IfcShellBasedSurfaceModel通过一组开放或闭合的壳来表示形状。壳内的连接面具有维数2，并且放置在维度3的坐标空间中。
注：定义根据ISO/CD 10303-42:1992
基于壳的曲面模型由一组维数为2的开壳或闭壳来描述。除边缘和顶点外，壳体不得相交。特别是，不同的面可能不相交。一个壳的完整面可以与另一个壳共享。壳体的重合部分应参考定义重合区域的相同面、边和顶点。至少应有一个外壳。壳可以独立于曲面模型而存在。
注：实体改编自ISO 10303-42中定义的基于外壳的表面模型。
IFC2x中增加的新实体。
非正式提议：
●基于壳的曲面模型的维数为2。
●壳体不得重叠或相交，除非在公共面、边或顶点处。
| # | Attribute | Type | Cardinality | Description | C |
|---|---|---|---|---|---|
| IfcRepresentationItem | |||||
| LayerAssignment | IfcPresentationLayerAssignment @AssignedItems |
S[0:1] | Assignment 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. | X | |
| StyledByItem | IfcStyledItem @Item |
S[0:1] | Reference to the IfcStyledItem that provides presentation information to the representation, e.g. a curve style, including colour and thickness to a geometric curve. | X | |
| IfcGeometricRepresentationItem | |||||
| IfcShellBasedSurfaceModel | |||||
| 1 | SbsmBoundary | IfcShell | S[1:?] | X | |
| Dim :=3 |
IfcDimensionCount | [1:1] | The space dimensionality of this class, it is always 3. | X | |
EXPRESS Specification
ENTITY IfcShellBasedSurfaceModel
SUBTYPE OF (IfcGeometricRepresentationItem);
SbsmBoundary : SET [1:?] OF IfcShell;
DERIVE
Dim : IfcDimensionCount := 3;
END_ENTITY;
原文：https://www.cnblogs.com/herd/p/13432477.html