Why Digital Enhancement of Rock Paintings Works: Rescaling and Saturating Colours

Article excerpt

Introduction

Rock paintings are found on all continents except Antarctica. Yet rock-art research is faced with a very real problem. Like other items of material culture, it 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 against rock walls; water damage; graffiti; and the accumulation of dust mantles and other mineral crusts over rock surfaces. The result is an increasing visual obstruction of the art through time. These problems are so acute that significant proportions of pictographs in many art sites around the world remain too unclear to record adequately, their forms often being relegated to the general category `indeterminate'. In Australia, this problem is well illustrated by various recording programmes in the tropical north. On the granite boulders of Torres Strait, exactly 50% of pictographs were identified as indeterminate by McNiven et al. (2001); and in Australia's Cape York Peninsula, 39% are indeterminate on the granites near Yarrabah (David 1994); 30% on the granite boulders of Bonney Glen Station (David 1991); 22% in the Mitchell-Palmer limestone zone (David, unpublished data); and 10% on the Ngarrabullgan sandstones and conglomerates (David 1992). In the vast majority of cases, it is fading (caused by a variety of processes, see below) that has rendered the motif forms indeterminate.

Digital image enhancement technology has been well known since it was developed for space exploration in the 1960s and 1970s (e.g. Castleman 1996; Pratt 1978). 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 (e.g. Clogg & Diaz-Andreu 2000; Henderson 1995; Mark & Billo 2000; Read & Chippindale 2000). Relatively cheap yet powerful computer enhancement programs such as Adobe Photoshop or Corel Photo-Paint are now easily available over the counter, ideal for rock-art applications. Yet few archaeologists understand why enhancement works -- what happens when an image is enhanced -- or how far enhancement can be taken. This paper aims to rectify the situation by explaining why digital enhancement of rock paintings works. We use one of our own case-studies from the islands of Torres Strait, between Australia and New Guinea, to illustrate two principles of image enhancement -- colour rescaling and manipulation of colour saturation -- to increase the visibility of rock paintings to the human eye.

Measuring the colour of rock images

What is light?

Light travels through space as electromagnetic waves of varying lengths. The entire spectrum of wavelengths is called the electromagnetic spectrum, but only a small section of this spectrum is capable of producing visual sensation (Jacbson et al. 1988: 8). Other parts of the spectrum include radio waves and infra-red waves that have wavelengths longer than visible red light, and ultra-violet light that has wavelengths shorter than visible violet light. Despite some experimental efforts to use these non-visible wavelengths of light to record rock-art images, results have been mixed and most recording is accomplished using visible light.

What is colour?

The human retina contains millions of light-sensitive cells that can be divided into two major types, rods and cones. Through variations in chemical composition, three types of cones enable us to discriminate among different colours.

The light that reflects off any surface (e.g. a rock background or a painting) consists of a `rainbow' or spectrum of electromagnetic waves of different wavelength (e.g. red, yellow, green, cyan, blue, magenta). These surfaces differ in colour when the reflected light has different amounts of light intensity at different wavelengths (FIGURE 1). …