3. Methods of Three-dimensional Representation

3.1 Why use 3-D technologies?

The use of three-dimensional technologies has developed very rapidly in the last ten years. In the field of Neolithic monumental archaeology, several tools have been used, such as digital photogrammetry or laser scanning (Cassen and Merheb 2005; Cassen et al. 2006). A software allowing three-dimensional modelling from digital photographs (Realviz ImageModeler®) has been applied to carved stones in Irish and Breton passage tombs. This work is not particularly innovative since similar software has been tested by a team from Durham University on a British rock art site (Simpson et al. 2004).

It is important to specify the type of use for which this three-dimensional technology is intended. Indeed, in the case of carved stones, this tool could not be used as a recording instrument. The identification of carvings and their accurate recording can be made only with oblique lighting and photographs and not with a three-dimensional modelling tool whose function is not to make visible the invisible (see the experiment carried out unsuccessfully on stele 11 in Castlerig stone circle – Díaz-Andreu et al. 2006). Three-dimensional technology applied to carved stones must not be considered as an investigation tool but as a representation tool. It is not a research instrument but a means to pass on the results of research. On the other hand, such technologies (particularly laser scanning), applied to other objects such as complex architecture or excavation sites, are absolutely relevant as recording techniques (for a recent example see Katsianis et al. 2008).

It is, then, as a representation tool that the Realviz® software has been used. The research carried out on the relationship between carvings and stone relief has given several results that are not well illustrated by the usual planimetrical representations. For example, in the case of the arcs carved against a line of relief, 2D drawings are inadequate and a description of them is needed to make us understand the stone's configuration and reliefs. Is the arc carved above a concave or a convex relief? Is the arris jutting out or inverted? Quite often a plan and section do not reproduce accurately enough the details and the nuances of a complex relief; therefore, only a 3D representation can fully render these elements. Two case studies have been made in order to appreciate the advantages and disadvantages of the software.

3.2 Realviz ImageModeler®

The software we used was ImageModeler® by Realviz to create a three-dimensional model from digital photographs taken from different viewpoints. There were three main steps in the process:

The calibration stage consists of recording common points on the stone for each picture. The software automatically calculates their 3-D position and creates a 3-D point cloud.
The modelling stage consists of creating a 3-D polygon-based model by triangulation of the point cloud.
The texturation stage is made automatically by the software: it extracts textures from the original photographs and it applies them to every surface of the 3-D model.

Figure 11: Three-dimensional modelling of orthostat 19 in Mané Kerioned B (Morbihan, Brittany)

The first experiment was made in Brittany on orthostat 19 in Mané Kerioned tomb B (Fig. 11). On this orthostat the carvings are organised around a large central bump whose volume and reliefs do not reproduce well in traditional planimetric drawings. Several shots have been made from four different stations, with direct and oblique lights. The whole main face (the only visible part of the stone) has been modelled and two types of texture extracted from the photographs: one model with direct light and one model with oblique lights in order to enhance the carvings. The second experiment was carried out on a smaller surface (Fig. 12). Orthostat C2 in Loughcrew U displays an arc sign carved in the lower part of its eastern face. The sign is on a surface delimited at the bottom and on the right by two significant arrises (see above). The carved surface has been shot from six different stations in daylight. The texture of the model has been extracted from the photographs used for the modelling and on which the carvings have been previously coloured red in order to make them visible.

Figure 12: Three-dimensional modelling of orthostat C2 in Loughcrew U (Co. Meath, Ireland)

The result of these experiments seems to show that three-dimensional modelling from digital photographs is a good additional tool to classical planimetric representations. The ease of use of the software and the minimal equipment required in fieldwork make its application interesting for archaeologists. To illustrate the relationship between artwork and stone relief, it is a relevant tool because it is the best way to reproduce the complexities of the stone surfaces.

3D models of orthostat 19, Mané Kerioned tomb B and orthostat C2, Loughcrew

Three-dimensional modeling of orthostat 19 at Mané Kerioned passage tomb (Morbihan, France) with Realviz ImageModeler - direct light

Three-dimensional modeling of orthostat 19 at Mané Kerioned passage tomb (Morbihan, France) with Realviz ImageModeler - oblique light

Three-dimensional modeling of a selected surface of orthostat C2 at Loughcrew tomb U (Co. Meath, Ireland) with Realviz ImageModeler

Three-dimensional modeling of a selected surface of orthostat C2 at Loughcrew tomb U (Co. Meath, Ireland) with Realviz ImageModeler with carvings highlighted

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