3D Visualization of a Complex Cave System

Through an initiative of the Austrian Alpine Club (ÖAV) Cave Exploration Group in Schladming (Styria, Austria) the Institute for Cartography of the Dresden University of Technology surveyed the Dachstein Southface Cave, generally known as “The Dachstein Hole". In cooperation with the Visualization Group of the Computer Center of this university a digital visualization of this cave has been produced. The objective was to acquire 3D data and generate a 3D model and textures as a prerequisite for the creation a lifelike walkthrough movie and a virtual reality model for interactive usage. The possibilities of the Visualization Group permitted to achieve satisfactory results by only using commercial software. Project Outline The aim of the project was to digitally reproduce the cave and its surrounding Alpine panorama as close to real-life conditions as possible. Figure 1 shows a flow chart of the data processing. The input data for the cave geometry model are measurements which were made during several expeditions. we purchased. the geometric model and the satellite image textures for the generation of the Alpine panorama. Photos of various cave passages served to produce textures of the interior of the cave. All the data together resulted in the complete 3D model. The data flow in Figure 1 goes from top to bottom. Due to the combination of different data it was not possible to use one software program for the scene visualization. We decided in favour of the animation software of Softimage and the virtual reality program of Realax which comprehends a modelmaker and real-time capabilities. Using these it is possible to realise a walk-through or fly-through. The same data served as a basis for the production of a video clip generated by means of Softimage. As equipment an SGI Onyx2 with 8 processors, one graphic pipe, stereo screen, shutter glasses and space mouse were at our disposal. An autostereoscopic display developed by the Department of Computer Science will furthermore be attached to this computer. A first “real 3D” overflight showing the Dachstein Alps has already been calculated for this display. Surveying The envisaged cave model, which was supposed to be as realistic as possible, required a high density of spatial data. In order to meet these high demands and to keep the effort within justifiable limits, the measurements of the cross-sections of the cave passages have been performed in such a way, that the characteristic shape is recorded as complete as possible, and to get it with a minimum number of cross-sections. This is why the cross-sections are notlocated in defined distances to the fixed reference points but according to the shape characteristics of the cave. Hence, portions with more dynamic profile characteristics, i.e. frequent chances in shape, are described by narrower cross-sections. Uniform or regular, tunnel-like corridors, on the other hand, require only a low density. For the measuring of the cross-sections the polygon segments of the basic survey have to be reconstructed by physical or optical measurements of the respective fixed point. In general, the profiles have been surveyed orthogonally to the polygon axis. Basically the cross-sections consist of eight measurements in well-defined directions with a constant angular interval of 45 degrees.