VRML -- a 3D language for 3D visualisation

1.1. Creating and sharing visualisation

The generation of 3D visualisations of scientific data is a comparatively mature field, and today's user can choose between a variety of public domain and commercial packages. Examples of popular general-purpose visualisation systems include IRIS Explorer [5], AVS [6], Data Explorer [7] and Khoros [8].

Classically, visualisation is used to (a) understand or explore data, and then to (b) publish the result in order to communicate this understanding to others. Following the rise in popularity of the WWW, traditional forms of publishing have been complemented by Web publication. The relative merits of the traditional media vs the Web are still under discussion, but one advantage that the Web has is the possibility of publishing and sharing data in new forms. To see this, consider for example a visualisation of a complex 3D object such as an oil reservoir model consisting of an assembly of coloured hexahedra.

A fixed view of the object may be chosen by the publisher, and shared as an image, or a selection of views may be chosen (usually by specifying the path a camera takes around the object) and published as a movie. Both of these forms only offer a limited number of views of the object which--if the object is complicated--may be over-restrictive for other users who wish to share in the understanding of the object. This leads us to consider the possibility of not sharing views, but a copy of the object itself. Here, the user can choose any view of the scene, and interact with it in the same way as the publisher. Moreover, individual views (i.e., camera locations and orientations) can be stored in the file, so the publisher has the ability to draw the user's attention to specific features in the scene, but the user still has the freedom to explore the object in whichever way they are inspired to do. Finally, the sharing of views as images or movies can take up more space on disk (and download time) than the sharing of the object, as Table 1 illustrates for the reservoir model. Clearly, the quantitative aspects of this comparison are dependent on the model, since its complexity determines the size of the 3D file, whereas the size of an image or movie file is related to its resolution in screen space, but we have found in practice that the trend illustrated in Table 1 has held for a wide range of visualisations that we have developed and used elsewhere.

1.2. Open Inventor and VRML 1.0

We have seen the advantages of interactivity that 3D publishing offers, although we have not yet discussed the character of the file format used to do this. The field of 3D geometry creation and storage is somewhat immature, and until recently, 3D formats were usually tied to a particular CAD application. Similarly, those scientific visualisation packages that allowed the user to save their results as geometry (not all did) would often use a proprietary--often undocumented--format.

This situation changed with the advent of Open Inventor [5], an object-oriented 3D graphics library which defines an simple file format for the description of 3D scenes. A scene in Open Inventor is made up of nodes; various classes of nodes implement geometry elements (primitive shapes, surfaces, text, etc.), properties (colour, lighting, texturing, transformation, etc.) as well as other behaviours.

Following the definition of VRML 1.0, a number of browsers and authoring tools appeared--for example, WebSpace [9], a browser developed by Silicon Graphics. The widespread acceptance of VRML makes it a natural choice for storing and sharing 3D geometry as output from a visualisation package. In the case of IRIS Explorer, converting its output to VRML is most easily done using the Inventor translator ivToVRML, which is part of the WebSpace distribution. A number of VRML files produced using IRIS Explorer are accessible on the WWW [10], together with a large variety of examples [11] from other application areas. Before going on to discuss the latest version of the language, we interpose a section describing three examples [12] of the way in which the connection between the visualisation system and the Web (effected via VRML) has been exploited recently.

[7]. Lucas, B., Abram, G.D., Collins, N.S., Epstein, D.A., Gresh, D.L. and McAuliffe, K.P. (1992) An Architecture for a Scientific Visualisation System, Proceedings of Visualization '92, IEEE Computer Society Press, 107.

[8]. Rasure, J. and Young, M. (1992) An Open Environment for Image Processing Software Development, Proceedings of 1992 SPIE/IS&T Symposium on Electronic Imaging, 1659

[9]. http://www.sgi.com/software/webspace_author.html

[10]. http://www.nag.co.uk/visual/IE/iecbb/VRML.html

[11]. http://www.sdsc.edu/vrml/

[12]. Walton, J. (1997) World Processing: data sharing with VRML, in The Internet in 3D: Information, Images and Interaction, Earnshaw, R.A., and Vince, J.A., ed., Academic Press, 237.