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IfcPolyLoop

Definition from ISO/CD 10303-42:1992: A poly loop is a loop with straight edges bounding a planar region in space. A poly loop is a loop of genus 1 where the loop is represented by an ordered coplanar collection of points forming the vertices of the loop. The loop is composed of straight line segments joining a point in the collection to the succeeding point in the collection. The closing segment is from the last to the first point in the collection. The direction of the loop is in the direction of the line segments.

NOTE This entity exists primarily to facilitate the efficient communication of faceted B-rep models.

A poly loop shall conform to the following topological constraints:

- The loop has the genus of one.
- Equation (4) shall be satisfied

NOTE: Corresponding STEP entity: poly_loop. Please refer to ISO/IS 10303-42:1994, p. 138 for the final definition of the formal standard. Due to the general IFC model specification rule not to use multiple inheritance, the subtype relationship to geometric_representation_item is not included. The derived attribute Dim has been added at this level.

HISTORY: New class in IFC Release 1.0

Informal propositions:

  1. All the points in the polygon defining the poly loop shall be coplanar.

EXPRESS specification:

ENTITY IfcPolyLoop
SUBTYPE OF ( IfcLoop);
Polygon  :  LIST [3:?] OF UNIQUE IfcCartesianPoint;
WHERE
WR21  :  SIZEOF(QUERY(Temp <* Polygon | Temp.Dim <> Polygon[1].Dim)) = 0;
END_ENTITY;

Attribute definitions:

Polygon  :  List of points defining the loop. There are no repeated points in the list.

Formal Propositions:

WR21  :  The space dimensionality of all Points shall be the same.

Inheritance graph

ENTITY IfcPolyLoop;
ENTITY IfcRepresentationItem;
ENTITY IfcTopologicalRepresentationItem;
ENTITY IfcLoop;
ENTITY IfcPolyLoop;
Polygon  :  LIST [3:?] OF UNIQUE IfcCartesianPoint;
END_ENTITY;