archaeological and cultural heritage: bringing life to an unearthed muslim suburb in an immersive...

Original article

Archaeological and cultural heritage: bringing life to an unearthedMuslim suburb in an immersive environment

Diego Gutierrez *, Francisco J. Seron, Juan A. Magallon, Emilio J. Sobreviela, Pedro Latorre

Grupo de Informática Gráfica Avanzada, Instituto de Investigación en Ingeniería de Aragón, Centro Politécnico Superior, Universidad de Zaragoza,C/María de Luna, 1, 50018 Saragoza, Spain

Received 7 April 2003; accepted 6 October 2003

Abstract

This paper describes the 3D digital reconstruction of Sinhaya, a X–XIIth century Muslim suburb in the city of Zaragoza. Accurate modelsand textures were obtained that capture all the wear and tear of a real suburb populated by real characters. The visualization is based onarchaeological evidence from excavations and accurate historical documents. Precise lighting algorithms developed by Grupo de InformáticaGráfica Avanzada (GIGA) provide a photorealistic look, while real actors composited into the synthetic scenes give life to the reconstruction.Images and animations can be viewed in a low-cost CAVE-like system (CLS) designed and developed by GIGA. Following a historicallyaccurate script, the audience is taken through the streets of the suburb and into its houses. This digital reconstruction will help to preserve partof the city’s archaeological and cultural heritage, giving life to a distant past. Visitors can experience what life was like in the suburb frommorning to sunset, and it can provide a new perspective for historians.© 2004 Published by Elsevier SAS.

Keywords: Virtual archaeology; Computer graphics; CAVE-like system; Global illumination algorithms

1. Introduction

Virtual archaeology [1] uses computer-assisted tech-niques to develop realistic three-dimensional replicas of an-cient objects and buildings. Normally these objects havedisappeared or are preserved in a way that makes it difficultor impossible to interpret their original shape.

Modern virtual reconstruction is quite realistic due toimproved computer systems and visualization peripherals,and a better understanding and implementation of geometricand visual modeling techniques. In this paper, visualizationis based on archaeological evidence found during excava-tions, and accurate and documented historical information.Each reconstruction involves a multidisciplinary team witharchaeologists, historians, computer scientists and script-writers.

The modern city of Zaragoza is in NE Spain on the shoresof the Ebro River. It was founded by the Romans (CaesarAugusta) in the first century BC on an Iberian settlement

(Salduie). The city has been subject to numerous invasions ofdifferent origins, which has left a rich and diverse strata ofarchaeological remains. Especially interesting are artifactsfrom the Roman and Medieval times and the Muslim domi-nation (714–1118), when the city was the capital of anindependent kingdom (or taifa) that included most of the NEquadrant of the Iberian Peninsula.

In 2001, while one of the main avenues in downtownZaragoza (the Paseo de la Independencia) was being exca-vated to build an underground parking lot, workers unexpect-edly unearthed the remains of Sinhaya, a X–XII centuryMuslim suburb (see Fig. 1, from Corral [2]). After consider-ing several alternatives, the local Cultural Heritage Commit-tee decided to carry out a complete archaeological study ofthe suburb, including a photogrammetric study of the exist-ing elevations.

Afterwards, the remains were reburied to reopen one ofthe city’s main arteries and all the information was digi-talized to be shown to the general public. In this way, visitorscould obtain information about their own heritage and expe-rience what was life like 1000 years ago.

The Grupo de Informática Gráfica Avanzada (GIGA, Ad-vanced Computer Graphics Group) of the University of Zara-

* Corresponding author.E-mail addresses: [email protected] (D. Gutierrez), [email protected]

(D. Gutierrez).

Journal of Cultural Heritage 5 (2004) 63–74

www.elsevier.com/locate/culher

© 2004 Published by Elsevier SAS.doi:10.1016/j.culher.2003.10.001

goza was given the task of providing a 3D digital reconstruc-tion of the suburb, including streets, houses and shops. Thisrequired accurate models and textures to capture all the wearand tear of a real suburb populated by real characters. Preciselighting algorithms give the images a realistic look, whilereal actors composited onto the synthetic imagery give life tothe reconstruction.

Images and animations were viewed in a low-cost CAVE-like system (CLS) designed and developed by GIGA. Thesystem has three screens (front 3 × 3 m, sides 4 × 3 m) wherethe images are rear-projected, creating an immersive envi-ronment with stereoscopy to accentuate the illusion of actu-ally being there. In this paper we describe the reconstructionprocess (work planning, team composition), the technicaldetails of geometric and visual modeling and the preparationof animations and actor integration. The illumination soft-ware (Advanced Lighting Environment for Photorealism,ALEPH) and the CLS are described in separate sections,followed by the results and conclusions.

2. The reconstruction process

The main goal of the reconstruction was to convey thesensation of a populated suburb with imperfections in thebuildings and objects and years of deterioration. Ironically, itis far more difficult to create imperfections with a computerthan perfect worlds, so this goal implied additional work atevery step.

2.1. The excavation

The process of digitizing the Muslim suburb began withthe excavation, which lasted 3 years and 6 months (see Fig. 2)and carried out by Grupo Entorno [3].

The first remains of a wall appeared on the north side. Itwas documented with pictures, drawings and topographicstudies and excavated to a depth of 1.60 m. The same processwas repeated for all subsequent findings and the whole sitewas leveled out. All spaces (defined as a union, either real orextrapolated, of several units that make up a room, an interiorcourtyard, etc.) were numbered (odd numbers, east side; evennumbers, west side) as well as streets and open areas.

Samples were taken of everything to be analyzed, frombricks and stones to earth and wood remains. An in-depthstudy was also carried out of all the pottery. The pictureswere later used as textures for the virtual models. The reso-lution of the pictures was 1600 × 1200 pixels in order tocapture as much fine detail as possible. An important premiseof the project was to make the suburb look populated andworn and avoid the sometimes extra-clean, brand-new lookoffered by computer graphics (CG).

A complete topographical study was also carried out (us-ing a LEICA TC307 station) to determine the position of allthe units and spaces. We chose the UTM coordinate systemas a reference to be able to compare this site with previous orfuture studies.

2.2. Building the suburb

Once the excavation was complete, the data were pro-cessed to adapt it to the needs of digital reconstruction. The

Fig. 1. Muslim Zaragoza, XI century.

64 D. Gutierrez et al. / Journal of Cultural Heritage 5 (2004) 63–74

result was a CAD image of the ground plan, which was usedto start building the suburb.

During the reconstruction, a team of archaeologists fromMálaga (Spain) complemented the work done on-site by thearchaeologists and historians in Zaragoza. This required acollaborative workspace based on the internet, so everybodycould access the updated information and images as theywere being created. Every aspect of the work had to beapproved by both teams. In some cases features were recon-

structed based on common sense or actual well-preservedbuildings. New iterations were carried out based on com-ments from the archaeologists and historians until the cycleconverged to a final solution. As a result, GIGA was notforced to take arbitrary decisions and every object that ap-pears on screen (from big houses to small dishes) is 100%accurate. A decision was made to model the whole suburbfrom the outside and two characteristic houses and two dif-ferent shops from the inside. Fig. 3 shows a flat-shaded

Fig. 2. Two different stages of the excavation process.

65D. Gutierrez et al. / Journal of Cultural Heritage 5 (2004) 63–74

Fig. 3. Flat-shaded version of the interior of a house and final render.


model of the interior of a house and the final version as seenin the CLS.

Texturing and shading was performed in a similar way,iterating proposals until both archaeologists and historiansagreed on a solution. For maximum historical accuracy thetextures were recreated from original pieces found during theexcavation, when possible. This usually implied careful re-construction work in 2D, since the whole texture and patternhad to be extrapolated and deduced from a small sample.Fig. 4(a) shows the original photographed piece of a plateand the digitally reconstructed texture. Some synthetic ob-jects, including the reconstructed plate, are shown inFig. 4(b,c).

For this production, animation was basically camera ani-mation of walking through the streets and into the houses andshops. We first built low-resolution models that were quicklyrendered in wireframe mode. The renders were used as ani-matics and were accepted by all the teams before sending theanimations down the rendering pipeline. By using animatics,all the necessary adjustments in the path or timing of theanimations could be completed in minutes, without much ofa rendering burden. Thus, we could try more possibilities pershot instead of settling for the second or third iteration due totime restrictions. It was very important for the camera to havea natural feel and to avoid the standard CG camera moves andsmooth interpolations. A hand-held camera test was per-formed for one of the animations to try to achieve a higherlevel of realism. The camera shake information was extracted

by filming reference marks while actually walking with acamera. This option was discarded during early tests whenthe animation proved unpleasant inside the CLS.

Secondary animation was also added on top of the mainanimation. This included dynamic simulations of flowers andplants swaying in the wind, curtains swinging slowly, waterin a fountain or smoke coming out of hot soup in the kitchen.These apparently irrelevant details helped bring the scenes tolife. The audience would not notice their presence explicitlybut their absence was quite evident in early tests.

The lighting was calculated using our in-house softwareALEPH (see Section 3). The goal was to use advanced globalillumination algorithms to make the images more realisticinstead of using simpler ray-tracing techniques. UsingALEPH, we could specify a time and a day, atmosphericconditions, and obtain a complete sun and sky model to lightthe scene (see Fig. 5). Only natural light was to be used,although at very different times of the day, from just afterdawn, to midday, to sunset and night. Given the complexityof the scenes and the wide variety of lighting setups, theALEPH system was much more accurate than adjusting thelighting by hand.

Rendering was done on a Beowulf system developed byGIGA. A Beowulf is a cluster of computers that work to-gether to solve a problem in parallel [4]. In order to keeprendering times as low as possible, several cinematographictechniques were also employed. In some shots where thegeometry was especially heavy, the scene were rendered in

Fig. 4. (a) Original photographed remain of a plate, (b) reconstructed texture and (c) test render of the plate and other utensils.


layers or passes, then composited by using matte channels.The swaying plants or the smoke from the pot were renderedusing straight ray tracing (see Fig. 6), and again added inpostproduction. Using ALEPH we could keep the shadows ina separate pass, as well as the sky. This provided more controlwhen it was time to tweak the images, adjust brightness andcontrast, or apply color correction to make them work in acontext. A separate depth pass was also used to blur certainobjects according to their distance to the camera, thus simu-lating the behavior of a real camera and enhancing the impactof certain static images.

As mentioned above, we wanted to avoid the dead-citylook that usually casts a shroud of gloom over computer-generated reconstructions. Since the streets had to be devoidof objects, doors had to be closed and windows had to besmall enough not to peek inside (all due to the X-centuryMuslim culture), the only viable solution was to populate thesuburb. Including humans in the synthetic world is essentialto create the illusion of presence. Computer-generatedpeople or avatars were ruled out in favor of real actorsperforming a carefully developed script against a bluescreen.To move the camera freely during all the shots, trackingmarks had to be rendered as another separate pass. The sametracking marks were used later to extract the CG-cameramoves for each shot and recreate them while filming theactors.Ateva Producciones, a Madrid-based company, filmedthe actors and composited them with the synthetic imagery.

3. The lighting calculation system

ALEPH [5] is a software system for lighting simulationover complex environments. It is used to accurately simulatethe interaction between light and materials in a given sce-nario, offering numerical results for illumination in the envi-ronment (predictive use in industrial designs) and photoreal-istic images.

The system combines elements and techniques from light-ing engineering and CG to improve the results in both fields.

A precise rendering system should be based on the simu-lation of light behavior, starting from real data and usingcomplex physical laws. It attempts to model all the phenom-ena in the light-material interaction, without neglecting anyimportant parts. Radiance is the physical magnitude used tomeasure all the possible interactions between light and sur-faces. The physical law that controls light transport in non-participating media is known as the Radiance Integral Equa-tion.

ALEPH solves the Radiance Integral Equation [6] byMonte-Carlo algorithms. Basic sampling is carried out bytracing rays into the scene; which can be very precise but alsovery time-consuming. Therefore, speedup algorithms areapplied, and parallelization is used when available (in multi-processor computers).

Fig. 5. Two renders of a street under different atmospheric conditions andtimes of the day.

Fig. 6. Secondary animation added in postproduction: (a) kitchen corner without smoke, (b) smoke layer rendered separately and (c) final composition.


Other general specifications of the system include:• Modularity, to be compatible with other software

projects or under various user interfaces.• A quality-computing time ratio that can be balanced.• Expandability: apart from a basis kernel, the rest of the

components of the system are runtime-loadable mod-ules, so new ones can be added without a completesystem rebuild.

• Multi-platform, so it can be used in low-end PC boxes orin multiprocessing ‘big iron’ systems.

3.1. ALEPH’s data handling

3.1.1. ColorThe basic magnitude in ALEPH is spectral radiance (i.e.

radiance with independent values over wavelength). Color ishandled by spectral representation. Since full in-memoryrepresentation of spectral data can be very expensive, asampled representation of a spectrum is used for internalcalculation and storage, as a balance between full spectraldata and simple RGB. Spectral sampling methods are opti-mized to maintain accuracy compared to full spectral data,and to allow the reconstruction of a spectrum from a color.

3.1.2. GeometryALEPH can handle simple and complex geometric primi-

tives. Triangles and polygons are among the former. Quadricsurfaces can be used directly by the renderer and splines andNURBS are read and adaptively meshed. Complex modelscan be imported from external sources if a suitable file formatreader exists. These file format readers (or model streamers,as they are called in ALEPH), are implemented as runtime-loadable modules.

In addition to these kind-of ‘classical’ primitives, ALEPHcan also handle procedural ones. Procedural primitives donot require explicit geometric information, but only have tooffer a suitable intersection method. Examples are fractalmountains based on Perlin’s noise algorithms [7].

3.1.3. MaterialsThe materials used in ALEPH can be quite complex and

powerful. As the system was developed to render the most ofphysical reality, the amount and types of data to handle isvery large.

A surface can have very different behaviors, from simplediffuse surfaces, to specular reflective, to fully glossy reflec-tive and transmissive. The surface reflectance properties aremodeled with a BRDF function. Transmission can be mod-eled in the same way with another function called a bi-directional transmission distribution function (BTDF). Amaterial can also have a complex emission pattern that can berepresented with a BEDF (E for Emission).

Properties that change over the surface are usually mod-eled with textures. So a BDF can just be a complex functionthat receives surface and viewing data and returns effectivereflectance for those conditions, evaluating both the textures

and the BDF. In ALEPH we wanted to separate BDFs fromtexturing, and to load BDFs and procedural textures at runt-ime as separate modules. As a result, adding new textures orBDFs to our library could be done without rebuilding thewhole system.

3.1.4. LightsLights in ALEPH are managed like materials; light geom-

etry is decoupled from light directional behavior, so both datacan be combined as required. Thus, the CIE or EULUM lightmatrix can be used with a point light to model street lightingor with an area light inside a building to model a luminairemade of a fluorescence tube array (which must be a distrib-uted light source to generate soft shadows...). Obviously,neither example was used during the reconstruction of theMuslim suburb.

ALEPH also defines special objects to deal with daylight.We combined data and procedures from several sources todefine very precise models for the sun and the sky, includingeffects like atmosphere turbidity or sky color variation.Physical data are used for the extra-terrestrial sun irradiance.Starting with the geographical location, date and time, wedefine all the required properties, such as the sun’s azimuthand elevation, atmospheric properties (optical path and at-tenuation), etc. Once the sun’s data is defined, we can alsomodel the appearance of the whole sky (color change alongsky hemisphere with daytime, intensity distribution...), andevaluate the total contribution from sun and sky to lightingfor outdoor scenes.

4. The CAVE-like system

4.1. System specification

A low-cost CLS was used as the output format [8,9] tobuild our system. It houses four to six people comfortably,although, due to the calculations, there is only one ideal pointof view where the perspective and stereoscopy look perfect[10].

Given the nature of the project, the notion of real time wasgiven less importance than high quality imagery. Thus, theimages have all been prerendered, which limits the interac-tion with the user but provide much better quality images.

4.2. Hardware

The CLS is a theater (4 × 3 × 3 m), with three rearprojection screens for the front (3 × 3 m), right and left walls(4 × 3 m) and six LCD projectors, two projecting on eachscreen, with a 1024 × 768-pixel resolution.

For stereoscopic vision we used polarized filters for eachprojector and their corresponding polarized glasses for theaudience. To control the projection we used two mid-sizedPC’s, one equipped with two MPEG-2 decoder cards (eachone offering four channels of simultaneous MPEG-2 decod-


ing) and the other with six SVGA output channels to sendstatic SVGA imagery to each projector.

The rough dimensions of the system are shown in Fig. 7and the actual CLS in Fig. 8. For simplicity we did notinclude a floor or ceiling screen. The room housing the CLSwas painted in matt black to absorb any light filtering in orbouncing off the screens. The system was built to be con-trolled from inside the CAVE, so a radio-based remote con-trol was used [11,12].

4.3. Software

Two main problems arose while designing the system:• The ability to deliver six (three stereo pairs) video

streams in real time from a single computer.• The need to play them synchronized at frame level.For the first issue we designed a program that controls the

PC that delivers video (the CLS-Video Display).For the second, we used the CLS-Slide Show program that

takes six images and sends them to the six projectors. Thisprogram is a caching–prefetching system that loads the nextframe that the user is going to see from the disk based on thecurrent frame. The computer is unusable during heavy read-ing from the disk (and perhaps decompressing JPEG or otherformats), but during this time the viewers are just payingattention to the specific details shown in the picture. We alsowanted the presentation to include some nice effects, likefading between slides.

The programs include the option of stepping forwards andbackwards, rewinding, fast-forwarding or jumping to a par-ticular image. The system can also run automatically byexecuting text scripts.

5. Tone reproduction and perceptual issues

The goal of creating photorealistic synthetic imagery is tocapture the visual appearance of the modeled scenes as ex-actly as possible. High levels of accuracy can be obtainedusing physics-based rendering methods to calculate the en-ergy distribution in the scene. However, this does not guar-antee that the visual appearance of the synthetic image willmatch the real scene. This is due to two problems. First, therange of luminance in a real scene usually surpasses thedynamic range of the output device by several degrees ofmagnitude. Pictures on paper have a maximum contrast of30:1, CRT monitors usually have no more than 100:1, andonly a few high-quality prints can reach 1000:1. However, inthe real world it is easy to find contrasts of over100 000 000:1.

Second, the conditions of visualization for the observer ofthe real scene are almost always different from the observerof the synthetic image. In addition, studies on the humanvisual system have not been able to build a definitive andverifiable model [13,14].

Tone reproduction provides a method of mapping (scal-ing) luminance values in the real world to a displayable

Fig. 7. Dimensions of the system.


range. The wide range of light in a real scene must beconveyed on a display with limited capabilities. In addition tocompressing the range of luminance, tone reproduction is

often used to mimic perceptual qualities of the human visualsystem. The goal is to generate an image that provokes thesame physiological responses as the real scene.

Fig. 8. The CLS system.


There is also another reason to study human perceptionmechanisms and their application to tone reproduction: tosave rendering times. By understanding how our brain inter-prets the image, solutions can be calculated with less preci-sion from a physics point of view if more physically-accuratesolution will not add anything to the image as perceived by ahuman observer (and more expensive in terms of renderingtime). This is especially interesting in the field of virtualreality or immersive systems such as CLS, where the ultimategoal is to produce photorealistic imagery in real time. Thegoal is not just to see an image through an output device(however photorealistic it might be), but to create the feelingof actually being there. This cannot be accomplished byignoring perceptual issues. Different perception-based met-rics can be found in [15,16].

Ad-hoc tone reproduction and perceptual issues weretaken into account in several images of the Muslim suburb.However, they are preliminary attempts and we cannot claimto have solved the problem. Initial tests inside the CLS havenevertheless proved that a correct tone mapping pays off interms of increasing the sensation of realism conveyed by theimages.

6. Results

The Muslim suburb found in the Paseo de la Independen-cia was successfully reconstructed in the digital realm. Twoversions were rendered: one for video display and another forthe CLS system, including immersion and stereoscopy.

The digital production follows a historically-accuratescript, which takes the audience through the streets of thesuburb in the morning and into a pottery shop, a carpet shop,a wealthy man’s house, into a more modest house where therest of the day passes and finally back into the streets at night.Actors perform the script filmed against bluescreen and com-posited into the synthetic world and serve as guides for theaudience.

The whole story takes a little over 5 min in the CLS andthe video lasts 20 min with synthetic imagery and real foot-age. Table 1 includes some relevant data about the produc-

tion, and several frames are shown in Fig. 9, both inside theCLS and as rendered images. The images were rendered in aBeowulf system with 5 Dual Pentium III 1 GHz processors.

7. Conclusions

This paper describes the complete digital 3D reconstruc-tion of Sinhaya, a X–XI century Muslim suburb of Zaragoza,including realistic interiors of some houses and shops. Accu-rate models and textures were needed to capture all the wearand tear of a real suburb populated by real characters as wellas a multidisciplinary team composed of archaeologists, his-torians, computer scientists and scriptwriters.

Precise lighting algorithms developed by GIGA give theimages a photoreal look and were used to specify the date, thetime of the day and various atmospheric conditions. A com-plete sun and sky model was prepared to light the scene. Onlysimulated natural light was used, although at very differenttimes of the day (from dawn to midday, to sunset and night).

Real actors were composited onto the synthetic imagery tobring life to the reconstruction. Good visual results were alsoachieved by including imperfections in buildings and ob-jects, dynamic simulations of the flowers and plants swayingin the wind, curtains swinging slowly, water in a fountain,etc.

The images and animations can be viewed in a low-costCLS designed and developed by GIGA. The system includesthree screens (one measuring 3 × 3 m and two sides 4 × 3 m)where the images are rear-projected. Inside this immersiveenvironment, the audience has the feeling of really beingthere, with stereoscopy accentuating the illusion.

The virtual reconstruction was quite successful and willhelp to preserve part of the city’s archaeological and culturalheritage, breathing life into a long-gone past. Visitors canexperience what was life like in the suburb, from morning tosunset, and archaeologists and historians can study and ana-lyze it in a whole new way.

8. Future work

Future work includes the technological improvement ofthe actual system and the development of new reconstruc-tions to help rebuild other parts of the city’s past. It will alsobe interesting to apply the technique in the field of industrialarchaeology, which helps to explain the inner workings ofold machinery and mechanisms through modern means. Fi-nally, tone reproduction and perceptual issues need to bestudied for CLSs while fully integrated in the renderingpipeline of the images.

In terms of the CLS, the main technological improve-ments would be to extend the actual control system to aclient–server configuration. With a new configuration andheavy code writing, we would be able to ray trace scenes inreal time.

Table 13D digital reconstruction of the Muslim suburb: technical data

Total rendering time 537 hNumber of generated animations 19 224 frames (12 min 49 s)Number of animations selected forsimulation

4030 frames (2 min 42 s)

Number of rendered static images,including low-resolution tests

1272

Number of static images selected forsimulation

54

Time of postproduction and file generation 63 h 18 minNumber of textures 613 files (226.7 Mb)Number of materials and shaders 5743Number of 3D objects 6030 (109 models)Number of polygons 1 502 691


Fig. 9. (a) A compound image of the three screens of the CLS. (b–d). More snapshots inside the CLS. (e–g) Renders of different views of the Muslim suburb.


The group is already involved in the pre-production offuture projects where we hope to achieve at least some of thegoals outlined in this section. One such project is the audio-visual architectonic recreation of the Roman theater of Zara-goza.

Even though ad-hoc tone reproduction techniques havebeen used for this reconstruction, the group has also devel-oped a tone mapping method based on the works in [17,18].Extensive tests will be performed inside the CLS to allow usto draw important conclusions and fine-tune the algorithmsto meet the demands of an immersive system. As thosealgorithms are validated and improved, they need to be inte-grated in the rendering pipeline to yield a stronger sensationof reality.

Acknowledgements

The authors would like to thank Jorge del Pico, AndrésMena, Beatriz Vicente, Eduardo Jiménez and Abel Hernán-dez for their continuous work throughout the whole project,as well the Comisión de Cultura of the Diputación General deAragón for their valuable contributions. This research waspartly done under the sponsorship of the Spanish Ministry ofEducation and Research through the projects TIC 2000–0426–p4–02 and TIC 2001–2392–CO3–02.

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