International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, V
ORB. An ORB is the software bus that connects all the objects
in the system. It enables objects to transparently make requests
to-and to receive responses from-other objects located locally
or remotely. The client is not aware of the mechanisms used to
communicate with, active, or store the server object. The
following figure illustrates the primary components in the
CORBA ORB architecture.
Object Implementation
Client
pud
LA
Dynaraic
Invocation
Skeleton
Dyraraic
Skeleton
Object
Adapter
ORB Core
Figure 1. CORBA ORB Architecture
2.2 CORBA-based Distributed Architecture
In this paper, the system architecture is designed on the basis of
the three-tiered Browser/Server pattern (See Figure 2).
Compared with the other distributed architectures of Web GIS,
such as J2EE * *DCOM, This architecture has several
advantages:
Browser Web Server
Java Applet i HTTP Server
(3D City
Scene) %
;
y
A ORB LIB
1
X
; I
1 1 :
i
rr" N CORBA
ORB LIB Connection Server
d — |- Pool
ee À
ol XXXV, Part B4. Istanbul 2004
a) It consists of four components: browser side, Web
server. application server and the data server, which can
improve the system's efficiency and stability.
b) The 3D city applet is downloaded from web server
and can communicate with CORBA server by ORB
gateway. Using this method, the limited that applet only
communication with the Web server will be resolved.
c) The CORBA server can allocate the computer tasks
to different computer servers in order to deal with load
balancing problems.
The key module of the system is the application server.
According to the bounding of the request city data, the
requested city data will be obtained from spatial database. Also
according the distance of viewpoint and the angle of view, this
application server will decide to use one of the LOD building
model and image resolutions.
In order to handle multi-users operation and network analysis,
a connection pool is designed using multi-threads technology.
The connection pool has some active connection threads. The
number of the active connection threads can be configured. If a
request is coming and the connection pool also has an active
connection thread, the request will be allocated an active
connection thread to respond the operation. Otherwise, the
request will activate a new connection thread to hand the
operation. If the operation finished, the connection thread
related to him will be a free connection thread. There is also a
database buffer to improve the speed of the data request from
spatial database.
B Image ere
= Server
UJ
= ORB LIB
2
zl CORBA Services
5
City Eae
Server
ORB LIB
Figure.2 Distributed Web Architecture Based on CORBA
3. LOD BUILDING MODEL
In recent years, the development in 3D city model is
- very rapid, especially in the fields of 3D data model
and 3D data structure, There are so many 3D city
models, which can be used to reconstruction the 3D city
landscape. (Wang. X H, Gruen A, 1998» Sun min, Chen Jun"
1999» Li Qingquan® Yang Bisheng 2000).But the data model
and data structure of these 3D city models are complex, it will
use many image data as building’s texture. If using there 3D
city models to visualization of Web-based 3D city
environments, it will take so long time to download the 3D city
data from server side and construct the 3D city landscape at
browser side.
In the 3D city model, the mainly model is 3D building model,
therefore, the 3D building model must be developed which is
suited to implement web-based 3D city visualization. It well
known that the LOD technique is useful to improve the speed
of operating large 3D city data set. Therefore, the data structure
of LOD building model is presented as follow (see figure 3):
In this LOD building model, the buildings can be decomposed
into five basic shapes: the roof, the bottom of buildings. the
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