vrml: today and tomorrow

Comput. 4 Graphics, Vol. 20, No. 3, pp. 427-434, 1996 Copyright 0 1996 Elswier Science Ltd

Printed in Great Britain. All rights n.wved 0097-8493/96 $15.00+0.00

Computer Graphics and the WWW

VRML: TODAY AND TOMORROW

WOLFGANG BROLL’ and TANJA KOOPZt

‘GMD-German National Research Center for Information Technology, Institute for Applied Information Technology, D-53754 Sankt Augustin, Germany

‘Computer Graphics Research Center (ZGDV), Wilhelminenstr. 7, D-64283 Darmstadt, Germany e.mail: [email protected]

Abstract-VRML (Virtual Reality Modeling Language) has already established itself as a standard for the exchange of 3-D descriptions on the Internet. In this paper we want to give an overview of the current state of VRML and show several areas, in which VRML will develop in the near future. A large number of people are involved in the development of the future VRML standard. We will give a general overview of the future capabilities of VRML without relying on any particular proposal. Copyright 0 1996 Elsevier Science Ltd

1. INTRODUCTION

Already a few months after the first Internet browsers were released, Virtual Reality Markup Language (VRML) is established as the standard 3-D format for the distribution of virtual worlds on the Internet. In its initial version it is still closely related to existing products. On the one hand the World Wide Web (WWW), which uses the same protocol (H’ITP [I]) to transmit data. On the other hand Open Inventor [2], since the initial draft specification was almost a subset of it.

However, the development of VRML has rapidly become independent from its foundations. Currently, VRML is still a static scene description language, which does not include interactive behavior. The aim of this paper is to show the current state of VRML and to introduce some of the key concepts for extending VRML in the future.

The idea of a virtual reality interface to the WWW was born and first discussed at the first WWW conference in Geneva, Switzerland, Spring 1994. There the term VRML was coined for the 3-D extension of the WWW because of the relationship to HTML. Since VRML enables the reproduction of work from an existing solution of 3-D scenes, the term was changed to Virtual Reality Modeling Language. After the conference a mailing list on VRML was established, which had an overwhelming number of participants in a short time. Mark Pesce announced a draft specification of VRML at the WWW fall conference. The VRML community decided to proceed from an already existing solution due to time pressures. A vote on the future features of VRML and the appropriate basis was held, where a majority voted for Silicon Graphics Open Inventor

+ Author for correspondence.

file format. Thus a subset of Open Inventor extended with network functionality became the main basis of VRML 1.0. After the fall conference Paul Strauss and Gavin Bell extracted the VRML parser library &Lib [3] from the Open Inventor source code, which enabled everybody to develop a VRML viewer. In spring 1995 at the third WWW conference in Darmstadt, Germany, the specification of VRML 1.0 [4] was presented and SGI introduced a first browser Webspace.

2. VRML 1.0

VRML is intended by its designers to be the standard language for exchanging virtual worlds (3-D scenes) including interaction possibilities and multi-user abilities via the Internet [5]. Therefore the main efforts were laid on the development of a platform independent, extensible and over low bandwidth connections transferrable description language. The first version of VRML, VRML 1 .O, meets only a subset of these demands but has the capacity to be extended to full capability in the near future. VRML 1.0 allows the creation of a static 3-D scene with limited interaction possibilities. Using a VRML viewer, it is possible to navigate freely through the scene and to follow hyperlinks to new 3-D worlds, HTML documents or other valid MIME types by selecting linked objects within the 3-D scene.

2.1. Viewers To make 3-D worlds described by VRML visible,

an additional application, the viewer or browser, that interprets the VRML data, is necessary. VRML is currently transferred over a network, using the HTTP protocol. VRML pages are usually accessed from World Wide Web pages. There are three different approaches to how this can be realized:

l helper applications l stand-alone applications

421

428 W. Broll and T. Koop

l integrated applications.

A helper application can only be used in combination with another application. The helper application then provides additional functionality. In the case of a VRML viewer, the viewer itself would be the helper application started by a Web browser such as Netscape Navigator. In contrast a stand-alone application does not need any support from another application. A stand-alone VRML viewer is able to display VRML files without the assistance of a Web browser. These viewers can not only interpret VRML syntax but are also fully capable of the network protocol (see Plate 1). In both cases it is necessary to configure the Web browser so that the VRML viewer is launched. The integrated application approach has the advantage that the VRML viewer is completely embedded within the main application (e.g. the Web browser). In this case no additional configuration of the Web browser by the user is necessary.

Meanwhile a large number of VRML viewers supporting most hardware and software platforms are available. These viewers provide different user interfaces for browsing and navigation through the

3-D scene. Most of them give the user the choice between several examination and navigation modes, e.g. walking and flying. Additionally they usually allow control over render speed and image quality. Up to now it is only possible to activate hypedinks to other worlds, HTML documents or other valid MIME types. Future enhancements of VRML will also allow browsers to provide a much richer and more flexible interface.

2.2. The scene graph VRML describes a 3-D scene as a graph of~objects.

These objects are called nodes. The scene graph consists of nodes arranged in a hierarchical structure. Displaying the scene is done by traversing the scene graph from top to bottom and left to right.

A state mechanism is used to influence nodes traversed later on. This state can be manipulated by properties such as material settings. Special mechanisms are defined to prevent unforeseen effects on nodes and to enable the separation of functionally independent parts of the scene graph.

Example 1 shows a simple VRML file, representing a blue cube.

Plate 1. Silicon Graphic’s VRML viewer Webspace showing parts of the city of Damutadt.

VRML: today and tomorrow

#VRML Vl .O ascii

Separtor

429

htatrial { diffuseColor 0 0 1

Cube { width 3 height 3 depth 3

1 vkwwhhe I

name ~‘hnp:/lvrml.test.org5orse.wrl”

Example 1. Simple VRML scene.

One can distinguish three different classes of nodes:

shape nodes (e.g. Cube) property nodes (e.g. Material) group nodes (e.g. Separator) hyperlink nodes (e.g. WWWInline)

Shape nodes define the geometry of the objects in the scene. Examples are Cone, Sphere, IndexedLine- Set, etc. Property nodes describe the appearance of the geometry on the monitor such as material, texture, etc. This class of nodes also includes transformation nodes, cameras and lights. Group nodes allow the handling of more than one node as a unit or guarantee that subsequent nodes are not influenced by the properties of child nodes. Some group nodes prevent the drawing from nodes according to conditions. Two special hyperlink nodes (WWWInline and WWWAnchor) are used. WWWInline nodes allow to assemble VRML worlds from different files or URLs, while WWWAnchor nodes specify links to new URLs.

Each node contains one or more fields. The names of the fields, the types and the default values are determined in the specification. VRML enables instancing of nodes using a DEF/USE construct. Each node can be labeled with a name. The programmer must ensure that the names are unique. If a node is labeled with a DEF keyword prefix, this structure can be reused by using the USE keyword and the name of the node (see example 2). This mechanism can also be used for group nodes, which allows the creation of instances of parts of the scene graph.

2.3. Current problems Beside several ambiguities, several problems arise

using the VRML 1.0 tile format. Most of them are based on the fact, that VRML was extracted from Open Inventor without adding sophisticated extensions.

t DEF first Cube {

width 3 height 3 depth 3

1

Translation { translation 0 5 0

USE first # add another instance of the cube above

Example 2. VRML instancing mechanism.

In this sub-section we will show some of the problems which have to be solved in future releases of the VRML specification, before new functional- ities such as behavior can be added.

Scene traversal. Since changes of properties are kept as part of the state during traversal, changing a single property can influence other branches of the scene graph. This makes it almost impossible for browsers to optimize the scene graph, if the properties are subject to change.

Level of detail. The original LevelOtDetail node, which realized the displayed level of detail according to the size of the bounding box on the screen was replaced by the much simpler LOD node. The level of detail selected by this node depends on the distance between the viewpoint and the center of the displayed object. This caused new problems, since including such objects into other VRML files might scale them, which will lead to inappropriate representations. Additionally most objects do not have similar extension into all directions and finally the field of view influences the size of the displayed objects. All these aspects are currently not considered.

No prototyping. The DEF/USE scheme as described above does not allow to specify a part of the scene graph without creating an instance.

No unique names. Names which can be attached to nodes by the DEF keyword do not have to be unique. For that reason they cannot be used to address a particular part of the scene graph as an object.

2.4. Applications The Fraunhofer Institute and the ZGDV use and

test VRML as an output and visualization format for 3-D data in different application areas. This includes the further development of VRML and VRML browsers to fit the requirements of special applications. On the one hand VRML offers the possibility of designing new user interface metaphors. The access on different kinds of data can be provided through a 3-D user interface. By this in many cases


the interaction with data becomes more intuitive for serve as much as possible of previous specifica- the user. On the other hand VRML can be used for tions. the visualization of data itself. An example here is the 3-D output of a 2-D graph. Often the nodes and There are three general design models to support

edges can be arranged more clearly in a 3-D interactive behavior in VRML worlds:

environment than in a 2-D representation. Another 1 ’ application is the visualization of simulation pro-

cesses in the areas of handling and order picking [6]. Here the 3-D representation of the simulation output 2

’ makes complex processes understandable and problems visible at once. For the integration of the 3. simulation process in VRML it is necessary to extend VRML with animation capabilities, a topic we are

Extending the VRML language specification, i.e. adding new nodes and keywords which realize behavior. Providing an interface to the scene graph and realizing behavior with external scripts. Embedding VRML scene descriptions within a scene behavior language.

working on.

3. VRML BEHAVIOR

The main issue facing VRML 2.0 is how to extend the currently static scene description language into a virtual world description language. This should include specifications for interactions and object behavior as well as multimedia extension, e.g. sound.

While the first two models can easily be integrated into an extended but open VRML specification, the third model relies on specifying all extensions outside VRML. For that reason the third approach is incompatible with the other approaches.

These extensions are necessary in order to realize VRML applications such as virtual shopping center, virtual communities, or MUDS. VRML applications will include interactive behavior and allow the user to influence the virtual environment.

Most of the proposals for VRML extensions were guided by one or several of the following goals. 0

l Simplicity-The mechanisms should be simple and efficient. Thus also non-programmers (designers/ artists) are able to use it.

l Reusability-It should be possible to define general behaviors once and to re-use them several times in different contexts without or with minor . modifications.

l Performance-To avoid some of the problems based on the structure of the scene graph, . extensions should not prevent browsers from optimizations of the VRML world.

l Authoring-It must be possible to realize interactive VRML worlds using a graphical user e interface. Thus it is possibIe to create compelling VRML worlds without programming or typing VRML code. .

D Interfa-VRML extensions should provide an interface (API) which allows to access and modify the scene graph by applications realized in scripting languages or external programs.

l Multi-user-Although currently no proposals ex- 3 ist, which include all aspects of multi-user extensions (synchronization, persistency, event rollback, dead reckoning, etc.), these issues have to be considered, to avoid problems later on.

D Scalability-Mechanisms to realize behavior should restrict the possibilities of creating large worlds by assembling a large number of interactive 3-D scenes.

scripting languages. Most of them are based on Java. The scripts modify the scene graph directly by an application interface provided by the appropriate browser. This approach does not require any changes or extensions to the specification or mod&&ions of the scene graph. However, since the API cannot be intluenced (restricted ore extended) within the scene descriptions, this approach is not upen to other external scripting languages or applications. Never- theless, this can be realized with minor extensions.

3.1, Extending the language specification Several proposals [7-lo] extend the VRML syntax

in order to specify interactive behavior as part of the scene graph. All these approaches have a certain number of common features. Thus any extensions that will be accepted in the future, will probably include them:

Prototypinglsubclassing-To create prototypes of behavior, a prototyping or subclassing mechanism is necessary. It allows the definition of new VRML nodes which can be used similar to built-in nodes. Additionally this mechanism can be used to encapsulate behavior and geometry, i.e. creating real world artifacts. Event detection-Events of input devices or external applications are detected by one or several new node classes. Scripting-To realize complex behavior by scripts, new nodes provide an interface between events, the scene graph and scripting language interpreters (such as Java [ 1 I]), or external applications. Built-in behavior-Some built-in behavior nodes provide some basic functionality for simple scene graph modifications (especially animations). Openness-New (more complex) mechanisms can easily be added, prototyping in con&m&ion with scripting can be used to evaluate these nodes first.

2. External scripting languages These proposals are completely based on external

D Compatibility-As VRML is an emerging standard for 3-D virtual worlds, radical changes to the syntax are unacceptable. Extensions should pre-

VRML: today and tomorrow 431

3.3. Embedding VRML The advantage of this approach [12] is that it

allows the integration of several types of medias, where 3-D VRML worlds are just one of them. The disadvantages are that it is not open to external scripts and applications and does not allow users to realize behaviors as part of the scene graph. Additionally properties of the scene graph, which will be modified, have to be redefined within the embedding language. Transformations, materials, textures, etc. which used to be part of the scene graph, have to be defined within embedding language constructs in order to modify them. This approach would reduce VRML to a geometry description language, which was definitely not the intention of the VRML community.

4. MULTI-USER EXTENSIONS

VRML will evolve to support distributed shared virtual worlds in the near future. After the first specification the VRML community focused on extensions to support interactivity. Thus there were no formal proposals on how to obtain multi-user functionality within VRML. Nevertheless this topic and its requirements were discussed extensively on the VRML mailing list. Some of the issues which will have to be considered when extending VRML to collaborative multi-user worlds will be introduced here:

l Scalability-Any approach extending local single user browsing to world-wide distributed virtual worlds must be scalable. This includes the number of users as well as the size of the virtual worlds and the distribution of the participants.

l Persistency-Persistency has to be guaranteed (at least on certain level) among the local copies of a shared world.

l Locking-To guarantee persistency, locking mechanisms might be necessary to prevent shared copies of a distributed world from unauthorized changes.

l Synchronization-Changes to local copies have to be distributed among all other participants sharing the same world. Suitable mechanisms have to be discovered to ensure synchronization at an accept- able update rate.

l Behavior-The behavior of objects within shared worlds has to be distributed and synchronized. Mechanisms to achieve this, without overloading the network connection may require smart tech- niques, such as deadreckoning.

l Protocol-The currently used HTTP protocol is not sufficient to transmit the required events for distributed multi-user VRML worlds. For that reason a new protocol has to be realized. One of the critical aspects of such a protocol is also the used network infrastructure.

l Avatars-User embodiments or avatars are used to represent the position and state of the participants of a shared virtual environment. In general each user should be able to select his or her avatar.

User representations depend not only on the different users, but also on the navigation mode and the virtual environment.

We now want to present two prototypes, which realize some basic multi-user functionality for VRML.

4.1. Simple multi-user extensions A multi-user extension as well as an according

protocol were realized by Sony as part of the Virtual Society project [13]. This system uses the HTTP daemon server to transmit the static scene description Afterwards the server is queried to receive the address of the Virtual Society server component. Each client connects to the appropriate server. The server keeps track of connected clients. All changes of the scene graph are distributed to the server which communicates them to the connected clients. Addi- tionally application objects can be located at the server.

Problems can arise when a large number of participants connect to a single server, since the network, as well as the server load, increases by the square of the number of clients.

4.2. Multicast approach An approach to realize multi-user capabilities for

VRML based on multicasting [14, 151 was developed at GMD [16]. Multicasting provides a flexible and powerful way of distributing data between a large dynamic group of participants. Nevertheless the current multicast protocol is not reliable and many users still do not have access to the MBone (multicast backbone).

Similar to the approach introduced before, this prototype uses the existing HTTP daemon servers to transmit the scene descriptions as well as the avatars of all current participants (users).

This allows even non multi-user-capable browsers to see other users’ embodiments. Plate 2 shows a virtual environment with three embodiments: two human-type avatars and a plane. The actual multi- user communication is performed by a second server daemon, which receives all changes of the VRML scene as well as information on joining or leaving avatars from the individual browsers via the multicast address. However, this daemon only updates the initial VRML world file, all updates on the local copies of the shared world are performed by the individual browsers (see Fig. 1). This is possible, since they receive the same messages from the multicast address as the server daemon. Thus the network load is minimized, since every message is only sent once (to the multicast address) and the server and the browsers have to handle the same number of messages. For that reason this approach scales very well until the number of messages is too high for at least one of the local browsers.


Plate 2. Three users represented by different avatars in a multi-user VRML world.

5. STANDARDIZATION PROCESS

In this section we will introduce the current standardization process of VRML and how it might be formalized in the near future.

5.1. The VAG The VRML Architecture Group (VAG) is a union

of researchers from different companies, research institutes and universities. The goal of the group is the extension of the VRML language specification to enable fully interactive virtual worlds. The basis for the work are whitepapers submitted by each particip- ant of the VAG. The papers cover different areas of interest for the extension of VRML because of the special research background each member has. The VAG considers also the demands of the VRML community and tries to integrate the ideas and prop&s of other researchers. In the end the VAG gives recommendations for a next extended version of VRML. There already exist suggestions for VRML 1 .l [17], but these do not cover the inclusion of object behavior and advanced interactions. There-

fore the VAG is also included in the specification process for VRML 2.0.

5.2. VRiUL consortium Up to now the specifi~tion of the VRML

language belongs to the VRML community. Every researcher can suggest improvements and additions for the specification which then will be discussed by the community in the VRML mailing list. At the VRML Symposium ‘95 in San Diego the danger arose that companies like Microsoft, SUN or SGI would determine the future of VRML Andy what it looks like. This was the reason that M&k Resee suggested the formation of a VRML Consortium in which companies, research institutes and universities could participate. The term VRML and the existing specification would belong to the consortium and the consortium develops VI&L further on.

This would guarantee, that a single company will not determine the standard exchange form-at for 3-D data and virtual worlds over the Internet in the

VRML: today and tomorrow

virtual world files

\ browser / \ browser) L browser)

4-b initial transfers

user updates

Fig. 1. File and message distribution using multicast groups.

future. But that a large number of researchers is included in the standardization process.

6. CONCLUSIONS

Although the specification of VRML 1.0 has been updated and some ambiguities have been removed, the need for clarification and improvement of VRML 1.0 still exits. These issues have to be addressed before new revisions of VRML, including more functionality, can be released.

VRML is a very powerful description language for exchanging 3-D scenes via the Internet. Since it is the first approach to create a 3-D user interface for the WWW, and is supported by a widespread community, it will also be accepted as the standard exchange format for virtual worlds over the Internet. Currently VRML only allows browsing through static 3-D worlds without interactive behavior. But the specification of VRML is designed for further extensions. On the one hand, the ongoing discussion is about integrating object behavior. Here the VAG started an initiative to consider different proposals for the extension of VRML 1.0. So in this area a widely accepted consensus will be reached in the near future. On the other hand multi-user support will be added to VRML. This topic is also discussed very intensively.

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1. T. Berners-Lee, R. T. Fielding and H. F. Nielsen, Hypertext Transfer Protocol HTTP 1.0. HTTP Work- ing Group. [www] http://www.w3.org/hypertext/ WWW/Protocols/Overview.html.

2. J. Wernecke, The Inventor Mentor. Addison Wesley, Reading, Massachusetts, USA (1994).

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4. G. Bell, A. Parisi and M. Pesce, The Virtual Reality Modeling Language, Version 1.0 Specification. [www] http://vrml/wired.com/vrml.tech/.

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434 W. Broil and T. Koop

10. D. R. Nadeau and J. L. Moreland, The Virtual Reality 14. D. P. Brutzman, M. R. Macedonia and M. J. Zyda, Behavior System (ORBS): a behavior language protocol Intemetwork infrastructure requirements for virtual for VRML. Proceedings of the VRML’95 Symposium, environments. Proceedings of the VRML’95 Symposium, San Diego, CA, Dec. 13-1.5 ACM (1995). San Diego, CA, Dec. 13-15, ACM, 95-104(1995).

11. A. van Hoff, S. Shaio and 0. Starbuck, Hooked on Java, 15. M. R. Macedonia, M. J. Zyda, D. R. Pratt et al., Addison Wesley (1995). See also: Java Language Exploiting reality with multicast groups: a network Documentation. Sunsoft (1995). [www] http://java.sun. architecture for large-scale virtual environments. Pro- com/docmnentation.html. ceedings of the Virtual Reality Annual International

12. Microsoft, A brief introduction to active VRML. Symposium (VRAIS ‘95) IEEE Computer Society VRML’95 Symposium, San Diego, CA, Dec. 13-15 Press, Los Alamitos, CA. pp. 2-10 (1995). (1995). 16. W. Broll and D. England, Adding multi-user support to

13. Y. Honda, K. Matsuda, J. Rekimoto and R. Lea, VRML. Proceedings of the VRML’95 Symposium, San Virtual society: extending the WWW to support a multi- Diego, CA, Dec. 13-15, ACM 87-94 (1995). user interactive shared 3D environment. Proceedings of 17. VRML Architecture Group, Meeting on VRML l.O/ the VRML’95 Symposium, San Diego, CA, Dec. 13-15, l.lj2.0 issues (August 20-23, 1995) [www] http:// ACM, 109-116 (1995). earth.path.net/mitra/papers/vag950823.html.

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