Visualization over the World Wide Web

Visualization over the World Wide Web
Ken Brodlie
School of Computer Studies
University of Leeds
Leeds, UK
E-mail: kwb@
Abstract
The Web has grown from being a global information repository into a worldwide distributed computing environment. This offers the opportunity to carry out data visualization as a Web-based application. In this paper we look at the different players involved in the creation of a Web-based visualization service, and hence build a reference model for Web-based visualization. We then use this model to identify three distinct system architectures for Web-based visualization. We illustrate each architecture with corresponding examples of visualization services.
1. Introduction
Undoubtedly the most significant development in computing in the last decade has been the emergence of the World Wide Web. The Web began as a publishing medium, with hypertext links used to connect documents at various locations on the Internet. The Web has now grown into a powerful infrastructure for distributed computing, with the Web browser as the ‘universal’ user interface. In this paper, we shall examine how the Web can be used as an environment for scientific visualization.
From an early stage, the Web has been used by scientists to publish the results of their research. These Web documents typically included 2D images (in GIF, JPEG and latterly PNG formats), captured from the output of some visualization system. The next development was a realisation that publication of 3D model representations of the visualization allowed the reader the opportunity to view the model from different angles, and generally provided a more exciting presentation format. The key enabling technology was the Virtual Reality Modelling Language, VRML, which is now the ISO standard for 3D graphics transfer across the Internet [8]. Leading visualization systems such as IRIS Explorer [4], AVS [11] and IBM Data Explorer (now Open Visualization Data Explorer) [1] have begun to provide modules which output VRML, thus making this form of publishing available ‘off-the-shelf’. Walton [18] describes the use of IRIS Explorer in this context; Jern [9] similarly describes the use of AVS.
Thus, scientists having completed their research, can use visualization and the Web to present their results to the world - this is visualization used in a very important dissemination role. However as the Web extends from being simply a publishing medium to being a powerful computing environment, so interest grows in using visualization and the Web in an analysis role. That is, scientists themselves carry out data visualization over the Web: from a Web browser, a dataset is specified, a visualization system selected, and - somehow - a visualization returned to the browser.
This paper focuses on visualization in this analysis context. There are exciting opportunities for the visualization community to provide Web visualization tools - with a widening market. Certainly, as just mentioned, the scientist and engineer - traditional users of visualization - will wish to carry out data analysis via the Web. But there is a much bigger community - the general public - for whom tools need to be developed. Already there are significant collections of numerical data on the Web: stock market data and meteorological data are just two examples. There is likely to be an increasing need for ‘visual interfaces’ to these large data repositories. The challenge for the visualization community is to determine how best to provide these tools.
In this paper, we begin by identifying the major players in a Web visualization service, and develop a reference model that allows us to compare and contrast different approaches. These approaches differ in their allocation of tasks to the client and server in the Web environment. We then use this model as a framework to describe and position some of the Web visualization systems that have been recently developed. Finally we look at some possible future developments, including means of
supporting collaborative visualization in the Web context.
2. A Simple Reference Model for Web Visualization
Who are the players then in a Web visualization service?In this section, we try to characterise the players and develop a simple reference model to help us understand their roles, and the possible system architectures for Web visualization. Two players are always present: the User and the Visualization Service Provider. In addition,there is a third player who may or may not be present: the Data Provider.
Player 1 : The User
This may be the specialist scientist or engineer, or just a member of the general public. The range of skill and experience can therefore be quite varied - in all cases, we can assume that they are familiar with a Web browser; in
international repository).
it will be visualized. There are three levels of service:• full service : in which the visualization is entirely created by the Service Provider, and returned as an image or 3D model to the User;
• software delivery : in which the software to create the visualization is downloaded to the User, to be executed by them;
• data only : in which the visualization software is assumed already resident with the User, and only data needs to be delivered. The role here of the Visualization Service Provider is managerial.
Player 3: The Data Provider
We have seen that the User may supply their own data.Alternatively the Service Provider might operate in a managerial role and provide a link to an external data provider. Typically this would be some organisation
providing a service by collecting and publishing data of particular interest (for example, the UK Atomic Energy Authority collect air quality statistics and publish these on the Web [2])
From this analysis, we can develop a simple schematic model of scientific visualization over the Web, as illustrated in Figure 1. The three players are placed at different locations on the Internet: ‘The User’ is the client; ‘The Visualization Service Provider’ acts as the server; ‘The Data Provider’ is logically at a third location, although as noted above, this player is not always present. The User runs a Web browser; the Visualization Service Provider hosts a Web page for the service. The parameters in the model are:
• visualization software : which can be provided by the Visualization Service, or by the User;
• compute power : which again can be provided by the Visualization Service or the User;
• data : which may be provided by an independent over the Web
3. Architectures
We see now how different arrangements for these parameters lead to different system architectures for Web visualization facilities. It is convenient to use as classification the different levels provided by the Visualization Service Provider, as described above.These three levels - full service, software delivery, and data only - map conveniently onto three different client-server architectures.
3.1 Full Service: Server-based Solution
In this architecture, the User downloads the service page, and enters (typically on a form) the location of the data, and a ‘recipe’ for the visualization. The Visualization Service Provider has responsibility for retrieving the data, executing the visualization, and delivering it back to the client. This can be regarded as a server-based solution - as illustrated in Figure 2.
internet
USER
web browser with VRML plug-in
VISUALIZATION SERVICE
html web pages
software compute power
DATA PROVIDER
data
Figure 2 - Full Service
An early example of this approach is the IRIS Explorer Web Visualization Service, developed by Wood, Brodlie and Wright [20]. This is shown schematically in Figure 3: the user enters the location of data as a URL, and specifies the technique to be used for visualization of the data; the form entries are processed by a CGI script running on the server; this generates a set of Skm commands (the IRIS Explorer scripting language) which execute IRIS Explorer, generating output as VRML for delivery back to the client. The skill requirement of the user is limited to navigation within a VRML browser; the
A demonstrator of this service has been created, to illustrate the air quality data provided by the Atomic Energy Authority as mentioned earlier. This data is collected hourly at a number of locations in the UK, and posted on the Web in numerical form. The demonstrator,shown in Figure 4, allows a user to select the site of interest, the pollutant, the time period, and the style of
visualization.
Figure 4 - Air Quality Visualization
Trapp and Pagendarm [15] have developed a very useful extension of the idea. Rather than the form-based interface, with its limited scope for interaction, they have created a Java-based user interface. Data can be loaded either from a URL as in the IRIS Explorer Web visualization service, or uploaded by the user themselves using a clipboard facility on the Java applet. The visualization facilities available are taken from the established HighEnd system [13].
Other examples have followed. The CurVis system [3],developed at the University of Rostock, is a specialist facility for 2D flow visualization. This returns 2D images rather than VRML, but an interesting feature is the choice of image compression technique offered - with
supporting information that reports transfer times from server to client.
Commercial visualization system suppliers are also developing products with a server-based architecture.One example is provided by the GSharp product from AVS [5]. The GSharp Web edition potentially allows a Data Provider to become a Visualization Service Provider: they install the GSharp Web Edition and allow access to their data via this software. IBM Open Visualization Data Explorer has also migrated to the Web [16], with thought given to issues such as geometry compression to enable the product to work well in a client-server setting.
3.2 Software Delivery: Server provides software,client provides compute power
This approach relies on Java technology to allow transport of software on demand, from server to client.The style of architecture is illustrated in Figure 5. The User downloads the service page, in which the location of the data and the style of visualization can be entered.A Java applet is then downloaded from the server in order to execute the visualization on the client. The location of the data is a major issue: security restrictions require that applets only read from files on the host from which they are downloaded. Thus an applet can only read data associated with the Visualization Service Provider. Fortunately there are solutions to this: the service provider can act as temporary host to the data for the period that the applet executes.
DATA PROVIDER
internet
USER
web browser with Java
compute power
VISUALIZATION SERVICE
html web pages
software
Figure 5 - Software Delivery .
One of the first examples of this style of working was developed at San Diego Supercomputer Centre by Michaels and Bailey [12]. This provides an applet for isosurface construction and rendering, and is illustrated in Figure 6. The solution to the data access security problem is to use the facility within HTML that allows uploads of files to the server: the user enters the filename on a form on the VizWiz page; the file contents are uploaded to the server where a CGI script receives them and generates a temporary file; this temporary file,residing on the same host as the applet, can then be accessed by the applet.
Figure 6 - VizWiz
Another example of this approach is the Java visualization service, developed at the University of Leeds by Kee [10], and illustrated schematically in Figure 7. This solves the data problem in a different way. It uses a ‘data server’ - a Java application - to fetch temporarily data across to the service provider, from any location on the Web. Java applications are not subject to the same security restrictions as Java applets. The end result is similar to VizWiz, but the use of a program to fetch the data offers additional flexibility. Firstly, data can be retrieved from any URL, not just the local filestore. Moreover, the server can be extended to include extra facilities: for example, Stanton [14] has
added a charging mechanism, which monitors access time to applets and charges accordingly; also, data
Wegenkittl and Groller [19], for the visualization of dynamical systems. This allows a user to enter a dynamical system as an algebraic expression, and to view the resulting vector fields using a variety of techniques,including new methods based on the Line Integral Convolution (LIC) method.
3.3 Data only: Client based solution
In this last approach, the responsibility for all processing falls on the client. The User downloads the
service page, and enters the location of the data. This is retrieved by the service provider, and wrapped in a MIME type that indicates the visualization application that should be called upon by the browser to process the data. This architecture is illustrated in Figure 8.
VISUALIZATION SERVICE html
web pages
USER
web browser with Vis software as helper appln
software compute power
DATA PROVIDER
data
internet
Figure 8 - Data Only
An example of this approach is the Web version of Vis-5D, developed at the University of Wisconsin by Hibbard [6], and illustrated in Figure 9.
Figure 9 - Vis-5D
A variation on this approach is possible with modular visualization systems: these can include a data reader which fetches data from a URL rather than (as normal) a local file. This is possible with both IRIS Explorer and AVS.
3.4 Evaluation
Each of the above approaches has its attractions, and its drawbacks.
Full service : The responsibility for providing the software (not only the licence, but also the skill in programming it) is off-loaded from user to service provider. The service provider can make the user interface accessible to novice users. The user simply needs to provide a browser. A concern might be the scalability: on the face of it, it seems an attractive approach to providing public service visualization - but too many simultaneous requests would impose a strain on the server. Another concern is the rigidity of this approach - the user has little flexibility to interact with the visualization running on the server. Intuitively there is concern that the data, and the processing of it, is rather remote from the user.
Software delivery : Here the scalability is better, because the processing is distributed to the client. Flexibility is also improved since a user can interact with the Java applet. It is reassuring that the data is local to the user and the visualization process. The concern here is the compute requirement on the client, and the network bandwidth requirement – to transfer the software initially, and (to circumvent the Java security restrictions)
to transfer data in a return trip to applet host and then back to user. This only makes sense for small datasets. Data only: This has the advantage of simplicity. Users already familiar with a visualization system can continue to use it, but accessing data over the Web. Data is processed locally, so interactivity is feasible. The disadvantage is that the user is required to provide the software and the skill to use it!
4. Conclusions and Future Work
This paper has set a general framework for visualization over the Web, allowing different approaches to be compared and contrasted. In particular, three classes of service by the Visualization Service Provider: full service, software delivery, and data only -are seen to correspond to different distributions of processing between Web client and Web server. VRML and Java are the key enabling technologies for this activity.
As we look to the future, we may expect to see at least two significant developments. The Internet is a powerful infrastructure for collaboration. Existing visualization systems such as IRIS Explorer are being extended to allow multi-user working, with several instances of the system running at different sites, but sharing data and process control [21]. Similarly work is beginning on collaborative visualization over the Web: specifically, the ‘full service’ approach of Wood et al has been extended to allow group working [22]. Users of the service can log their results in a data store, accessible to subsequent users of the service. This offers an asynchronous form of collaboration. For synchronous collaboration, Isenhour et al [7] have developed a prototype Java-based system called Sieve: this can be seen as an extension of the ‘software delivery’ approach to collaborative working.
Another likely development is extension of these ideas to use the Web as a framework for combined simulation and visualization. In the examples described above, the focus has been on data visualization - with an understanding that the data will have been generated by some other process. The advantage of linking simulation and visualization to allow computational steering has long been advocated, and we can expect developments soon in this direction. This would enable, for example, scientists to run computational fluid dynamics simulations from a Web browser, passing results automatically for visualization - a step towards using the Web as a ‘problem solving environment’.Acknowledgements
I should like to thank a number of colleagues and students who have worked closely with me in the area of Web visualization. Jason Wood developed the ‘full service’ IRIS Explorer Web service, and together with Helen Wright has been an invaluable source of ideas in all areas of this work. Abraham Kee, Peter Stanton and Edward Teong developed the ‘software delivery’ Java service described above. Thanks also to Stuart Lovegrove for his help with both VRML and Java.
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(See also /dx)
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