Definition from ISO/CD 10303-42:1992: An open shell
(IfcOpenShell) is a shell of the dimensionality 2. Its domain, if present, is a
finite, connected, oriented, 2-manifold with boundary, but is not a closed
surface. It can be thought of as a closed shell with one or more holes punched
in it. The domain of an open shell satisfies 0 < X < 1. An open shell is functionally more general than
a face because its domain can have handles.
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.
NOTE Note that this is slightly different from the
definition of a face domain, which includes none of its bounds. For example, a
face domain may exclude an isolated point or line segment. An open shell domain
may not. (See the algorithm for computing below.)
Definition from IAI: In the current IFC Release only poly loops
(IfcPolyLoop) are defined for bounds of face bound (IfcFaceBound). This will
allow for faceted B-rep only. For further specification, including the Euler
formulars to be satisfied, please refer to ISO 10303-42:1994.
NOTE Corresponding STEP entity:
open_shell, please refer to ISO/IS 10303-42:1994, p.148 for the final
definition of the formal standard.
HISTORY New class in IFC Release 2.x
Informal propositions:
- Every edge shall be referenced exactly twice by the face bounds of
the face.
- Each oriented edge shall be unique.
- No edge shall be referenced by more than two faces.
- Distinct faces of the shell do not intersect, but may share edges or
vertices.
- Distinct edges do not intersect but may share vertices.
- Each face reference shall be unique.
- 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.
- The closed shell shall be an oriented arcwise connected 2-manifold.
- The Euler equation shall be satisfied. Note: Please refer to ISO/IS
10303-42:1994, p.148 for the equation.
