IFC 4.3.x development

8.20.3.2 IfcClosedShell

8.20.3.2.1 Semantic definitions at the entity

NOTE  Definition according to ISO/CD 10303-42:1992
A closed shell is a shell of the dimensionality 2 which typically serves as a bound for a region in R3. A closed shell has no boundary, and has non-zero finite extent. If the shell has a domain with coordinate space R3, it divides that space into two connected regions, one finite and the other infinite. In this case, the topological normal of the shell is defined as being directed from the finite to the infinite region.

The shell is represented by a collection of faces. The domain of the shell, if present, contains all those faces, together with their bounds. Associated with each face in the shell is a logical value which indicates whether the face normal agrees with (TRUE) or is opposed to (FALSE) the shell normal. The logical value can be applied directly as a BOOLEAN attribute of an oriented face, or be defaulted to TRUE if the shell boundary attribute member is a face without the orientation attribute.

The combinatorial restrictions on closed shells and geometrical restrictions on their domains are designed to ensure that any domain associated with a closed shell is a closed, orientable manifold. The domain of a closed shell, if present, is a connected, closed, oriented 2-manifold. It is always topologically equivalent to an H-fold torus for some H ≥ 0. The number H is referred to as the surface genus of the shell. If a shell of genus H has a domain within coordinate space R^3^, then the finite region of space inside it is topologically equivalent to a solid ball with H tunnels drilled through it.

The Euler equation applies with B=0, because in this case there are no holes. As in the case of open shells, the surface genus H may not be known a priori, but shall be an integer ≥ 0. Thus a necessary, but not sufficient, condition for a well-formed closed shell is the following:

Image

NOTE  Entity adapted from closed_shell defined in ISO 10303-42.

HISTORY  New entity in IFC1.0

Informal Propositions:

  1. Every edge shall be referenced exactly twice by the loops of the face.
  2. Each oriented edge shall be unique.
  3. No edge shall be referenced by more than two faces.
  4. Distinct faces of the shell do not intersect, but may share edges or vertices.
  5. Distinct edges do not intersect but may share vertices.
  6. Each face reference shall be unique.
  7. The loops of the shell shall not be a mixture of poly loop and other loop types. Note: this is given, since only poly loop is defined as face bound definition.
  8. The closed shell shall be an oriented arcwise connected 2-manifold.
  9. The Euler equation shall be satisfied. Note: Please refer to ISO 10303-42 for the equation.

# Attribute Type Description
IfcRepresentationItem
LayerAssignmentSET [0:1] OF IfcPresentationLayerAssignment FOR AssignedItemsAssignment 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.
IfcTopologicalRepresentationItem
IfcConnectedFaceSet
1 CfsFaces SET [1:?] OF IfcFace The set of faces arcwise connected along common edges or vertices.

8.20.3.2.1 Entity inheritance

dot_inheritance IfcClosedShell IfcClosedShell IfcConnectedFaceSet IfcConnectedFaceSet IfcClosedShell->IfcConnectedFaceSet IfcOpenShell IfcOpenShell IfcOpenShell->IfcConnectedFaceSet IfcTopologicalRepresentationItem IfcTopologicalRepresentationItem IfcConnectedFaceSet->IfcTopologicalRepresentationItem IfcTopologicalRepresentationItem_children 6 more... IfcTopologicalRepresentationItem_children->IfcTopologicalRepresentationItem IfcRepresentationItem IfcRepresentationItem IfcTopologicalRepresentationItem->IfcRepresentationItem IfcRepresentationItem_children 3 more... IfcRepresentationItem_children->IfcRepresentationItem

8.20.3.2.2 Formal representations

ENTITY IfcClosedShell
 SUBTYPE OF (IfcConnectedFaceSet);
END_ENTITY;