# Methodology for the Efficient Progressive Distribution and Visualization of 3D Building Objects
## 6 Conclusions  
This paper focused on the generalization and progressive visualization of three-dimensional (3D) city models. A multiple representation structure of four levels, denoted as BuildingTree, was specified. This structure is designed for the visualization of a city model at different levels of detail, using boundary representation for the most detailed levels and parameterized prototypes (constructive solid geometry (CSG) models) for the less detailed levels. We also developed a framework to derive a BuildingTree from a city model for the most detailed level using building cross-section detection, segmentation and an automatic prototype generation method. The methodology was implemented based on open standards (CityGML, X3D, etc.) and tools. The experimental results indicate that the proposed framework can be implemented on mainstream web browsers based on online 3D city model  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Frames per Second** & **Size (KB)** & **Loading Time (ms)** \\ \hline Original LOD3 & Null & 48,000 & unloadable \\ LOD3 ES & 3.22 & 2183 & 35 \\ CLOD(CSG) & 3.76 & 77 & 15 \\ LOD1 & 7.22 & 22 & 15 \\ BuildingTree & 5.66 & 397 (without LOD3 ES models) & 15 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Test results of the generalized 3D city models.  
visualization without plugins. Furthermore, the experimental results confirm the data compression rate of BuildingTree, as well as progressive data transformation visualization. A reason for the latter was the comparatively small data volume due to the use of parameterized prototypes to represent the buildings. Also, it was observed that the current Frame per Second (FPS) improvement is not as great as the data volume deduction. Therefore, for progressive data transmitted city models, a 3D rendering engine which supports basic prototypes is essential for visualization efficiency. In future studies, we will try to support more prototypes to deal with complex buildings and improve the visualization efficiency by selecting or updating a rendering engine which is optimized for predefined prototypes.  
This work was supported by National Science Foundation of China (41671457), Natural Science Foundation of Jiangsu (BK20151551), Natural Science Foundation of the Higher Education Institutions of Jiangsu (16 KJA170003), Jiangsu Province Postdoctoral Research Funding (1402120C), National Key Technologies R&D Program of China (2015 BAD18B02 and 2015 BAK36B02), National Center for International Joint Research on E-Business Information Processing (2013B01035), China Special Fund for Grain-Scientific Research in the Public Interest (201513004) and Project of the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Key Laboratory of Geological Information Technology, Ministry of Land and Resources, Beijing and Lund University.  
[PERSON] and [PERSON] conceived and designed the experiments; [PERSON] performed the experiments; [PERSON] and [PERSON] analyzed the data; [PERSON] wrote the paper; [PERSON] revised the paper.  
The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.