In the beginning…
As an archaeologist, I have always been interested in rock art. My first contact with prehistoric art was during my undergraduate years, in a fieldwork campaign at the UNESCO World Heritage site of Foz Côa (Portugal). I was completely immersed in the amazing landscape in which thousands of Palaeolithic animals were carved in open air contexts. The fine lines that make these images are often very difficult to see and the recording of the motifs was mostly done during the night, through direct tracing and with artificial raking light. The powerful lamps attracted a number of insects, which would draw a number of other animals that we could not see in the black. The complete darkness of the night upon us was majestic, in addition to the flickering animals on the rocks and the fluttering nerves for the unknown wild life that surrounded us. The experience was remarkable and the following year I went back for another season.
Unintentionally, my professional path developed hand in hand with rock art studies. I surveyed, recorded and studied several sites of different types of rock art in various countries. My experience informed me that traditional rock art recording methods are time-consuming and can be controversial. It is now mostly agreed that direct tracing or other methods that involve physical contact with the rocks may compromise potential future dating methods or other analysis of micro-organisms embedded within the pores of the rocks, which may bear relevant information for further understanding and dating of rock art.
Recording Atlantic Rock Art
In 2013, I started my PhD project with the aim of understanding the phenomenon of Atlantic Rock Art, a type of prehistoric carving common to a number of modern countries in western Europe (namely Iberia and the British Isles). In order to assess the relationship between the engravings in these regions beyond their motifs, I developed a system of comparison that would provide unbiased data and robust comparative terms. In general, the iconography of this rock art style is based cup-marks (shallow holes cut into the rock), cup-and-rings (concentric circles surrounding a cup-mark) and other derivatives. Although a number of variables were included in this study, I was particularly interested in reviewing the details of the motifs, tracing the gestures and making processes of these carvings. To do so, I needed to focus on the shapes of the motifs, the carving techniques, their interaction with the rock surface on which they were engraved and their relationship with each other, namely through the identification of superimpositions that could provide chronological evidence.
Plans and drawings were available for some of the rock art in the study areas chosen for this project. Nevertheless, it was clear that the existing records were often inaccurate or not detailed enough for the intended level of interrogation. The whole project included the analysis of 200 open-air carved rocks of various sizes, and it became apparent that recording them digitally was the best way forward.
From Reflectance Transformation Imaging (RTI) to Structure from Motion (SfM)
Having joined the University of Southampton, which strongly contributed to the development and use of RTI in archaeological contexts, this initially seemed to be a good option to explore and digitally record the rock art. RTI was initially developed by Tom Malzbender at Hewlett Packard Laboratories, and in recent years it has been widely applied to Cultural Heritage. In essence, it is an image recording method in which the reflectance properties of a surface are transformed through contrast enhancement improving the perception of micro-topographies, by using Polynomial Texture Maps (Malzbender et al 2000, 2001). Polynomial Texture Maps (PTM) are produced through the capturing of a number of photographs from a stationary point towards a fixed object, under specific lightning conditions, allowing for the documentation of subtle surface information. The light is projected from various directions, rotating around the object, creating a ‘light dome’. The photographs are then combined, using a specially designed algorithm, into one single file that compiles the entirety of the captured data. The result is an accurate record of the surface’s shape. An interactive, and freeware software (available here), facilitates the re-lightning of the object, that can be viewed under different renderings and light conditions.
RTI was widely used in the first fieldwork campaign of my project, particularly to document the carved rocks of the study area of the Machars, a peninsula in the southwest of Scotland. Nevertheless, the nature of the rock art and the technicalities of RTI raised a number of obstacles, such as lighting and weather conditions, since all rocks are located in the open landscape. The size of the rocks and their predominant horizontal format also posed a number of logistical difficulties. Although quite versatile and forgiving, RTI was not the ideal method to record the entirety of the rocks, but became very useful to document details of the panels.
Since then, I adjusted the recording methodology and Structure from Motion (SfM) photogrammetry became the preferential technique to document the rock art. This was then the preferred technology used in the remaining researched study areas. My engagement with photogrammetry began with my PhD project and resulted from a self-taught learning process, in addition to some mentoring provided by colleagues who were already regularly using digital technologies in their work.
Photogrammetry became essential in the visualisation of rock art, which can be particularly difficult in the open landscape, where motifs are in constant weathering due to contact with the elements and other risks. The lighting conditions also interfere greatly with the identification of the engravings. The 3D modelling enabled unbiased reproduction of the carved surfaces, contrasting with other subjective manual methods, and allowed for the integral documentation of the rock art along with their geological contexts (i.e. boulders or outcrops), regardless of their sizes which can vary considerably, from just a few centimetres to more than 10 meters long (see Figure 2 and link below). The contextualization of the motifs on the rock surface (and, when possible, the whole rock), often missing from manual drawings, created a detailed perception of the relationship between rock art and the natural features of the rocks that were often incorporated into the compositions.
Due to the simplicity of the method, SfM photogrammetry was used during field surveys to easily document rocks which we suspected were carved, but of which the in-situ inspection was inconclusive. In many occasions the 3D models provided great surprises while revealing extensively carved rocks when the grooves were mostly weathered and hardly visible (see Figure 3 and 4). Besides an accurate visualisation of the rock art, photogrammetric models were capable of capturing very fine details of the carved motifs but also allowed an exploration of the prehistoric artist’s gestures and the creation processes – through the study of the carving techniques, preferences and behaviours, superimpositions and the use of natural features incorporation into the compositions. (Figure 5).
The digital recording of Atlantic Art presented a few challenges. On one hand, most of the surfaces were horizontal or approximately horizontal, a characteristic of this prehistoric style of rock art. As such, the position of the camera needed to be perpendicular to the outcrop or boulder and sometimes, depending on the size of the panel and the level of detail required, having it at an appropriate height was difficult. On most occasions, the aim was to produce high resolution 3D models. In some cases, particularly with smaller rocks, as little as 20 to 30 photographs were taken, whilst in other cases the surveys required c. 1000 pictures for the requisite level of detail and to ensure that there were no holes in the models. Of course, the capture of the photographs was also sometimes limited due to vegetation and the micro-topography of the outcrops. Recording outdoors requires specific manipulation of the camera settings, and on occasion other additional accessories, in order to control shadows, water on the surfaces, unwanted reflections, etc.
The comparison between the 3D models produced and plans drawn by other researchers, enabled the verification of the latter’s accuracy. In some cases, the plans were quite precise whilst in others they missed many important details. This confirmed the importance of producing new records that would meet more closely the objectives of my project and provide effective answers to the enquiries. Furthermore, photogrammetry models enable more efficient monitoring of rock art, a better assessment of how rocks and carvings suffer with weathering and other threats.
Scotland’s Rock Art Project
Recently I have been working as a Postdoctoral Researcher with Scotland’s Rock Art Project, hosted by Historic Environment Scotland. This is a 5-year AHRC (Arts and Humanities Research Council, UK) funded project, with the main aim to enhance the understanding and awareness of Scottish prehistoric rock art through community engagement and research. This project involves the training and support of community teams to record rock carvings across Scotland according to a methodology devised for the purpose. This involves the production of 3D models, a technology that we are training the volunteers to use.
Since the prehistoric rock art in Scotland is the same as the one I studied during my PhD, I brought my previously acquired experience into this project, including that relating to the digital recording and reproduction of the carvings.
Rock art and Sketchfab
Sketchfab has been an essential tool to disseminate the 3D models created for both projects. During my PhD project I had access to a number of drawings and plans of rock art, that were either published or kindly shared by other researchers. For me, making data available to others is an important part of any research, in order to widen our understanding and knowledge. I want to return the generosity of all those who shared their work with me, and those who helped me with my project, by sharing my results. Sketchfab provides a great platform for this, being an accessible tool to all. In addition, it is great to be able to display the 3D models with different lighting conditions and annotate any important details.
For Scotland’s Rock Art Project Sketchfab is also being widely used particularly as an integral tool of the project’s methodology. In fact, not only is it used to share the 3D models resulting from the volunteer’s work with a wider audience, but considering that internet connections are often very slow in remote areas of Scotland, it is also a great means to share models that would otherwise be quite heavy. It contributes to a greater autonomy for the various teams of volunteers, since each of them has their own Sketchfab page in which they upload the 3D models, then included in the online database that we have created. Once again, Sketchfab has an important role in the dissemination of rock art, contributing to a greater awareness of this type of archaeological site.
I would like to thank Abby Crawford, Mieke Roth and Jasmin Habezai-Fekri for inviting me to contribute to this amazing series of Women on Sketchfab.
- Malzbender, T.; Gelb, D.; Wolters, H.; Zuckerman, B., 2000. Enhancement of Shape PErception by Surface Reflectance Transformation. HP Labs Technical Reports, 38R1.
- Malzbender, T.; Gelb, D.; Wolters, H., 2001. Polynominal Texture Maps. In SIGGRAPH 01: Proceedings of the 28th annual conference on computer graphics and interactive techniques. New York: ACM Press, pp. 519–528.