Digital retouching of rock paintings


The new digital era of photography has created new opportunities for documenting and interpretating ancient rock art. In the following I will write about my experiences on documenting and processing Finnish rock paintings, a subject close to me since 1994. I mainly work as an independent artist, but I have also co-operated with the Finnish National Board of Antiquities. (2019: Please notice that the text is originally written in 2003 and things have changed a bit. I have done some remarks in 2015 and 2019 to keep the text updated.)

Finnish rock paintings have been dated 5000–1500 BC while Finland still lived Stone Age. Ceramics decorated with comb-like pits was in use, but people still hunted, fished and gathered their food. Agriculture was in a minor role. All known paintings in Finland are painted on cliffs with red ocre.

For me a good quality colour photograph is an ideal document of a rock painting. It shows the things that can be seen with a bare eye. It does not miss anything or see anything extra. A photograph also shows the surface of the rock with all the damage and lichens. I think this is a great benefit that is difficult to achieve in any other method of documentation. (2019: Although the new 3D methods seem to offer very promising opportunities.)

Using digital picture processing increases the possibilities of the photography. It is possible to strenghten the colour of the paintings and make the paintings more visible. Even some new figures can be found. The methods are quite simple, and they are possible for anyone who has basic knowledge of digital picture processing. However three steps should be taken care of before any results can be expected: 1) Photographing the paintings. 2) Scanning the photographs and making the basic adjustments. 3) Processing the photograph.

Field work: photographing

Making a good quality photograph of a rock painting is probably the most difficult task in the list. Processing can be done with a less quality pictures as well, but the later processing is easier and the results more realible if the photograph is good.

Nowadays it is possible to use a digital camera for the documentation. The resolution of the newest portable models (5 million pixels) (2019: This is ancient!) is good enough for serious field work. Good results can be obtained with the cheaper models if the photographs are taken close enough and maybe combined later with a computer. The best results can be achieved with a high resolution digital camera. However they are difficult to use in field conditions and they also need a computer next to them. The prize of such an equipment is also a problem. Anyway, I still use film and my old reliable 35 mm and medium format cameras. I use colour negative film because of its flexibility, and scan the negatives for later processing. (2015: The rapid evolution of digital cameras in the past 12 years has opened new possibilities. Nowadays I mainly use a high resolution 36 megapixel digital SLR camera. Using RAW file format offers the best possible picture quality for later processing.)

In the field the first thing to be taken care of is the weather and the light conditions. For paintings an even light without shadows is the best. Cloudy sky and no rain is ideal. Also the temperature and the humidity of the air may effect the way paintings can be seen. In winter, after a cold and humid period the paintings are often more visible than in summer. I have noticed the difference in several places.

For maximum sharpness a tripod and a small aperture should be used. There is no idea to use unsharp or fuzzy pictures for documentation. If possible photographs should be taken straight towards the painted surface for minimizing perpective distortions. Of course if the paintings are high in the cliff this may be difficult to arrange. At times I have had to take pictures from the distance and use a telephoto lens.

If the pictures are going to be used for measurements a scale is needed. I use a one and a half meter long stick with the camera in the other and the scale in the other end. In the field I place the scale to the rock and take several overlapping pictures so that the whole rock is covered. Because the camera is in the other end of the stick every picture is taken from the same distance. I often place a gray card next to the scale. It doesn't make the picture more beautiful, but it usually makes the later colour adjustments easier.

figure 1

Scanning and basic adjustments

Digital basics

Digital photograph is a mosaic of tiny squares, pixels. Every mosaic plate – pixel has a colour, that is marked by certain numbers. There is many different methods of making a colour in a computer (RGB, CMYK, HSB, Lab...). The most common method is to use three basic colours (red, green, blue). The method is the same with the one in our eyes. In computer this is done by using three colour channels, that can have (usually) values from 0 to 255. In the computer screen or the print the values of colour cannels are mixed, and they show as different tones. More than 16 million colour tones can be produced in this method. (2015: It is highly recommendable to use 16 bit colour depth instead of the normal 8 bit. This multiplies the amount of colour tones and gives remarkably better results in processing. Of course, this requires that the original photographs – whether scanned or taken with digital camera – are already in a file format that supports higher colour depth (such as Raw file formats).)

Because the colours of the pixels are based on numbers, it is easy to select similar colours (= similar numbers) and change them. You can for instance, select certain red tones and strenghten them.

Scanning (2019: No need to scan, if a digital camera is used.)

I always scan the photograps from the film. A good quality film scanner is expensive, and it needs also a trained person to do the job before good results can be obtained. It is also possible to let the laboratory to do the scanning. If I need a big amount of photographs to be scanned I usually let the laboratory to do the job. The problem is that the automatic colour adjustments they use are usually not ideal for pictures of rock surfaces, and I have to make new adjustments by myself. If I scan myself I can make all adjustments needed while scanning, which is better for the final result.

With 35 mm film I usually scan the pictures to the size of about 2000 x 3000 pixels, which makes about 18 MB files. If the picture is printed with a photo quality printer (300 dpi) a size of 17 x 25 cm is obtained.

The basic correction every picture needs is to make the darkest point of the picture black (0, 0, 0 in RGB colour system), lightest white (255, 255, 255) and gray areas gray without any colour cast (all colour channels have same values, for example 130, 130, 130). (2015: The numbers refer to 8 bit colour depth. However in practice they are often used when working with 16 bit colour depth as well.) If this can be done in the scanner usually no later corrections is needed. If I do the correction in picture processing software (Photoshop) I use curves -control (Image – Adjust – Curves) and its eyedroppers. I use the black eyedropper for the darkest point in the picture (deep shadow or the black area of the measuring scale), the white eyedropper for the whitest point in the picture (light stone or the white area of the measuring scale) and finally the gray eyedropper for the gray card. After this the contrast and the colour balance of the picture should be correct, and the picture is a good digital document of the rock surface and the paintings. (2015: If the original picture is in RAW file format, the basic adjustments can be done in RAW converter, such as Photoshop's Camera Raw or Lightroom.)

Making the paintings more visible

For digital picture processing there are several programs available. I have always used Adobe's Photoshop (nowadays version 6.0) and I haven't had any reason to try another. For adjustments I use adjustment layers rather than change the picture itself. With layers it is always possible to go back and look, what the picture was before any adjustments were done. (2015: The current version is Photoshop CC. Nowadays there are also special picture processing softwares for rock art photographs. The best known is DStretch, a plug-in for ImageJ. It is an excellent, easy to use tool for processing rock art images. DStretch makes big changes to the colours of the rock. In my work I prefer natural colours, and I still use Photoshop and the methods described below. However I often check the images with DStretch as well, for reference.)

The first thing needed is a good quality picture taken with a digital camera or scanned from a photograph. Colour balance and contrast should be corrected before any other adjustments are made.

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A good and simple way to make the paintings more visible is to increasing the saturation of the painting. The Hue/Saturation -control of Photoshop is ideal for this. So at first a new adjustment layer is created (Layer – New adjustment layer – Hue/Saturation). From the Hue/Saturation -window red colours are selected and the saturation level is pushed from zero to +50 – +80. All the red areas of the picture increase in saturation. You can also limit the effect to exactly the right tones with the spectral controls in the bottom of the window. If the preview is turned on the difference is seen while working, and it is easy to see the effect. (2015: The tools of Photoshop look a bit different in the current version, but the functions are about the same.)

figure 3

Increasing the saturation is quite ideal way to make the paintings more visible. The computer selects all the tones you want. It does not miss any detail or add anything to the painting. It also leaves the backgroung untouched. It is possible to check if a missing part of the painting is due to later damage or if a crack on the rock has effected the way paintings are presented.

figure 4

With the Hue/Saturation -control it is also possible to increase the difference of tones. For example, at times different red tones have been used in the same area of the painting. Probably the other being earlier than the other. In the figure below two different Hue/Saturation adjustment layers are made. One for increasing the saturation of purple-red paint and another for increasing the saturation of orange-red paint. The hue of the purple-red is changed towards magenta so that the difference in tones is more clear. (Take the curser over the picture, and you see the original photograph).

figure 5

Selecting the Red Colour

Another method to strenghten the colour of the paintings is to select the colour of the paint and for example darken it. I usually first use the Hue/Saturation -control anyway. It is much easier to find the right colours if they are already strenghtened.

The Color Range -tool (Select – Color Range) is a nice way to select similar tones of the picture.

figure 6

When any colour of the picture is clicked, all similar pixels are selected. Increasing fuzziness selects more pixels close to the clicked. I usually use fuzziness amount 10 – 40. If another colour is clicked with the Add to Sample -tool all the pixels of the two colours are selected. When selecting the colour of the rock painting all the tones of the painting should be selected. The progress of the work can be followed in the little preview of the Color Range -window. If Selection Preview: White Matte is selected it is possible to see the result in the original picture as well. When all the tones from the painting (and none from the background) are selected OK is clicked and the final selection is made.

Nothing has changed yet, but with the active selection you can do changes that effect only to the selected pixels. So if you want to see only the paintings and remove the background, you inverse the selection (Select – Inverse) and clear the background with the Clear -key.

figure 7

I often use the Curves -tool to darken the paintings and after inversing the selection to lighten the background. The relation between the painting and the surface is still visible, but the contrast is stronger and the paintings became more visible.

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The paintings I have used above (figures 2, 4, 7 and 8) are in Pakanavuori in Kuusankoski, Finland. Pakanavuori paintings are like most cases quite easy to process. Also most of the paintings in Astuvansalmi, Ristiina (figure 5) are quite easy to work with. However there are some rock paintings that need more work if they are wanted to be digitally strenghtened. For example the geometric figure by the lake Vitträsk (figure 1) in Kirkkonummi is quite easy to see, but difficult to process.

If the colour of the rock is close to the colour of the painting or if the painting is very faint it may be difficult to find the right adjustments. Either some parts of the painting are not strenghtened or also parts of the background streghtenes as well. This makes the picture fragmentary or fuzzy. With a careful action good results can be usually achieved. Also a rock with a lot of lichen is a difficult one. The picture becomes fragmentary and defective. A good example is the left part of the painting in Verla, Valkeala.

Some words on landscape

I believe that if you really want to document rock art, it is not enough to document only the figures, but also their environment. My first visit to Astuvansalmi 1994 was a great example. I expected to see faint marks in the rock, but what I really experienced was totally different. A huge cliff and silent landscape. It was almost frightening. Of course the landscape has changed a lot during the past 6000 years. Water level, vegetation and climate have changed. But still the landscape carries marks and memories that may have been important to the people who made the paintings. For me the environment is almost more important than the paintings themselves.


© Ismo Luukkonen, 2003 (updates 2015 and 2019)

Senior lecturer of photography, Turku University of Applied Sciences / Arts Academy
Turku, Finland