The Key Technologies and Solutions to 3D Digital City Visualization

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The Key Technologies and Solutions to 3D Digital City Visualization

Kevin J.L. Chen, David X.F. Jiang
Supresoft Inc. (Beijing), P.R.China.
Jlchen@supresoft.com.cn Davidjiang@supresoft.com.cn

ABSTRACT
The technologies for 3D Digital City Construction is becoming increasingly popular in recent years. Supresoft automatic technologies on 3D Digital City Construction facilitate fast 3D Digital City Construction in a cost-effective way. This paper firstly analyzed the characteristic and application of digital aerial photo and review the key technologies utilized for DEM generation, DOM (Digital Orthoimage) production, 3D City Model fast construction, and online 3D fast visualization. Secondly it introduced one of the critical integrated technologies on 3D Digital City construction - automatic fast building facade texture acquisition and processing. Finally, detail of Supresoft total solution on implantation of Digital City is introduced.

INTRODUCTION
“3D Digital City Model” is not only a hot topic in academic research [Debeve, et al., 1996; Cipolla, et al, 1999], but also plays an increasingly important role in urban development and management, especially in city planning, construction and risk assessment. The implementation of 3D Digital City Model requires the combination of digital photogrammetry and 3D virtual reality technologies. This paper discusses the technologies developed by SUPRESOFT for 3D Digital City Model construction.

2. 3D CITY MODEL CONSTRUCTIONS
The 3D virtual reality solution includes 3D Digital City Model construction and 3D visualization The model is constructed based on the true horizontal and vertical geo-coordinate of the city features, such as building, road, hills, etc. Generally, two steps are needed to construction the model: data acquisition and model set up.

Digital photogrammetry system developed from analogue photogrammetry system and analytical photogrammetry system is the most efficient way for acquiring data from terrain and building in three dimensions. The computer automatically does a considerable amount of man labor, such as image orientation, image matching, and DEM extraction. However, in practical applications, building boundary can only be measured manually in digital photogrammetry system. Even though many algorithms have been developed for building extraction from aerial imagery or airborne laser scanning altimetry data (Alharthy, 2004; Oda, 2004), the results are not yet satisfied due to low detection accuracy. (Schenk, 2000).

In recent years, satellite imageries acquired from SPOT5, QuickBird, and IKONOS and the digital aerial photography taken by DMC, VEXCEL is taking place of film-based photos. The digital aerial photo gains popularity because of its fast image development, high resolution and non-loss for data copy. In the image processing of aerial photo, GCP and stereo pairs are needed to generate DEM model or ortho-rectification. For satellite image processing the parameters of rcp are applied for exterior orientation. The area is usually overlapped by a pair of stereoscopic images in film based photogrammetry or satellite imagery. However in digital aerial photogrammetry the area is overlapped by more than one pair of stereoscopic images. Therefore some digital photogrammetry systems take advantages of multi-overlapped aerial photo for aerial triangulation. This innovative technology improves the accuracy of the output results.

The geospatial information such as DEM, DOM and DLG generated by photogrammetric technologies has speed up the process of 3D city model construction. Technology developed by SUPRESOFT is able to automatically generate 3D building polygon model provided sufficient geo-coordinate parameters, to acquire building texture quickly using digital video camera and render the texture onto the polygon models. The typical output of 3D digital city model consists of terrain represented by overlapping digital ortho-image (DOM) with digital elevation model (DEM), and the buildings with true texture on the terrain, showed in figure 1.

Fig.1 (a) building without texture

Fig.1 (b) building with texture
Fig.1 3D model overlay on DEM

The 3D city model is geo-referenced. Therefore, it is possible to perform measurement in the 3D Model with acceptable accuracy. It is very useful for city planning purposes. The city planner is able to design the target area taking account rich information from the nearby region. Fig.2 is building height and silhouette analysis of a region in Shanghai city, China.

Fig.2 Building height and silhouette analysis of a Region in Shanghai City, China

3. BUILDING FAÇADE TAXTURE
ACQUISITIONS
Building texture is indispensable in the city scenes visualization. The traditional technique is to acquire the building texture by hand-held digital camera. And paste it onto the building facade manually. This process used to be one of the most labor-intensive tasks. SUPRESOFT technology offers an alternative way to acquire building façade texture with digital video camera. It reduces a considerable amount of time in data acquisition compare with digital photography, while the rich information of texture is preserved in the video frame. Correction of the lens and geometry distortion is required for the digital video records:

a. Camera calibration to eliminate lens distortion
b. Removal of occlusion of tree
Two criteria are adopted to detect the vegetation occlusion. The first is using the color similarity of trees. Hue value in the range of 80-200 is counted as vegetation pixels. The second one calculates the density of lines with regular orientation in a

Fig.3 Occlusion Removal Result (Zhang, et al., 2004)

predefined grid. The grid of low-density value are consider as vegetation region because the vegetation has irregular lines. [Zhang, et al., 2004]
c. Building façade texture is ortho-rectified by calibrating DC angular parameters. Then they are mosaic by selecting mosaic point determined by correlation coefficient and geometric constraint. [Zhang, et al., 2004] Finally Tone adjustment is applied using algorithm of maximum intensity. The building texture is rendered onto the building model providing a realistic view. One example is shown in Fig4. The image mosaic and tone adjustment is implemented with ImageXuit RS technologies developed by SUPRESOFT. It is specialized for remote sensing image processing with accurate and efficient algorithm, and it takes a leading position in the industry.

4. VISUALIZATION OF 3D CITY
The output of Digital City Model usually has a data capacity of Giga bytes. Therefore data compression technique is essential to allow fast roaming and management of the data in a medium performance PC or through the Internet. 3D Browser developed by SUPRESOFT is able to tackle this challenge. It is implemented by using multilevel texture swap, multi-level geometric model and TILE (Data Management Tile).

The roaming of 3D scenes through the web is implemented by video streaming technologies, scenes real time reconstruction and lossless compression. The feature is characterized as below:

Fig.4 (a) real scene

Fig.4 (b) building models

Fig.4 an example of building model

a. The system adopts symmetric compression method the compression ratio can be as high as 20:1.

b. Scenes roaming based on LOD (level of detail) technology and real time model reconstruction. The graph can be displayed from raw to detail, avoiding the long time buffering.

c. The data transfer based on streaming technologies together with multi-thread download technology and encryption algorithm to enable a fast and secure communication between client and server.

d. The data access of Web3D utilized B/S three level system. It doesn’t require high performance hardware configuration, allowing user with different band-wide to view the quality-comprised scenes in real time.

e. Users are predefined to have different level of data access permission.
f. This 3D roaming and data management functionalities are encapsulated as ArcMap objects, which can be easily embedded into the other GIS systems. Fig.5 is an example of 3D roaming and data management.

Fig. 5 an example of 3D roaming and management
system developed by SUPRESOFT used in
Tsinghua University for land and construction management

5. SUPRESOFT’S TOTAL SOLUTIONS
SUPRESOFT developed total solutions to fulfill the requirement of 3D digital city model implementation. These include: remote sensing image processing, full digital photogrammetry systems for data generation, 3D GIS for 3D city model construction and visualization. The solution is shown in figure 6:

VirtuoZo – flagship product of SUPRESOFT is a highly automatic and integrated spatial data processing system. It is able to perform aerial triangulation and generate DLG, DEM and DOM. It has the following features:

Fig.6 Supresoft’s total solution

a. The advanced matching algorithms: bridge mode image matching and global image matching, utilizing a relaxation method. This allows a fast image matching of 1000 points/sec on a normal PC.

b. Highly automatic process for image orientation, DEM extraction and DOM generation

c. Able to handle high-resolution satellite data such as SPOT5, IKONOS, QUICKBIRD, etc, digital aerial photography such as DMC, VEXCEL, ADS40 etc, and close range photogrammetry data, which is well demonstrated in the reconstruction and modeling of the great Buddha statue in Bamiyan, Afghanistan. [Gruen et al., 2003]

d. Support various format of Data, i.e. Soccet Set, Imagestation, Albany etc. And seamless embedded into microstation and Autocad.

IMAGIS is 3D Visualization GIS developed by Supresoft Inc. It has the feature as follows:

a. Support various common 2D/3D data exchange, easy to exchange data with other systems

b. Provide powerful 3D entity modeling tools to create 3D model according to the requirements;

c. Provide SQL Server, Oracle database interface, manage attribute data with them

d. Provide building façade texture fast construction technology; reduce the amount of time and man effort in 3D city model set up. MagiXity - one of the modules of IMAGIS, is a professional 3D modeling and visualization system based for 3D digital model construction. The main functions are shown in figure 7.

Fig.7 The main functionalities of IMAGIS fro 3D digital model construction (Supresoft Inc., 2003)

Developed by Supresoft, 3Dbrowser is an ideal system to realize 3D landscape creation and real time roaming for huge amount of image data on PCs. 3Dbrowser, one of the IMAGIS product module, is a powerful and flexible tool for real time roaming of huge amount of 3D landscape data. 3Dbrowser’s another key advantage is real time query of attributes in 3D roaming environment.

Web3D, developed by Supresoft Inc., is a data browse platform based on web, in which the 3D scene can be shared and browsed via Internet, shown in figure 8. With the functions of these products, 3D digital city model can be set-up with acceptable man-effort and visualized in real-time on a normal PC. These products can also be widely applied in many application fields such as: fundamental surveying and mapping, urban planning, land resources management, military survey, satellite remote sensing, railroads, highways, irrigation works, electric power and other energy applications, environmental protection, agriculture, forestry, oceanography, telecom applications etc.

Fig.8 Data share model of Web3D

6. CONCLUSION
The implementation of 3D digital city model is practicable with integrated technology of remote sensing image processing, digital photogrammetry systems, and 3D visualization GIS. It is demonstrated that the digital video camera provides an efficient way for building texture acquisition. And tune adjustment and mosaic technique is able to generate seamless building texture close to the reality. However, the automatic building boundary extraction technology is still not applicable, due to the low detection accuracy. Technologies for automatic building boundary extraction need further development in the future 3D digital city research to reduce the man-effort in building such models.

7. REFERENCES

Alharthy, A., and Bethel, J., (2004), Detailed Building Reconstruction from Airborne Laser Data Using a Moving Surface Method, XXth ISPRS Congress, 3 commissions
Cipolla.R. Robertson. D.P and Boyer.E.G. Photobuilder, (June) (1999). 3D models of architectural scenes from uncalibrated images. Proc. IEEE International Conference on Multimedia
Computing and Systems, Firenze, volume I, pp.25-31. Debevec.P Taylor.C.J Malik.J., (1996). Modeling and Rendering Architecture from Photographs: A Hybrid Geometry- and Image-Based Approach, Proc. Of SIGGRAPH 96.
Gruen, A., Remondino, F., Zhang, L., (2003): Image-based Reconstruction and Modeling of the Great Budda Statue in Bamiyan, Afghanistan. International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol. XXXIV Part 5/W12.
MagiXity Manual, (2003), SUPRESOFT INC. Oda, K., Takano, T., Doihara. T., Shibasaki, R., (2004), Automatic Building Extraction and 3-D City Modelling From Lidar Data Based on Hough Transformation, XXth ISPRS Congress, 3 commissions
Schenk, T., (2000). Object Recognition in Digital Photogrammetry. The Photogrammetric Record, XVI(95), pp. 743-759.