Improved Visualization of Rock Carvings

IT 16 089 Examensarbete 15 hp November 2016 Improved Visualization of Rock Carvings Filip Hedman Institutionen för informationsteknologi Department of Information Technology Abstract Improved Visualization of Rock Carvings Filip Hedman Teknisk- naturvetenskaplig fakultet UTH-enheten Digitizing rock carvings is a way to archive historically important sites and to make the data available for interpretation all around the world. Laser scanning has become Besöksadress: a very useful tool to capture the details of the rocks, and this thesis aim to answer Ångströmlaboratoriet Lägerhyddsvägen 1 how we can use different filters on the captured 3D data to visualize patterns from Hus 4, Plan 0 the rock carvings, while minimizing the noise from the surrounding geometry. Different coloring methods are evaluated to accentuate the rock carvings, while a Postadress: median filter is implemented to reduce the noise of the renderings. The results show Box 536 751 21 Uppsala that it is indeed possible to perform this kind of visualization, and that some methods are more suitable for the task than others. The results of this thesis will hopefully Telefon: make the choice of method easier for other researchers in forthcoming projects. 018 – 471 30 03 Telefax: 018 – 471 30 00 Hemsida: http://www.teknat.uu.se/student Handledare: Filip Malmberg Ämnesgranskare: Anders Hast Examinator: Olle Gällmo IT 16 089 Tryckt av: Reprocentralen ITC Contents 1 Introduction 8 2 Background 10 2.1 Galician rock carvings . 10 2.2 Traditional methods used in archeology . 11 2.3 Laser scanning . 11 2.4 Surface reconstruction from point clouds . 12 2.5 Visualization Toolkit . 13 2.6 PLY 3D format . 14 3 Methods 15 3.1 Altering the mesh . 15 3.1.1 Mesh decimation . 15 3.1.2 Mesh smoothing . 16 3.2 Shading and coloring . 16 3.2.1 Median filter . 16 3.2.2 Elevation coloring . 17 3.2.3 Curvature coloring . 18 3.2.4 Triangle area coloring . 20 4 Results 25 5 Related work 27 6 Discussion 29 6.1 Visualization of 3D scanned rock carvings . 29 7 Summary 31 7.1 Conclusion . 31 7.2 Future work . 32 7.2.1 A more robust visualization application . 32 7.2.2 Machine Learning . 32 7.2.3 Tactile feedback system . 33 7.2.4 Point light method . 33 References 34 5 Acknowledgements I would like to acknowledge my supervisor Filip Malmberg for all the help and direction given to me in the beginning of the project. I would also like to thank my reviewer Anders Hast for carefully reviewing my thesis multiple times, and helping me finish the whole project. 7 1 Introduction In recent years, work has begun to digitize rock carvings for archiving and further interpretation. Digitization of rock carvings may include more traditional methods such as simply photographing the carvings [1], but more recently laser scanning have become a prevalent method in the area [2]. With the help of a laser scanner, researchers can create a high definition 3D scan of the rock carving. The benefits of this method are clear; researchers can create a 3D scan that can be shared between multiple research teams for further interpretation without the need for each team to make an excursion to the actual site of the carvings. This lowers the barrier for research, and it keeps the physical wearing of the site to a minimum [3]. Problems occur if the carving is heavily weathered and the rock surface is far from smooth, since it can be visually difficult to discern the actual carvings. This problem can to some extent be handled by applying different kinds of shading in order to accentuate the pattern which has been carved on the rock. However, we think that different computer graphics techniques could be used to improve the visualization and give better representations of the actual pattern. The idea of using techniques from the field of computer graphics to achieve better visualization results have been explored before, for example by Trinks et al. [4]. Results can vary a lot between different types of data sets, and in this thesis we will take a look at scans of carvings obtained in the northwest part of Spain. The first method that will be evaluated for this purpose is elevation coloring, which highlights the difference in elevation between two 3D models. The next one is curvature coloring, which colors each point in the model according to curvature at that specific point. The last coloring method is triangle area coloring, which simply colors a triangle according to its total area. A median filter will be implemented and evaluated as well, to enhance the renderings in conjunction with the methods mentioned before. While the coloring itself is the main topic of this thesis, different methods to alter the mesh itself will be evaluated. Mesh decimation will be used to reduce the number of total polygons, and thus make the data easier and faster to work with. Mesh smoothing is used to reduce small details in the mesh data, which is a crucial part of the elevation coloring process. The main questions we aim to answer is how different filters can be used on 3D data to visualize patterns from rock carvings, and how we can reduce the noise from the surrounding rock in the renderings. The results of this thesis show promising results from curvature and elevation coloring on the data sets, while the implementation of triangle area coloring yielded only minimal improvements over the default shading. The results from these methods are improved 8 even further by the implemented median filter, which could prove to be a useful tool for other researchers in the area of visualization. 9 2 Background 2.1 Galician rock carvings The data sets that serves as the base for this thesis is obtained from Galicia in the northwest part of Spain. The typology of the carvings are varied and very rich, and it is possible to advert different themes and different execution techniques on the rock surfaces. Geometric motifs are common in Galicia, which is basically simple circles, circle combinations, labyrinths and bosses. Naturalistic motifs can also be seen, with common motifs being humans and animals [5]. Figure 1 and 2 shows the Abelairas and Rotea de Mendo data sets rendered with standard computer graphics methods. The laser scans includes color data as well, which in this case showcase some imperfections in the surface caused by moss. The color data will be excluded when visualizing rock carvings, since it is easier to discern simple black and white patterns. Figure 1: Abelairas stone carvings with color data. 10 Figure 2: Rotea de Mendo stone carvings with color data. 2.2 Traditional methods used in archeology The problem of finding and interpreting rock art has existed for a long time in the field of archeology, and there is already a vast array of different methods available to handle it. Some of the traditional methods include rubbing and scraping on large sheets of paper laid over the rock surface. This method can only be conducted by a professional on site, and there is a risk of physical wear on the surface [6]. Taking photographs of the petroglyphs and then improving them with the help of digital photo editing software is a more modern approach that was popular during the 1990’s [1]. That method works to some extent, but there is only so much detail you can get from a photograph. The final result will depend a lot on the quality of the original photograph, and the lightning of the scene. 2.3 Laser scanning With the use of a laser scanner it is possible to get a very detailed picture of the rock structure. In addition to being very precise, it does not alter the physical structure in any way, which is preferable since the rock formations are often very old and fragile. There are several types of laser scanners, but in general they emit a light that bounces 11 off the object and back to the scanner. Since the speed of light is a known constant, it is possible to calculate the distance to a certain point of an object by taking half the round-trip time and multiplying it with the speed of light [7]: C = Speed of light R = Round-trip time Distance = R/2 · C The scanner will gather information about a large set of points, which will then form a point cloud. To be able to render the rock as an object with a surface, we need to triangulate the points. Triangulation is the process of creating triangles from the point cloud. 2.4 Surface reconstruction from point clouds An important step in the process of digitizing rock carvings is the triangulation of the data captured by the laser scanner. The scanner merely returns a point cloud of the surface topology, and an extra step to reconstruct the actual rock surface with triangles is needed. There exists multiple methods for surface reconstruction, but one of the most widely used is Delaunay triangulation. In two dimensions, this is a triangulation DT(P) of a set of points P, such that no point P is in the circumcircles of the triangles in DT(P) (See figure 3). This extends to three-dimensional data as well, with the use of circumscribed spheres instead of circles [8]. The visualization framework used for this thesis, VTK, implements a Delaunay-based triangulation algorithm [9]. Figure 3: A Delaunay triangulation for an example set of points with the corresponding circumcircles. 12 2.5 Visualization Toolkit We will make use of the Visualization Toolkit [10], henceforth called VTK, which is a library created and maintained by Kitware Inc.

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