Virtual Dunhuang Art Cave: A Cave within a CAVE

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  • EUROGRAPHICS 99 / P. Brunet and R. Scopigno(Guest Editors)

    Volume 18 (1999), Number 3

    Virtual Dunhuang Art Cave:A Cave within a CAVE

    B. Lutz and M. Weintke

    (Fraunhofer Institute for Computer Graphics, Darmstadt, Germany)

    AbstractVirtual Reality can present historical places in a three-dimensional and interactive way, giving visitors a pho-torealistic impression of objects. Not only existing scenarios can be shown, but VR can also be used to rebuildscenarios that were damaged or destroyed a long time ago, giving new life to the cultural heritage.We used Virtual Reality to present the Mogao Grottos in Dunhuang. This cave site is one of the most importantcultural and religious places by the ancient Silk Road. The presentation is to give visitors the impression of visitingthe cave site and provide information about the caves, paintings and statues in an interesting way. To achieve this,we developed a new, intuitive interaction paradigm, which enables the user to explore the caves. To give observersa photorealistic impression of the caves and to create a feeling of immersion, innovative rendering techniqueswere integrated.The resulting presentation combines Virtual Reality and archaeology to give tourists a realistic experience of thiscave site and to support scientists in their research work.

    1. Introduction

    The city of Dunhuang is an oasis town located in the north-west of China. Formerly, Dunhuang was an important pointby the Silk Road, which splits in Dunhuang to follow thenorthern and southern borders of the Taklamakan Desert.Nowadays, the town is famous for the Mogao Grottos, a cavesite which is one of the major and best-preserved cultural re-licts of the ancient Silk Road. The site consists of about 500caves (see Figure 1, more than 3,000 statues and over 45,000m2 of painted murals. These caves were dug and decorated

    Figure 1: A part of the cavesite

    by Buddhist monks between the fourth and the fourteenthCentury. The most famous cave is the library cave (cave17), which was discovered in 1900 by Wang Yuanlu, a Daoistpriest. Inside that cave more than 40,000 ancient documentswere found1. These documents contain important informa-tion about religion and everyday life in former times.Sand, humidity, time, and tourists have destroyed parts of the

    cave site. Today only about 40 of the 500 caves are open tothe public. To protect the paintings and statues, shields wereinstalled in the caves. In 1943, the Dunhuang Academy wasfounded. This academy is responsible for the protection andstudy of the Dunhuang relicts and runs a museum close tothe cave site. The cave site is under the protection of theChinese government. In 1987, the Mogao Grottos were de-clared a World Heritage by the UNESCO.In a cooperation between the Zhejiang University inHangzhou (China) and the Fraunhofer-IGD in Darm-stadt/Rostock (Germany), a computer-based environmentfor the preservation, restoration and propagation of Dun-huang art will be developed.A new approach to combine archaeology and Virtual Realitywas developed, giving us the possibility to present historicalplaces to tourists and to support researchers in their researchwork. A new interaction device enables visitors to explorehistorical places in an intuitive and interesting way.

    2. Goals of the ProjectThe goal of this project is to use computer technology to helppreserve, restore and propagate Dunhuang art. The Germanpart of the project is divided into three main topics2:

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    Dunhuang InfoWeb Internet-based VR High-end VR presentation

    This paper focuses on the high-end VR presentation. Twodifferent kinds of users, tourists and researchers, were iden-tified. In the first phase of this project, we mainly developeda system for the propagation of the Mogao Grottos. The VRpresentation will give tourists a photorealistic impression ofthe beauty of the caves, they can virtually visit the Dunhuangcaves. They even can enter those which are normally closedfor the public and explore all the statues and paintings in-side. Additionally, it is possible to obtain more informationabout these paintings and statues and about the constructionof the caves.To support research processes, a visualization system forrestoration results was developed.

    3. Generation of the Models

    3.1. Acquisition of Data

    For the generation of the models and the presentation, dif-ferent kinds of data were needed. It was not possible to getoriginal data from the Dunhuang Academy, due to the exportregulations of the Chinese government. It is also forbiddento take photographs at the cave site. Thus, we had to focuson those caves which provide enough information (caves 428and 303) to create the virtual models (see colorfigure 15).The following data was used for the modeling process:

    Plans of the caves and the cave site. Photographs of the interior and exterior of the caves. Historical information about Dunhuang. General information about the caves and the cave site. Personal experience.

    In March 1998, our team visited the cave site to get fur-ther information and to get an impression of the caves. Thishad great influence on our realization of the presentation andhelped us to obtain a realistic representation of the caves.

    3.2. Modeling the Caves and Statues

    The first step to take was the generation of the 3D geometri-cal models of the caves.The modeling of the cave geometry was done with formZ,a 3D modeling program, by using plans as well as picturesof the cave. All the missing measurements were calculatedusing known values and pictures of the objects.The same base models were used for the web-based and thehigh-end VR presentation. For the web-based presentation,the limitations of the Internet (like the limited bandwidth)and the limitations of the soft- and hardware (PC / VRMLplayer) had to be considered. Therefore, the modeling startedwith a simple representation of the cave interior. Only themost important geometries, like the walls and the middlepillar, were modeled (see figure 2). For details, like statues,

    textures were used to reduce complexity. Replacing 3D ob-jects by textures proved to be sufficient if only 2D projec-tions were used and if the object could only be seen fromnearly the same direction as the picture had been taken. InDunhuang, statues often were placed inside niches, so thatthe viewing angle was limited. The simple geometry of thecave 428 was built with less than 100 polygons. These mod-els were generated for the web-based presentation, but werealso used as a basis for the detailed models for the high-endVR presentation. The objects were exported as VRML mod-els and included into the Dunhuang Info-Web.For the high-end VR presentation, the limitations of the

    Figure 2: Simple model of the middle pillar

    Figure 3: More detailed model of the middle pillar

    VR system (see chapter 4) and the hardware had to be con-sidered, but due to the use of a high end-graphics work-station, the models could be more complex. The high-end model consisted of 14,560 polygons, while the VRMLmodel only consisted of less than 100 polygons. Taking intoaccount that we used stereo projection, more details had tobe added, especially where textures were used instead ofthree-dimensional objects. Otherwise these textures wouldlook flat and this would destroy the feeling of immersion.

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    Three-dimensional representations of objects were neces-sary to achieve a realistic illumination and to generate shad-ows. Only for some of the smaller objects textures were usedinstead of detailed geometry to reduce the complexity of thescene.

    During this phase, more details were added (see figure 3)to the base models. The textures used for the statues in theVRML models had to be replaced by 3D objects. These ge-ometries are the most complex objects inside the cave 428;the Buddha statue in cave 428 consists of 4,719 polygons.Statues were very difficult to model because of their organicform. We used Softimage to generate these organic formswith Metaballs. Photographs were used to control the mod-eling process. Furthermore, no plans or other data of the stat-ues were available, therefore all the measurements had to becalculated from photographs3; 4 of the statues.Finally, the geometric models were converted to the FHS(Fraunhofer Standard) data format that is used by the Vir-tual Design II VR system5 (see chapter 4.1).The models were built using plans of the cave, giving us themost important measurements, like the height of the ceilingor the dimensions of the floor. Using this information, somedetails, which may be important for scientist, could not bemodeled. For example, all the walls were modeled as pla-nar faces, which is not true for the real walls. The qualityof the virtual caves is good enough to give tourists a realis-tic impression of the caves. These models can also be usedby scientists to compare the different styles of caves duringthe different dynasties, however, there are not enough de-tails for doing scientific research on a specific cave. Withoriginal data available (exact measurements of the caves,photographs of all the paintings and statues, etc.), modelswith the necessary level of detail for research work could bebuilt. The use of a 3D scanner or photometric modeling toolswould speed up the modeling processes of these statues.

    Figure 4: The real grotto

    3.3. Texturing

    The Dunhuang Caves are very famous for their paintings,which include:3

    Buddhist icons

    Figure 5: The virtual model

    Buddhist stories Illustrations of Buddhist Sutras Portraits of donors Daoist deities and mythological figures Apsaras and music goddesses Decorative patterns

    Like the documents found in the library cave, these imagesgive researchers important information about Buddhismand the everyday life in former times. For a tourist, thesepaintings with their small details and the variety of scenesare very impressive. One can spend hours inside one caveexploring all these artworks. Almost every wall are painted,and the visitor is surrounded by the mural paintings. Topresent this variety of images and to achieve a realisticimpression of the cave interior, high-resolution textureswere used.The generation of the textures was difficult because ofthe lack of original data. There were only a few cavesthat provided us with enough pictures to get the necessarytextures, and even in those caves some images were notavailable. In locations where we had no photographs, similarpictures from other places inside the cave were used instead.Because of the varying lighting conditions and perspectivedistortions, the images had to be postprocessed after scan-ning. To use the textures for the web-based presentation,the images were reduced in resolution to lower the loadingtime.To support the restoration of the caves, we added thepossibility to visualize restored images. A virtual cavegives researchers the opportunity to see the results ofthe restoration process in advance. The different renewedimages can be viewed in their original surrounding and inrelation to neighboring images. To show the possibilitiesof VR for the visualization of restoration, images of thecave in its original state were needed. Since such imageswere not available, an image processing program has beenused to renovate the textures. We removed scratches,included missing parts and enhanced the faded colors (see

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    colorfigure 15). We did not take into account any seriousscientific methods for restoring these images. Our goalwas to demonstrate new methods and approaches for thepreparation of the restoration process . Our partners at theZhejiang University in China will develop an expert systemfor color restoration. As soon as this system is finished, itwill be connected to the VR system and the images will bereplaced by images generated by this expert system (seechapter 6). The resulting virtual model presents geometricalsimplicity with high-quality textures, thus appearing to bevery realistic in comparison to the pictures of the real cave428 (see figures 4 and 5).

    4. Generation of the Presentation

    4.1. Technical Description of the VR System

    The VR system Virtual Design II5 is used for the presenta-tion. This VR system was developed by the Fraunhofer-IGD.Virtual Design II allows us to use many different output de-vices. A five-sided CAVE6 was mainly used for the genera-tion of the presentation. Alternatively, a large-screen stereoprojection can be used, which is a good way to show the Vir-tual Dunhuang Art Caves as part of an exhibition or a mu-seum. If needed, a monitor can be used or video tapes can begenerated.It was very important for us to have the ability to developspecial interaction devices, and to achieve special renderingeffects. Both are possible with Virtual Design II.

    4.2. Principles of the CAVE Presentation

    Whenever presenting a digital architectural model in aCAVE, one has to take into account the characteristics andlimitations of this particular output device from the point ofarchitecture. For VR presentations, the actually simple roomof a CAVE turns into a corridor to an apartment or an en-trance to a more complex world. You have to make the visi-tors curious to see the whole apartment.Overcoming the boundaries of the CAVE (2,4 m x 2,4 m x2,4 m), to create the feeling of being in the virtual worldmeans widening the room inside the CAVE to the extensionsof the scenario. Attracting the visitors whole attention willmake him/her forget the fact, that he is actually surroundedby five projection walls presenting realtime stereo images.Enabling the visitors to satisfy their expectations of free in-teracton and corresponding to their behavior in real environ-ments, we integrated a new interaction device to the virtualworld of the Dunhuang Grottos.

    5. Interaction

    5.1. The Flashlight

    To achieve a realistic impression of the caves, we appliedradiosity calculations to the scene using Genesis7. The

    Figure 6: Presenting the cave within a CAVE

    result was a well done three-dimensionality with diffusereflections, emphasizing the mural paintings and the supe-riority of the sculptures. The lighting conditions inside thereal caves were simulated very well, but just like inside thereal caves, it was too dark to recognize the details of thepaintings. To show the beauty of the colorful paintings andstatues to the visitor, additional light sources are necessary.Until now, it is not possible to recalculate radiosity values inrealtime in complex scenarios with moving light sources.With the demand of showing all treasures and exhibits wellilluminated, we used the normal hardware shading, but thatadded an unrealistic brightness to the scenario. The reallight conditions at the site are completely different. Enteringthe caves means coming into absolute darkness; which iswhy these approaches were dismissed.Inside this dark cave we felt like being an archaeologist,exploring dark caves, armed with a mere flashlight, in thehope of discovering secret treasures.This idea quickly transformed into the solution: By accept-ing the situation of total darkness, a geometrical model ofa flashlight was integrated into the virtual environment ofthe cave 428 to create a highly realistic impression and anauthenticcave-feeling.We first tried to use an OpenGL spotlight, but that did notlook convincing enough. The OpenGL spotlight lightingcurrently available is only calculated at vertices, whichcreates pumping artifacts for coarse geometry. Furthermore,these spotlights do not cast shadows, which are an importantcue for spatial relations and realism in general.Thus, we had to come up with a different solution. The

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    only way to create the effects we desired involved multiplerendering passes: Render the fully lit scene first, darkeneverything that is shadowed and finally multiply in theimage of the flashlight (see figures 7 and 8).

    The shadows themselves are realized using the SGI

    Figure 7: Flashlight activated, without shadows

    Figure 8: Flashlight activated, with shadow

    OpenGL shadow extensions. To use these extensions, animage is rendered from the viewpoint of the flashlight. Thedepth buffer of this image is used as a texture. In a secondpass, the whole geometry is rendered again using auto-matically generated 4D texture coordinates that representthe geometrys coordinates in the coordinate system of theflashlight8. The shadow extensions allow the comparisonbetween a texture coordinate and the texture value. Thiscomparison decides whether there is something closer to theflashlight than the point in question. If there is, the point isin shadow, otherwise the point is visible from the flashlightand thus lit. For points that are in shadow, their color valueis multiplied by an ambiance factor in order to avoid theabsolute blackening of the shadows. This method createsonly hard shadows, a point is either in shadow or not. Aflashlight typically has a bright spot in the center and getsdarker the further you get to the edge. Furthermore, there issome structure in the light created by the projection of thefilament. To create that effect, we used a separate pass that

    projects an image of the light distribution of the flashlightinto the scene (see figure 9)What is left after the shadowing is multiplied by that image

    Figure 9: Flashlight texture

    to darken the parts that are close to the outer edge of the litareas.

    Thus, the overall geometry has to be rendered 4 times: onceto render the lit scene as a basis, once to create the depthbuffer as seen from the flashlight, which is used in anotherpass to darken the points in shadow, and finally to add theintensity drop-off and structure seen in a typical flashlight.The effect of a second flashlight is also interesting, but theframe rate would drop too much for a realtime application.For the rendering of the CAVE presentation, an SGI Onyxwith 3 Infinite Reality Graphic Pipes is used.Once implemented, the effects of the described feature werequite surprising:The screens of the cave cube seemed to disappear.Realizing the fact that the light of the flashlight stopped infront of details of the virtual cave, the real projection wallswere overcome.

    This interplay between light and resulting shadows, enablesthe observer to get the feeling of becoming a part of thevirtual world.Facing the behavior of the beam of light, both the cave cubeand the virtual cave seem to be merged together, resulting inbeing one cubature.There was still a restriction that had to be removed. At

    this stage, a data glove was used to handle the virtual lamp.Apart from the fact of constantly putting the glove on andoff, handing the flashlight over to the next explorer, theforce feedback was missing, too:The feeling to hold a real lamp in your own hand wasmissing. To control the light, using a data glove, was notvery intuitive. Many visitors needed further explanations onhow to handle it.That is why a physical flashlight, with a Polhemus tracker

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    Figure 10: The virtual flashlight

    Figure 11: A flashlight as an interaction device

    attached inside, was connected to our VR system. Enablingthe switch of the lamp made its natural functionalityaccessible to the explorer, he can use this button to switchbetween the two different lighting situations (see figures 12and 8).The resulting gadget and its impact on the virtual caveenvironment are so convincing that some people look intothe flashlight and are surprised they are not blinded by thebeam they expect but do not find.However, the most impressive circumstance is pretending tobe an explorer searching for secret treasures, by discoveringthe cave step by step with only a flashlight at hand (seecolorfigure 17).As a result of this, the explorer feels totally unrestricted.Up to now, when entering the fully lit scenario (see figure12), he would get dizzy after a short time, caused by thenumerous exhibits and the various mural paintings.

    With the light turned off and being armed with theflashlight, he is able to discover detail after detail and toexperience the whole beauty of the cave. In addition, theexplorer can stop at will, at arbitrarily chosen positions,in order to thoroughly inspect the various cave arts. Thespot of the light helps him to concentrate on single partsof the paintings. This way, the perception of the observeris sharpened for the artifacts around him, and equally theexhibits get the attention they deserve. To get an overview,

    Figure 12: The full lighted model

    he can switch to the full-lighted model (see figure 12) atanytime.

    5.2. The Virtual Tour Guide

    In order to provide people visiting the virtual caves withadditional information about the meaning of the paintingsand the statues, a virtual tour guide was integrated into thepresentation. In order not to disturb the visitor with 3D textfloating in his field of view, distracting his attention from theobjects he likes to view, speech output was used. The visitorcan view the scenario and listen to the information providedby the virtual voice, just like he would listen to a real tourguide.

    5.3. Seeing the invisible

    The museum run by the Dunhuang Academy provides visi-tors with information that is not displayed in the real caves:the building process of the caves, the different materialsused, and how the statues were constructed. In the virtualcaves, the paintings can be removed (see figure 13 and col-orfigure 16) to reveal the underlying layers. All the layerscan be removed step by step to show the visitors the struc-ture of the caves.

    6. Results and Future Work

    In this first part of the project, we realized a Virtual Realitypresentation of the Mogao Grottos. The focus was mainly onthe propagation of the caves. We showed that it is possibleto give tourists a very realistic view of these historical im-portant caves. Visitors can explore the virtual cave with theflashlight, getting a convincing impression of the size andbeauty of the real caves. Using the flashlight as an interac-tion device makes it easy to interact with the virtual envi-ronment, even for people who never visited a virtual worldbefore. The virtual flashlight does not only generate a pho-torealistic representation of the scenario, but also makes the

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    Figure 13: One painting removed

    presentation more interesting for the visitors. Tourists canget more information about the caves with the help of thevirtual tour guide, and they can see things which cannot beseen in the real caves. It is possible to see objects from view-points which are impossible to be reached in reality.This presentation can be used for exhibitions or in muse-ums to make the Dunhuang caves accessible to many people.Even the closed and endangered caves can be visited. Withthe help of restaurateurs and expert systems for restoration,tourists can see the caves in their old beauty, just as if theywere newly built. Researchers can benefit from the virtualcaves. Different styles of caves, paintings and statues caneasily be compared. It is possible to present and to discussthe results of restoration processes in advance.Virtual Reality has proved to be a valuable tool to presentcultural heritage in a realistic way. The three-dimensionalprojection, the feeling of immersion, and the possibility tointeract with the virtual world in realtime let the visitors be-come a part of the presentation. Tourists can discover histor-ical places, places which are in danger of being destroyed bytoo many visitors, and places which already are destroyed.In the next months, the presentation will be further devel-oped. First, an evaluation of the existing system will takeplace. For this purpose, we will develop a questionnaire toobtain information about the quality of the presentation. Thenext step will be to include more information in the virtualenvironment. A new concept for the virtual tour guide has tobe developed. This concept shall allow us to include differ-ent kinds of media in the information system, and to adaptthe provided information to the demands of different usergroups (researchers, tourists, school kids, etc.).The first part of our work showed us that it is possible topresent the Mogao Grottos using Virtual Reality. During thework, we have had many ideas for the further developmentof this presentation. In our opinion, the most important top-ics are:

    the inclusion of more caves (at least one of each dynasty), the connection to a multimedia information database,

    the connection to the expert systems for restoration devel-oped at the Zhejiang University,

    the generation of detailed models for research purposesusing original data,

    the inclusion of other intuitive interaction metaphors (vir-tual camera, speech recognition, etc.).

    the implementation of more tools to support future re-search work.

    Acknowledgments

    The presented work is the result of a project funded by theDLR. We thank Dr. Koepke and Mrs. Hongsernant for theirhelp. We also thank our colleagues at the Zhejiang Univer-sity in Hangzhou (China) and at the Fraunhofer-IGD in Ro-stock (Germany).We thank our colleagues and students who implemented thetools and modeled parts of the virtual caves. In alphabeticalorder they are: Jochen Ehnes, Stefan Hoehne, Henry Kohtz,Claudia Pilo, Gerrit Vo, Silvia Zamai and Heike Ziegler.Special thanks to Dirk Reiners who implemented all the ren-dering effects.

    References

    1. S. Whitfield. The international Dunhuang Project.http://idp.bl.uk/idp/idp1.html, 1998.

    2. E. Berndt, S. Hambach, S. Mader, and B. Lutz. VirtualDunhuang Art Cave. Computer Graphik topics, 10:14 15, 1998.

    3. N. Qiang. Dunhuang studies - A Introduction.http://www.silk-road.com/dunhuang.shtml, 1997.

    4. Guo Lian Fu and Wang Wei Zhong. Dun Huang Shi KuYi Shu. ISBN 7-5344-0558-0/J-559, 1993.

    5. P. Astheimer, F. Dai, W. Felger, M. Gbel, H. Haase,S. Mller, and R. Ziegler. Virtual Design II - An ad-vanced VR System for Industrial Applications. In Vir-tual Reality World 95, Febuary 1995.

    6. Prof. J. Encarnao, Chr. Knpfle, S. Mller, andM. Unbescheiden. Evaluation of VR-Technology: Ex-periments and Applications with 3-, 4- and 5-sidedCaves. In The Future of the WWW, April 1998.

    7. S. Mller, F. Schffel, N. Gatenby, T. Grosch, S. Krake,W. Kresse, O. Liesenfeld, A. Pomi, D. Reiners, andM. Unbescheiden. Genesis Manual Version 2.4, 1997.

    8. M. Segal, C. Korobkin, R. van Widenfelt, J. Foran, andP. E. Haeberli. Fast shadows and lighting effects usingtexture mapping. In Edwin E. Catmull, editor, Com-puter Graphics (SIGGRAPH 92 Proceedings), vol-ume 26, pages 249252, July 1992.

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    Figure 14: [Lutz and Weintke] Cave 428: Full lighted model of the caves 428 and 303

    Figure 15: [Lutz and Weintke] Cave 428: Mural with normal and restored paintings

    Figure 16: [Lutz and Weintke] Cave 428: Different materials below the paintings

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    Figure 17: [Lutz and Weintke] Cave 428: Exploring the cave 428 with the flashlight

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    Virtual Dunhuang Art Cave: A Cave within a CAVEAbstract1. Introduction2. Goals of the Project3. Generation of the Models3.1. Acquisition of Data3.2. Modeling the Caves and Statues3.3. Texturing

    4. Generation of the Presentation4.1. Technical Description of the VR System4.2. Principles of the CAVE Presentation

    5. Interaction5.1. The Flashlight5.2. The Virtual Tour Guide5.3. Seeing the invisible

    6. Results and Future WorkAcknowledgmentsReferences

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