The 5th International Symposium on Virtual Reality, Archaeology and Cultural Heritage
VAST (2004)
K. Cain, Y. Chrysanthou, F. Niccolucci, N. Silberman (Editors)

The Ancient Olympic Games: Being Part of the Experience
A. Gaitatzes, D. Christopoulos and G. Papaioannou
Foundation of the Hellenic World, Greece

Abstract
This paper presents the virtual reality applications that the Foundation of the Hellenic World has
produced associated with the Olympic Games in ancient Greece. The separate virtual reality shows arepresented in terms of interactivity and educational value. The technical aspects of the productions arenext explained in detail, with an emphasis on character animation, dynamics and occlusion culling forsurround screen projection environments. These techniques were mostly utilised in the recent production
regarding the ancient pentathlon, where much effort has been made to recreate the feeling of the games
and help the user/spectator be an interacting part of the edutainment activity.
Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: Animation,
Virtual Reality. I.3.8 [Computer Graphics]: Applications. I.6.8 [Simulation and Modelling]: Animation,
Gaming, Visual. J.2 [Physical Sciences and Engineering]: Archaeology.
Additional Key Words and Phrases: Character Animation, Dynamics, Occlusion Culling.

1.

Introductionth

The undertaking of the 28 Summer Olympiad byAthens, the capital city of Greece, has spurred anincreased interest for shows and complementary
edutainment contributions to the games themselves. The
Foundation of the Hellenic World (FHW) has prepared aseries of thematic productions related to ancient
Olympia and the Olympic Games, culminating with thehighly interactive and accurate representation of ancient
Olympia and the pentathlon (running, long jump,
javelin, discus throwing and wrestling), whose final
version was released just before the beginning of the
Olympic Games. The high demand for interactive andentertaining productions, apart from simple walkthroughapplications, however eye-catching [PGC03], has driven
us to pursue an interaction model different from theclassic navigator/inspector one. The virtual reality (VR)productions should be educational through creativity andactive participation in events.
Some of the Olympic Games thematic applications
have been running for quite some time and the feedback
FHW has received has confirmed the preference of thespectators toward more “playable” environments whereexperimentation and first-hand experience is the mostimportant channel leading to knowledge.

 The Eurographics Association 2004

In the rest of the paper, section 2 provides thelinking background and a brief insight to each one of thethematic applications, while section 3 explains the key
points of the scientific and technological features related
to the interactive shows and our effort to implement
these in a surround screen projection environment.

2.

The productions

In 2000, as the Olympic Games were returning to theirbirthplace, we decided to focus the new projects on thethematic region of the Olympic History and Games forthe virtual reality productions that would open in thetime period before the Olympic Games in Athens.
Instead of focussing on only one production, where
every concept and aspect of this glorious event would be
explained and presented, a different approach waschosen. The event and its history would be approachedby the creation of 4 projects. Each project would becomplementary to the previous one and lead onto thenext to finally conclude in a full-scale interactive
representation of ancient Olympia and its Games.
A new production was released annually to the
public, leading into a deeper understanding of thehistory of the Olympic Games and presenting thedifferent aspects and impact in Greek culture and art.

Gaitatzes, Christopoulos and Papaioannou / The Ancient Olympic Games: Being Part of the Experience

2.2.

Olympic pottery puzzle

The study of ancient pottery has been a very importantsource of historical and archaeological information forunderstanding the life and culture in ancient Greece.
Apart from the functional significance of the various
types of pots, which give us clues about rituals andeveryday life, the ink-paintings that cover many pot
shards found at excavation sites, are priceless since they
allow us to have an inside look into history. The goal of
the Olympic pottery puzzle application was to educate
the visitors about all this information, emphasizing on
Olympic Athletic events in an entertaining andunderstandable way [RE00].

Figure 1: The Temple of Zeus at Olympia

Starting from the first project, which was a simple
presentation walkthrough, the following projects
introduced new interactivity features and real-timegraphics techniques to engage and captivate the visitor.

2.1.

The "Olympic Pottery Puzzle" exhibit was createdusing the above aspects as guidelines and aconstructivist’s approach to interaction [PRS*94](Figure 2). The user must re-assemble a number ofancient vases putting together pot shards. The users arepresented with a colour-coded skeleton of the vessels
with the different colours showing the correct positionof the pieces. They then try to select one piece at a timefrom a heap and place it in the correct position on thevase. When they finish the puzzle, the painting on thevase comes to life, presenting an animation of one of theancient Olympic contests. Being a highly interactive
exhibit, different object selection mechanisms had to be
employed to make the process as natural and simple touse. From a technical perspective, much effort was
directed towards the realistic, highly detailed
representation of vases and the simulation of their

The temple of Zeus at Olympia

In the first production of the series the "Temple of Zeus
at Olympia", visitors have the opportunity to admire thesplendid temple itself as well as the sheer glory of thefamous statue of Zeus, one of the seven wonders of theancient world, of which nothing remains today (Figure1). On the east pediment of the temple the myth of theorigins of the Olympic Games is depicted, the chariots
race between two kings. As the visitor approaches the
temple the metopes come into view, portraying thetwelve labours of Hercules, the famous hero son of
Zeus. Walking on the backside of the temple on the west
pediment, the visitor can marvel the battle between thepeople of Lapithes and Centaurs; the fight between
Reason and Instinct.
In order to highlight places of interest in the virtualenvironment, an alternate navigation model was also
employed. Even though the users have the freedom tomove freely in the environment they also have thechoice of a predefined path navigation model that assists
them in making the experience more meaningful as thepath highlights points of historic significance.

Figure 2: The Olympic pottery puzzle The Eurographics Association 2004

Gaitatzes, Christopoulos and Papaioannou / The Ancient Olympic Games: Being Part of the Experiencematerial properties like specularity and glossiness, using
multi-pass rendering techniques.
Although no large stunning worlds or ancient cities
were simulated, this exhibit still captivates the visitors of
all ages, giving them the opportunity to interact with thevases intuitively. Completing the 3D assemblage puzzle
with the help of VR equipment, helps them learn some
important historical facts about the vases and the
Olympic contests, as well as have a glimpse of therestoration procedure of earthenware. The reanimationof the depicted athletes was made in 2D (planar video
overlays), preserving the colours and character of theancient Greek ink paintings. This, in turn, helped thevisitors perceive the Olympic contests and any
information regarding the Olympic Games in antiquityfrom the perspective of an ancient Greek artist,
something the public responded enthusiastically to. The
Olympic pottery puzzle is suitable for presentation inboth single-screen and surround screen projection
systems.

2.3.

Feidias’ Workshop

After the successful launch of the first interactive
application regarding the Olympics, it was important tobuild on the positive feedback and winning concepts andcreate an experience, which would bring the visitor one
step closer to Olympia. In this VR exhibit, visitors
engage themselves in another marvel of ancient Greek
art, the creation of golden ivory statues. Golden ivory
statues are regarded as the masterpieces of Greek
sculpture, and were admired as such even at the time of
their creation. Only statues of Gods and heroes weremade using this technique.
“Feidias' Workshop” (Figure 3) is a highlyinteractive virtual experience that takes place at theconstruction site of the 15-meter-tall golden ivory statueof Zeus, one of the seven wonders of the ancient world.
The famous sculptor Feidias and his team of artisans
built the statue in this workshop. Zeus’ statue wasafterwards dismantled and transferred to its designatedlocation in the most important temple at Olympia, theTemple of Zeus.
The visitors enter the two-storey-high workshop andcome in sight of an accurate reconstruction of an
unfinished version of the famous statue of Zeus andwalk among the sculptor's tools, scaffolding, benches,materials, and moulds used to construct it. The visitors
take the role of the sculptor's assistants and actively help
finish the creation of the huge statue, by using virtual
tools to apply the necessary materials onto the statue,
process the ivory and gold plates, apply them onto thewooden supporting core and add the finishing touches.
Interaction is achieved using the navigation wand of the The Eurographics Association 2004

Figure 3: Feidias’ workshop: A creative approach tointeractive edutainment

VR system, onto which the various virtual tools areattached. Using these tools the user helps finish the workon the statue, learning about the procedures, materials
and techniques applied for the creation of thesemarvelous statues. The various workers and Feidias
himself are displayed using image-based rendering
techniques in the form of animated impostors [PGC03].
This technique was chosen because of its low polygon
count since the detail of the other models in the scene
was high. In order to provide additional depth cues forthe interaction and better image realism, shadows wereincorporated using precomputed lightmaps.
This VR production presents an accurate
reconstruction of a populated and active workshop, withinteractive and educational value. The learning by doing
concept worked well and increased the visitors’ interest
and active participation in the VR show. Visitors areamazed with the techniques and level of craftsmanshipinvolved in the building of such large-scale statues, facts
that are mostly unknown to people at large. The use of
the CAVE-like environment helps support the large

Gaitatzes, Christopoulos and Papaioannou / The Ancient Olympic Games: Being Part of the Experience

scale and proportions of the workshop and the statue ofZeus. The positive feedback we got from theincorporation of characters, led us to the conclusion thatthe public was especially interested in the way of life
and working habits of people in ancient Greece. The useof animated representation of characters helped theaudience understand, participate and immersethemselves into the experience, giving them a reference
point of interest.

2.4.

A walk through Ancient Olympia

In our most recent VR production “A Walk throughAncient Olympia”, the user, apart from visiting thehistorical site, learns about the ancient games themselves
by interacting with athletes in the ancient game ofpentathlon (Figure 4).
We are at the end of the 2nd century BC. The day
breaks and in front of us appears the majestic sanctuaryof Zeus in ancient Olympia. In antiquity the Olympic
Games took place here, while today it hosts the lightingof the Olympic flame. The visitors can wonder aroundand visit the buildings and learn their history and theirfunction: the Heraion, the oldest monumental building of
the sanctuary dedicated to the goddess Hera, the templeof Zeus, a model of a Doric peripteral temple withmagnificent sculpted decoration, the Gymnasium, which
was used for the training of javelin throwers, discus
throwers and runners, the Palaestra, where the wrestlers,
jumpers and boxers trained, the Leonidaion, which was
where the official guests stayed, the Bouleuterion, where
athletes, relatives and judges took a vow that they would
upheld the rules of the Games, the Treasuries of various
cities, where valuable offerings were kept, thePhilippeion, which was dedicated by Philip II, king of
Macedonia, after his victory in the battle of Chaeronea
in 338 BC and the Stadium, where most of the events
took place.
In addition, the public can interact virtually with 3D

Figure 4: Screenshots and live capture from the production “A Walk through Ancient Olympia”.
 The Eurographics Association 2004

Gaitatzes, Christopoulos and Papaioannou / The Ancient Olympic Games: Being Part of the Experience

digital representations of Olympian athletes in theancient pentathlon, including the 200-meter sprint, thediscus throwing, the long jump, the javelin throwing and
wrestling. Instead of just observing the games thevisitors take place in them. They pick up the discus or
the javelin and they try their abilities in throwing them
towards the far end of the stadium. Excited about theinteraction they ask when will they be able to interact
with the wrestler one on one. A role-playing model ofinteraction with alternating roles was tried here withpretty good success as the visitors truly immersed in theenvironment wish they could participate in more games.
Finally, decorated with red ribbons in his hands andlegs, the glorious winner makes the tour of triumph
holding a palm leaf in his hand while the spectators givehim a standing ovation. The Games come to an end.
3.

Production technology

From a technical point of view, the productions of FHW
are presented on two VR systems, both using projective
viewing technology [GCR01]. The larger system is a
CAVE-like ReaCTor immersive display, consisting offour 3m × 3m projection walls (3 rear-projected viewing
walls and a top-projected floor). An 8-processor SGI
Onyx2 drives the four projectors through four
InfiniteReality2 graphics subsystems, in active stereo.
A maximum number of 10 visitors and a museum
educator enter the immersive cube, all wearing stereo
shutter glasses and the show is controlled by the guidevia a 6-degrees-of-freedom tracked joystick (wand). The
museum educator also wares a hat with an attached
6DOF sensor for the head position and orientation
tracking, which affect the perspective projection on eachwall.
The smaller VR system consists of an
ImmersaDesk R2 rear projection tilted screen driven in
active stereo by an SGI Octane2. 6DOF head andhand tracking are provided via a head-mounted tracker
and a tracked joystick. This system, having a lesser
immersive effect and also a much slower computing
system, is used mostly for lightweight applications. The
large interactive VR shows are run in the surroundscreen projection environment.
In terms of software technology, we have built ourown game engine, Enhanced Visualisation System(EVS) [PGC03], [PIA*98], [GCR01], which uses SGI’s
OpenGL Performer scene-graph library [RH94], butalso pure OpenGL for the rendering and VRCO’s
CAVELib for the tracking input. As natural interaction
requires the presence of familiar and realistic
representations not just of visual aspects of the virtualworld, but also of actions and procedures, important
techniques and features such as character animation and The Eurographics Association 2004

dynamics have been incorporated in the VR engine andare discussed below.

3.1.

Character animation

Real-time character animation in simulated virtualenvironments has progressed rapidly over the last years.
This development has also been reflected by the changesof the visual style in virtual environment applications.
Originally, game characters have been either handdrawn or computer generated 2D images. For simple
walkthroughs or small applications these techniques
might be satisfactory. When the experience is to beenriched with interaction in active environments where
people should explore and participate in closeinteraction with virtual people, the use of 3D animatedcharacters, with recognizable and natural features,
gestures and fluid motion, is essential.
The animation technique, which has establisheditself as the defacto way in bringing articulated models
to life in real-time 3D environments, is skeletal
animation combined with soft skinning. Skeletal
animation was developed for the animation ofarticulated (e.g. humaniform) objects, in order tosimplify the animator’s work and make the animatedforms more life-like and naturally moving [Wood00]. It
is an improvement over previously used animation
techniques like kinematics on hierarchical articulated
objects [WW93] or morphing of character meshes.
These techniques had a number of drawbacks likevisible seams and gaps or high memory usage, absenceof flexibility and interpolation problems.
Skeletal animation uses an endoskeleton, ahierarchic structure of joints and bones, which drives askin, a vertex mesh representing the outer shell of theobject. A bone is simply a transformation matrix,
determining its position in relation to its parent bone towhich it is connected via a joint (Figure 5). All the bonesof the articulated object form together the skeleton. Only
the skeleton is explicitly animated by recursively
traversing the hierarchic data structure starting from theroot bone, passing down the transformation of the parent
objects to their children and concatenating the passeddown and local rigid transformations. The bones in turnimplicitly animate the skin and its vertices. Using soft
skinning, each vertex of the mesh can be influenced bymore than one bones [Wood00], effectively mimicking
the way a bone in a real body would affect the skin of aliving being. Memory usage for skeletal animation issmall and requires a significantly lower amount of
information to be stored when compared to the othertechniques. Animation data can also be generated on the
fly using techniques like inverse kinematics [WW93],
and applied to the model in real-time.

Gaitatzes, Christopoulos and Papaioannou / The Ancient Olympic Games: Being Part of the Experience

action has ended or specify the number of times theanimation should cycle, since only "cycle constantly" or"cycle only once" modes are currently supported.

3.2.

Figure 5: Character animation using bones and soft
skinning

For our recent VR production "A Walk throughAncient Olympia" the skeletal animation technique was
implemented for animating the various characters andathletes (Figure 4). Using the Cal3D animation
framework library [Cal3D], specialized EVS renderingand simulation nodes were created which allowed us toincorporate skeletal animations from commercial
modelling and animation software into our framework.
Besides the playback of animations, the ability to blend
and mix multiple animations was implemented foradvanced animation control, generating smoothtransitions between the animations and dispensing withextra transition animation cues.
Automatic generation of LOD versions of the Cal3D
mesh are generated during the export phase, which canbe shown at runtime for any animated model. Cloth andhair simulation, although still experimental, can beincorporated using the mass-spring model. In order tomake memory usage more efficient, model instancing
was implemented in cases where animation, mesh, andmaterial data are shared. Initially, a set of core models iscreated, from which instance models, representing one
specific instance of the model with its own animation
state, are subclassed. A core model consists of all mesh,skeleton and animation data, while an instance model
has its own animation state but inherits all other datafrom the core model.
Due to the fact that models also have to interact
with the environment by picking and dropping otherobjects, a mechanism for attachment and detachment ofother scene objects on the characters was implemented.
Although the animation system worked perfectly forthe last project there are some shortcomings that have tobe addressed concerning control of the animation
sequences. Currently, there is no option to specify
automatically what action should be taken when an

Newtonian dynamics

One of the challenges in the “Walk through Ancient
Olympia” was to be able to actively participate in atleast one of the games (Figure 4). Considering thedifficulty to move in a CAVE when surrounded byspectators, we decided to implement such interaction forthe discus and javelin throwing games, which are
relatively static. Both games required that the user takehold of an object and send it flying through the scene
under the influence of forces in a controllable manner.
The object would collide, bounce off and exchange
energy with other geometry. For this purpose we used
Newtonian dynamics, a well-known and extensively
studied motion model for rigid bodies and their response
to collisions [Ebe04].
3.2.1 EVS dynamics implementation. In the EVS
implementation of dynamics, special geometric nodes,
the dynamics objects, can be “hooked” to or “unhooked”
from force fields at run-time. These objects govern theirmotion themselves under the influence of the forces they
are presently attached to, unless they take part in one ormore dynamic simulations. In the second case, thesimulation node reports collisions, manages friction anddumping constraints, calculates the exchange of energy
between objects (not necessarily all dynamic)
participating in the simulation and estimates new linear
and rotational speed and momentum for the objects.
For the moment, dynamic objects are controlled only
by one hard constraint vital to our applications:grabbing. Once a dynamic object is grabbed, thedifferential rotational part of the user’s hand (wand)transformation is applied at each frame to the dynamic
object first. The force-related differential matrix is thenapplied and finally, the positional part of the wandmotion. In this order, the trajectory defined by thecurrent forces and the wand influence are disassociated.
EVS supports various types of force fields, including
wand-controlled and transformable ones. The actual
throwing action uses a wand-controlled force field(duration of application and direction), combined with aconventional linear field for the gravity.
The collision detection is handled by the dynamics
simulation node, which uses the ColDet freeware libraryfor this purpose [ColDet]. This collision detection
system cannot report multiple contact points or edges,
but it still works sufficiently well for our demands and isvery fast. In some cases though, it causes instabilities ora wrong behaviour when handling marginal contacts
 The Eurographics Association 2004

Gaitatzes, Christopoulos and Papaioannou / The Ancient Olympic Games: Being Part of the Experience(objects may stop instead of pivoting on the contact edge
and falling away).
3.2.2. Aerodynamics. An issue that has arisen in the
javelin throwing game was the inability of the spear’s
body to align itself correctly with its trajectory. The
airflow has little effect on the discus, being heavy anduniformly dense. This is not the case for the javelin
though, which is heavily influenced by the air. In orderto address this problem, we have added an adjustableaerodynamics bias to the rotational part of the dynamics
object transformation. What it does is that it slowly
corrects the object’s rotational matrix so that its primary
axis becomes aligned with the sum of applied forces.
In more detail, at initialisation time, the principal
axes of an object are extracted from a random samplingof the object’s surface vertices and they correspond to
the eigenvectors of the covariance matrix of the sample
set (second order central moments) [LD89], [Ebe04].
The largest primary axis is the eigenvector that
corresponds to the largest eigenvalue. At each frame, acorrective transformation matrix R Bias is multiplied tothe time-differential rotation matrix (Figure 6):
R Bias = Rθ , rGG
a1 × Fnorm
2
G G
G G
a ×F
, θ ≠0G  a × F
r =  1 norm 1 normθ=
(1,0,0),0θ = bias ⋅

π G

(1)

G
where bias is the amount of aerodynamic influence, rGis the axis of rotation, a1 is the transformed primary axis
Gand Fnorm is the total applied force direction.

When objects travel a long way airborne, the above
adjustment helps rectify the orientation of a lightweight
object relative to its trajectory.

3.3.

Occlusion culling for open environments

As graphics cards have a limit to the fill rate and triangle
count the application can pump into them, culling – or
non-visible geometry elimination - is used extensively inlarge 3D worlds in order to minimize the geometry sent
for rendering in the graphics card. In occlusion culling,[CCSD03], geometry that is hidden behind objects
closer to the camera point, is discarded before being
subject to depth sorting algorithms. Shadow culling[HMC*97], uses predefined occlusion proxies(occluders), which consist of simplified (and usuallyconvex) polygonal versions of actual rendered geometry,such as rectanglular barriers, in order to efficiently blockgeometry hidden behind them. Shadow culling is best The Eurographics Association 2004

Figure 6: Aerodynamics correction computation
suited for our outdoor sparse environments as comparedto other techniques, most of which are targeted for dense
or indoor scenes.
In each frame, a semi-infinite convex frustum iscreated for each (convex) occluder polygon, the cap of
the semi-infinite frustum being the occluder polygon
itself and the sides connecting the each edge with the
viewpoint. The bounding boxes of the geometry to berendered are compared for containment with thesefrusta. If a bounding box resides completely within all
frusta, then the object it contains is hidden. This process
is performed in a hierarchical manner, discarding whole
sub-trees of a 3D scene graph prior to forwarding thegeometry to the rendering engine. Careful construction
and placement of the occluders [PGC03], results in highhidden geometry elimination and a considerable
speedup, especially for enclosed environments.
3.3.1 Solid occluders. In the case of outdoor scenes, like
Olympia, where the static geometry is often blocky(buildings) but scattered and sparse, simple occlusion
culling is not very effective due to the partial occlusion
phenomenon (see Figure 7): Two or more adjoining
planar occluders may partially hide a distant object but
the combined occlusion area of them may hide it
completely. This object cannot be eliminated if thefrustum for each occluder is created separately, and thejoining of frusta is an expensive operation.
To overcome this taxing limitation, we decided touse convex solid occluders, such as boxes and cylinders
as proxies for large isolated structures (Figure 7). A
convex solid when projected on a plane is guaranteed to
produce a convex polygon. The convex frustum of the
projected polygon is the union of the frusta that would
be generated from the individual planes of the solid
occluder, thus bypassing the need to merge frusta inorder to avoid partial occlusion.
For each frame, the solid occluder frustum isgenerated as follows. The view dependent silhouette of
the solid occluder is extracted by connecting the edges
belonging to adjacent polygons, which are not both
visible or hidden simultaneously:Gaitatzes, Christopoulos and Papaioannou / The Ancient Olympic Games: Being Part of the Experience

them. In our case, Olympia contains more than 200
occluder planes and solids. Therefore, an optimal set ofoccluders has to be selected for each frame at run-timein order to keep the number of “active” occluder
primitives to a minimum. For this task, a “score” oroptimization function f planar has to be devised that takes
into account the solid angle of the frustum. Hudson et al.
use the area-angle approximation presented by Coorg
and Teller [CT97]:
f planar =Figure 7: Comparison between polygonal and solid
occluders in shadow culling.

edge(tri , trj ) ∈ Silhouette ⇔G
G N i ⋅ ( Pi 0 − C )  ⋅  N j ⋅ ( Pj 0 − C )  ≤ 0

The Silhouette edges do not lie on the same plane ingeneral. Therefore, we must select a cap (near plane) for
the semi-infinite frustum, based on the relative positionof the viewer and the silhouette points. If the near plane
is too close, the frustum solid angle may become toolarge, resulting in the false elimination of partiallyhidden geometry. Similarly, choosing a plane near the
average of the silhouette points can be a disastrous
selection for elongated solid occluders in the view
direction. We chose to fix the near plane of the frustum
to the furthest point of the silhouette from the viewer.
This clipping plane’s normal vector is the average
directional vector between the viewpoint and thesilhouette points:

i=0

K −1

) ∑(S − C )
i=0

i

G
where A is the area of a planar occluder, N is its normalG
vector and V is the vector from the viewpoint to thecentre of the occluder. We use the criterion in eq. (4) for
planar occluders. For solid occluders we use anapproximation formula which depends on the projection
on the viewplane of the solid occluder’s volume Vol andthe squared distance of the occluder from the viewpoint:

(5)

(2)

triangle’s first point and C the viewpoint.

((4)

Vol G −2 Vol
f solid = G ⋅ V = G 3
VV

Gwhere N k is the normal vector of triangle trk , Pk 0 the

K −1
G
N near = ∑ Si − C

G G− AN ⋅ V
G2
V(3)

where Si is the i-th point out of the K silhouete vertices.
3.3.2 Occluder selection. As Hudson et al.[HMC*97]
suggest, a scene may contain too many occluders for theengine to be able to test each object against each one of

Keep in mind that the optimisation function for thesolid occluders does not depend on angular attributes as
the near plane of the constructed frustum always faces
the centre of projection (see eq. (3)). f planar and f solid
are balanced and do not need further biasing to becomecompatible.
The effectiveness of the solid occluders becomesapparent when moving among the buildings, especiallyat ground level and at inspection distance (near). Planaroccluders would mostly produce partial occlusion when
not facing the main sides of the buildings straight on.
Most of the time we view the blocky buildings from odd
angles and that is where the solid occluders provide a
unified contiguous frustum to take into account all sides
at once.

3.4. Skylight illumination model

Most realistic rendering in VR has dealt with indoor
scenes or has focused on objects and buildings.
However, outdoor scenes differ from indoor scenes in
two important aspects, other than geometry: most of
their illumination comes directly from the sun and sky;and the distances involved make the effects of air "aerial
perspective" visible. A Skylight Illumination Model,
once incorporated into the framework, captures andsimulates the visual aspects and results of these
phenomena. It changes sunlight position and colour from The Eurographics Association 2004

Gaitatzes, Christopoulos and Papaioannou / The Ancient Olympic Games: Being Part of the Experience

the pale red of dawn to the bright yellow of midday andback again. It determines the colour and brightness ofthe sky throughout the day and it cues us to the distanceof objects by shifting their colours. All these effects vary
not only based on time of day, but also depending on
weather, pollution and other factors.
Traditionally, outdoor VR applications would model
these phenomena using Skydomes or Skyboxes, texturemapped with static high-resolution sky and cloud
renderings. In our latest outdoor simulation of ancient
Olympia, a sky simulation from sunrise to sunset andnightfall was needed. Therefore, the implementation of a
Skylight model was necessary.
The analytic model of [PSS99] was implemented
with some modifications regarding the colour
conversion algorithm to produce more dramatic
sunsets/sunrises and the incorporation of a nightfall
simulation with rendering of stars and moon (Figure 4).
An EVS node was created which constructs thedome geometry takes as input the sun position andweather conditions and finally, draws the dome,
computing the colours for every vertex with analytic
formulas. The colour and intensity of the sun is also
computed and all underlying geometry is lit by its
computed light attributes. The sun is implemented as astandard OpenGL light source. Indirect skylightillumination, caused by the atmospheric scattering oflight, is simulated by additional light sources placed in
the perimeter of the scene. These light sources are linked
to the simulation and their intensity and colour iscontrolled to match their respective position in the
skylight simulation.
In some cases, one might be satisfied with the colour
and intensity of the skylight but the lighting on theobjects might be too low. Brightening the skylight,
causes the lighting on the objects to get higher, but theintensity and colour of the skylight also changes.
Manually adjusting the lighting is achieved byintroducing more peripheral light sources into thesimulation, which only affect underlying geometry.
Unfortunately, the vertex colours computed from theinitial illumination model are in Yxy CIE colour spaceand had to be converted first to XYZ tristimulus values
and then to RGB [WS00]. By tweaking the original
conversion matrix from XYZ to RGB to a different
colour whitepoint setting, an increase in the vividness ofthe colours at sunset/sunrise is achieved, providing more
dramatic results. The initial implementation proposed in[PSS99] only computed values during the day. To
overcome this limitation, at nightfall, the sky andsunlight colours get interpolated to a standard nightbluish colour. During the night, stars and the moon are
faded in, implementing a fast and easy night sky model.
 The Eurographics Association 2004

All the weather parameters and the position of thesun can be animated to produce smooth animations ofsunset or sunrise along with changes in weather
conditions.
The skylight model enhanced greatly the natural andrealistic appearance of the scenery and decreased
production time and resources on the graphics card sinceno time consuming rendered images had to be generated
and loaded into texture memory. Since the specificmodel was created for real-time use, it had little impact
on the frame rate of the application. Not only were thevisitors excited with the virtual world by its vivid andbeautiful imagery but the skylight effect also allowed
cinematic introductions and endings to be realized.

4. Conclusions

As the curtain of the ATHENS 2004 Olympic Gamescame down, the odyssey of creating this series came toan end, confirming and outlining once again theimportance of combining interactivity, storyboard with
culture, advanced graphics and social interest points. All
these key factors lead to the success and embracement of
this series of applications from the public. Encouraged
from the visitor numbers (approx. 7500) during thesummer of the Olympiad and their positive feedback, wefeel convinced that we succeeded in the challenge ofbringing Ancient Olympia to the public and comfortablein undertaking and pushing future projects into new
directions.
Finally, below follow a few highlight quotes fromour guest book:“What a great exhibition – both the real and the virtual.
Keep up the good work” – T. Berners-Lee (inventor of
the world-wide web).
“Today, on my third visit, I admired very much the new
3D program of Olympia” – J. Rogge, President ofInternational Olympic Committee.
“I was astounded with my visit to the Temple of Zeus. I
would like to express my admiration and sincere
congratulations” – I. Skoularikis, mayor of Olympia.

5. Acknowledgements

We would like to thank Aimilia Voulgari and Hara Sfyri
for their great effort in modelling, animating andpainting the textures for most of the 3D scenery.

Gaitatzes, Christopoulos and Papaioannou / The Ancient Olympic Games: Being Part of the Experience

The authors gratefully acknowledge the support forthis paper by the EU Network of Excellence on Virtual
Reality and Virtual Environment Applications for FutureWorkspaces, INTUITION IST 507248-2. With this work
extensive knowledge has been gained on VR concepts
and methodologies. State of the art software and tools
were developed to support this work that can be used forfurther advancing the ease of creation of other Virtual
Reality applications. It can contribute to the standards
used in VR as many of the difficulties were addressedand solved so valuable experience has been gained
regarding the specific technologies used. It can also be
exchanged with other cultural organizations to study theease of porting to different setups and environments thus
giving feedback to produce better standards.

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 The Eurographics Association 2004