IfcRepresentationContext 与 IfcRepresentation
1. IfcRepresentationContext
IfcRepresentationContext 定义了成果（Product）的IfcRepresentation相关上下文。
The IfcRepresentationContext defines the context to which the IfcRepresentation of a product is related
representation上下文是representation项相关的上下文。
A representation context is a context in which a set of representation items are related
IfcRepresentationContext属性
| 项 | 类型 | 描述 |
|---|---|---|
| ContextIdentifier | IfcLabel | 项目中representation context可选的标识. |
| ContextType | IfcLabel | representation context类型描述. |
| RepresentationsInContext | IfcRepresentation@ContextOfItems S[0:?] | 同一个representation context中所定义的所有shape representations. |
1.1 IfcGeometricRepresentationContext
IfcGeometricRepresentationContext定义了针对项目中成果的 shape representations 应用的上下文
The IfcGeometricRepresentationContext defines the context that applies to several shape representations of products within a project.
内容：
- 定义了shape representation类型
It defines the type of the context in which the shape representation is defined
- 定义了数值精度
he numeric precision applicable to the geometric representation items defined in this context
- 定义了WorldCoordinateSystem 属性，可用于偏移从项目坐标系的全局原点偏移
In addition it can be used to offset the project coordinate system from a global point of origin, using the WorldCoordinateSystem attribute
- 提供真北方向
The main representation context may also provide the true north direction
关于the true north
示意图如下：
a.如果WorldCoordinateSystem 属性的y轴不指向全局北向，应该提供TrueNorth 属性
The TrueNorth attribute should be provided, if the y axis of the WorldCoordinateSystem does not point to the global northing
b.如果提供了HasCoordinateOperation 作为转换到地理坐标系统的变换，TrueNorth 仅仅作为一个信息展示不作他用。不会加到IfcCoordinateOperation任何变换运算中
If a conversion to a geographic coordinate system is included by virtue of the HasCoordinateOperation attribute, then the TrueNorth attribute shall be omitted or included only for informational purposes. It shall not be added to any transformation already applied by an IfcCoordinateOperation.
细节
当呈现model (3D) view时，IfcGeometricRepresentationContext是必须的；当呈现plan (2D) view 时是可选的（但是如果plan view是与比例尺相关的，就必须给定）。
The use of one instance of IfcGeometricRepresentationContext to represent the model (3D) view is mandatory, the use of a second instance of IfcGeometricRepresentationContext to represent the plan (2D) view is optional (but needs to be given, if there are scale dependent plan views)
示例：
1.2 IfcGeometricRepresentationSubContext
IfcGeometricRepresentationSubContext是 IfcGeometricRepresentationContext子上下文。共享IfcGeometricRepresentationContext的属性：WorldCoordinateSystem, CoordinateSpaceDimension, Precision， TrueNorth
IfcGeometricRepresentationSubContext defines the context that applies to several shape representations of a product being a sub context, sharing the WorldCoordinateSystem, CoordinateSpaceDimension, Precision and TrueNorth attributes with the parent IfcGeometricRepresentationContext.
内容：
IfcGeometricRepresentationSubContext用于区分【与representation view和比例尺相关的信息内容】的呈现形式。
The IfcGeometricRepresentationSubContext is used to define semantically distinguished representation types for different information content, dependent on the representation view and the target scale
可以控制最适合与此geometric representation的lod的上下文。
It can be used to control the level of detail of the shape representation that is most applicable to this geometric representation context.
可以控制在plot 视图中的IfcShapeRepresentation的形式。
In addition the sub context is used to control the later appearance of the IfcShapeRepresentation within a plot view.
注意：如果IfcShapeRepresentation所用的子上下文中，有与某一项相关的IfcStyledItem，IfcStyledItem所呈现的样式信息(例如IfcCurveStyle，IfcTextStyle)是需要被应用到plot中的。
NOTE If the IfcShapeRepresentation using this sub context has IfcStyledItem’s assigned to the Items, the presentation style information (e.g. IfcCurveStyle, IfcTextStyle) associated with the IfcStyledItem is given in target plot dimensions. For example, a line thickness (IfcCurveStyle.CurveWidth) is given by a thickness measure relating to the thickness for a plot within the (range of) target scale.
每个IfcProduct类型可能存在多种IfcRepresentation的子类型实例，每种子类型都对应到不同的IfcGeometricRepresentationSubContext的上下文。
Each IfcProduct can then have several instances of subtypes of IfcRepresentation, each being assigned to a different IfcGeometricRepresentationSubContext).
IfcGeometricRepresentationSubContext的ContextIdentifier 属性值 可以标识 IfcShapeRepresentation的RepresentationIdentifier属性。
The applicable values for the inherited ContextIdentifier attribute shall be identical to the RepresentationIdentifier attrubute defined at IfcShapeRepresentation.
2.IfcRepresentation
IfcRepresentation定义成果(Product)属性的表达方式和形状的表达方式
The IfcRepresentation defines the general concept of representing product properties and in particular the product shape.
针对每个IfcRepresentation，公有定义属性：
ContextOfItems：引用 IfcGeometricRepresentationContext 作为其表达上下文Reference to an IfcGeometricRepresentationContext as agreed for this representation.
RepresentationIdentifier：表达形式，例如Body代表3D形状，FootPrint代表2D平面视图，Axis代表参考轴
Name of the representation, for example, ‘Body’ for 3D shape, ‘FootPrint’ for 2D ground view, ‘Axis’ for reference axis.
RepresentationType：几何体、拓扑关系的构建表达类型，例如SweptSolid表示3D扫掠建模实体，Brep表示边界约束建模实体
Name for the geometric, or topological representation type, for example, ‘SweptSolid’ for 3D swept solids, ‘Brep’ for boundary representation.
2.1.IfcShapeRepresentation
IfcShapeRepresentation 是为了表达一个成果的几何呈现方式，或者与特别地几何呈现上下文相关的成果部件。
The IfcShapeRepresentation represents the concept of a particular geometric representation of a product or a product component within a specific geometric representation context.
RepresentationType 属性定义了用来表达形状呈现方式的几何建模方式（比如’SweptSolid’, 'Brep）
The inherited attribute RepresentationType is used to define the geometric model used for the shape representation (e.g. ‘SweptSolid’, or ‘Brep’).
RepresentationIdentifier 属性用于表达IfcShapeRepresentation 定义的呈现类别（比如’Axis’, ‘Body’）
the inherited attribute RepresentationIdentifier is used to denote the kind of the representation captured by the IfcShapeRepresentation (e.g. ‘Axis’, ‘Body’, etc.).
一些用于标识shape representation的呈现标志的值，属于RepresentationIdentifier属性预定义值的集合。
Several representation identifiers for shape representation are included as predefined values for RepresentationIdentifier.
一些用于定义shape representation的呈现类型的值，属于RepresentationType属性预定义值的集合
Several representation types for shape representation are included as predefined values for RepresentationType.
2.2. IfcRepresentationItem
IfcRepresentationItem 是 IfcRepresentation用于呈现IfcProductRepresentation的具体项。主要内容包含：
几何体或者拓扑实体表达项
实体表达样式信息
The IfcRepresentationItem is used within an IfcRepresentation (directly or indirectly through other IfcRepresentationItem’s) to represent an IfcProductRepresentation. Most commonly these IfcRepresentationItem’s are geometric or topological representation items, that can (but not need to) have presentation style infomation assigned.
关联样式可以通过 IfcStyledItem和IfcPresentationLayerWithStyle进行定义，如下图所示
可以通过IfcStyledItem的样式覆盖IfcPresentationLayerWithStyle的样式，如下图：
2.2.1.IfcGeometricRepresentationItem
IfcGeometricRepresentationItem 作为表达项的所有几何对象的基类
An IfcGeometricRepresentationItem is the common supertype of all geometric items used within a representation.
2.2.1.IfcMappedItem
IfcMappedItem 是源定义的插入实例（为了与block 、shared cell 、macro 定义进行比较），这个实例是被用于笛卡尔变换。
The IfcMappedItem is the inserted instance of a source definition (to be compared with a block / shared cell / macro definition). The instance is inserted by applying a Cartesian transformation operator as the MappingTarget.
