In the Geo3View model each triangle mesh is stored in one geometry file. The file name refers to the location in the mine. To prepare the virtual silver mine as an application to the VRML visualization server this complex segmentation has been simplified. All meshes are combined in groups in respect to the different mining levels. Vtk is used to store related meshes in combined geometry files. These files build the geometry data base of VRengine, which serves the requests to the silver mine.

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Fig. 2: Visualization of the upper part of the silver mine. The mine is located near Freiburg (SW-Germany) in the Black Forest mountains. The subvolume shows the galleries above the so called ‘Kappler Stollen’. The bounding box is 800 m long, 400 m wide and 420 m high.

The Sandbox Experiment

The second geological data set represents the final stage of a tectonic sandbox experiment [Ebert95]. Physical simulations are used to prove theories about long term deformation processes in the earth crust. The experiment mentioned here focused on the collision of three continental plates. In the model different crustal layers are represented by colored sand. To investigate the internal deformation style, the deformed sand body has been cut into slices. To reconstruct the spatial relationships a geometric model is developed. Layers and fault boundaries were digitized from the sketched slices. Shape blending techniques were used to interpolate intermediate slices [LindeXX]. The surface of selected layers and faults is reconstructed by triangulation between the slices. The derived data set is a boundary represented geometric model of the deformed sand body. The triangulation program of the Geo3View package stores the meshes as geometry files. The resulting geometry data set consists of a stack of deformed layers, disrupted along deformed fault planes. The VRengine allows the combination of layers and faults in one VRML scene. Therefore, the server interface provides a HTML form and clickable image maps. The requested subset of meshes is sent to the client's browser, where a VRML viewer is used to visualize the scene. Fig. 3 shows a VRML scene including two selected layers and the fault planes.

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Fig. 3: Visualization of the sandbox model. Two layers and the fault planes are selected. The experiment focused on the development of the transpressional Ribeira Belt in south-eastern Brazil (3).

ELEMENTS OF VRENGINE

The visualization server is implemented on an Apache⁽⁴⁾ web server running on a UNIX host. VRengine is built of several components, which are related to one of the following topics:

Fig. 1 shows these components and the data flow schematically. The three groups are sketched as a stack of cards, corresponding to their model specific contents (here the virtual silver mine).

The User Interface

The first step to generate a visualization is a HTTP request to get the model specific user interface. The discussion of the interface is based on the silver mine application. One part of this interface is shown in Fig. 4. Two methods for the selection of elements are implemented: (1) Visual selection can take place on an image map. This approach is used to select larger units like the upper part of the silver mine. (2) Tables with radio buttons are provided for a more detailed selection.

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Fig. 4: User interface to the virtual silver mine. Displayed is the lower part of the HTML form. The radio buttons are used to select different elements.

The HTML form of the silver mine interface includes tables corresponding to the upper and lower part of the mine. Table rows represent the different levels of the mine. The columns separate various element types: galleries, shafts and the planes of fractures and sills. A third table refers to additional elements of the model like orientation points or the topography. A button is used to start the request for a visualization including only the marked elements.

The CGI Programs

A model specific CGI program is invoked by the interface. In the example session, the program silvermine is bound to the 'generate button' of the HTML form. Fig. 5 shows the program code with some comments. It is written in C++. The communication between the program and the calling interface is based on the CGI environment. All elements of the HTML form are defined in this environment. The program uses functions of the C++ CGI Library⁽⁵⁾ to parse the environment. All accessible parts of the mine are listed in an array of elements (Fig. 5). The names of these elements match the names of the radio buttons in the HTML form. Using this name convention, the state of each radio button can be accessed by a simple function call. If the radio button is marked, the name of the element is used to identify the corresponding geometry mesh file. Vtk methods are used to read the mesh and add it as an actor to the scene. An individual color is set to the actor using the entries from the element list. If the loop has passed all elements of the list, the resulting scene is exported as a VRML file. While sending the VRML scene to the client's browser, the VRML header is exchanged. A predefined header is sent to provide some model specific viewpoints and lights.

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Fig. 5: Source code of the CGI program used within the silver mine application.

The data base

Each model has a data base consisting of a set of geometry files, stored in MOVIE.BYU display format [Chris86]. This format is used in the Geo3View system to save geometric models as sets of single mesh files. For the visualization server purpose, the meshes are grouped into larger entities. Shell scripts written in Tcl are used to generate the different files by using the methods of vtkBYUReader and vtkBYUWriter classes. The grouped files are used by the CGI programs to generate the VRML scenes.

RESUME

The virtual silver mine is developed to provide a more natural way to investigate space relationships inside the mine. Geologists doing research on water flow paths [Linde96] or on the nature of basement rocks can use VRengine in addition to the maps to get a three-dimensional impression of the mining area. The model also provides a good frame for other fields of interest, for example archaeology. In 1997 a small part of the mine was re-opened for visitors. The virtual silver mine application is a good starting point for people who want to prepare - or retrace - a visit.

The sandbox application interfaces a different field of geological research. The physical experiment took place in Brazil, the geometric model was prepared in Germany. The visualization server simplifies the exchange of information about the experiment and allows a vivid discussion of the model by the distant research groups. It is also used to demonstrate the results to related working groups who are not necessarily familiar with the applied methods.

Work in progress and future vision

At this time we are extending the functionality of VRengine, using methods of vtk. First tests on applying an algorithm to reduce the number of triangles [Schro92] were successful. The meshes can be reduced using the methods of the vtkDecimate⁽⁶⁾ class. The interfaces will be extended by an option to set different parameters for the reduction. Using this option enhances the access to the generated VRML scenes significantly.

Beside this work already in progress we think about the extended possibilities of VRML 2.0. Using script nodes to invoke executable code, will open another way to navigate through the models. Perhaps once we will be guided by a robot [Greha97], who is programmed with model specific knowledge. So we can follow him along flow paths of water, or on his way to points of special interest.

ACKNOWLEDGEMENTS

First we would like to thank Will Schroeder, Ken Martin and Bill Lorensen for providing the Visualization Toolkit. This really great piece of software is very well supported by their Web server site, a mailing list and their textbook [Schro96]. There are lots of other useful things like Apache, Mesa, Glut and the GNU suite we are using from the Internet community. Thanks to all of them.

REFERENCES

Christiansen,H: MOVIE.BYU training text.- Provo, Utah, Community Press, 300 p., 1986

Ebert,H, Neves,M, Hasui,Y, Lopes,J, Guerra,M: Compartimentacão crustal e evolucão cinemática da Província Mantiqueira através de modelagem fisica.- 5. Simp. Nac. Estudos Tectônicos, Bol. Resumos Expandidos, Gramado-RS, SBG-RS, São Paulo, pp. 26-28, 1995

Ganter,S: 3D-Modellierung der Grube Schauinsland, Südschwarzwald.- 80 pp., unveröffentlichte Diplomarbeit, Geol. Inst. Univ. Freiburg, 1995

Ganter,S, Lindenbeck,C, Pflug,R, Ulmer,H: Geometrische Modellierung der Grube Schauinsland bei Freiburg im Breisgau.- Schriftenreihe des BDG, 14, pp. 101-103, 1995

Grehan,R: Write Cosmic Code.- Byte, 8, 22, pp. 113-114, 1997

Klein,H, Ramshorn,C: 3D-Computergraphik zum Visualisieren von geologischen Strukturen und simulierten geologischen Prozessen.- 140 pp. + 43 pp. Anhang, Dissertation Geowiss. Fakultät Universität Freiburg, 1991

Lindenbeck,C, Ulmer,H: Entwicklung und Anwendung von Computerprogrammen zur Visualisierung geologischer Strukturen und Prozesse.- Freiburger Geowissenschaftliche Beiträge, 9, 280 pp., 1995

Lindenbeck,C, Wirsing,G: Hydrogeologische Untersuchungsergebnisse und 3D-Visualisierung des Grubengebäudes Schauinsland.- Arbeitshefte Geologie, Hannover, 1, pp. 60-65, 1996

Lindenbeck,C, Ulmer,H, Ebert,D, Pflug,R: Geometric Modeling and Visualization of a Sandbox-Experiment: Transpressional Tectonics in the Precambrian Ribeira-Belt (SE-Brazil).- Zentralblatt für Geologie und Paläontologie, in press

Schroeder,W, Zarge,J, Lorensen,W: Decimation of Triangle Meshes.- Computer Graphics (SIGGRAPH '92), 26, pp. 65-70, 1992

Schroeder,W, Martin,K, Lorensen,B: The visualization toolkit: an object-oriented approach to 3D graphics.- Prentice Hall (London), 826 pp., 1996

Webpage created by Ch. Lindenbeck & H. Ulmer