In this paper, we present our work in designing, implementing, and evaluating a set of 3D interactive spatial measurement tools in the context of Cultural Heritage Toolbox (CH Toolbox), a framework for computer-aided cultural heritage research. Our application utilizes a bi-manual, spaceball and mouse driven user interface to help the user manage visualized 3D models digitized from real artifacts. We have developed a virtual radius estimator, useful for analyzing incomplete pieces of radial artifacts, and a virtual tape measure, useful in measurement of geodesic distances between two points on the surface of an artifact. We tested the tools on the special case of pottery analysis

Balcısoy, Selim

Balcisoy, Selim

Ozmen, Yilmaz Can

Özmen, Yılmaz Can

English

Sabanci University Research Database

A Framework for Working with DigitizedCultural Heritage ArtifactsCan Ozmen1 and Selim Balcisoy1Sabanci University, Turkey,canozmen@su.sabanciuniv.edu, balcisoy@sabanciuniv.edu,http://graphics.sabanciuniv.eduAbstract. In this paper, we present our work in designing, implement-ing, and evaluating a set of 3D interactive spatial measurement tools inthe context of Cultural Heritage Toolbox (CH Toolbox), a frameworkfor computer-aided cultural heritage research. Our application utilizes abi-manual, spaceball and mouse driven user interface to help the usermanage visualized 3D models digitized from real artifacts. We have de-veloped a virtual radius estimator, useful for analyzing incomplete piecesof radial artifacts, and a virtual tape measure, useful in measurement ofgeodesic distances between two points on the surface of an artifact. Wetested the tools on the special case of pottery analysis.1 IntroductionUsing digital models of cultural heritage artifacts, obtained through 3D scannerand modeling technology, has the following advantages. It is faster, cheaper, andsafer to store, transport, search, sort, and re-arrange digitized models instead ofthe real pieces. Digitization is also beneficial for the computational analysis ofthese artifacts as it makes it easier to compute and verify the results.Although the use of digitized models in cultural heritage research is beneficial,the adoption of such techniques is problematic because nearly all experts aretrained for using the traditional methods. Thus, practitioners of the field cannoteasily transfer their expertise with traditional tools to new software based toolswithout further education [1]. Similar problems arise in the medical field as well,where the disparency between computer tools and formal education methods isacknowledged. The generally preferred solution to this problem is to present afamiliar interface to the underlying software system.In the following subsections we will present two traditional tools, the tapemeasure and the bordimeter, and discuss their usage in the special case of ar-chaeological pottery classification.1.1 Tape MeasureThe tape measure is used for determining the dimensions of a sherd and thesurface distance between any two points on the sherd. These measurements are2used for classifying a piece, along with its weight and thickness. It is also usedfor measuring fracture lengths to aid in reconstruction. The main purpose of thetape measure is to find geodesic distances either on the surface, along the rim,if the sherd is part of the rim, or along the fractured edges.The tool is used with two hands. The protrusion on the edge of the tape isfastened to one of the end-points of the distance to be measured, then it is heldin place using one hand while the other hand pulls the measure and extendsthe tape to the other end-point. The measurement can be more easily read bylocking the tape at the desired length. The tape measure is, in essence, an easierto use metered rope.1.2 BordimeterA bordimeter, also called a rim chart, is a set of concentric circles drawn on apiece of paper or cardboard. It is used for estimating the rim radius of a vesselgiven a sherd belonging to the vessel’s rim. The rim radius is used in calculatingthe rim size, which in turn helps archaeologists to assess a pot’s usage. Radiusestimation is also used in volume, and consequently capacity estimation. Bothassessments help in the classification of the vessel.The tool is placed on a flat surface, then a rim sherd is put rim-side downto find the best fitting circle, which is then used to estimate the radius. Becausethe bordimeter is a solid piece of paper, radius estimation cannot be done withpieces other then rim sherds.2 Related WorkThe most common approach in working with digitized cultural heritage artifactsis to use existing 3D modeling software. The problem is that effectively usingthese programs needs expertise with the software and user interface which theresearcher may lack. They are also general purpose programs, mostly gearedtoward digital content creation, architecture, or technical drawing and thus maynot mimic the traditional way cultural heritage researchers work with artifacts.Applications targetting specific archaeological tasks exist, but they are limitedby their tight focus and cannot be easily extended to other domains in analyticalcultural heritage research.2.1 GeodesicsA geodesic curve is the shortest path between two points on a surface. Commonareas of use include navigation, path-finding, motion planning and network op-timization. It is also an important step in many computer graphics algorithmssuch as mesh parametrization, mesh segmentation and mesh editing [4–6]. Cur-rent graphics hardware all use a triangular mesh format to process and visualize3D geometries, thus most geodesics research is done for the discrete case [7].3The Djikstra shortest-path algorithm is not sufficient to solve the discretegeodesic problem because the shortest path on a surface does not always followalong the edges of the mesh. The simplest solution is to augment the originalmesh with extra points before running Djikstra. Lanthier et. al. [9] comparedifferent ways of populating a mesh with additional vertices on existing edges,called Steiner points, and show that a bounded approximation to the geodesiccan be obtained with this method. The error bound and complexity of the al-gorithm depends on the number and distribution of these Steiner points. Andescription of the algorithm using a fixed-point distribution scheme, which weused, will be presented in the next section. A survey of approximate algorithmscan be found in [8].Several algorithms giving an exact solution to the discrete geodesic problemhave been proposed. The “single source - all destinations” algorithm described byMitchell, Mount and Papadimitriou (MMP) uses the continuous Djikstra methodand it has a worst case running time of O(n2 log n), where n is the number ofvertices. The MMP algorithm was recently implemented by Surazhky et. al. [11],who conclude that it performs much better than the worst case analysis suggests.An exact algorithm with O(n2) running time based on surface unfoldings wasproposed by Chen and Han [12].2.2 Circle FittingThe problem of fitting a circle to a given set of co-planar points is called 2D circlefitting. It is a nonlinear least squares problem, which can be solved iteratively byreducing it to a set of linear least squares problems [3]. The numerical solutionwe used will be presented in the next section.3 CH ToolboxCH Toolbox is a 3D application framework for analytical cultural heritage re-search. It visualizes digitized models of artifacts in 3D and allows the user to an-alyze the pieces using a spaceball and mouse driven interface. It has a bi-manualuser interaction scheme, where the mouse is used to switch between tools, togglevisualization modes and select the artifacts to be manipulated and the spaceballis used to translate and rotate the selected artifacts. Bi-manual interaction is afamiliar human trait, and increases productivity in 3D manipulation tasks [13].CH Toolbox is developed in C++, using the open-source scenegraph libraryOpenSceneGraph [14] that helps visualizations in OpenGL by providing an or-ganizational hierarchy. The CH Toolkit and the interactive spatial measurementtools developed for it are available for all the operating systems that OpenScene-Graph supports. Our application runs in real-time on desktop PCs.We developed a radius estimation and a surface distance measurement toolfor the CH Toolbox. They are used interactively inside the framework and allowquick and precise measurements. The next subsections detail their implementa-tion and usage.43.1 Virtual Tape MeasureThe virtual tape measure is used for measuring the surface distance betweentwo points on a model. The user fixes a point then moves the mouse to interac-tively visualize and measure geodesics originating from the start point as seenin Figure 1.We solve the geodesic problem in the pre-processing stage before the arti-fact model is visualized inside the CH Toolbox environment. The approximategeodesic solution proposed by Lanthier et. al. [9] is used. The algorithm is asfollows:1. The original mesh is loaded and converted to a triangle mesh.2. Original edges are sub-divided to create extra vertices, called Steiner points.3. New edges are created between two Steiner points if they are adjacent onthe same triangle edge or they lie on different edges of the same triangle.4. The single-source all-destinations Djikstra is run for each vertex.We used a fixed scheme to evenly add two Steiner points per edge, therefore6 vertices and 27 edges are added to a triangle in the pre-processing stage.Lanthier et. al. [9] proved that the algorithm runs in O(n5) for the single-sourceall-destinations problem, where n is the number of triangles in the original mesh.The geodesic is visualized by traversing vertices on the shortest path betweenthe two end points of the tape measure. We choose to visualize the extra edgesonly if they lie on the shortest path solution, thus the tool has negligible impacton the interactivity of CH Toolbox. The bi-manual interaction scheme of theframework is also conserved while using the virtual tape measure.Fig. 1. Screenshot of the tape measure in action3.2 Virtual BordimeterVirtual Bordimeter is a radius estimation tool for digitized artifacts. A best fitcircle is computed and displayed interactively as the user moves the mouse overvisualization of the artifact model as seen in Figure 2.The tool works as follows:51. The plane defined by moving the mouse is intersected with the model toobtain a set of points on the plane.2. The average of the points is taken as the initial estimate for the circle.3. The circle is fit iteratively using least squares fitting to these selected points(see Figure 3).4. The center and the radius of the circle is computed.Even if the least squares solution does not converge, the iteration for fittingthe circle is stopped after a certain number of steps to maintain interactivity. Wefound that the solution converges in sufficient time if the points are not nearlylinear. Otherwise, a warning message is displayed. This is not a problem withour test case since archaeological pottery has a curved surface.The circle and its center is visualized in addition to the text display of thelocation of the center and its radius because it helps the expert to visually verifythe suitability of the numeric solution as the rotational axis of the artifact.During the measurement process the expert can use the spaceball to move andorient the model. She can also change the transparency of the circle visualizationto prevent it from obscuring the artifact.Fig. 2. Screenshot of the virtual bordimeter in actionFig. 3. Circle fitting64 ResultsWe tested the surface distance algorithm on a Pentium IV 2.6GHz PC with 1GBRAM and a dual Xeon 3.4GHz workstation with 4GB RAM. For a sherd meshwith 17K triangles, our algorithm takes 16 seconds on the Pentium IV and 14seconds on the Xeon as seen in Table 1. The number of extra vertices and edges,and its impact on performance is also reported. Although the perfomance is notadequate for real-time interaction, our interviews with cultural heritage expertsshow that the it is acceptable for real world use cases [1].Table 1. Performance of approximate geodesic algorithmModel Vertices Faces Stn. points Stn. edges time 1 (s) time 2 (s)Sherd 9183 17696 62967 427304 16.2003 14.0507Sphere 25502 50986 178502 1230534 47.5673 41.2052Sphere 102302 204552 716102 4938138 202.2870 175.5850We also compared rim estimations and surface distances measured on a realpot with the approximate results we got using our virtual tools on a sherd mesh(Table 2). The error rate of the geodesic is very small, and it is consistent with thebound proven by Lanthier et. al. [9]. Again, the error was found to be acceptableby experts [1] although it includes the error from the 3D scanning process aswell.Table 2. Error rate of virtual toolsTool Real (cm) Virtual (cm) Rel. errorTape 15.35 15.44 0.6%Bordimeter 12.74 12.17 4.5%5 Conclusion and Future WorkOur contribution is the development of a platform for interactive computer-aidedcultural heritage tasks. We implemented measurement tools that archaeologistsdaily use in real life by adapting algorithms developed for other computationalgeometry tasks. The algorithms we chose are suitable for use in a real-timeinteractive environment.7Accuracy problems of the radius estimation tool must be further researched.We suspect the relatively high error rate is due to errors in the 3D scanningtechnology we used. Our tools should be tested with models obtained from otherscanning technologies.CH Toolbox was approved by the cultural heritage experts who have seen it.We plan to release CH Toolbox as an open-source application to aid researchersin the cultural heritage domain.6 AcknowledgementsThis research was supported by TUBITAK EEAG 104E155.References1. Personal communication with Prof. Bernard Frischer, Univ. of Virgina. (2006)2. Preim, B., Spindler, W., Oldhafer, K. J., Peitgen, H.-O.: 3d distance measurementsin medical visualizations. Interactive Medical Image Vis. and Analysis (2001), 31-363. Gander, W., Golub, G. H., Strebel, R.: Least squares fitting of circles and ellipses.BIT 34 (1994), 556-5774. Krishnamurthy, V., Levoy, M.: Fitting smooth surfaces to dense polygon meshes.Proc. of SIGGRAPH 96 (1996) 313–3245. Funkhouser, T., Kazhdan, M., Shilane, P., Min, P., Kiefer, W., Tal, A., Rusinkiewicz,S., Dobkin, D.: Modeling by example. Proc. of SIGGRAPH 2004 (2004) 652-6636. Floater, M. S., Hormann, K.: Parameterization of triangulations and unorganizedpoints. Tutorials in Multiresolution in Geometric Modeling (2002) 287-3157. Mitchell, J. S. B., Mount, D. M., Papadimitriou, C. H.: The discrete geodesic prob-lem. SIAM J. of Computing 16(4), 647-6688. Mitchell, J. S. B.: Geometric shortest paths and network optimization. Handbookof Computational Geometry (2000) 633-7029. Lanthier, M., Maheshwari, A., Sack, J.-R.: Approximating weighted shortest pathson polyhedral surfaces. Proc. 13th Annu. ACM Symp. Computational Geometry(1997) 274-28310. Martinez, D., Velho, L., Carvalho, P. C.: Geodesic paths on triangular meshes.Proc. of SIBGRAPI/SIACG. (2004) 210-21711. Surazhsky, V., Surazhsky, T., Kirsanov, D., Gortler, S. J., Hoppe, H.: Fast exactand approximate geodesics on meshes. ACM Trans. on Graphics 24(3) (2005), 553-56012. Chen, J., Han, Y.: Shortest paths on a polyhedron: part I: computing shortestpaths. Int. J. of Computational Geometry and Applications 6(2) (1996) 127-14413. Hinckley, K., Pausch, R., Profitt, D., Kassel N. F.: Two handed virtual manipula-tion ACM Trans. on Human Computer Interaction, Vol 5(3) (1998) 260-30214. OpenSceneGraph: www.openscenegraph.org

A framework for working with digitized cultural heritage artefacts

W. Gander

V. Surazhsky

J. Chen

K. Hinckley

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A Framework for Working with Digitized Cultural Heritage Artifacts

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A framework for working with digitized cultural heritage artefacts

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