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The recording of Bet Giorgis, a 12th century Rock-Hewn church in Ethiopia Model Rendering In order to realise the goal of a fine-detail visually realistic model of Bet Giorgis, a fully rendered solid model is required. Two approaches were adopted for this phase, one at UCT-Cape Town and the other at ETH-Zurich. The former essentially comprised the following steps: exterior orientation determination, image rectification/ortho-rectification, image registration and texture mapping. The latter employed a self-developed, view-dependent texture mapping procedure, as described in more detail in Visnovcova et al. (2001). In this object-oriented technique, the texture of each object face is mapped from the original images directly, with a selection of the geometrically optimal source image being included. Also, in order to avoid checkerboard-type texture artifacts, mapping can be from multiple images with the texture elements being combined from several adjacent images in a weighted average procedure. The visualisation program disp3D imports object faces for texture mapping from Photomodeler. At this writing (January, 2001), texture mapping is in the process of being completed. Shown in Figure 5 is a very crude preliminary textured model of Bet Giorgis generated by 'rubbersheeting' imagery onto the implicitly assumed planar walls of the church. This model, when scrutinised closely, highlights the deficiency of rubbersheeting techniques for accurate solid model renderings of structure with significant relief.  Figure 5: Screen capture from preliminary VRML model created by 'rubbersheeting' of imagery. Visualisation A primary aim of building a visually realistic digital model is to make it available to as wide a group of interested parties as possible. This naturally implies dissemination of the model in an international standard format that support 3D interactive viewing, for example VRML for web delivery. As an example of the functionality of visualisation software suited to this task, the program disp3D, which uses C, Motif and OpenGL graphics libraries, supports DXF, VRML, V3D (an internal format, used by CyberCity Modeler), TIFF, JPEG and PPM for input; and DXF, VRML, TIFF and JPEG for output. This means that most of the relevant commercial visualisation packages can be used with the generated Bet Giorgis model. Disp3D delivers the following visualisation functions: point cloud; wireframe with hidden lines; shaded surface; and texture map in mono and stereo display, via anaglyph projection and polarized stereo. Image streams for MPEG-1/MPEG-2 videos can also be created, and a fly-over and walkthrough path definition capability is currently under development. These features, as applied to the Bet Giorgis model, will be shown at the time of this paper's presentation. The model will also be made available in the near future on the Web. Conclusions Patias & Peipe (2000) have spoken of an ever-changing paradigm in the photogrammetric recording and documentation of cultural heritage sites. They have highlighted recent enhancements in the photogrammetric approach. These include a trend away from stereomodels to multi-image surface reconstruction; the use of low-cost, amateur digital cameras and user-oriented image measurement and data processing systems; the potential for multi-sensor integration; the widening acceptance of image-enhanced information systems; and the widespread use of 3D modelling, visualisation, web-authoring tools and information delivery via the Internet. The aims and conduct of the Bet Giorgis documentation are very much in accordance with this new paradigm, and it is the hope of the authors that once the visually realistic digital modelling of this church and others in Lalibela are complete, a valuable information source will have been generated for use in site conservation, historical and architectural studies, education related to Ethiopia's rich heritage and history, and in visualisation for general enquiry and tourism. Acknowledgements The authors gratefully acknowledge the support and assistance of the Ethiopian Mapping Authority in the conduct of the Lalibela Project. The assistance of Mr Hadgu Medhin and the encouragement of Mr Tsegaye Denboba Wolde from the EMA have been much appreciated. The authors also acknowledge assistance given in the fieldwork and data processing stages by M.C. Biers and Alemu Nebebe. Li Zhang is thanked for providing his texture mapping and visualisation program disp3D. References
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