Enhancing Virtual Reality Walkthroughs of...

4th International Symposium on Virtual Reality, Archaeology and Intelligent Cultural Heritage (2003)D. Arnold, A. Chalmers,F. Niccolucci (Editors)

© The Eurographics Association 2003.

Enhancing Virtual Reality Walkthroughs ofArchaeological Sites

G. Papaioannou, A. Gaitatzes and D. Christopoulos.

Foundation of the Hellenic World

AbstractThis paper describes the methodological aspects of the application of various established and newgraphics techniques in virtual reality applications, in order to visually enrich conventional walkthroughsand extend the common capabilities of virtual environment visualization platforms. The paper describesthese techniques and goes to the extent of explaining various practical implementation issues. Examplesand application case studies are provided to demonstrate the enhancements.

Keywords:Virtual reality, lightmaps, image-based rendering, culling, video.

1. Introduction

Virtual reality is usually associated with large surroundscreen projection screen installations, stereo displays,head mounted displays and other high-tech hardwareencountered in theme parks around the world. Butvirtual reality is in essence the art of illusion ofsimulating a fake environment where the spectator isimmersed and cut-off from reality as effectively aspossible, no matter the means to achieve this. Eachapplication area has its own hardware and softwareplatforms to support this alternative experience,depending on the role of the VR environment and theamount of interactivity the users are involved in.Naturally, VR is an attractive and intuitive means tovisualise and explore the past and is thus exploited ineducation, tourism and research in the cultural heritageand archaeology domain.

Most archaeological applications of virtual realityare focused on exploring virtual reconstructions of sites,buildings, outdoor scenes and individual artefacts, withan educational motivation, although other archaeologyrelated applications such as excavation planning andautomated reconstruction tools do exist as well. Throughthe use of intriguing and visually appealing virtualworlds, people and especially young children, caneffortlessly study and learn while being entertained. Inour experience, Immersive environments have asignificant impact on the memorization of informationand the contextual linking of entities and meaningsdiscovered in a virtual tour, rendering VR an effectiveeducational tool.

Using cultural heritage content in a VR application

implies that a minimum level of representation validitymust be conveyed from the historical sources to thevisual content.1 Unfortunately, historical validity,although of crucial importance to an educational /edutainment / research scenario, is a limiting factor tothe amount of visually stunning computer graphics andthe nature of interactive story-telling. Visualisingancient worlds is a vivid example of this type oflimitation due to the uncertainty about the exactappearance of buildings, clothing etc, due to lack ofconcrete and detailed evidence for many elements thatappear in a virtual world. Imaginative creation in suchan environment is not always allowed, because bringingthe extremity and exaggeration of Hollywood-likeproductions to a cultural heritage application can anddoes spoil the value of the historical research behind theVR walkthrough.

Therefore, by avoiding building on too littleinformation and distorting facts, archaeologicalwalkthroughs can only highlight and visually enhancethe virtual world by pursuing the convergence ofphotorealism and real-time rendering. It is true, thathardware technology in haptics, displays and graphicsprocessing, reaches new levels of perfection and speedevery day, but at the same time, the demand of thespectators is increasing with equal or more rapid rates.High detail representations encountered in pre-renderedgraphics are too slow to render in real-time systems andthe same holds true for the light interaction and visualeffects. Instead, VR and real-time visualization requirescarefully simplified models and fast and cleveralgorithms to brute force rendering of off-line systems.Textures replace geometric details, models are presentedin multiple levels of detail, surroundings are displayed

2 Papaioannou, Gaitatzes and Christopoulos / Enhancing Virtual Reality Walkthroughs of Archaeological Sites


only when they are visible and so on 2. Evidently,programming and computer graphics theory techniquesfrom the realm of consumer-level game industry have tobe utilised even in large-scale visualisation installationsto bridge the gap between the world of cinematicproductions and real-time VR applications.

This paper summarises the attempt to visuallyenhance archaeological walkthroughs through the use oftechniques already applied individually in the gameindustry and broadcast applications in an immersiveVRplatform. At the same time, many optimisationtechniques are presented as well, which are used tocounterbalance the processing overhead of the visualenhancements. The reader should also bear in mind thatan immersive VR application has the processing cost ofa conventional 3D shoot-em up game with largepolygonal environments multiplied by the number ofdisplay pipes involved in the surround screen setup, andall that doubled for stereo vision.

2. Enhancing Visual Credibility

When VR was first utilised for walkthroughs, thestate of technology only allowed for single-pass-texturedsimplistic polygonal environments with simple triggeredinteraction. In the meantime, games and low-endcomputer systems have evolved, but these advanceswere not ideally followed on the more cumbersome VRplatforms, mostly due to lack of competitiveness in therespective market.

Visual realism in computer games comes in manyflavours but mostly relies on heavy texturing, specialillumination effects, pre-computed illumination anddetailed animation. A VR application, being essentiallya game engine, can incorporate the technologiesassociated with these enhancements. Manycommercially available visualisation libraries for high-end systems, like SGI’s OpenGL Performer™ andOpenGL Shader™, already use standard techniques likemultiresolution models and image-based rendering.

2.1. Pre-calculated illumination

Recently, new advances in graphics hardware havemade possible the use of real-time shadows in computergames, although still at a severe computational cost.Fortunately, virtual archaeological worlds mostlycomprise of inanimate environments and the interactionwith the scenery is non-destructive. This way, dynamicillumination, including shadow generation is limited tomoving characters and a few props (e.g. carts) that movein the vicinity of the virtual camera.

Most archaeological walkthroughs involve outdoorscenes with reconstructed buildings. Pre-calculated

illumination in the form of illumination maps(lightmaps) 3 as double pass to standard material texturesor standalone textures like baked or combined textures,has more visual impact and is cheaper than the shadowvolumes shadow generation technique. Dynamicshadows are only needed for the few moving objects,when these shadows cannot be approximated byprojective textures. Furthermore, outdoor scenes are litby sunlight, which can be simulated by a combination ofsharp-shadow, infinite point light source and a simpleskylight direct illumination model. Most commercialmodelling and rendering software supports baked texturegeneration and export mechanisms, although customsolutions can be adopted (see Figure 1 and the casestudies section).

2.1.1. Memory and Texture Space Utilisation

The biggest issue with baked textures is the memoryutilisation. Even in expensive graphics subsystems,texture memory is a valuable resource, which the

Figure 1: Skylight illumination on a low detail model ofthe Temple of Zeus at Ancient Olympia, Greece. Belowit, the lightmap used for texturing the column ring of thetemple.

Papaioannou, Gaitatzes and Christopoulos / Enhancing Virtual Reality Walkthroughs of Archaeological Sites 3


programmer quickly runs out of. Texture baking is agood alternative to multi-pass rendering because itcombines the material decals with the otherwise blendedillumination second pass texturing. Unfortunately, theadvantage of using tiled textures for repetitive patterns islost, as the single pass textures that incorporate theillumination information need unique texturecoordinates. This obviously leads to poor texturememory utilisation and a data transfer bottleneck whentransferring textures to and from the display hardwarememory. The apparent advantage of having a single passinstead of a double one is very frequently outbalancedby this texture swapping, especially in complex scenes.

Bad memory utilisation often occurs from poortexture packing. Lightmaps are built for individualpolygon clusters, after segmenting the initial geometryinto patches of adjacent triangles, using the triangles’normal vector direction as a splitting criterion 4. Thelightmaps are then combined into a single texture andthe mesh vertices are assigned unique texturecoordinates. This process involves “packing” or stackingthe lightmaps into a single image using an image spacesubdivision method 5. An initial estimate of the size ofthe individual lightmaps can be determined using theratio of the area of each lightmapped polygon cluster (orthe axis-projected area, if axis-oriented splitting is used)against the total surface area. Unfortunately, the texturepacking procedure may leave unused space between thelightmaps in the final image, resulting in a failure toinclude all maps into a single texture. One solution is touse a second texture map and store the remainingtextures there, but as texture memory is a limiting factor,it would be preferable if all lightmaps could be packedwithin a single, user-constrained texture.

We achieve this by incorporating in our customlightmap renderer an iterative lightmap resizingmechanism, which operates before actually renderinginto the lightmaps: Relative lightmap sizes aredetermined using the projected polygon area of the

polygon clusters as mentioned above. Then thelightmaps are repeatedly scaled and packed into a singetexture until they fit as tightly as possible withoutexcluding any lightmap (Figure 1). This way,illumination maps get the maximum crispness possiblefor a given texture size and no valuable texture space iswasted in blank image space.

2.2. Video Source Overlays

Merging 3D worlds and real action is a commonpractice in cinematography. Chroma-keyed videosequences of actors and props filmed in front of a bluescreen are easily overlaid and mixed with pre-renderedanimation sequences and imaginary scenes. But what ifthis experience should be transferred to the VR set?Real actors may participate as virtual guides into thereconstructed world or be a part of the backgroundactivity of a marketplace and give an impression offamiliarity, natural movement and look to the polygonalscenery 6.

The biggest problem is that most video formats donot provide an alpha channel (transparency) and, worse,most media libraries do not support this extrainformation, not to mention cross-platform portabilityissues. Therefore, it is difficult to infuse a blendingmechanism in the video itself. Porting the video post-process notion of chroma-keying to a real-timerendering system is also a poor solution. An acceptablequality video source, especially for close-up inspection(e.g. a virtual guide) cannot be of a resolution less than256X512 pixels wide for a figure or object covering 1/4-1/3 of the viable screen area. Filtering this image toderive an alpha value from the RGB hue at a frame rateof 15-30 fps is too demanding a process to have on a VRsystem already struggling to render all the texturedgeometry and handle the game engine events.

In our implementation, we have opted for anOpenGL® blending mode that uses double passrendering to simulate masking 7 (Figure 2). The maskingoperation is as follows: An inverted mask is generatedfrom the original source, which is white where the imageshould appear transparent, and black in the solid areas.Additionally, the background colour of the originalimage (e.g. blue for a blue screen) is blackened. Themask is rendered first with a disabled depth test and ablending function of (GL_DST_COLOR, GL_ZERO).Then, the normal image is rendered with a blendingfunction of (GL_ONE, GL_ONE) on top of the mask.

This technique was originally applicable to low-level immediate OpenGL® rendering, but the idea wasported to a scene-graph-based environment, based onOpenGL Performer™. The original video is postprocessed to fit a power-of-two size, as the video sourceis used as a texture. Using chroma-selection, an inverted

Group node

Mask Video Billboard

Image Video Billboard

Rest of scene graph

Traversal order

Figure 2: Video Overlay on billboard geometry in ascene graph.



masking video is produced and saved in a separate videofile. In each frame draw call, a time-synchronised frameis loaded from the buffer of both channels (mask andimage) and placed on an identical piece of geometry. Inthe case of our VR engine, a billboard was adopted asthe underlying geometry (Figure 3), as the original usescenario demanded an image of a real actor alwaysmoving in front of the user, in a guided tour.

2.3. Image-Based Modelling

High geometric detail is a key factor for thegeneration of a convincing virtual world. For parts of thescene that are far away from the observer or objects thatthe user does not interact with, it is a common practiceto replace the geometric complexity with textures 8. Animportant psychological aspect of the VR walkthroughis that by constantly moving through the virtual world,the representation accuracy is mostly felt rather thanmeticulously observed. Therefore, if heavy, life-liketexturing replaces detailed architectural models in anarchaeological site, the image is going to “fit in” thesenses and the impact is going to be greater thanoverloading the system with modelled details that no oneis going to get too close to notice.

Extending this idea, image-based modelling can beused for the generation of simplified geometry fromphotographs of real archaeological sites or objects. Thebig advantage is that one gets at the same time a low

polygon representation of the scenery and properlyregistered appearance and illumination maps, directlyfrom the recorded imagery (Figure 4).

Image- or photogrammetry-based modelling workswell with both distant outdoor entities, like tree lines andwhole buildings, and closely inspected crude objects likeslabs and columns. The main disadvantage is the viewdependency of the texture maps, which implies that in

Figure 3: A Video source (girl) is superimposed as abillboard in the scene. The partial silhouette haloing isa result of the chroma-keyed transparent cloth and hair.

Figure 4: Image-based modeling. Above: Visualdetail is acquired with conventional photography andimprinted as textures on simple geometry. Below: Themonument of Philopappou in Athens. Textures basedon real pictures of the site cover a roughapproximation of the monument’s geometry.



order to avoid distortion or exposing unregistered anduntextured parts of the scene, one has to restrict thenavigation activity close to certain paths. Evidently,image-based modelled objects have as low a polygoncount as the modeller wishes but on the other hand, theysuffer from the same restriction that applies to theillumination maps: The textures derived from the imagecapture and warping procedure cannot be tiled, thusincreasing the memory utilisation of the graphicssubsystems.

The procedure of extracting the geometry from asequence of images itself is a fairly known but tedioussemi-automatic task that involves defining andcalibrating virtual cameras in 3D space that correspondto the photographs taken and the detection of commonreference points in the original photographs 9. Usingcomputer vision methods, these points are back-projected in 3D space and can be used as buildingreference points to model a crude approximation of theobserved geometry. By projecting, merging and clippingthe original photographs on the polygons generated byhand, one can get the textures required to display theoriginal objects in a convincing manner.

2.4. Detail Textures

Another technique for adding visual detail in avirtual scene and more precisely to the terrain and tiledmaterials, is the use of detail textures. They are specialtextures used to give some sense of detail when viewedfrom very close. When the main texture starts to getblurry from being viewed too close to it, the detailtexture gradually blends in with the coarse layer. Adetail texture is a very small, fine pattern which is fadedin as you approach a surface, for example wood grain, orimperfections in stone. Detail textures modulate thesurface colour they are applied to, this way scaling thesurface's brightness up or down. Colour components inthe range 0-255 with RGB brightness values from 0-127darken the surface; 128 has no effect; and 129-255brighten the surface. Therefore, when drawing detailtextures, it is important that you design them with theirbrightness in the proper range so they affect surfacesnaturally. If a detail texture's average brightness deviatestoo far from 128, then surfaces will appear to brighten ordarken as you approach them.

A second rendering pass of the detailed geometry isusually required, unless multi-texturing is nativelysupported and this affects frame-rate, so it is ajudgement call when to use them and when not to. Foroutdoor terrain however, where, because of thewalkthrough navigation mode, the viewpoint is alwaysclose to the textured ground surface, detail texturessubstantially improve the crispness and natural look ofthe image; the base texture alone, no matter how large,cannot stand a very close examination. In practice, a

small 64X64 or 128X128 detail texture with Gaussianmonochromatic (greyscale) noise samples centred at 128RGB intensity has a very good visual impact.

3. Speed Optimisations

3.1. Image-Based Rendered Characters

The rendering of outdoor and indoor environmentsrequires the synthesis of what are often considered twoseparate problems: the real-time visualization of largescale static environments, and the visualization ofcharacters and probs.

In archaeological and walkthrough applications mostof the bandwidth and rendering capacity is used for theoutdoor and indoor environments and buildings, whichare usually the points of interest. In order to enrich theseenvironments characters and other props might be used.To render many animated characters and props ascomplex polygonal models will result in non interactiveframe rates, therefore a balance has to be made betweenthe amount of models used and their complexity.Unfortunately using exclusively low polygon modelscomes into contradiction with the rest of theenvironment, which might be of better quality and istherefore bound to break the suspension of disbelief.

In order to minimize geometric complexity andachieve a balance between amount and quality ofmodels, image based rendering and impostors are oftenused for specific models. Especially humanrepresentations have been successfully modelled usingimage-based approaches 10. The principle of imagebased rendering techniques is to replace parts of thepolygonal content in the scene with images. Theseimages can be computed dynamically, as is the case withrelief texture mapping 11, or a priori and are then placedas impostors into the environment using simplegeometry like planes, which always face the viewer.Usually the images for the impostor are taken fromdifferent positions around the original object and areselected at run-time depending on the viewpoint positionand the frame of animation.

In our recent application "Feidias' Workshop" weimplemented IBR techniques for visualization ofanimated characters and discovered their usability andimpact in VR and educational applications. Eachcharacter was replaced by an impostor, whose texturewas changed depending on the viewpoint position andthe frame of animation (Figure 5). Thus there wasinformation for all the possible views of the impostor.The geometry used for the impostor was a quadrilateralfacing always the user (billboard). Pre-rendered view-dependent textures were preferred instead of dynamictechniques like relief textures due to the seriouscomputational overhead imposed on the processors,especially in the case of multiple IBR nodes.



3.1.1. IBR advantages

Using IBR impostors has various positive impactson the application if implemented properly. Since therendering of the images is usually done offline therendering speed in real-time applications is significantlyfaster in comparison to using polygonal models.

The original models from which the images aretaken can be detailed, high-polygon-count models.These models, although being too expensive for real-time use, are perfect candidates for IBR techniques,since the geometric complexity does not deter the offlinerenderer and provides convincing visual details for theillumination interplay. This possibility of high-detail hasbeen very beneficial in educational and archaeologicalapplications. The human eye is accustomed to detectdetails on depicted human figures and therefore,complex pre-rendered imagery increases their photo-realism and natural appearance.

Modelling human characters for real-time use is adifficult and daunting job, which only skilled modellersand artists can perform efficiently. A low polygoncharacter aims at being efficient in structure, polygoncount and texture usage, while at the same time avoidssacrificing too much quality. This trade-off involvescareful and skilled modelling. Models needed for IBRdo not have these restrictions since they are renderedoffline. Often commercial packages can be used toconstruct an initial high-polygon human model.Alternatively, the source for the images used on the

impostor need not necessarily be modelled, but instead,they can also come from real photographical images orvideo captures.

Using impostors has proven to be ideal forcharacters that perform an isolated action in thebackground and which the user does not interact with orgets too close to. View-dependent impostors exploitframe-to-frame coherency efficiently and require lessviewing angle instances if they are always far enoughfrom the viewer. This way, there are slow andconstrained changes to the visible sides of the depictedsubject, thus requiring a sparse sampling of the viewingdirection space.

Animation for characters rendered using IBRtechniques can be realized faster and incorporated easierinto an existing engine or framework than other full 3Danimation techniques like morphing or kinematics. It ishighly applicable for filling the environment withactivity making it interesting and avoiding the typicalVR walkthrough in empty environments with buildingsand static trees.

3.1.2. IBR pitfalls and disadvantages

Although the use of IBR impostors may seem easyat first, it also has certain pitfalls and difficulties, whichalso apply to the video textures. The most annoyingartifact when using impostors is the popping that occursin the transition from on image (view direction) sampleto another. Because the image samples were taken fromcertain positions there is no information about thedepicted geometry for viewing positions in betweenthem. This results in sudden and abrupt changes.

When getting too close to the impostor, pixelisationoften occurs. The bigger the texture used the better theimage quality and the more the texture usage on thegraphics system. Using too small textures makes theimage look blocky and using too big textures drains thebandwidth with texture downloads.

Because there is no information for the Z-bufferwhen rendering the impostor, rendering artifacts wherethe impostor plane intersects 3D geometry can occur, socareful scene design is important, in order to leaveenough space around the impostor to allow it to freelyrotate without bumping on the surrounding objects.

3.1.3. Overcoming IBR disadvantages

To mitigate the problem of popping between theimage samples, a single-pass multitexturing algorithm isapplied which blends the closest impostor viewinstances together.

Figure 5: Image-based rendered characters from one ofour recent applications.



In order to minimize texture usage and texturebandwidth, tests have to be performed about whattexture sizes to use, so as to maximize the useful areathat an image object occupies. The texture size dependson the object itself and the usage of the impostor in thescene. Impostors further away can have smaller texturesizes.

For easy production and preview of the impostors aspecial tool was developed. This IBR model pre-processor takes into account the desired texture size, the3D model extends and the number of positions andelevations (rings) around the model from where theimages would be captured. The software enables theuser to interactively position and scale the model insidea window that has the size of the specified texture andalso preview the model in various positions from wherea capture will be made. This way, it is easy to place themodel so that it occupies the maximum area of theimage window without intersecting the imageboundaries.

When rendering the images of the impostors, multi-sample antialiasing is used in order to reduce thepixelisation in the image texture and enable the usage ofsmaller, smoothed-out textures.

The view-dependent image sample density can beoptimised with respect to the viewable range andnumber of samples for each impostor separately. Thecharacter or depicted object may be sampled fromcertain views from which it is known to be observedmore frequently. For instance, if the character is to beviewed only upfront and rarely from above, viewpointelevation ranges between -20 and 20 degrees should bemore densely sampled than the rest, which, in somecases (see section 5) may be not sampled at all. Thistechnique imposes some complexity on the renderingand transition between samples since each elevation canhave different amount of views.

3.2. Culling

In order to represent a VR Cultural Heritage scene ingreat detail as to show its attributes, a great deal ofpolygons is required. Simple rendering algorithmsthough cannot cope with the plethora of such polygonsfor a real time walkthrough of the archaeological site.Techniques are required to eliminate polygons even-though they would normally be rendered since they existin the viewing field of the user. Different intelligentculling techniques exist that remove polygons from therendering pipeline.

3.2.1. Shadow Culling

Hudson et al. 12 proposed a geometry culling methodwhere the main idea is to select a small set of largeoccluders and discard the objects behind the occluders,i.e., objects that are shadowed by an occluder withrespect to a certain viewpoint. These occluders may notbe regular visible scene geometry, but also invisiblemattes purposefully inserted in the virtual world forculling other geometry. Culling is performed bycalculating the shadow volume pyramid that stems fromthe viewer position, passes from the occluder polygonboundaries and extends to infinity and determiningwhich sets of objects fall completely inside this volume(Figure 6). These objects are not drawn at all for theparticular frame. In average, the extra cullingcalculations performed by the rendering engine take farless time than rendering the hidden geometry and about20% is gained in rendering speed.

3.2.2. Contribution Culling

Another culling technique is contribution culling 13,in which objects whose visual contribution is smallerthan a user-supplied threshold are discarded fromconsideration even though they may be visible. Thefactors defining contribution are generally object sizeand range, field-of-view, display window size, andmonitor resolution.

Generally, contribution culling can be thought of aspart of the level-of-detail simplification process. Themodeller - scene designer determines manually thedistance from the viewer that certain objects couldvanish completely from sight, according to the object’ssize and set an empty LOD target at this range. Treesand other scenery parts of trivial contextual importancecan be treated with contribution culling. This techniqueis very valuable when exploring outdoor archaeologicalsites, because there are many locations of interest thatdraw the eye and help vanishing geometry to passunnoticed.

Figure 6: Shadow Culling technique for eliminatingnon- visible polygons in the viewing frustum.



4. Implementation

Employing VR components that are user-friendlyand easy to use creates an important base for thesoftware developed to use this hardware. The softwareprovides a layer of mediation between the hardware andthe final programmer and user; VR applications areusually developed using object-oriented languages ontop of graphics libraries such as SGI's OpenGLPerformer™ and OpenGL®.

All enhancements and extensions discussed aboveare built on top of the VR framework initially developedby Pape et al 14 and adapted for the cultural heritageapplications of the Foundation of the Hellenic World 1.The engine development was done in C++ and wasbased on OpenGL Performer™ and OpenGL® for thegraphics, on the CAVElib™ library for transparentaccess to handling virtual reality hardware andcomponents and a custom sound library for playingpositional audio (Figure 7). These libraries provide theabstraction layer necessary for the execution of theruntime environment on all VR hardware platforms usedby FHW.

The system is divided into two major components:the virtual world authoring interface, consisting of ascripting language parser and conversion tools, and thecore VR engine which runs the compiled scene-graph ofthe VR world and its environment and behaviours in realtime.

The new engine extends the capabilities, such asparticle systems, morphing and dynamic texturing, ofthe original one with lightmaps, real-time videooverlays, view dependent textures and object-to-objectcollision detection. New productions make use of theaforementioned culling techniques and introduce imagemodelled elements and view-dependent sprites in thevisual content.

In truth, not all enhancements affect the core engine.Lightmaps are anyway incorporated as a multi-texturing

stage in the rendering process, regardless of the nature ofthe depicted information. Commercial software oftenprovide the illumination maps along with the geometry,which can be readily displayed by the engine.Sometimes though, custom illumination (e.g. skylight orcartoon models) must be deployed, or older – or thirdparty - models must be illuminated without affecting theoriginal geometry. For these situations, a customlightmap generation tool, has been developed. The toolis built on top an extensible shader and illuminationarchitecture to accommodate future needs. It uses aniterative, adaptive lightmap-space ray-tracing algorithmfor tracing incident irradiance from the sampled lightsources and the environment. Currently, directillumination with a wide range of light sources,including a skylight model, is supported, while futureversions will include radiosity, dust and corrosionsimulations.

5. Case Study: The Workshop of Feidias

Feidias' Workshop (Figure 8) is an interactive virtualexperience in the laboratory of the sculptor Feidias. Ittakes place in ancient Olympia, amongst an accuratereconstruction of the famous statue of Zeus and thesculptor's tools, materials, and moulds used to constructit. Visitors become the sculptor's helpers and activelyparticipate in the creation of the huge statue, by usingvirtual tools to apply the necessary material onto thestatue and add the finishing touches.

Since this application reconstructs the interior of aworkshop the technique of lightmapping, which is very

Figure 7: VR engine layersFigure 8: Image from Feidias' Workshop where most ofthe techniques described are visible.



well suited for indoor environments, was used forshadows and lighting simulation on the staticenvironment. The statue of Zeus, the tools and theinteraction with them had to be exact and detailed. Thisresulted in high polygon counts and high texture usage.Although all the necessary models and tools wereincorporated in the scene, the absence of humancharacters was evident.

We chose to incorporate characters depictingworkers and Feidias, the sculptor itself, using IBR in theform of animated impostors. This technique was chosenbecause of its low polygon count since the detail of themodels in the scene was high. Another reason wasbecause it would bind nicely with the existinglightmapped shadows. Lightmaps use static lightsources. By using the same light sources for theimpostor images, the detail of shading in the imageswould match completely with the shading in theworkshop producing consistent and realistic results.

The models and some animations were created usingCurious Labs POSER© and Discreet 3Dstudio Max© in afraction of the time compared to morph target 3Dmodels we created for full 3D character animations inprevious projects. The texture sizes used was 128x256and 256x256 depending on the pose of the character.Despite this being a very low texture size, because of theblending and antialiasing techniques used to create thefinal images, the impostors looked quite satisfying evenfrom up close.

By careful positioning the characters the possibilitiesof artifacts when viewed from various angles, whichmay be caused by intersection of the impostor planewith the 3D models, were avoided. Furthermore wenoticed that usually the characters were seen fromupfront. Therefore the views perpendicular to thecharacter (elevations between -20 and 20 degrees) wereoptimised using more sample images. In some cases itwas sufficient to have sample images only for theseviews, thus providing no sample images for otherelevations. Due to the fact that each impostor had a verysparse set of images associated with it, we opted for aclosest neighbour selection criterion of the view-dependent textures rather than a blending mechanism.Interpolation between inconsistent views tended toproduce blurry results and, as the characters were mostlyviewed from arbitrary angles, they always had aninsubstantial appearance.

The result was a reconstruction of a populated andactive workshop in which the visitors would alsoobserve and learn about the habits and appearance ofworkers during that time. The use of characters even asimpostors enriched the environment tremendouslymaking it an interesting experience for all ages and typesof visitors.

References

1. A. G. Gaitatzes, D. Christopoulos and M. Roussou,"Reviving the past: Cultural Heritage meets VirtualReality", VAST2001: Virtual Reality, Archaeology,and Cultural Heritage, November 2001.

2. T. Akeninne-Möller, E. Haines, Real-TimeRendering, A K Peters, 2002.

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4. G. Papaioannou, E. A. Karabassi and T. Theoharis,“Segmentation and Surface Characterization ofArbitrary 3D Meshes for Object Reconstruction andRecognition”, Proceedings of the IEEEInternational Conference on Pattern Recognition‘2000, pp. 734-737, 2000.

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8. P. Sander, X. Gu, S. Gortler, H. Hoppe and J.Snyder, “Silhouette Clipping”, Proceedings ofSIGGRAPH 2000, ACM, pp. 327-334, 2000.

9. O. Faugeras, Three-Dimensional Computer Vision,MIT Press, 1993.

10. F. Tecchia, C. Loscos and Y. Chrysanthou, “Image-Based Crowd Rendering”, IEEE ComputerGraphics & Applications, 22(2):36-43, 2002.

11. M. M. Oliveira, G. Bishop, D. McAllister, “ReliefTexture Mapping”, Proceedings of SIGGRAPH2000, ACM, pp. 359-368, 2000.

12. T. Hudson, D. Manocha, J. Cohen, M. Lin, K. Hoffand H. Zhang, "Accelerated Occlusion Cullingusing Shadow Frusta", Thirteenth ACM Symposiumon Computational Geometry, June 1997.

13. DirectModel 1.0 Specification. Hewlett-Packard,October 1997.

14. D. Pape, T. Imai, J. Anstey, M. Roussou and T.DeFanti, “XP: An Authoring System for ImmersiveArt Exhibitions”, Proceedings of VSMM ’98,November 1998.

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