Digital Classicist: Developing a RTI System for Inscription Documentation in Museum Collections and the Field

In the first of this summer’s Digital Classicist Seminar Series, Kathryn Piquette and Charles Crowther of Oxford discussed Developing a Reflectance Transformation Imaging (RTI) System for Inscription Documentation in Museum Collections and the Field: Case studies on ancient Egyptian and Classical material . In a well-focused discussion on the activities of their AHRC DEDEFI project of (pretty much) this name, they presented the theory behind RTI and several case studies.

Kathryn began by setting out the limitations of existing imaging approaches in documenting inscribed material. These include first hand observation, requiring visits to archives sites, museums etc. Advantages are that the observer can also handle the object, experiencing texture, weight etc. Much information can be gathered from engaging first hand, but the costs are typically high and the logistics complex. Photography is relatively cheap and easy to disseminate as a surrogate, but it fixed light position one is stuck with often means important features are missed. Squeeze making overcomes this problem, but you lose any sense of the material, and do not get any context. Tracing has similar limitations, but there is the risk of other information being filtered out. Likewise line drawings often miss erasures, tool marks etc; and are on many occasions not based on the original artefact anyway, which risks introducing errors. Digital photography has the advantage of being cheap and plentiful, and video cann capture people engaging with objects. Laser scanning resolution is changeable, and some surfaces do not image well. 3D printing is currently in its infancy. The key point is that all such representations are partial, and all impose differing requirements when one comes to analyse and interpret inscribed surfaces. There is therefore a clear need for fuller documentation of such objects.

Shadow stereo has been used by this team in previous projects to analyse wooden Romano British writing tablets. These tablets were written on wax, leaving tiny scratches in the underlying wood. Often reused, the scratches can be made to reveal multiple writings when photographed in light from many directions. It is possible then to build algorithmic models highlighting transitions from light to shadow, revealing letterforms not visible to the naked eye. The RTI approach used in the current project was based on 76 lights on the inside of a dome placed over the object. This gives a very, very high definition rendering of the object’s surface in 3D, exposed consistently by light from every angle. This ‘raking light photography’ takes images taken from different locations with a 24.5 megapixel camera, and the multiple captures are combined. This gives a sense not only of the objects surface, but of its materiality: by selecting different lighting angles, one can pick out tool marks, scrape marks, fingerprints and other tiny alterations to the surface. There are various ways of enhancing the images, all of which are suitable for identifying different kinds of feature. Importantly, as a whole, the process is learnable by people without detailed knowledge of the algorithms underlying the image process. Indeed one advantage of this approach is it is very quick and easy – 76 images can be taken in around in around five minutes. At present, the process cannot handle large inscriptions on stone, but as noted above, the highlight RTI allows more flexibility. In one case study, RTI was used in conjunction with a flatbed scanner, giving better imaging of flat text bearing objects. The images produced by the team can be viewed using an open source RTI viewer, with an ingenious add-on developed by Leif Isaksen which allows the user to annotate and bookmark particular sections of images.

The project has looked at several case studies. Oxford’s primary interest has been in inscribed text bearing artefacts, Southampton’s in archaeological objects. This raises interesting questions about the application of a common technique in different areas: indeed the good old methodological commons comes to mind. Kathryn and Charles discussed two Egyptian case studies. One was the Protodynastic Battlefield Palette. They showed how tools marks and making processes could be elicited from the object’s surface, and various making processes inferred. One extremely interesting future approach would be to combine RTI with experimental archaeology: if a skilled and trained person were to create a comparable artefact, one could use RTI to compare the two surfaces. This could give us deeper understanding about the kind of experiences involved in making an object such as the battlefield palette, and to base that understanding on rigorous, quantitative methodology.

It was suggested in the discussion that a YouTube video of the team scanning an artefact with their RTI dome would be a great aid to understanding the process. It struck me, in the light of Kathryn’s opening critique of the limitations of existing documentation, that this implicitly validates the importance of capturing people’s interaction with objects: RTI is another kind of interaction, and needs to be understood accordingly.

Another important question raised was how one cites work such as RTI. Using a screen grab in a journal article surely undermines the whole point. The annotation/bookmark facility would help, especially in online publications, but more thought needs to be given to how one could integrate information on materiality into schema such as EpiDoc. Charlotte Roueche suggested that some tag indicating passages of text that had been read using this method would be valuable. The old question of rights also came up: one joy of a one-year exemplar project is that one does not have to tackle the administrative problems of publishing a whole collection digitally.

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