Torres Strait rock-art: an enhanced perspective.

McNiven, Ian J. ; David, Bruno ; Brady, Liam 等

Torres Strait, strategically located between the mainlands of Australia and New Guinea, has long fascinated anthropologists and archaeologists as a 'bridge and barrier' to the movement of peoples and cultural traits. Of particular interest has been the place of Torres Strait as a 'horticultural frontier' (Harris 1977) and a diffusionary pathway for material culture (McCarthy 1940; Rowland 1987; Walters 1988). All of these investigations require detailed knowledge of the development of island communities across the Strait and an understanding of long-term changes in social relationships (political alliances, trade relationships etc.) between separate island polities. While considerable information exists on historical trade relationships and social alliances (Haddon 1935; Lawrence 1994; Vanderwal in press), little archaeological knowledge is available about long-term changes in social relationships over the past 2500 years of known human occupation of the region. One avenue of research we are exploring to shed light on this area is stone tool exchange and changing patterns of raw material use and movement (McNiven 1998; McNiven et al. in press a). Another is through investigation of spatial and temporal trends in stone and shell arrangement types, and in rock-art styles and the use of pigments (ochres) across Torres Strait. However, Torres Strait rock-art presents us with a challenge. Many rock paintings have faded due to weathering caused by their harsh maritime and tropical setting. As such, often only the most recent artistic phases appear to have survived. In many instances bare traces of earlier art, sometimes superimposed by more recent pictographs, are apparent, although the shapes of individual motifs are too obscure to be determined with the naked eye. This article outlines our recent rock-art investigations in the northern Strait adjacent to the Papua New Guinea coast. We focus on the site of Kabadul Kula on the island of Dauan where we have applied digital enhancement techniques with remarkable success to help see faded and nearly effaced paintings.

Torres Strait rock-art

Available records reveal the presence of thirty-three rock-art sites (paintings, stencils and engravings) on Torres Strait islands (for overviews see Cole and David 1992; McNiven et al. in press b; McNiven and David in press) (Figure 1). However, our recent surveys have nearly doubled the known number of rock painting sites, indicating that a rich and elaborate rock-art heritage exists for the region. Alfred Haddon made the earliest recordings of rock-art over a century ago during the 1898 'Cambridge Anthropological Expedition to Torres Straits' (Haddon 1904:4-5,358-9, Figs 1-3,76-7, Plates 1.2 and 2.1; see also Herle and Rouse 1998). While no obvious 'contact' paintings with 'European' motifs have been recorded to date, it is clear paintings were made during the nineteenth century. For example, Haddon photographed a charcoal hand stencil site on Pulu Islet in 1898 that has since been completely effaced, suggesting production soon before Haddon's visit. Rock paintings of totemic symbols executed on Pulu by Mabuiag Islanders in November 2001 and October 2002 reveal that rock-art is a continuing tradition in the region (McNiven et al. 2002).

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Digital enhancement of rock paintings

Like all material culture, rock-art is subject to taphonomic processes and various forms of destruction over the long term. Such problems include granular disintegration of decorated rock surfaces; varied biological problems such as lichen, algal or fungal growths, insect nests and the rubbing of cattle and pigs against rock walls; water damage; fire and smoke damage; graffiti; and the accumulation of dust mantles and other mineral crusts (e.g. amorphous silica) over rock surfaces (David et al. 2001; Watchman 1992, 1993). The result is an increasing visual obstruction of the art through time. To help overcome this problem, numerous archaeologists are now using digital enhancement techniques to help them better see the rock paintings that are or were once obvious on rock walls. Relatively cheap yet powerful computer enhancement programs such as Adobe Photoshop or Corel Photo-Paint are commercially available, ideal for rock-art applications.

In short, the aim of the enhancement process is to increase the colour contrast between pixels making up the foreground (paintings) and background (rock surface) of a digital photograph so the paintings are perceptible by the human eye (see David et al. 2001 for a detailed overview of the digital enhancement process). The process is based upon the theoretical notion of 'colour space', where an infinite range of colours can be produced along three axes representing endless combinations of the three primary colours of red, green and blue. Individual colours seen by humans mostly represent clusters ('clouds') of specific points in 3-D colour space. The line from black to white along the diagonal of the colour cube is called the 'grey line' and consists of all shades of grey from black at the origin to white at the far diagonal corner. Two colours nearby each other in colour space will appear similar. Points in colour space near the grey line appear greyish or pastel or unsaturated. The less grey contained within a colour, the more that colour is 'saturated'. Thus colours near the three primary colour corners of the colour cube appear deep and pure and are called saturated.

The two key ways of increasing colour contrast (distances in terms of colour space) on a digital image of a rock painting is by colour rescaling and colour saturation. Rescaling works by moving colours apart in a colour space parallel to the grey scale. This action is equivalent to increasing the brightness of colours. Saturation works by moving colours away (perpendicular) from the grey line. This decreases the amount of grey in colours and increases the purity and thus the distinctiveness of the targeted colours. Thus, for example, any slightly reddish pixels will become redder, any slightly greenish pixels will become greener etc. The end result is a simplification of the range of colours across an image and increasing (visual) differences (distances) between colours. If the correct colours are saturated, the end result can be an image that has the paintings represented by a narrow range of bold colours and the background rock represented by a vastly different range of colours. In some cases, the paintings can be made to stand out further on the screen by making the background (rock surface) appear more uniform by desaturation (i.e. moving the background colours so they are closer to the grey line) and then removing all black colour so the background becomes essentially white.

These two techniques, increasing contrast by colour rescaling and altering the saturation of colours, are usually undertaken manually. Neither of the two methods is inherently better than the other, and indeed various combinations tend to produce the best results. A method's performance depends on the quality of the image being enhanced (lighting, focus, etc.), its particular colour-cloud distribution, and the colour of the rock wall. The performance is usually evaluated by human perception so the different enhancement methods must be evaluated and modified interactively. For these and other reasons, simple methods such as colour rescaling and enhancing colour saturation tend to be used more frequently than other, more complex methods.

The results of computer enhancement can be presented as is (i.e. digital photograph). However, increased clarity can be achieved by tracing areas of apparent painting, or otherwise creating a black and white drawing showing painting against a white background. Tracing can be done manually from a photo print, on a computer using a mouse and drawing program, or by systematically convering the highly contrasted foreground-background pixels into a black and white image with a computer drawing program.

Kabadul Kula (Dauan Island)

Kabadul Kula is a large granite boulder with a shallow overhang under which can be seen numerous red paintings (David et al. 2001; McNiven et al. 2000; McNiven et al. in press c). It remains the most complex rock-art site recorded to date in Torres Strait and the only site with an associated story of its creation (see McNiven et al. in press c for details). Islander history recalls that the paintings were made some time ago (possibly nineteenth century) by a group of Kiwai Island raiders (Kupamal) who had canoed from the mouth of the Fly River 140km to the northeast. After their secret landing, the raiders drew pictures with red ochre (parma) on the granite boulder now known as Kabadul Kula. The next morning they attacked the village of Buli killing many with their stone-headed clubs (gabagaba) and cutting off their victims' heads with bamboo knives (upi). The raiders managed their escape, but not before a number were killed by Dauan warriors (Lawrie 1970:143-7).

Many of the paintings at Kabadul Kula remain clear today. However, faint traces of earlier artistic episodes are barely visible across the two major panels (boulder sites) forming the site. In April 2000 we visited the site, recording each painting with Nikon Coolpix 950 digital cameras (Figure 2). Sixteen pictographs were clear enough to the naked eye to make out their motif forms. They included anthropomorphs, marine animals (e.g. crayfish, dugong) and geometrics (e.g. circles) (Figures 3 to 5). Since initial recording of the site (David et al. 2001; McNiven et al. 2000), detailed digital enhancement (principally colour re-scaling and saturation) of the entire wall surface using Adobe Photoshop has revealed eight previously unknown pictographs. At least some of these faded motifs appear to represent the earliest artistic phase(s) (pre-dating the Kiwai raid of oral tradition) at the site. The enhanced images include underlying non-figurative motifs (Figure 3), an insectlike motif (Figure 6), an anthropomorphic face and possible nearby shield (Figure 7), and an anthropomorph with a fish head-dress of a type used historically in Torres Strait (Figures 8 and 9).

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One important implication of the enhancement work is that the rock was already decorated when the Kiwai raiders decided to inscribe it with their own marks. To shed light on the antiquity of these painting events and possible early Papuan contacts, discussions are continuing with the Dauan community to undertake excavation of deposits at Kabadul Kula to obtain buried pieces of ochre, exfoliated fragments of painted rock-wall and charcoal for radiocarbon dating (see David et al. 1992 and Watchman et al. 2000 for examples of this technique). A collaborative community-university (doctoral thesis research by LB) program of systematic recording, including digital enhancement and computerised tracing of all known painted surfaces in Torres Strait, is also currently in progress.

Conclusion

Digital enhancement is a low cost and relatively easyto-use technique that opens a new window on Australian rock-art studies. However, the technique is much more than an analytical aid for rock-art investigations. We found that one of the most rewarding consequences of digital photography and computer enhancement is the ability to immediately communicate results back to local community members (particularly those physically unable to visit sites) whilst in the field. The immediacy of feedback allows community members to see the daily progression of research and to discuss the cultural significance of new finds. Most significantly, digital enhancement allows community members to see previously unknown paintings and an opportunity to see even well known rock-art sites in a new light, and in this way to be involved in aspects of analysis previously not commonly accessible to local community members.

ACKNOWLEDGEMENTS

This project was kindly funded by a grant from the Australian Institute of Aboriginal and Torres Strait Islander Studies, Canberra. Logistical support was provided by Garrick Hitchcock and Kevin Murphy (both formerly of the Native Title Unit, Torres Strait Regional Authority, Thursday Island). Special thanks to the Dauan Island community for hospitality and support.

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Ian J. McNiven, Bruno David and Liam Brady School of Geography and Environmental Science, Monash University