CIPA 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
In order to solve for the issues described in previous section,
a VRML split-browser has been developed. Its working is
based on a computer network model, where a central
processor (the server) provides different kind of services to a
number of hosts (clients). In practice, this kind of viewer is
obtained by application of a client-server model to a generic
VRML viewer. As shown in figure 2, the proposed model
clearly distinguishes between server-side and client-side
tasks. The server manages the whole virtual environment
computing new scene views, processing user’s commands
and executing scripts or animations. From the client-side, an
interactive GUI is provided to the user by which the virtual
views sent form the server are displayed. Moreover, the user
can interact with the server by means of a number of
navigation commands available on the client’s GUI. After an
initial setup phase, needed to negotiate the parameters of the
communication protocol (e.g. maximum throughput accepted
by the client), a bidirectional communication loop begins
between server and client. In practice, the user activates a
command in the client’s GUI, through mouse and/or
keyboard. Such command is transmitted to the server as
event, i.e. as a request of scene change. Then the server
intreprets the event and, if possible, sends back to the client
the new view, which will displayed on the user’s GUI. The
described loop is repeated according to the user interaction.
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T 7
Graphical
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VRML
Model
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Figure2: server/client-based VRML viewer architecture
This client-server approach shows following advantages:
1) The amount of data sent to the client is greatly reduced
respect with the transmission of the whole 3D model in
VRML format. Indeed the server sends to the client an
image sequence only: adopting an image compression
technique, the throughput could be further reduced;
2) The client doesn’t need high computational capabilities,
as the world management step is fully performed on the
server;
3) Since server and client are independent each other, the
LOD (Level of Detail) of each scene can be interactively
changed by the client;
4) The system features a certain degree of portability, as
the source codes can be modified accordingly to the
Operative System on which the server-side and the
client-side applications should run.
5) The copyright on the 3D model is keeped, as the user
will never receive on his PC the whole VRML model,
but only a set of images corresponding to the actual
scene displayed on the monitor screen;
6) The user’s GUI is managed by the client only,
independently from the server. In this way a higher
effectiveness of the whole system is ensured, since the
server-side of the viewer doesn’t need to dedicate
computing resources for the user’s GUI management.
In order to allow the client to navigate the 3D model as fluent
as possible, regardless the size and complexity of the model
and client’s computing capabilities, above mentioned features
of our proposed split-browser are still not sufficient. An
image compression technique has to be implemented to get
an optimized client/server communication protocol. To this
aim two different compression algorithms have been
investigated, the LZ77 and the JPEG, comparing the results
of their applications to our split-browser transmitting simple
geometric 3D shapes. However obtained results revealed a
limited compression capability (higher for the LZ77 respect
with the JPEG) for our goals. This can be explained
considering that those algorithms are “general purpose”: they
can be applied to whatever kind of file, regardless its content.
Therefore we developed an “ad hoc” compression technique
which would exploit the “knowledge” about the context
where it should be applied. The description of this technique
will be the topic of the following section.
4. THE FRAME COMPRESSION ALGORITHM
The alternative procedure we developed in order to further
reduce the throughput between server and client is based on
the application of projective transformations. The underlying
idea is that subsequent views, processed by the server and
displayed by the client to the user, are joined each other
through such kind of transformations. Since the knowledge
about the geometry of the virtual environment (e.g. eye-point
position, viewing direction, etc) is needed, projective
transformations don’t allow to preserve the same level of
flexibility and versatility as LZ77 and JPEG image
compression algorithms. Therefore our method can be
applied to specific cases only, where geometry related images
are handled. As shown in figure 3, the server/client communi
cation procedure works as follows:
Server-side operations
1) Given the user input (scene change) at time n, the server
processes the request generating the corresponding new
view of the 3D model (view B).
2) As the geometrical parameters describing the 3D scene
at time n-I and n are known, the server can determine
the projective transformation, mapping view A to view
B. Computed parameters are then sent to the client.
3) The view A, describing the 3D scene at time n-1, is
applied the previous computed projective transforma
tion. Consequently, a predicted view is generated, which
should represent a good approximation ov view B. the
prediction is generated by transforming the reference
frame (view A) on the basis of the displacement of the
user point of view (known from the selected command
of the GUI) and the distance from the image pixel
projection plane (known from the content of the Z
buffer).
