# An Indoor Space Model of Building Considering Multi-Type Segmentation
## 3 Indoor Space Extension Model Based on IFC
### Entity Extension  
Due to the diversity of components and spatial complexity of indoor scenes, it is necessary to perfect the spatial entity types of the current IFC4 standard to meet more complex indoor space expression and information exchange. The granularity of indoor space objects proposed in this work can be divided into five levels: building, floor, household, room, and functional subspace. Establishing the mapping relationship between the indoor space in this article and the entities in IFC, it can be seen that there are some indoor space objects that cannot use the entity description provided in IFC, as shown in Table 2.  
The IfcSpatialStructureElement class defines the common attributes of all spatial structure units in IFC. It is often used to organize indoor spatial structures. It includes four hierarchical units: IfcSite, IfcBuilding, IfcBuildingStorey, and IfcSpace. IfcSite is a defined area of land in IFC4, on which the building project construction is to be completed.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Indoor Space Objects** & **IFC Spatial Structure Entities** \\ \hline Build space & IfcBuilding \\ Floor space & IfcBuildingStorey \\ Household space & / \\ \hline \multirow{3}{*}{Room space} & Single-floor space & IfcSpace \\ \cline{1-1}  & Split-level space & / \\ \cline{1-1}  & Cross-floor space & / \\ \hline \hline \end{tabular}
\end{table}
Table 2: Mapping relationship between indoor space objects and entities defined in IFC.  
Figure 8: Classification of indoor functional subspaces.  
This paper focuses on the multi-level indoor space framework based on the IFC standard. Therefore, the IfcSite element is not mentioned specifically. As well, the mapping relationship between indoor space objects in this study and IFC spatial structure entities are shown in the table. Among them, the single space (IfcSpace) does not distinguish between single-floor space, split-level space, and cross-floor space. At the same time, household space and indoor functional subspace objects cannot adopt the description of space objects provided in IFC.  
The method of new entity extension based on the current release of IFC mainly depends on the IfcProxy class. The extension based on proxy entity uses the existing IfcProxy entity in IFC to extend the space model. The IfcProxy is an instantiable entity type which is intended to be a kind of container for wrapping objects. According to the needs of different users, the entity is instantiated, and the newly defined entity information is described by the attribute Name, Tag, ProxyType, and attribute set, so as to extend the IFC data model. Therefore, this article adopts the agent entity-based extension method to expand the indoor space.  
The IFC standard uses the EXPRESS language (ISO 10303-11:2004) to describe the entity. The example of the EXPRESS definition of the indoor functional subspace extended by the IFCProxy entity is as follows:  
**ENTITY** IfcProxy  
**SUBTYPE OF** IfcProduct;  
PredefinedType: **OPTIONAL** IfcSubSpaceEnum;  
BoundedBy: **OPTIONAL** IfcReSpaceBoundary;  
ElevationOfFloor: **OPTIONAL** IfcLengthMeasure;  
**END_ENTITY**;  
**TYPE** IfcSubSpaceEnum = ENUMERATION OF (  
RECEPTIONAREA,  
STORAGEAREA,  
SLEEPAREA,  
USERDEFINED,  
NOTDEFINED);  
**END_TYPE**;