Dauan, one of the Topwestern Islands (close to Papua New Guinea), is home to around 200 people and to Australia's northernmost rock art. One sunny day in 2000 we met on this remote tropical isle to test an exciting idea.

Less than 50 m from shore is Kabadul Kula. Here a granite boulder almost 6 m high creates a shallow rock overhang, home to paintings from a number of different phases in the Torres Strait's past. Some of these paintings have faded so much that they are now invisible. In a number of places later images have been painted directly on top of earlier ones.

Art provides one of the best avenues for studying the interactions that once took place between Australia and Melanesia via the Strait. Unfortunately rock art is fragile: dust and salts can form an opaque crust over paintings; the paint may absorb electromagnetic radiation, causing oxidation and changes in the pigments; or the paint itself may disintegrate and fall off.

Our plan was to see if digital image enhancement might allow us to view otherwise invisible art. The portability of digital cameras, and the quality of images they provide, make them ideal for fieldwork. We took along a standard laptop computer and two Nikon Coolpix 950 digital cameras. Elderly members of the Islander community, unable to come to the site, could see the results on the laptop when we returned to the village later that day.

Face with spiked hair

Digital image enhancement has been around since early secret spy satellites and space missions like the 1960s Ranger lunar programme. Now the technique is available to anyone with a home computer and a camera, and commercially available software such as Adobe Photoshop or Corel Photo Paint.

How does it work? Visible light consists of a 'rainbow' or spectrum of electromagnetic waves of different wavelengths. Colours of an object - such as a rock or a painting - differ when the light that reflects off the surface has different amounts of intensity in different parts of the spectrum.

A digital camera records the light reflected from a rock surface on a sensor consisting of a rectangular array of pixels, or picture elements. For a colour camera, each pixel is really three different sensors, each sensitive to a respective colour corresponding to red, green and blue (R, G, B). Three sensors are considered adequate because they are roughly equivalent to the way the human eye perceives colour using its three different types of cone receptor in the retina. So three numbers, the R, G and B intensities, determine the colour at a point or pixel in the image.

In addition to the individual colours themselves, there are shades of grey from black to white, which emerge from a combination of colours. The less grey is contained within a colour, or the 'purer' it is, the more that colour is said to be 'saturated'.

As images fade, the differences between paintings and rock reduce. At the same time the colour R, G, B values move closer to one another, and also to grey, so that differences - contrast - may eventually disappear. If colours are close to one another - if the contrast is poor - the differences may not be noticeable to a human observer. Colour scientists define the term 'Just Noticeable Difference', the point at which the figure-background difference becomes just perceptible by a human viewer. The task of the digital enhancer is to increase the noticeable difference between the colour of the rock and the colour of the art.

We realised that if we placed a digital photo of the Kabadul Kula rock wall into the computer and instructed it to recolour the grey or brown pixels (mostly background), we could increase the distinctiveness of the rock. Likewise, intensifying the reddish colour would affect the foreground more than the background (because most rock paintings in Torres Strait are red) and bring out the contrast of all coloured pixels and thus coloured regions on the rock surface.

Increasing the saturation of colours would remove the impact of greys, further increasing the colour contrasts.

So we took pictures with the digital camera and entered them into our laptop using the compact flash adapter card. Then we adjusted the photos' brightness and contrast to get the best picture (like adjusting the 'brightness' and 'contrast' controls on a TV). Next we adjusted the image saturation, intensifying the colours and making them look more distinct (like the TV 'colour' control). As we made these adjustments, we looked at the results and tried, by repeating previous steps, to get the best results.

We then adjusted the hue in the image, because the human eye can sometimes distinguish some colour (hue) differences better than others (like adjusting the 'tint' control on a TV). Next we tried to expand the contrast between the average colours of the regions. This is something we cannot do on a TV, but can do in some image processing programs. Finally, we experimented with these tools on the software, until we produced the best contrast between the rock and the paintings.

We will never forget what happened at Kabadul Kula. With our eyes fixed on the computer screen, we adjusted the scaling, saturation and contrast of each of the digital photos in succession. One by one, previously invisible motifs appeared.

First came a large face with spiked hair and striking jagged teeth, and next to it a long shield. The shield may be a record of contacts from the Fly River mouth 120 km to the east, for no such item of material culture has ever been recorded from the Strait.

Nearby lay a painting of a hut complete with occupant. Further away was a dancer with elaborate fish head-dress, characteristic of Torres Strait ritual paraphernalia and stylistically akin to drawings and paintings also known from islands 68 km to the south.

Headhunting raid

A canoe with an in-curling sail may be a 'crab claw' canoe, foreign to Torres Strait and foreign also to neighbouring Papuan regions such as the Fly River mouth, but common in the Papuan Gulf some 200 km to the east. We are not certain of this latter identification, but it alerts us to the possibility of cultural influences from further afield than previously imagined.

You can see 16 scenes at Kabadul Kula with the naked eye. By the end of the day, with the aid of digital technology, we had identified 28 more - a total of 44 paintings. Oral tradition says the art was created by Kiwai warriors of the Fly River mouth during a headhunting raid, probably in the 19th century. The digital enhancements showed us the Kiwai were not the first artists: they re-inscribed a marked rock.

No wonder that Saila Savage, Chairman of the Kubin Community Council on the island of Mua some 70 km to the south of Dauan, has invited us to examine Mua's rock art.

David and McNiven are archaeologists at Monash University, Melbourne (Australia). Brayer is a computer scientist at the University of New Mexico, Albuquerque (USA). They thank the Dauanalgaw (Torres Strait Islanders) Corporation, the Dauan Island Council and the Dauan Island community for their hospitality.

* There are many possible applications of this technique, from studying colours in stone tools to recording church wall paintings or fading adverts on street frontages. Send us your images, and British Archaeology will consider publishing a selection

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