IfcSectionedSpine

IfcSectionedSpine

Definition from ISO/DIS 10303-42-ed2:1999: A sectioned spine is a representation of the shape of a three dimensional object composed of a spine curve and a number of planar cross sections. The shape is defined between the first element of cross sections and the last element of this set.

NOTE A sectioned spine may be used to represent a surface or a solid but the interpolation of the shape between the cross-sections is not defined. For the representation of a solid all cross-sections are closed curves.

Definition from IAI: A sectioned spine (IfcSectionedSpine) is a representation of the shape of a three dimensional object composed by a number of planar cross sections, and a spine curve. The shape is defined between the first element of cross sections and the last element of the cross sections. A sectioned spine may be used to represent a surface or a solid but the interpolation of the shape between the cross sections is not defined.

For the representation of a solid all cross sections are areas. For representation of a surface all cross sections are curves. The cross sections are defined as profiles, whereas the consecutive profiles may be derived by a transformation of the start profile or the previous consecutive profile.

The spine curve shall be of type IfcCompositeCurve, each of its segments (IfcCompositeCurveSegment) shall correspond to the part between exactly two consecutive cross-sections.

New: The IfcSectionedSpine combines the functionality of the previous entities IfcAttDrivenTaperedExtrudedSegment, IfcAttDrivenMorphedExtrudedSegment, IfcAttDrivenTaperedRevolvedSegment, IfcAttDrivenMorphedRevolvedSegment.

NOTE Corresponding STEP entity: sectioned spine. Please refer to ISO/DIS 10303-42-ed2:1999, p. 282 for the definition of the formal standard. The cross sections are defined in IFC as IfcProfileDef. The position coordinate systems are added.

HISTORY New entity in IFC Release 2x. The definition has changed after version 2x beta 1.
ISSUE: See issues and change log for changes made in IFC Release 2x.

Example of an IfcSectionedSpine

The SpineCurve is given by an IfcCompositeCurve with two Segments. The Segments[1] has a ParentCurve of type IfcPolyline and a Transition = CONTSAMEGRADIENT. The Segments[2] has a ParentCurve of type IfcTrimmedCurve and a Transition = DISCONTINUOUS.
Each CrossSectionPosition lies at a start or end point of the Segments.
Each CrossSections are inserted by the CrossSectionPositions. The first two cross sections are of type IfcRectangleProfileDef, the third is of type IfcDerivedProfileDef.

Final result of the IfcSectionedSpine. The body (shown transparant) is not fully defined by the exchange definition.

Informal propositions

non of the cross sections, after being placed by the cross section positions, shall intersect
non of the cross sections, after being placed by the cross section positions, shall lie in the same plane
the local origin of each cross section position shall lie at the beginning or end of a composite curve segment.

EXPRESS specification:

ENTITY IfcSectionedSpine

SUBTYPE OF (

IfcGeometricRepresentationItem);

SpineCurve

IfcCompositeCurve;

CrossSections

LIST [2:?] OF IfcProfileDef;

CrossSectionPositions

LIST [2:?] OF IfcAxis2Placement3D;

DERIVE

Dim

IfcDimensionCount := 3;

WHERE

WR1	:	SIZEOF(CrossSections) = SIZEOF(CrossSectionPositions);
WR2	:	SIZEOF(QUERY(temp <* CrossSections \| CrossSections[1].ProfileType <> temp.ProfileType)) = 0;
WR3	:	SpineCurve.Dim = 3;

END_ENTITY;

Attribute definitions:

SpineCurve	:	A single composite curve, that defines the spine curve. Each of the composite curve segments correspond to the part between two cross-sections.
CrossSections	:	A list of at least two cross sections, each defined within the xy plane of the position coordinate system of the cross section. The position coordinate system is given by the corresponding list CrossSectionPositions.
CrossSectionPositions	:	Position coordinate systems for the cross sections that form the sectioned spine. The profiles defining the cross sections are positioned within the xy plane of the corresponding position coordinate system.
Dim	:	The dimensionality of the spine curve is always 3.

Formal Propositions:

WR1	:	The set of cross sections and the set of cross section positions shall be of the same size.
WR2	:	The profile type (either AREA or CURVE) shall be consistent within the list of the profiles defining the cross sections.
WR3	:	The curve entity which is the underlying spine curve shall have the dimensionality of 3.