ISPRS Int. J. Geo-Inf. 2018, 7, 393 3 of 12
Most recently, Cesium offered BIM to the 3D Tiles conversion service for the community.
The converted 3D Tiles are hosted on Cesium servers and are exposed as a URL from access.
Still, the conversion process is a black box to users, as it does not offer any control options to customise
the 3D Tiles generation. More importantly, this is an infeasible solution when the BIM data are sensitive
to the restricted public access requirement (which is usually the case), meaning the conversion has to
be performed in-house.
2.2. Mago3D (IFC)
Mago3D, created by Gaia3D (Seoul, South Korea), can seamlessly integrate AEC (Architecture,
Engineering, and Construction) models and 3D GIS using Cesium [
16
]. It comes with an open-sourced
tool called F4DConverter for converting popular 3D model formats (including .ifc, .3ds, .obj, and .dae)
into F4D format, which was devised for Mago3D. This converter splits an original 3D model data
into smaller-sized models divided by octrees, applies the Net Surface Mesh (NSM) method on each
octree to reduce data size, and creates rougher data [
16
]. When rendering 3D scenes (Figure 2 shows
an example), compared with the 3D Tiles format advocated by Cesium, the F4D format does not yield
better performance or visual effect in Mago3D. The LOD (Level of Detail) controlled by NSM generates
many surface mapping glitches and conflicts and weigh down the user experience.
ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW 3 of 12
Most recently, Cesium offered BIM to the 3D Tiles conversion service for the community. The
converted 3D Tiles are hosted on Cesium servers and are exposed as a URL from access. Still, the
conversion process is a black box to users, as it does not offer any control options to customise the 3D
Tiles generation. More importantly, this is an infeasible solution when the BIM data are sensitive to
the restricted public access requirement (which is usually the case), meaning the conversion has to be
performed in-house.
2.2. Mago3D (IFC)
Mago3D, created by Gaia3D (Seoul, South Korea), can seamlessly integrate AEC (Architecture,
Engineering, and Construction) models and 3D GIS using Cesium [16]. It comes with an open-
sourced tool called F4DConverter for converting popular 3D model formats (including .ifc, .3ds, .obj,
and .dae) into F4D format, which was devised for Mago3D. This converter splits an original 3D model
data into smaller-sized models divided by octrees, applies the Net Surface Mesh (NSM) method on
each octree to reduce data size, and creates rougher data [16]. When rendering 3D scenes (Figure 2
shows an example), compared with the 3D Tiles format advocated by Cesium, the F4D format does
not yield better performance or visual effect in Mago3D. The LOD (Level of Detail) controlled by
NSM generates many surface mapping glitches and conflicts and weigh down the user experience.
Figure 2. IFC model converted into F4D format and rendered in Mago3D using Cesium.
3. Proposed Conversion Approach
This work aimed to explore a feasible and transparent solution for porting the IFC BIM into
Cesium. The 3D Tiles format is most attractive for three reasons: (1) it is created and advocated by
the Cesium core development team and the Cesium engine has been actively and continuously
maintained and optimised to support this format; (2) it is built upon the prevalent glTF™ (GL
Transmission Format), which is designed for the efficient transmission and loading of 3D scenes and
models by applications. The glTF minimises both the size of 3D assets and the runtime processing
needed to unpack and use those assets; (3) currently, it is a proposed OGC (Open Geospatial
Consortium) Community Standard. Given these advantages, 3D Tiles was selected as the target
format of the BIM conversion.
The proposed conversion approach from IFC BIM to 3D Tiles consists of four key steps: IFC
decomposition, IFC to OBJ conversion, OBJ to glTF conversion, and glTF to b3dm (Batched 3D Model,
which is a data format defined in 3D Tiles specification) conversion. The first three steps break the
BIM into constituent components and convert each component, stored as a small IFC file, to OBJ
format and then to glTF format. The last step merges the glTF files to a single constituent or multiple
b3dm files containing a batch table hierarchy with a tileset.json file, which collectively defines 3D
Tiles. Figure 3 shows the overall conversion workflow. Multiple open-source tools and libraries
(highlighted in yellow in Figure 3) were used during the conversion (Appendix A).
Figure 2. IFC model converted into F4D format and rendered in Mago3D using Cesium.
3. Proposed Conversion Approach
This work aimed to explore a feasible and transparent solution for porting the IFC BIM into
Cesium. The 3D Tiles format is most attractive for three reasons: (1) it is created and advocated by the
Cesium core development team and the Cesium engine has been actively and continuously maintained
and optimised to support this format; (2) it is built upon the prevalent glTF
™
(GL Transmission
Format), which is designed for the efficient transmission and loading of 3D scenes and models by
applications. The glTF minimises both the size of 3D assets and the runtime processing needed
to unpack and use those assets; (3) currently, it is a proposed OGC (Open Geospatial Consortium)
Community Standard. Given these advantages, 3D Tiles was selected as the target format of the
BIM conversion.
The proposed conversion approach from IFC BIM to 3D Tiles consists of four key steps:
IFC decomposition, IFC to OBJ conversion, OBJ to glTF conversion, and glTF to b3dm (Batched
3D Model, which is a data format defined in 3D Tiles specification) conversion. The first three steps
break the BIM into constituent components and convert each component, stored as a small IFC file,
to OBJ format and then to glTF format. The last step merges the glTF files to a single constituent or
multiple b3dm files containing a batch table hierarchy with a tileset.json file, which collectively defines