IfcVector是一个既有大小又有方向的几何表示项。矢量的大小仅由“幅值”属性定义，方向仅由“方向”属性定义。
注意“方向”属性的“方向比率”不用于定义幅值。
注：定义根据ISO/CD 10303-42:1992
矢量是根据矢量的方向和大小定义的。“幅值”属性的值定义向量的幅值。从方向属性的分量计算矢量的大小是不可靠的。选择这种表示形式是为了减少数值不稳定的问题。例如，大小为2.0 mm且与坐标轴相等倾斜的向量可以用方向属性（1.0,1.0,1.0）表示。
注：实体改编自ISO 10303-42中定义的矢量。
IFC1.0中增加的新实体
Formal Propositions
| Rule | Description |
|---|---|
| MagGreaterOrEqualZero | The magnitude shall be positive or zero. |
| # | 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 | |||||
| IfcVector | |||||
| 1 | Orientation | IfcDirection | [1:1] | The direction of the vector. | X |
| 2 | Magnitude | IfcLengthMeasure | [1:1] | The magnitude of the vector. All vectors of Magnitude 0.0 are regarded as equal in value regardless of the orientation attribute. | X |
| Dim :=Orientation.Dim |
IfcDimensionCount | [1:1] | The space dimensionality of this class, it is derived from Orientation | X | |
EXPRESS Specification
ENTITY IfcVector
SUBTYPE OF (IfcGeometricRepresentationItem);
Orientation : IfcDirection;
Magnitude : IfcLengthMeasure;
DERIVE
Dim : IfcDimensionCount := Orientation.Dim;
WHERE
MagGreaterOrEqualZero : Magnitude >= 0.0;
END_ENTITY;
原文：https://www.cnblogs.com/herd/p/13974508.html