RESEARCH AND IMPLEMENTATION ON THE WEB3D VISUALIZATION
OF DIGTAL MOON BASED ON WEBGL
Yi Lian1 2 Long He1 Jinsong Ping2 Hu Zhang1 Xiaoming Zeng1 Chenglei Wang1 Lei Chen1
1.Collage of Urban And Environment Science Of Tianjin Normal University
2.The National Astronomical Observatories of the Chinese Academy of Sciences
ABSTRACT
Into the 21st century, the human exploration of the moon
increasingly frequent, China, the United States, Russia, India,
Japan and other countries have joined the ranks of lunar
exploration which provided a lot of data. And the platforms
of Digital Moon are rewritten based on third-party plug-ins,
which is poor in real-time, operability and interaction.
On this research the web platform of Digital Moon is
built based on WebGL specification for rendering 3D
graphics within any compatible web browser without the use
of plug-ins. So that the paltform is suitable for many kinds of
browers, and it is convenient to the public to understand the
moon and help the scientist to explore the universe.
Index Terms：Digtal Moon, Chang-E, Web GIS, WebGL
1. INTRODUCTION
The moon has longstanding questions such as lunar
environments,
origin,
formation
and
evolution,
magnetization of crustal rocks, internal structure and possible
life. Into the 21st century, the human exploration of the moon
increasingly frequent, the United States, Russia, India, Japan
and other countries have joined the ranks of lunar exploration.
The recent lunar missions, Selenological and Engineering
Explorer(SELENE),
Chandrayaan-1,
and
Lunar
Reconnaissance Orbiter/Lunar Crater Observation and
Sensing Satellite (LRO/LCROSS), have provided new data
and got some achievement about these issues.
There have also developed Digital Moon Platforms,
such as Google Earth, World Wind in foreign countries and
UniGlobal, IMAGIS in domestic market. But at present,
many of the three-dimensional GIS softwares are rewritten
based on third-party plug-ins, which is poor in real-time,
operability and interaction. The web platform of Digital
Moon is built on this research based on B/S architecture,
which is used of Javascript scripting language, and WebGL
specification for rendering 3D graphics within any
compatible web browser without the use of plug-ins. The
detection data obtained from Chang'E satellite would be
displayed in the platform with digital elevation model. At the
same time platform would enhance its real-time, operability
and interactivity and provides a more convenient way to use.
Besides, the paltform is suitable for many kinds of browers,
such as Google Chrome without the use of plug-ins, which is
978-1-5090-4951-6/17/$31.00 ©2017 IEEE
convenient to the public to understand the moon and help the
scientist to explore the universe.
2. DATA AND PROCESSING
With the development of lunar exploration in China,
scientific data resources of lunar exploration is gradually rich,
which has laid a solid foundation for the digital lunar research
and construction of digital lunar platform. "Chang'E-1" lunar
exploration satellite has completed the detection of full-moon
spherical terrain and texture data, including three-line CCD
stereo camera (CCD) ，which is mainly used to obtain threedimensional images of the lunar surface and draw lunar threedimensional images ; The laser altimeter (LAM) is mainly
used to obtain the data of the terrain elevation under the
satellite and provides the satellite relative lunar height data
for the adjustment of the CCD stereo camera working
parameters; The interferometric imaging spectrometer (IIM)
is mainly used to obtain the spectral information of the lunar
surface and analyze the material composition of the lunar
surface. In addition, there are gamma-ray spectrometer
(GRS), X-ray spectrometer (XRS), microwave detectors
(MRM), solar energy particle detector (HPD), solar wind ion
detector (SWID) and other detection data.
PDS (Planetary Data System) is a data storage standard[2]
proposed by NASA and widely used in international deep
space exploration field. Data products are generally
composed of a label and a data object and more complex data
products may have multiple independent data objects[3][4].
The data objects mainly refer to a series of images and tables.
Through the C # programming language to read the PDS data
header files, in order to obtain image-related spatial
information.
3. METHOD AND THEORY
3.1 WebGL Specification
The WebGL is an extension of HTML5 canvas element,
which is now widely used for developing web applications
requiring 3D visualization. It is a 3D graphics API, written in
low level language and is based on OpenGL ES 2.0. Besides,
the WebGL has a broader scope of application, and can play
rich client-edge features powerful and ease the pressure
effectively on the server side.
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The WebGL is a 3D drawing standard. Firstly, it uses
JavaScript script to achieve the production of Web interactive
3D animation without any browser plug-in support; Secondly,
it implements graphics rendering by taking advantage of the
underlying graphics hardware acceleration capabilities with a
unified, standard, cross-platform OpenGL interface. The
WebGL technology not only eliminates the need for the
development of Web page rendering plug-in trouble, but also
creates more complex 3D structure. As shown in the WebGL
technology rendering process[6] in Figure 1:
3.2 The Quadtree Structure
The organization of data directly affects the speed of
data operations. At present, there are regular grid, quadtree[7],
KD tree[8], KDB tree[9], BSP tree[10], and R tree[11] in the
organization and management of image data. The Quadtree,
as one of the most common spatial indexes, is easy to
implement and has better operability, and is more suitable for
mass data organization.
When using the quadtree detail level model, the terrain
is divided constantly into four regions of equal size from the
whole terrain. The deeper the segmentation is, the higher the
resolution is. The node accuracy of the layer is half that of the
next layer. Each node holds information about a certain area,
including latitude and longitude, the height of the center point,
the height of the edge node.
The level of detail (LOD) model is a graphics
generation acceleration method. It refers to the object in the
same scene, with different details of the description method
to get a set of models to choose for use when drawing. The
use of LOD model can reduce the pressure of network
transmission and computer, and realize the rendering of 3D
scene in real time. The LOD technology can achieve the
smooth display of 3D terrain when combined with
hierarchical data generated by quadtree structure of the digital
elevation model. The higher the LOD level, the higher the
resolution of the terrain data and the three-dimensional
display accuracy, and vice versa[12].
3.5 The 3D visualization Technology
Technology of View-driven three-dimensional transfer
allows users to see the three-dimensional scene is
continuous[13]. In general, the parcels around the viewpoint
are automatically loaded into the buffer, and the isoparms are
transferred when the viewpoint moves to that range. At the
same time, it determines the movement of the viewpoint and
locates it to the next parcel. In the process of dynamic
rendering, as the viewpoint moves, it is necessary to update
the data block in the data page. If the view height changes
during the move, it needs to recalculate the field of view
range and switch to the data layer of the response scale to
update the data.
3.3 The Algorithm and Rule of Rectangle Tiling
The Quadtree and vector tile maps can be organically
combined by tile numbering, enabling map slicing to be
dynamically invoked during map zooming and roaming
operations. Since these tile numbers have regularity, an index
relationship can be established based on tile numbers and
coordinates. Based on this index relationship, the required
geographic layer and geographical feature are located, and
the geographic data not within the display range is not read.
The Slice processing was performed by using a tile map
method of TMS (tile map service). According to its standard,
you can get through the formula to display tile index after
determining the latitude and longitude and the level of the
case. When you display the map, you can determine which
tiles you need to load based on the center coordinates and the
levels and bounds. And then an index relationship is
established between the number and coordinates. We can find
the other in the tile number and the latitude and longitude
interval. The specific formula is as follows:
n = 2zoom
log_deg+180
xtile =
×n
360
1
log(tan(lat_rad) + sec(lat_rad))
ytile = (1 −
)×n
2
π
Where, zoom is the level, lat_rad is the latitude of the
display area, lon_deg is the longitude of the display area, xtile,
ytile is the number index of the tile.
3.4 The Level of Detail Expressions
4 THE WEB PLATFORM OF DIGTAL MOON
4.1 The Architecture of Digtal Moon
In this paper, the realization of the data access and
transmission is mainly by JavaScript as a client-side
development technology, window built IIS as a WEB server.
As shown in the system architecture in Figure 2, the user can
operate the mouse or keyboard in a browser with embedded
WebGL to achieve the three-dimensional display of the moon
data. In the first step, the user action is transmitted to the
server through the network, and then the network server
processes the HTTP protocol and transmits the operation
requirement to the map server; In the second step, the map
server analyzes the user request, and then retrieves the data to
be analyzed from the database, and returns the processing
result to the network server; In the third step, the network
server follows network protocol and sends the results of the
data to the browser client to display.
4.2 The Projection Parameter of Digtal Moon
Projected coordinate system(D_MOON_2000): The
Moon-2000 coordinate system is the lunar coordinate system
for the IAU 2000 Cartesian Coordinate System (IAU,
International Astronomical Union)[5]. The coordinate system
of D-moon-2000 is the universal coordinate system in the
world lunar research field, which sets the spherical
parameters that the length and the short axis are 1737400m,
and the reference level deviation parameter is [0,0,0].
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Projection mode: According to the way of latitude and
longitude record used in the spatial information of 2C source
files, Geographic Lat/Lon latitude and longitude geography
projection is adopted in the whole month image stitching.
This projection method can preserve the relationship between
the original data and the spatial information as much as
possible, which also can realize the re-projection under
retaining the characteristics of the data accurately.
Map units: The degree is used as the map unit used by
the Geographic Lat/Lon projection. The spatial resolution of
the satellite under Chang'e-1 CCD camera is 120m and
0.003957346 Degree/Pix.
4.3 Web Publishing
As shown in Figure 3, the Digital Moon is published by
Internet Information Services(IIS) on this research. And the
web platform can display the 3D moon in the web browser
without the use of plug-ins. The Web context is characterized
by: a dynamic environment where the three-dimensional
display must be expressive; a large number of users and a
high rate of events to process; users can browse in any
browser according to their needs at any time; and distinct
users which may assess similar kinds of information at the
same time. The main characteristics of our system are:
(i)A simple interface for browse and operation,
supporting the HTTP protocol, which enables an easy
integration of the system in the Web. User can define event
types and attributes on the system and these modications are
immediately taken into account by the system;
(ii)A semi-structured event model which permits an easy
representation of the information published on the Web, and
is exible enough to support an easy integration of information
published by distinct publishers;
(iii)An efficient matching which allows the system to
handle a large number of operation (several millions of
operation).
4.4 System Testing
The system testing mainly includes two parts: Web
browser testing and Web server stress testing.
Web browser testing refers to testing its cross-browser
performance by running the system on different browsers. In
this paper, the effect testing is carried on the Google browser,
Firefox browser and IE browser. The results show that the
system in the three major browsers are released successfully
which is proved its cross-browser features.
Web server stress testing mainly refers to the use of HP
LoadRunner 12.53 Community Edition load testing software
to detect the load when the platform is running. The test
object is a three-dimensional data visualization scenario. The
number of virtual users is 50 and the duration is 12 minutes
and 57 seconds. All the services are passed through. The
server runs stably and the response time is fast enough to
satisfy the user's fast feeling time.
The test results show that the WebGL-based 3D data
visualization system can not only achieve good visualization
effect, but also show excellent cross-platform interactive
ability. At the same time it has a rapid response, normal
operation, high stability of the fine features.
5 CONCLUSIONS
China's lunar exploration project has made great
scientific and engineering achievements, while obtaining a
large number of different types of lunar exploration data,
which have significant scientific research value.
Based on the advantage of WebGL, the Digital Moon is
established, which is composed of quadtree data organization,
tile technology, LOD technology and 3D visualization
technology. And the paltform is convenient to the public to
understand the moon and help the scientist to explore the
universe.
6 ACKNOWLEDGEMENT
We gratefully thank Prof. Ping for his careful reading
and valuable suggestions. This work was supported by
National Science Foundation of China(41601348,41471314)
and the Foundation of Tianjin Normal University (52XB1502,
52XK1604, 52XB1617, 52XB1619, 52XQ1602).
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UniformVarible
Varying Varible
JavaScript
Vertex Shader
Element Rasterization
Array Object
The Processed Varying
Attribute
Variable
Depth mixing, testing and so on
Frame Buffer
Slice Shader
Processing Slices
Figure 1 The Rendering Flow Chart
Client Browser（Embedded WebGL）
Http Protocol
Presentation layer
Internet
The Web Server
Service layer
The Map Server
The Database
Figure 2
Data layer
The Logical structure of 3D lunar system
Figure 3 The web platform of Digtal Moon
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