Very Realistic Graphics for VisualisingArchaeological Site Reconstructions

Alan Chalmers1University of Bristol, UK

AbstractRecent developments in computer graphics areproviding powerful tools for modelling multi-dimensional aspects of data gathered byarchaeologists. Computer graphics techniquescan now be used to reconstruct and visualisefeatures of a site which may otherwise bedifficult to appreciate. This new perspectivemay enhance our understanding of theenvironments in which our ancestors lived.However, if we are to avoid misleadingimpressions of a site then we need to be able toevaluate the “realism” of the resultantreconstructions.

This paper investigates the problemsassociated with reconstructing archaeologicaland heritage sites on computer.

K e y w o r d s : Very Realistic Rendering,Archaeology, Global Illumination, VisualPerception.

1 IntroductionComputer graphics techniques are increasinglybeing used to reconstruct and visualize featuresof archaeological sites [1]. While this newperspective may enhance our understanding ofthe environments in which our ancestors lived,if we are to avoid misleading impressions of asite, then the computer generated imagesshould not only look “real”, but must simulateaccurately all the physical evidence for the sitebeing modeled. The aim of realistic imagesynthesis is the creation of such accurate, highquality imagery which faithfully represents aphysical environment, the ultimate goal beingto create images which are perceptuallyindistinguishable from an actual scene.

Before the advent of modern lighting,illumination within ancient environments was dependent on daylight and flame. The lack ofwindows in ancient environments(certainly thecase in caves) and later, glass if available wasexpensive, windows were inappropriate fordefense etc, meant that even during daylighthours, some form of firelight was necessary for

interior illumination. The fuel used for the firedirectly affects the visual appearance of thescene. Furthermore, flames are not static andtheir flicker may create patterns and movingshadows that further affect how objects lit bythese flames might look. Thus any realisticreconstruction of an archaeological site musttake into account that these would have beenflame-lit environments and thus thereconstructions should not only incorporate theaccurate spectral profile of the fuel beingburnt, but also the manner in which the flamemay move over time.

2 Luminaires“Command the Israelites to bring you clear oilof pressed olives for the light so that the lampsmay be kept burning”

[Exodus 27:20]

The luminosity of flame is due to glowingparticles of solids in laminar flux, the colour ofwhich is primarily related to the emission fromincandescent carbon particles. In this paper weare interested in the perception of objectsilluminated only by the flame from candlesmade using different fuel types and the impactmoving the candle to different positions in anenvironment has on visual perception. Wehave built upon the simplified candle flamemodel of Inakage [5], which Raczkowskiextended to include the dynamic nature of theflame[10].

2.1 Building the Luminaires

The acquisition of valid experimental data is ofvital importance to realism as the fuel burntmay have had a significant influence on theperception of the ancient environment. Thefirst stage was to build a variety ofreconstruction candles in collaboration withour archaeology department. Figure 1 (a)shows a selection of the candles. From top leftthey are:• Two processed beeswax candles (such as

might be used today), tallow candle (ofvegetable origin), unrefined beeswax

7

candle, and a selection of reeds coated invegetable tallow

• We also used a rendered animal fat lamp,1(b) and an olive oil lamp, 1(c).

Figure 1: Experimental archaeology: toreconstruct ancient light sources

Detailed spectral data was gathered using aspectoradiometer. This device is able tomeasure the emission spectrum of a lightsource from 380nm to 760nm, in 5nmwavelength increments. This gives an accuratebreakdown of the emission lines generated bythe combustion of a particular fuel. This datacan be used to create an RGB colour model foruse in rendering the scene.

2.1.1 Converting Luminaire Data

It is essential, when converting the detailedspectrum data from the spectroradiometer intovalues representing the red,1 green and blueportions of the spectrum, that this conversionis calculated in a perceptually valid way, asdefined by the CIE (Commission Internationalde l’Eclairage) 1931 1-degree standardobserver1.

Figure 2 shows the functions for the X, Yand Z channels. The Y channel measures theluminance of a source, and the X and Zchannels measure the chromaticity. Thisinformation is more useful when broken downas follows:

1 This system specifies a perceived colour as atristimulus value (i.e. three coordinates)indicating the luminance and chromaticity of astimulus as it is perceived in a 1-degree fieldaround the foveal centre.

Figure 2: CIE Tristimulus functions

If we let xX

X Y Z=

+ +( ) a n d

yY

X Y Z=

+ +( )

Then we can calculate the exact colourvalues for the red, green and blue sections ofthe spectrum, disregarding luminance. For acanonical set of VDU phosphors;

RED(x,y) = (0.64, 0.33)GREEN(x,y) = (0.29, 0.60)BLUE(x,y) = (0.15, 0.06) (Ward98)

2.1.2 Modelling the flame

Candle flame may be defined as diffusion wickflames, in which heat transfer from the flamecause a steady production of flammable vapour[4, 7]. Observations from the reconstructedlight sources showed that all of the flameswere small and (ignoring the effect of airturbulence for now [13]) fairly steady.

Rather than attempting to model the shapeof the flame mathematically, video footage ofthe real reconstructed flames was processedusing computer vision techniques, the shape ofthe flame extracted and the real flameincorporated in the virtual scene which wasrendered using Radiance [14]. Theillumination of the flame in the environmentwas achieved by approximating the shape ofthe flame by a series of illum spheres [14], asshown in figure 3 (a) and included in thevirtual scene 3(b). The number and size of thespheres can of course be varied to achieve abetter “fit” to the shape of the flame for eachframe of the video sequence.

8

Figure 3: (a) Simple luminaire model (b) realflame in virtual environment

Conversion of the spectral data to RGBdoes of course lead to an approximation of thecolours present. In future, for more accuracywe will need to consider calculating theconvolution of the emission spectrum of thelight source with the reflectance curve of thematerial under examination. However, evenwith the approximation, significant perceptualdifferences related to fuel type are achieved.These simulations can be validated with realscenes [8, 9]. Figure 4 shows a test sceneincluding a Macbeth colour chart illuminatedwith (a) a 55W electric bulb, (b) an olive oilcandle and (c) a candle made from animal fat.

Figure 4: The effect of different fuel types (a)modern (b) olive oil (c) animal fat

3 Example applicationsValidated flame lighting has now been appliedto a number of archaeological sites toinvestigate the perception of objects withinthese sites.

3.1 Cap Blanc

The rock shelter site of Cap Blanc,overlooking the Beaune valley in theDordogne, contains perhaps the most dramaticand impressive example of Upper Palaeolithichaut relief carving. A frieze of horses, bisonand deer, some overlain on other images, wascarved some 25,000 years ago into thelimestone as deeply as 45cms, covers 13m ofthe wall of the shelter. Since its discovery in1909 by Raymond Peyrille severaldescriptions, sketches, and surveys of thefrieze have been published, but they appear tobe variable in their detail and accuracy In1999, a laser scan of was taken of part of the

frieze, figure 5, at 20mm precision [12]. It wasobviously of utmost importance that an “eyesafe” laser was used to ensure there was nodamage at all to the site.

Figure 5: Part of the frieze from Cap Blanc

Some 55,000 points were obtained in twoscans of the upper and lower part of theselected area, figure 5 (the MDL laser scannerused did not have sufficient memory to storeall the points in a single scan). These pointswere stitched together and converted into atriangular mesh.

Figure 6: (a) Clouds of points from the scan(b) The reconstructed horse

Detailed photographs of the frieze weretaken, each one of which included a standard“rock art colour chart”, just visible in figure 5.As the exact spectral data for the colour chartis known, this enabled us to compensate forthe lighting in the photograph and thus obtainapproximate “illumination free” textures toinclude with the wire-frame model.9

3.1 Results

Images were rendered using Radiance. Figure7(a) shows the horse illuminated by asimulated 55W incandescent bulb (as in a low-power floodlight), which is how visitors viewthe actual site today. In figure 7(b) the horse isnow illuminated by an animal fat tallow candleas it may have been viewed 25,000 years ago.As can be seen the difference between the twoimages is significant with the candleillumination giving a “warmer glow” to thescene as well as more shadows.

Figure 7: Frieze lit by (a) 55W incandescentbulb (b) animal fat candle

This shows that it is important forarchaeologists to view such art work under(simulated) original conditions rather thanunder modern lighting. (It is of courseimpossible to investigate these sensitive siteswith real flame sources).

For this site, we wanted to investigatewhether the dynamic nature of flame, coupledwith the careful use of three-dimensionalstructure, may have been used by ourprehistoric ancestors to create animations inthe cave art sites of France, 25,000 years ago.

To investigate this hypothesis, images wererendered with the simulated flame in a numberof different positions relative to the carving ofthe horse. Figure 8 shows some stills of theanimation that was created [2].

Figure 8: Stills from the animation: Firelight:Graphics and Archaeology

The video which illustrates this work canbe seen at:

www.cs.bris.ac.uk/~alan/capblanc.htmlThe shadows created by the moving flame doindeed appear to give the horse motion.

3.2 The House of Vettii, Pompeii

The House of the Vettii in Pompeii is one ofthe best-preserved and decorated buildings inthis World Heritage site, and is the mostfrequently visited building in Pompeii [9]. Therich colours and extensive use of artistictechniques such as trompe l'oeil, along with itsmagnificent state of preservation drawsmillions of visitors through its rooms eachyear. However, the impact of time and tourismon such a site has led to serious deterioration.A very realistic computer reconstruction of theHouse of the Vettii allows us to view the siteas it might have been when it was in use beforethe eruption of Vesuvius in AD 79 [3].

The room chosen for the study was areception room, which opens onto acolonnaded garden. The room’s three walls arerichly decorated by intricate frescoes, figure 2.

Figure 2. The room as it appears today

The frescoes in the room were recordedphotographically. Once again, a colour chartwas included at either side of each photo topermit calibration, identify illumination levelsand allow us to obtain illumination free

10

textures to incorporate into the computermodel.

Figure 7 shows a visualisation of thefrescoes using simulated modern and olive oillighting. It is noticeable that the lamp-litscenes (Figures 7b – 7d) can be perceived aswarmer in appearance when compared to themodern light (Fig. 7a), with the yellow and redpigments particularly well emphasised. Theappearance of the three-dimensional trompel’oeil art is also influenced.

Figure 7. Clockwise from top left: (a) modernlighting (b) olive oil lamp (c) olive oil lampwith salt (d) olive oil lamp with water

4 ConclusionsWe will of course never know for certainwhether the artists of the Upper Palaeolithicwere in fact creating animations 25,000 yearsago, however the reconstructions does showthat it certainly is possible. There is otherintriguing evidence to support this hypothesis.As can be seen in the figures, the legs of thehorse are not present in any detail. This haslong been believed by archaeologists to be dueto erosion, but if this is the case, why is therest of the horse not equally eroded? Could itbe that the legs were deliberately not carved inany detail to a c c e n t u a t e any motion?Furthermore traces of red ochre have beenfound on the carvings. It is interesting tospeculate again whether the application of redochre at key points on the horse’s anatomymay also have been used to enhance anymotion effects.

Future work will investigate thesepossibilities as well as consideringreconstructions of other painted (rather than

carved) sites in the region to see if a similarhypothesis could apply equally to the use ofpainting on complex three-dimensional rocksurfaces.

Work on the House of Vettii is also only inits preliminary stages. Current work involvesthe addition of models of appropriate Romanfurniture and artefacts to the scene. This willnot only create a more realistic scene, but willallow archaeologists to investigate theappearance of these objects under their originallighting conditions. It is important toremember that a reconstruction is only oneglimpse of many valid interpretations. Variousconfigurations of lamps are also beingmodelled to provide a number of possiblescenarios.

Very realistic computer reconstructions ofpast environments provides a safe andcontrolled manner in which the archaeologistcan test hypotheses regarding perception andpurpose of colour in decoration and artefacts.Computer generated imagery indistinguishablefrom the real physical environment will be ofsubstantial benefit to the archaeologicalcommunity, and this research is one method ofmoving beyond the current trend of photo-realistic graphics into physically andperceptually realistic scenes which areultimately of greater use to those investigatingour past.

Acknowledgements

We would like to thank Jean Archambeau, theowner of Cap Blanc, for his permission towork at the site and his interest and Francescod’ Errico, Kate Robson Brown, Ian Robertsand Michael Hall for their input to this projectMany thanks also to the Bristol/BordeauxTwinning Association and the ALLIANCE /British Council programme (Action integréefranco-britanique) for their financial support.

5 References[1] Barcelo, J.A., Forte, M. and Sanders,

D.H., eds. Virtual Reality in Archaeology.(2000) ArchaeoPress.

[2] Chalmers A., Green C. and Hall M.“Firelight: Graphics and Archaeology”,Electronic Theatre, SIGGRAPH 00, NewOrleans, July 2000.

11

[3] Devlin K., Chalmers A.G. “Realisticvisualisation of the Pompeii frescoes”,AFRIGRAPH 2001, ACM SIGGRAPHPublications, November 2001.

[4] Gaydon, A.G. and Wolfard, H.G. Flames:Their Structure, Radiation andTemperature. (1979) Chapman and Hall.

[5] Inakage M., “A Simple Model of Flames”,Computer Graphics Around the World, ed.T.S.Chua, T.L.Kunii, Proceedings ofComputer Graphics International, (1990)Springer-Verlag, pp. 71-81.

[6] Mavrodineanu R. et al., “Analytical FlameSpectroscopy, Selected Topics”. (1970)MacMillan.

[7] McNamara A., Chalmers A., Troscianko T.and Gilchrist I. “Comparing real &synthetic scenes using human judgementsof lightness”. In B. Peroche and H.Rushmeier, editors, Rendering Techniques2000, Springer Wien.

[8] McNamara, A. and Chalmers, A., ImageQuality Metrics, Image Quality MetricsCourse Notes, SIGGRAPH 00, (July2000).

[9] Nappo, S., Pompeii: Guide to the LostCity. (1998) Weidenfeld and Nicolson.

[10] Raczkowski J. “Visual Simulation andAnimation of a laminar Candle Flame”.International Conference on ImageProcessing and Computer Graphics,(1996) Poland.

[11] Roberts, I. Modelling Realistic Flame.Project Thesis submitted in support of thedegree of Bachelor of Science inComputer Science, University of Bristol(2001).

[12] Robson Brown K.A., Chalmers A.G.,Saigol T., Green C., d'Errico F. “Anautomated laser scan survey of the UpperPalaeolithic rock shelter of Cap Blanc”,Journal of Archaeological Science, 28,pages 283-289, 2001.

[13] Stam J., Fiume E., "Turbulent Wind Fieldsfor Gaseous Phenomena". Proceedings ofSIGGRAPH 93 (1993) pp.369-376.

[14] Ward Larson, G. and Shakespeare, R.,Rendering with RADIANCE: The art and

science of lighting simulation. (1998)Morgan Kauffman.

Permission to make digital or hard copies of all or part of this work forpersonal or classroom use is granted without fee provided that copiesare not made or distributed for profit or commercial advantage and thatcopies bear this notice and the full citation on the first page. To copyotherwise, to republish, to post on servers or to redistribute to lists,requires prior specific permission and/or a fee.Spring Conference on Computer Graphics, Budmerice, Slovakia Copyright © ACM 2002 1-58113-608-0/02/009...$5.00

12