An ontology for 3D visualisation of cultural heritage

To date, 3D computer graphics and modelling techniques have been used in the study of the ancient world mainly as a means to display traditional research. The value of these digital techniques has been often assessed merely on the degree of graphic aesthetic quality (Favro 2006).
The pursuit of photorealism has proven ineffective in engaging the audience (Champion and Dave 2007) but also misleading, as it suggests that is possible to reproduce an artefact or a scene «exactly as it was» in the past (Baker 2012). Behind every scholarly 3D visualisation is a thorough study of excavation records, iconographic documentation, ancient literary sources, artistic canons and precedents (Hermon 2008). However, this valuable research is not always detectable in the final visual outcome.
The London Charter for the Computer-based Visualisation of Cultural Heritage1 made a huge step forward in the regulation of scholarly 3D visualisation—prescribing that researchers’ choices and motivation must all be documented. No 3D model can be considered a scholarly resource if its research method is not «transparent» (Forte 2012). The London Charter presents methodological guidelines for recording this data, but does not go as far as to offer a formal framework in which to place this information; each modeller is left to simply follow their own style. Moreover, the clients who commissioned the 3D model (such as museums or other cultural institutions) are frequently more interested in the final product than in its rationale, which is often completely overlooked and not circulated (or dropped from the budget line altogether). Time and resource constraints not only affect the accuracy and availability of the documentation, but also make it very unlikely that a researcher, or even a team, develops more than one visualisation of the same cultural heritage place/object, perpetuating the naive idea that only one visualisation is possible or correct.
The growing compatibility between 3D content and web browsers2 allows the use of RDF technology to, potentially, connect the 3D model and its parts, internally with each other—identifying and defining relationships—, and externally with online information about the material remains, previous publications, primary and secondary sources, and with available alternative visualisations of the same object (that share the same controlled vocabulary).
Ontologies for cultural heritage are already commonly used in the management of museum collections and databases3. However, they tend to focus on material artefacts and to meet the specific needs of museum curators and cataloguers. Therefore, they do not seem the most suitable means to deal with digital objects (that are hypothetical representations of material objects), to state methodological relationships, or describe a scholarly process.
The proposed ontology for 3D visualisation for cultural heritage will:
• Describe the 3D digital object. After assigning a specific URI to each element of a 3D digital visualisation (@prefix “obj”: <http://hypothetical3donthology.kcl.ac.uk/objects/>), they will be associated to metadata (such as creator(s), software(s) used, formats available) and to a formal description of the cultural heritage object they represent.
For example obj:001 rdf:type art:shaft. Where @prefix “art”: <http://hypothetical3donthology.kcl.ac.uk/ArtVocabs/>4
• Describe the 3D digital object’s relationships with other 3D digital objects5. Through a dedicated namespace (@prefix “tdvo”: <http:// hypothetical3donthology.kcl.ac.uk/threedvisontology/>) it will be possible to state and describe properties, values and relationships of the 3D digital objects such as
the relationship between a 3D digital object and the file it belongs to (obj:001 tdvo:isPartof obj:3Dfile.max);
the relationships between different objects within the same file (obj:001 rdf:type art:shaft. obj:010 rdf:type art:column. obj:001 tdvo:isPartof obj:010).
• Describe 3D digital object’s relationships with its physical referent. Through a digital geographical gazetteer such as Pleiades6, the 3D digital object will be linked to the place where the visualised building (or other referent) is located. For example, for a file “3Dfile.max” visualising the Odeon in Aphrodisias, we will have: 3Dfile.max gawd:depicts pleiades:638753/odeon. Different 3D visualisations could be connected to the physical building and be available alongside the photographic documentation linked to Pleiades via Flickr.
• Assess and represent the level of speculation involved in the creation of each element, presenting 3D visualisation more as a scientific hypothesis than an «exact reconstruction». For example obj:001 tdvo:hasCertainty tdvo:certainty6
where the level of certainty from 6 (maximum) to 0 (minimum) would be defined as follow:
tdvo:c6 rdfs:label “Certainty 6”; rdfs:comment “the ancient element is still in situ, and its dimensions and position can be measured”.
tdvo:c5 rdfs:label “Certainty 5”; rdfs:comment “the ancient element is not in situ but it has been visually documented in the past and the documentation is still available”. tdvo:c4 rdfs:label “Certainty 4”; rdfs:comment “the ancient element is not in situ but it can be geometrically derived from the surviving elements”.
tdvo:c3 rdfs:label “Certainty 3”; rdfs:comment “the ancient element is not in situ but it can be visualised according to well accepted standards and precedents”.
tdvo:c2 rdfs:label “Certainty 2”; rdfs:comment “the element is not in situ but it can be visualised according to the modeller’s experience, knowledge, intuition”.
tdvo:c1 rdfs:label “Certainty 1”; rdfs:comment “the element is not in situ and it has been added for communicative purposes”.
tdvo:c0 rdfs:label “Certainty 0”; rdfs:comment “the element has not been created for scholarly purpose and does not aim to historical accuracy. However, some characteristics of an original referent can still be recognised”.
Represent the relationships between the 3D digital visualisation, its sources, referents and interpretations.
For example: tdvo:isBasedOn rdfs:label “is based on”; rdfs:comment “the shape, dimensions or decoration of the element is based on visual or written information contained in a relevant document describing established practices, standards and rules”.
The object of the predicate could be traditional bibliographical references and/or the digital URI of the source and/or the URL of a digital edition of the source such as the ones available on open digital archives (obj:001 tdvo:isBasedOn dbpedia:De_architectura).
tdvo:hasEvidenceIn rdfs:label “has evidence in”; rdfs:comment “the 3D element can be compared with specific verbal or visual evidence such as video/photographic documentation or official excavation records”.
The object of this predicate would be archive numbers or bibliographical references identifying physical documents or artefacts, and/or URIs of digital reproductions of them, available on digital databases such as Arachne7 or CLAROS8. For example, if obj 002 was an element of the 3D visualisation of the Basilica in Pompeii:
obj:002 tdvo:hasEvidenceIn <http://arachne.uni-koeln.de/item/marbilder/2015507>.
tdvo:isMentionedIn rdfs:label “is mentioned in”; rdfs:comment “the visualised building
(or part of it) is mentioned in a ancient (or modern) text”.
tdvo:isDescribedIn rdfs:label “is described in”; rdfs:comment “the visualised building (or part of it) is described in a ancient (or modern) text”.
The latter predicates could link to bibliographical references and/or to the digital version of ancient texts such as the ones available through Perseus Library9.
The main goal of this proposal is not to present a detailed ontology, but to show the potential of the application of Open Linked Data to 3D visualisation, and how such an interaction will change the way 3D visualisation is applied in the study and understanding of cultural heritage. The ontology itself is not meant to be the work of a single researcher, but the collaborative effort of the different communities of practitioners.
To summarise, the suggested ontology will:
constrain and standardise the documentation, making it synthetic instead of verbose;
speed up the recording process thus reducing time/cost and making the documentation more likely to be retained in projects’ budgets;
allow 3D visualisations to join and enrich the growing network of linked digital resources to study the past;
make 3D visualisations human- and machine-searchable, connecting them with the literary and historical sources that mention the visualised artefact or building;
allow and encourage comparison of different visualisations and interpretations of cultural heritage, as the same resource (historical, archaeological, literary) will be connected to all the related visualisations that share the same vocabulary;
allow citations, re-use and peer-review of 3D visualisations, as every 3D element (and its author) will always be identifiable and linkable through the URI;
contribute to transform 3D visualisation from a univocal display of traditional research to a collaborative virtual environment where different scholars work together not only to implement the content but also to refine the ontology itself.
References

1. www.londoncharter.org
2. Thanks, for example to APIs such as OpenGL and WebGL
3. Cf., for example, CIDOC CRM or FRBRoo.
4. There are a few examples of formal vocabularies dedicated to art and architecture that can be used to describe the represented object. The Thesaurus developed by the Getty foundation, for example, has recently been released in Open Linked Data format (February 2014).
5. An interesting and useful precedent in using RDF to describe aggregations of files can be found in the open archive initiative.
6. pleiades.stoa.org
7. arachne.uni-koeln.de/drupal
8. www.clarosnet.org/XDB/ASP/clarosHome
9. www.perseus.tufts.edu/hopper
Baker, D. (2012). Defining Paradata in Heritage Visualization. In Bentkowska-Kafel,
A., Denard, H. and Baker, D. (eds) (2012). Paradata and Transparency in Virtual Heritage. Farnham: Ashgate
Champion, E. and Dave, B. (2007) Dialing Up the Past. In Cameron, F. and Kenderdine, S. (eds) (2007). Theorizing Digital Cultural Heritage. A critical discourse. Cambridge, MA: MIT Press
Favro, D. (2006) In the eyes of the beholder: Virtual Reality Recreations and academia. Journal of Roman Archaeology Supplementary Series Number 61(2006): 321-334
Forte, M. (2012) Cyberarchaeology: a Post-Virtual Perspective. www.academia.edu/3573281/Cyberarchaeology_a_Post-Virtual_Perspective (accessed 6th March 2014)
Hermon, S. (2008) Reasoning in 3D: a Critical appraisal of the role of 3D modelling and virtual reconstructions in archaeology. In Frisher, B, and Dakouri-Hild, A. (eds) Beyond Illustration: 2D and 3D Digital Technologies as Tools for Discovery in Archaeology. British Archaeological Reports International Series.
Online Resources:
The CIDOC Conceptual Reference Model:www.cidoc-crm.org (accessed 6th March 2014)
Getty Vocabularies as Linked Open Data:www.getty.edu/research/tools vocabularies/lod/index.html (accessed 6th March 2014)
The London Charter:www.londoncharter.org (accessed 6th March 2014)
Open Archives Initiative:www.openarchives.org (accessed 6th March 2014)
Pleiades Gazetteer:pleiades.stoa.org (accessed 6th March 2014)