Visualising ancient Egyptian artefact surface transformations

These past several days have been largely occupied with preparing and delivering a conference paper at ‘The Computer Applications and Quantitative Methods in Archaeology’ (CAA) 2012 at the University of Southampton.

As mentioned in a previous post, I am researching early Egyptian (and Mesopotamian) image bearing artefact surfaces in order to understand how they were formed and transformed through scribal and artistic activities. Taking the example of elaborate carving on a Late Predynastic (c.3100BCE) Egyptian mudstone palette, namely the so-called Hunters Palette, I am investigating evidence for its creation through close analysis of the carved surface using Reflectance Transformation Imaging (RTI) data*.

 

Hunter's Palette: relief carved mudstone, said to come from Tell el-Amarna.  Photograph © Trustees of the British Museum; drawing from Smith 1949; both from Davis 1992.

My aim is to answer questions such as: What tool and carving technique types can be discerned? What stages in surface preparation / carving can be identified? To what extent was the carving planned out prior to execution?

While beginning to unpick these issues -- studying my RTI data in RTIViewer and slowly moving the virtual light around -- I found intriguing new evidence erasure and recarving!**

This 'diffuse gain' RTI photographic output is a detail of the the lower right side of the upper right palette fragment (above). It shows the 'ghosts' of previous carving and what may be the original path of the rope (indicated by three parallel arrows), and the original position of the hunter's 'tail' (indicated by a horizontal arrow) which is attached to the back of his kilt and abuts the rope (Piquette and MacDonald Forthcoming).

In order to visualise the palette’s surface further and characterise these transformations more fully, I’ve teamed up with Lindsay MacDonald, a researcher in Geomatic Engineering at UCL. Using his mathematical and computing expertise (Lindsay specialises in colour imaging, vision science, digital photography and computer graphics) and my raw RTI data (i.e. the series of photographs I took at the BM in late 2010 using the RTISAD project's Oxford RTI lighting dome), Lindsay has been experimenting with new visualisations of the erased/recarved area using different kinds of algorithmic rending or AR (fear not - if mystified, click the link and become de-mystified via a neat and tidy description with examples courtesy of the good folks at Cultural Heritage Imaging!).

Depth map of Hunters Palette with section profiles (Piquette and MacDonald Forthcoming).

One result of Lindsay's algorithmic rending (AR) work was the depth map, presented above as a grey-scale image. The deepest areas are darkest and the highest are lightest. The red lines indicate the positions of a vertical and a horizontal cross-section. The graphs at the left and bottom show the corresponding elevation profiles (scale in mm). In the vertical section it can be seen that the height drops by about 10mm in the area below the hunter’s rib cage -- possible evidence for recarving that was not apparent from the amalgamated RTI image. In the horizontal section there can be seen a slight furrow of depth approximately 3mm in the background region above the 'original tail’, further supporting the observation that this area of the surface has been recarved.

These and additional results from our collaborative work on the Hunters Palette and its surface transformations formed the topic of our paper at CAA 2012 in Southampton (check out the abstract). We're now in the process of finalising this piece of research and writing it up for publication, but we have plans to undertake the further study of early Egyptian inscribed and decorated artefacts surfaces using both RTI and AR -- with many exciting results anticipated!

* Thanks are due to the British Museum and the Arts and Humanities Research Council’s Digital Equipment and Database Enhancement for Impact (DEDEFI) scheme.

** Not quite clear on what I am describing? You can have a go easily enough using RTIViewer on your PC or Mac. Download the Open Access ( = free) software and 'test drive' it on the papyrus or rock art examples provided. Try it...you'll like it! :-) And then let me know what you think!

Reflectance Transformation Imaging (RTI): A taster

In my previous post I referred to the way in which the digital imaging technique Reflectance Transformation Imaging (RTI) can make visible surface marks which may not be apparent with the naked eye -- or at least enhance features that are difficult to see depending on lighting conditions and how you look at an object (more on seeing/looking in another post, but just to highlight here that this is not necessarily a neutral activity!).

I am exploring the possibilities of inserting an RTI viewer into this blog, or otherwise linking to one in order to show examples of my RTI results. In the meanwhile, if any reader comes across a solution -- please do let me know. For those unfamiliar with the kinds of results one can achieve with RTI, here is one of my favourite examples from the AHRC-funded University of Oxford and University of Southampton RTISAD project I worked on in 2010-2011 (for more information check out our interim report).

Here is a detail of a stunning portrait from the mummy of a Roman woman who lived around c.160-170AD. She may have been buried at El Rubaiyat in Egypt where the portrait is thought to have been found (and now housed in the British Museum). Her portrait is made with encaustic on limewood and added gilding. The top image is an RTI output with even 'default' lighting; the middle image shows the 'diffuse gain' rendering mode which enhances the perception of surface shape features for interpretive purposes; and the lower image exemplifies the 'specular enhancement' mode which renders the shape-based reflections to also enhances the perception of the surface shape.

 

(EA 65346. All © Trustees of the British Museum)

I hope this example gives some idea of what RTI can do to aid research of material surfaces. The aspect of RTI that I cannot show here on the blog (yet...) is the ability to reposition the light virtually. This re-lighting feature together with the different rendering modes constitute a powerful toolkit for studying the details of material surfaces, whether one is a conservator, art historian, text scholar, archaeologist or even...undertaking criminal forensic investigation!!