The article presents the main results of the implementation of the project "Virtual Denisova Cave in Altai", which was aimed at creating virtual 3D models of the cave and some of the most characteristic finds, as well as developing software for interactive visualization of the results of archaeological research in the virtual space of the model. In addition to preserving information about a unique natural and archaeological site, the virtual 3D model of Denisova Cave is designed to solve a number of scientific and practical tasks. The project is the first Russian experience of virtual 3D modeling of a Paleolithic cave site.

Keywords: virtual archeology, laser scanning, 3D model, visualization, Denisova cave, Paleolithic.

Introduction

Currently, one of the most promising archaeological sites for studying the most ancient human culture and the surrounding natural environment in Northern Eurasia is the multi-layered Paleolithic site in Denisova Cave in the north-west of Gorny Altai. The loose sediment column of the cave contains more than 20 lithological layers that contain archaeological material from various epochs-from the Middle Paleolithic to the late Middle Ages. The collection of finds from the central hall, two galleries and the entrance area of the cave totals tens of thousands of items. The most important discoveries are related to the study of the Paleolithic layers of Denisova Cave. The anthropological remains of a previously unknown subspecies of man found in the layer of the initial stage of the Upper Paleolithic became a world-class scientific sensation.

The preservation of information about Denisova Cave as a unique natural and archaeological site using the entire arsenal of modern computer methods, in particular, the creation of its virtual (digital, computer) 3D model, is of undoubted importance and relevance.

The term "virtual archeology" was coined in 1990 by P. Reilly, one of the pioneers of the use of 3D computer modeling and visualization for studying archaeological sites (Reilly, 1990). The main tasks of this area include:

The publication was prepared with the support of a grant from the Ministry of Education and Science of the Russian Federation (Resolution No. 220), received by the Altai State University (project N 2013 - 220 - 04 - 129 "The oldest settlement of Siberia: formation and dynamics of cultures in Northern Asia").

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This includes recording information about research objects (3D documentation), their virtual reconstruction, visual data analysis and hypothesis testing, providing wide access to this information, and creating virtual museums. Virtual archeology has been actively developing in the world for more than 20 years. Major international conferences on this topic are regularly held*, and articles on completed projects are published in leading scientific journals**. The total number of 3D models of archaeological sites created is estimated at thousands [Bawaya, 2010; Borodkin and Zherebyatiev, 2012].

3D modeling of Paleolithic cave sites using laser scanning can be considered as a separate area of virtual archeology. One of the first works of this kind was performed in 1994. A laser scanner was used to create a textured 3D model of the hard-to-reach Cosquet Cave in France (Thibault, 2001). Later, 3D models of Arago Caves were created in France, Grotta dei Cervi in Italy, Parpalho [Lerma et al., 2010], Altamira [Donelan, 2002], Las Caldas, Peña de Candamo [Gonzalez-Aguilera et al., 2009] in Spain, and Wonderwerk in South Africa (Ruther et al., 2009). A detailed review of projects for modeling Paleolithic cave sites has shown that the main motivation for most of the work is to preserve three-dimensional information about cave paintings and engravings. Much less attention is paid to the visualization of archaeological finds in the virtual space of created 3D models. This task was fully implemented in the project "Virtual Denisova Cave in Altai"***, which is the first domestic experience of virtual 3D modeling of a Paleolithic cave site.

Virtual 3D model of Denisova Cave

In August 2012, laser scanning and detailed photography of the cave were performed. 37 scans were made, and the total point cloud is about 50 million pixels. Then, based on the results of laser scanning and photographing, a textured polygonal model was created. It contains 88,254 polygons, of which 86,000 are directly occupied by the cave, and 2,254 are artificial objects (decking, railings, stairs). The texture resolution ranges from 30 to 100 thousand pixels per 1 m2 of the model's surface for different sections. Thus, a detailed 3D model of the cave was created, which captures its geometry and appearance with high accuracy (Figure 1).

The created 3D model was linked to the archaeological coordinate system (ASK) used during excavations to fix the location of finds. The ASCII ordinate (Y) axis is rotated 62° clockwise relative to the northward direction along the true meridian, roughly coinciding with the cave entrance line and the long axis of the central hall. The abscissa (X) axis is perpendicular to the ordinate axis in the plane tangent to the Ground at the origin. The applicator axis (Z) is directed vertically upwards and complements the coordinate system to the right triple. The origin of the ASK coordinates is a conditional point in space located near the drop line of the cave. ASK is fixed on the object using a series of reference points fixed in the root wall of the cave. All reference points were captured during scanning. When processing the data, the cave model was converted to ASK coordinates, and the binding accuracy was approx. 5 cm (Fig. 2). Thus, it is possible to compare the created 3D model with archaeological schemes and directly transfer the array of available archaeological data to its virtual space (Fig.3).

Laser scanning, photographing, and 3D modeling were performed separately.

* EUROGRAPHICS Workshop on Graphics and Cultural Heritage-Seminar on Graphics and Cultural Heritage of the European Association for Computer Graphics; International Symposium on Virtual Reality, Archeology and Cultural Heritage (VAST) - International Symposium on Virtual Reality, Archeology and Cultural Heritage; International Conference on Virtual Systems and Multimedia (VSMM) - International Conference on Virtual Systems and Multimedia. virtual systems and multimedia; 3D Virtual Reconstruction and Visualization of Complex Architectures (3D-ARCH) - International Seminar "Three-dimensional virtual Reconstruction and Visualization of Complex Architectural Structures" of the International Society for Photogrammetry and Remote Sensing; Electronic Imaging & the Visual Arts (EVA) - International Conference "Electronic Images and Visual Arts" ; Conference "Archeology and Geoinformatics" (AGIS); International Conference "Virtual Archeology" (State Hermitage Museum).

** Journal of Cultural Heritage; Virtual Reality; IEEE Computer Graphics and Applications; The Photogrammetric Record; Sensors; ISPRS Archives; IEEE Multimedia; Journal on Computing and Cultural Heritage; Science and Technology for Cultural Heritage; Applied Geomatics; Remote Sensing.

*** The project was implemented by the Virtual History of Science and Technology Center of the Institute of History of Natural Science and Technology of the Russian Academy of Sciences together with the Institute of Archeology and Ethnography of the Siberian Branch of the Russian Academy of Sciences with the support of Trimetari LLC as part of the RAS Target Program "Development of the Permanent Exhibition of RAS Achievements" 2012.

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Figure 1. Textured polygonal 3D model of Denisova Cave. a, b - pre-entrance area; c - central hall.

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Figure 2. Cross-section of the 3D model of the cave at the level of 0 m with a superimposed coordinate grid.

Figure 3. Visualization of the location of finds in the virtual space of the 3D model.

4), marking various cultural and chronological stages of the cave's settlement. The created item models contain about 50 thousand polygons. When texturing them, displacement maps obtained from more highly polygonal models (500 thousand polygons per object) are used. Thus, a high visual realism and detail of virtual objects is achieved with a relatively small volume of the model and texture, which is especially important for interactive visualization tasks in stereo mode.

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Figure 4. Textured polygonal 3D models of stone artifacts.

a-skreblo (layer 12); b-Levallois point (layer 11).

5. "Cloud of finds" in the excavation of the eastern gallery.

Figure 6. Cross-section of the 3D model of the cave at the +1 m level.

An interactive 3D presentation (software)was developed to visualize the created 3D model of the cave. It supports both mono and stereo display modes and provides visualization of point clouds, 3D models of caves, finds, wooden decks, railings and stairs, spatial location of finds in various archaeological horizons (Figure 5), as well as additional data: orientation to the cardinal directions of the axes and the grid of ASK, the location of reference points. The possibility of horizontal cutting of the virtual model at any level is implemented for the convenience of visual analysis of the cave geometry and location of finds (Fig. 6).

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Figure 7. Presentation interface. The Cave window.

The presentation has four windows. The user can switch from one to the other in any order. The "Globe" window displays a virtual globe with a three-dimensional model of the cave - so you can study the location of the cave on the ground and the surrounding terrain. The Cave window (Figure 7) displays models of caves and other objects. In the "Discovery Cloud" window, only the location of finds in archaeological horizons is visualized. The "3D model of a find" window displays models of individual items with the ability to switch between them.

Conclusion

The created 3D model of Denisova Cave and software can be used to solve various scientific problems. In particular, the function of horizontal sections with a step of 10 cm allows you to draw the most accurate plans of the entire karst cavity or its specific section at any height. You can also create vertical sections of the cave, calculate volumes, and measure distances between objects inside it. The model can be used to refine existing archaeological schemes and create a new topographical basis. The model allows you to visualize the relative positions of lithological layers. A wide set of filters in the finds database allows you to quickly search for any artifact (or group of finds) of interest and determine its location in the layer relative to other objects. A significant addition is the ability to visualize and analyze the horizontal and vertical orientation of a find in a layer. The model allows you to perform planigraphic analysis at any scale-from a square to a sector or a cave as a whole. Also, the 3D model can be adapted for viewing via the Internet, posted on a public site to create a virtual tour. This is especially true for Denisova cave, which is quite difficult to visit object.

Acknowledgements

The authors express their gratitude to S. Baykova, A. Ivanov, D. Linovsky (Trimetari LLC) for participation in the work on laser scanning and creation of 3D models, as well as S. Kotelnikov (Navgeokom LLC) for consulting on the methodology of work.

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Riither H., Chazan M., Schroeder R., Neeser R., Held C, Walker S.J., Matmon A., Horwitz L.K. Laser scanning for conservation and research of African cultural heritage sites: the case study of Wonderwerk Cave, South Africa // J. of Archaeol. Sci. - 2009. - Vol. 36, N 9. - P. 1847 - 1856.

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The article was submitted to the Editorial Board on 11.02.14.

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