcTrimmedCurve agrees with or is opposed to the direction of the BasisCurve.

curve parameterization — Figure 8.9.3.75.A shows the four arcs (dashed blue and green lines with arrow showing different orientations) that can be defined by the same *BasisCurve* (of type *IfcCircle*) and the same trimming points (given by Cartesian points and parameter values) by using different assignments to *Trim1* and *Trim2* and *SenseAgreement*.

At least one of these shall be specified at each end of the curve. The SenseAgreement makes it possible to unambiguously define any segment of a closed curve such as a circle. The combinations of sense and ordered end points make it possible to define four distinct directed segments connecting two different points on a circle or other closed curve. For this purpose cyclic properties of the parameter range are assumed; for example, 370 degrees is equivalent to 10 degrees.

The IfcTrimmedCurve has a parameterization which is inherited from the particular basis curve reference. More precisely the parameter s of the trimmed curve is derived from the parameter of the basis curve as follows:

if SenseAgreement is TRUE: s = t - t~1~

if SenseAgreement is FALSE: s = t~2~ - t

In the above equations t~1~ is the value given by Trim1 or the parameter value corresponding to point 1 and t~2~ is the value given by Trim2 or the parameter value corresponding to point 2. The resultant IfcTrimmedCurve has a parameter ranging from 0 at the first trimming point to |t~2~ - t~1~| at the second trimming point.

Informal Propositions:

Where both the parameter value and the Cartesian point exist for Trim1 and Trim2 they shall be consistent. (i.e., the BasisCurve evaluated at the parameter value shall coincide with the specified point).
When a Cartesian point is specified by Trim1 or by Trim2 it shall lie on the BasisCurve.
Except the case of a closed BasisCurve where both parameter 1 and parameter 2 exist, they shall be consistent with the sense flag, i.e., (sense = parameter 1 < parameter 2). Or, for every open curve where both parameter 1 and parameter 2 exist, they shall be consistent with the SenseAgreement, i.e., SenseAgreement = (parameter 1 < parameter 2).
If both parameter 1 and parameter 2 exist, then parameter 1 <> parameter 2. For a closed base curve, e.g. IfcCircle or IfcEllipse, this also applies to the cyclic properties, as 360' is equal to 0', parameter 1 = 360' and parameter 2 = 0' are treated as being equal and therefore violating this proposition.
When a parameter value is specified by Trim1 or Trim2 it shall lie within the parametric range of the BasisCurve.

8.9.3.75.2 Entity inheritance

8.9.3.75.3 Attributes

#	Attribute	Type	Description
IfcRepresentationItem (2)
	LayerAssignment	SET [0:1] OF IfcPresentationLayerAssignment FOR AssignedItems	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.
	StyledByItem	SET [0:1] OF IfcStyledItem FOR Item	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.
IfcCurve (1)
*	Dim	IfcDimensionCount	This attribute is formally derived. `IfcCurveDim(SELF)` The space dimensionality of this abstract class, defined differently for all subtypes, i.e. for IfcLine, IfcConic and IfcBoundedCurve.
Click to show 3 hidden inherited attributes Click to hide 3 inherited attributes
IfcTrimmedCurve (5)
1	BasisCurve	IfcCurve	The curve to be trimmed. For curves with multiple representations any parameter values given as Trim1 or Trim2 refer to the master representation of the BasisCurve only.
2	Trim1	SET [1:2] OF IfcTrimmingSelect	The first trimming point which may be specified as a Cartesian point, as a real parameter or both.
3	Trim2	SET [1:2] OF IfcTrimmingSelect	The second trimming point which may be specified as a Cartesian point, as a real parameter or both.
4	SenseAgreement	IfcBoolean	Flag to indicate whether the direction of the trimmed curve agrees with or is opposed to the direction of the basis curve.
5	MasterRepresentation	IfcTrimmingPreference	Where both parameter and point are present at either end of the curve this indicates the preferred form.

Table 8.9.3.75.B

8.9.3.75.4 Formal propositions

Name	Description
NoTrimOfBoundedCurves	Already bounded curves shall not be trimmed.
`NOT('IFC4X3.IFCBOUNDEDCURVE' IN TYPEOF(BasisCurve))`
Trim1ValuesConsistent	Either a single value is specified for Trim1, or the two trimming values are of different type (point and parameter)
`(HIINDEX(Trim1) = 1) OR (TYPEOF(Trim1[1]) <> TYPEOF(Trim1[2]))`
Trim2ValuesConsistent	Either a single value is specified for Trim2, or the two trimming values are of different type (point and parameter)
`(HIINDEX(Trim2) = 1) OR (TYPEOF(Trim2[1]) <> TYPEOF(Trim2[2]))`

Table 8.9.3.75.C

8.9.3.75.5 Formal representation

ENTITY IfcTrimmedCurve
 SUBTYPE OF (IfcBoundedCurve);
	BasisCurve : IfcCurve;
	Trim1 : SET [1:2] OF IfcTrimmingSelect;
	Trim2 : SET [1:2] OF IfcTrimmingSelect;
	SenseAgreement : IfcBoolean;
	MasterRepresentation : IfcTrimmingPreference;
 WHERE
	NoTrimOfBoundedCurves : NOT('IFC4X3.IFCBOUNDEDCURVE' IN TYPEOF(BasisCurve));
	Trim1ValuesConsistent : (HIINDEX(Trim1) = 1) OR (TYPEOF(Trim1[1]) <> TYPEOF(Trim1[2]));
	Trim2ValuesConsistent : (HIINDEX(Trim2) = 1) OR (TYPEOF(Trim2[1]) <> TYPEOF(Trim2[2]));
END_ENTITY;