# 8.20.3.12 IfcOpenShell

## 8.20.3.12.1 Semantic definition

The shell is defined by a collection of faces, which may be oriented faces. The sense of each face, after taking account of the orientation, shall agree with the shell normal as defined below. The orientation can be supplied directly as a BOOLEAN attribute of an oriented face, or be defaulted to TRUE if the shell member is a face without the orientation attribute.

The following combinatorial restrictions on open shells and geometrical restrictions on their domains are designed, together with the informal propositions, to ensure that any domain associated with an open shell is an orientable manifold. > * Each face reference shall be unique. * An open shell shall have at least one face. * A given face may exist in more than one open shell.

The boundary of an open shell consists of the edges that are referenced only once by the face - bounds (loops) of its faces, together with all of their vertices. The domain of an open shell, if present, contains all edges and vertices of its faces.

Informal Propositions:

1. Every edge shall be referenced exactly twice by the face bounds 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.

## 8.20.3.12.4 Formal representation

ENTITY IfcOpenShell
SUBTYPE OF (IfcConnectedFaceSet);
END_ENTITY;