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Reflectance Transformation Imaging                


The project utilized advanced digital imaging technologies to support the best visualization possible of medieval coins. The primary application used was Reflectance Transformation Imaging (RTI) which in conjunction with Superzoom imaging capacity can provide exceptional clarity, texture details and the ability to manipulate the lighting source on the surface of coins. As a result, each coin of the collection can be visually explored within a dedicated 'window' featuring an array of helpful tools and instructions. The overall imaging effort was developed and materialized by the The Cyprus Institute's Imaging Cluster for Archaeology and Cultural Heritage (ICACH) 

What is Reflectance Transformation Imaging?

RTI is a computational photography method. It relies on extracting reflection information from several photographs captured from a fixed point under a “known” hemispherical coordinate lighting environment. An algorithm re-synthesizes that information into a new photorealistic representation that has dynamic light properties based on empirical mathematical illumination models engender from the fields of computer graphics, computer vision and optics. The final RTI image examined on a screen is not a three-dimensional model but rather a two-dimensional image that looks-alike a 3D representation with important advantages over it in terms of texture accuracy. Among other uses such as in medicine and forensics, the application of RTI in archaeology has proved to be particularly effective. An indicative example was the application of RTI technology in the successful decoding of the Antikythera Mechanism inscriptions on the metal fragments of the famous ancient device.   

RTI was originally invented by Tom Malzbender and Dan Gelb at Hewlet-Packard Labs. Their 2001 publication presented their research results, methodology and tools described as Polynomial Texture Mapping (PTM), a term still used as an alternative to RTI to describe the algorithm used in the imaging process. Further developed and enhanced by the West Semitic Project at the University of Southern California (USC) and Cultural Heritage Imaging (CHI), RTI offers remarkable possibilities in mapping the relief qualities of objects’ surfaces. Inscriptions, incisions, decorative relief designs and even painting brushstrokes on an array of archaeological material and surfaces such as stone, clay, marble, coins, paintings, mosaics, sculpture, jewellery and other minor objects, can be documented with accuracy.  

RTI has key advantages in comparison to other methods of digital documentation, such as 3D laser scanning, as it is able to provide high-fidelity photorealistic textures on surface geometry. Additionally, it is a user-friendly method in terms of compactness, speed and interoperability. It can be also applied on 3D objects instead of conventional “texture maps” and “bump maps” that luck photographic accuracy and quality.            
How does it work?
A custom-build RTI dome that consists of a hemispherical domical device with a hole at the apex, and thirty-six halogen lights embedded at randomly fixed intervals around the dome and controlled by a by using a pre-scripted programmable microcontroller. An artefact is placed at the base of the dome, while a camera is positioned looking downward focusing, through the hole at the top, on the aforementioned object. Thirty-six photographic images are sequentially taken, each with a single light shining on the artefact, thus creating thirty six images with different light angles. Then, the PTM algorithm synthesizes the data from these images to create a single image that can be examined on a RTI viewer with a “virtual torch.” The viewer allows the user to move the light angle intuitively in real time, so that the combination of light and shadow representing the relief features of the object’s surface can be freely altered. RTI also permits the enhancement of the subject’s surface shape, color and luminance attributes, which extracts detail out of the surface that cannot be otherwise derived.
Imaging Cluster for Archaeology and Cultural Heritage (ICACH)
Over the past decade the remarkable progress of high-fidelity digital imaging and visualization technologies have significantly contributed to research in archaeology and cultural heritage. The Cyprus Insitute is developing a new imaging laboratory cluster focused on archeological and cultural heritage research at the local and regional level. Imaging Cluster for Archaeology and Cultural Heritage has primarily focused on the use, promotion and development of computational-based photography technologies such as Reflectance Transformation Imaging.