Reflectance Transformation Imaging Research Articles

LightBot: A Multi-Light Position Robotic Acquisition System for Adaptive Capturing of Cultural Heritage Surfaces.

Multi-light acquisitions and modeling are well-studied techniques for characterizing surface geometry, widely used in the cultural heritage field. Current systems that are used to perform this kind of acquisition are mainly free-form or dome-based. Both of them have constraints in terms of reproducibility, limitations on the size of objects being acquired, speed, and portability. This paper presents a novel robotic arm-based system design, which we call LightBot, as well as its applications in reflectance transformation imaging (RTI) in particular. The proposed model alleviates some of the limitations observed in the case of free-form or dome-based systems. It allows the automation and reproducibility of one or a series of acquisitions adapting to a given surface in two-dimensional space.

Heritage textiles – an integrated approach for assessment and future conservation

The digital revolution has been present in our lives more and more since the beginning of the third millennium and until now, has affected the way in which cultural heritage is valued, preserved and passed on. In this context, the present study aims to use digital technologies (digital radiography and Reflectance Transformation Imaging) to contribute to assessing the conservation status of a heritage textile fabric; a traditional women’s shirt about 100 years old from Bihor County (ie). The investigations concerned both the fine details that are not visible to the naked eye, regarding the internal structure of the material and external (damaged areas, embroidery with traditional motifs, etc.). The results obtained show high suitability of the methods used for the analysis performed on historical textiles, having major importance in identifying the visual representation of the weave, the fibres and the surface of the fabric, the deterioration and also the current state of preservation and contributing to the identification of optimal restoration-conservation solutions.

Andean pre-Hispanic pottery forming 3D analysis: a pilot study from Quebrada de Humahuaca (Argentina) using digital methods

Digitization, three-dimensional (3D) documentation and visualization of archaeological materials are processes in increasing development that are used for the enhancement of heritage. These tools have multiple uses for the analysis and research of archaeological objects, although their use in pottery forming techniques studies is less explored. In this paper, diverse digital methods are explored in the study of pottery forming macro-traces, using the 3D model of an archaeological vessel from South-Central Andes. This case is proposed as a pilot study, aiming to reveal the potential of digital techniques for understanding pottery forming techniques. The particular case analysed corresponds to a globular pot of the Humahuaca Black-on-Red style, recovered at the Pucara de Volcán archaeological site, in Quebrada de Humahuaca (Jujuy, Argentina). Initial studies of macro-traces on the pot suggested the use of paddling as the forming technique. The workflow used to contrast this hypothesis included the generation of a 3D model by close-range photogrammetry; and the analysis of the resulting point-cloud and mesh using Morphological Residue Model (MRM) and Virtual Reflectance Transformation Imaging Visualization (V-RTI), with diverse open-source software packages, such as AliceVision Metashape and CloudCompare. These methods increased the micro-topography visibility of the pot surface. As a result, the presence of sub-circular depressions in the body of the pot -similar to percussion cupules-, horizontal pressure lines in the collar, and micro-pull-outs in the maximal diameter of the pot were described. These macro-traces were interpreted as corresponding to the paddling technique used for the elaboration of the pot body —a technique not previously identified in pre-Hispanic traditional pottery manufacturing in the north of Argentina—, and of coiling for manufacturing the collar. The digital methods explored have great potential in the study of pottery forming techniques, although their scope depends on the accuracy of the 3D model analysed.

Quantitative evaluation of the Reflectance Transformation Imaging and Normal Integration technique in profilometric application.

Abstract This article provides a quantitative analysis of the ability to faithfully reproduce the profile of objects using the Reflectance Transformation Imaging (RTI) coupled with Normal Integration technique. The study was conducted using ad hoc designed and built reference targets. It was possible to compare the profiles obtained using RTI technique with those realized using more conventional profilometric and 3D techniques such as micro photogrammetry, laser scanner and optical profilometry. It has been seen that the RTI technique is especially valid in the reproduction of high spatial frequencies and that it is very suitable for the study of cultural heritage, as it is portable, fast and low-cost.

Traveling Salesman Problem with neighborhoods on a sphere in reflectance transformation imaging scenarios

In this paper, we propose a solution to the non-Euclidean variant of the Traveling Salesman Problem with Neighborhoods on a Sphere (TSPNS). The introduced problem formulation is motivated by practical scenarios of employing unmanned aerial vehicles in the Reflectance Transformation Imaging (RTI). In the RTI, a vehicle is requested to visit a set of sites at a constant distance from the object of interest and cast light from different directions to model the object from the images captured from another fixed location. Even though the problem can be formulated as an instance of the regular traveling salesman problem, we report a significant reduction of the solution cost by exploiting a non-zero tolerance on the light direction and defining the sites as regions on a sphere. The continuous neighborhoods of the TSPNS can be sampled into discrete sets, and the problem can be transformed into the generalized traveling salesman problem. However, finding high-quality solutions requires dense sampling, which increases the computational requirements. Therefore, we propose a practical heuristic solution based on the unsupervised learning of the Growing Self-Organizing Array (GSOA) that quickly finds an initial solution with the competitive quality to the sampling-based method. Furthermore, we propose a fast post-processing optimization to improve the initial solutions of both solvers. Based on the reported results, the proposed GSOA-based solver provides solutions of a similar quality to the transformation approach while it is about two orders of magnitude less computationally demanding.

RTI metoda jako způsob digitalizace kulturního dědictví

Current technological developments offer an inexhaustible number of procedures for digitization. However, technological equipment intended for teams in professional laboratories reaches price levels that cultural institutions often do not have enough finances. Procedures and methods using equipment, which institutions often already own or are not so expensive to acquire, come to the fore. One of them is the RTI (Reflectance Transformation Imaging) method, which creates a spatial relief composed of a sequence of images with visible lighting. The RTI method is very successful in depicting illegible or wiped details and is already widely used in archaeology.

The Conservation of a Mummified Child from the Australian Institute of Archaeology

The paper discusses a child mummy’s past restoration, pre-treatment condition, and the minimally invasive conservation interventions undertaken. The mummy was purchased in 1965 by the Australian Institute of Archaeology and has been displayed and stored since that time. Conservation was made possible by a grant. Images taken soon after the mummy arrived in Australia guided some conservation decisions. Analytical methods used to identify materials associated with previous restorations and the mummy’s deteriorating condition included Fourier transform infrared spectroscopy, Energy-dispersive X-ray spectroscopy fluorescence and Reflectance Transformation Imaging.

A Case for Coexistence of Different Potting Practices – Baltic Ware in Latvia

Latvia in the 11th–13th century poses a curious case for the coexistence of two different practices of Baltic ware production. The Baltic ware pots from lower reaches of the River Daugava and from the Courland region look not just stylistically, but also technologically different. Our paper assessed the production traces by using macro-observations, Reflectance Transformation Imaging (RTI) and extensive ethnographic material of Slavic pottery production of the 1900s, as well as using modern replicas as visual aids to assist in the identification of the principal coil attachment methods. The results showed that potters from the lower reaches of Daugava used the wheel’s rotation extensively during the shaping process of Baltic ware. The production of the pots required the potter to possess a level of technical skill which implied a level of professionalisation. Baltic ware from Courland was less technically complicated and used comparatively more of the methods of handmade pottery production.

The Cañaveral de León stela (Huelva, Spain). A monumental sculpture in a landscape of settlements and pathways

A newly discovered prehistoric stela from Cañaveral de León (Huelva, Spain) is studied through a combination of scientific methods, including thin section petrography and lithological contextualisation, various state-of-the-art digital imaging techniques for the analysis of the engraved motifs (3D modelling and Reflectance Transformation Imaging), and detection of pigments on its surface (Principal Components Analysis, HSI-contrast stretch, ferric pigments index and algebraic operations between bands), as well as archaeological surveys aimed at establishing the landscape context the stela was part of. The results reveal this stela is analogous to a larger series of late prehistoric sculptures portraying personages with ‘headdresses’, largely concentrated in the Iberian south-west and often connected to Bronze Age settlements and burial sites. In addition, the Cañaveral de León stela is closely associated to an old pathway that has had a historical prominence in terms of long-distance mobility, connecting various regions of western Spain in a South-North direction.

An elegy of stone and light: Applying digital imaging methods for the recording of the Aegean stone kernoi in the context of Akrotiri, Thera

Stone slabs with circularly arranged cup-marks (kernoi) in the Bronze Age Aegean have been interpreted as grave marks, offering tables, astronomical instruments, and recently as gameboards. This paper, in the context of the Bronze Age Akrotiri settlement, at Santorini (Thera), proposes the use of digital imaging methods for the recording of these objects. Reflectance Transformation Imaging and Structure from Motion Photogrammetry is used for the optimal recording of these objects and is proposed as a standardised method for the recording, analysis and interpretation of similar objects accross the Aegean and Eastern Mediterranean.

Reflectance Transformation Imaging (RTI) Investigation of Engraved Chalk Plaques from the Stonehenge Region

Newly discovered and previously documented Late Neolithic chalk plaques from the Stonehenge locality have been subjected to new, non-invasive techniques which allow access to previously unseen elements of archaeological evidence. The application of these methods – involving Reflectance Transformation Imaging (RTI) and Polynomial Texture Mapping (PTM) – has revealed detail of the surface preparation and allowed methods and sequence of the compositions to be unpicked, clarifying their complexities. The results reveal a range of approaches to the compositions, some of which demonstrate planning, order, and intention while others include less systematic, rapidly executed sketches. Investigations of lines and surfaces have been made, supplemented by preliminary studies of replicated test pieces, to examine potential implements used in their creation and remark on plaque biographies and surface attrition following manufacture. Furthermore, detail revealed by RTI provides indications of the orientations in which some of the plaques should be viewed and – in one instance – suggests a ‘reflected’ element that may not be entirely abstract. Results from improved radiocarbon determinations place the plaques in the early part of the 3rd millennium bc which, together with identification of individual motifs, allows the plaques and the designs to be reconsidered within the corpus of Neolithic art in the British Isles.

Documenting Functional Use-Wear on Bone Tools: An RTI Approach

Bone tools have a long archaeological history, and have recently been shown to retain use-traces distinctive of different perishable crafting practices. When examined in a controlled way, these diagnostic use-traces can serve as proxies for the crafted forms the bone tools were used to produce (e.g., baskets, leather goods, etc.). However, a number of methodological stumbling-blocks have hindered the sharing of bone tool use-wear results in a consistent standardized format. We suggest the application of Reflectance Transformation Imaging (RTI), and provide details for how to construct an RTI system which resolves most problems related to reproducibility in use-trace analysis.

Making a Mark: Process, Pattern and Change in the British and Irish Neolithic

This paper presents key results of theMaking a Markproject (2014–2016), which aimed to provide a contextual framework for the analysis of mark making on portable artefacts in the British and Irish Neolithic by comparing them with other mark-making practices, including rock art and passage tomb art. The project used digital imaging techniques, including Reflectance Transformation Imaging (RTI), and improved radiocarbon chronologies, to develop a new understanding of the character of mark making in the British and Irish Neolithic. Rather than considering this tradition in representational terms, as expression of human ideas, we focus on two kinds of relational material practices, the processes of marking and the production of skeuomorphs, and their emergent properties. We draw on Karen Barad's concept of ‘intra-action’ and Gilles Deleuze's notion of differentiation to understand the evolution and development of mark-making traditions and how they relate to other kinds of social practices over the course of the Neolithic.

Reflectance transformation imaging for documenting changes through treatment of Joseon dynasty coins

We present reflectance transformation imaging (RTI) as a documentation tool for visualizing and recording the treatment of coins. RTI—a computational photograph technique that calculates light positions—allows interactive relighting for vision. Virtual light enhances surface details for examining morphological difference. By applying Dome RTI method, stages of conservation treatment were recorded to enhance the overall characteristic features of the relief upon the coin surface, and then detect and identify weathered characters. Patina removal and consolidation were documented along with the original state; a significant difference in the coin’s surface was observed using different filters of the RTI viewer. Specular enhancement and normal visualization results were most effective for detecting the change in morphology and reflectivity. Microscopic RTI was applied to visualized minimal changes of characters between treatment stages. Character “常” of coin 1 showed changes in the cleaning of dirt and removal of alumina powder. The character “元” of coin 3, originally covered by a thick patina, revealed clear strokes using virtual relighting through RTI. These documentation images indicate that RTI is a promising tool to support manual recording of conservation stages and, furthermore, allow detection of areas difficult to visualize through the human eye.

Visualization, Documentation and Non-Destructive Investigation of an Ancient Egyptian Corn Mummy in a Falcon-shaped Wooden Coffin

Corn mummies’ were funerary objects in ancient Egypt that were served as sacred images of the god Osiris during the Khoiak feast. They are grain-filled linen sacs, usually filled with an emmer or barley specimen, the ingredients for staple food in ancient Egypt (bread and beer). In the fertility cult of Osiris, the function of Corn mummies was presumably for continuous prosperity of the land and healthy crop growth. This paper focuses on the use of nondestructive methods for the documentation and visual examination of a Corn mummy coffin (No. 1242) dating back to the late period; but its original inscriptions are partially lost. Digital imaging and Reflectance Transformation imaging (RTI) were used to document all surface aspects. D-stretch was used to enhance surface details that were not clearly visible by digital imaging and RTI. Infrared imaging was used to reveal the details of hieroglyphic inscriptions and outline drawings on the wooden coffin. X-ray radiography was used to non-destructively verify the inside contents of the wrapped mummy inside the wooden coffin, and whether it includes any human remains or not. The methods provided a comprehensive documentation and examination of the various parts of the artifact.

HD-RTI: An adaptive multi-light imaging approach for the quality assessment of manufactured surfaces

Reflectance Transformation Imaging (RTI) is a technique for estimating surface local angular reflectance from a set of stereo-photometric images captured with variable lighting directions. The digitization of this information fully fits into the industry 4.0 approach and makes it possible to characterize the visual properties of a surface. The proposed method, namely HD-RTI, is based on the coupling of RTI and HDR imaging techniques. This coupling is carried out adaptively according to the response at each angle of illumination. The proposed method is applied to five industrial samples which have high local variations of reflectivity because of their heterogeneity of geometric texture and/or material. Results show that coupling HDR and RTI improves the relighting quality compared to RTI, and makes the proposed approach particularly relevant for glossy and heterogeneous surfaces. Moreover, HD-RTI enhances significantly the characterization of the local angular reflectance, which leads to more discriminating visual saliency maps, and more generally to an increase in robustness for visual quality assessment tasks.

Technical and Scientific Imaging for Archaeological purpose – A Brazilian case.

This paper is about Technical and Scientific Imaging for caving painting (archeological site in Brazil). High quality imaging with color management has been used, in addition to scientific procedures to generate digital images, capable for recording not visible light (ultraviolet and infrared radiation). In addition, the RTI (Reflectance Transformation Imaging) technique was used. Rock Art is a kind of Cultural Heritage, extremely fragile archaeological remains and susceptible to a series of factors (physical, chemical, anthropic and others) that can cause different impacts for their preservation. The Rock Art can suffer an irreversible damage, which can greatly compromise research and relative studies, then, requiring great scientific and technical procedures for imaging.

The Palermo Stone and Its Associated Fragments: New Discoveries on the Oldest Royal Annals of Ancient Egypt

Historical royal sources concerning Old Kingdom Egypt are rather scarce. One of the most important is a group of seven inscribed stone fragments also known as royal annals, the most famous of which is certainly the so-called Palermo Stone. These annals have been the subject of countless studies over more than a century since their initial discovery. However, the reading and interpretation of the hieroglyphic inscriptions engraved on them is still partial and often obscure. In recent years, however, the annals underwent a complete re-examination by means of the most up-to date technologies of 3D photographic documentation and reproduction – first of all, the so-called ‘Reflectance Transformation Imaging’ (RTI). In this article we will provide some insights on the new reading of selected parts of the fragments, especially the two major pieces: the Palermo Stone and the so-called ‘Cairo Fragment 1’.

Web-based Exploration of Annotated Multi-Layered Relightable Image Models

We introduce a novel approach for exploring image-based shape and material models registered with structured descriptive information fused in multi-scale overlays. We represent the objects of interest as a series of registered layers of image-based shape and material data. These layers are represented at different scales and can come out of a variety of pipelines. These layers can include both Reflectance Transformation Imaging representations, and spatially varying normal and Bidirectional Reflectance Distribution Function fields, possibly as a result of fusing multi-spectral data. An overlay image pyramid associates visual annotations to the various scales. The overlay pyramid of each layer is created at data preparation time by either one of the three subsequent methods: (1) by importing it from other pipelines, (2) by creating it with the simple annotation drawing toolkit available within the viewer, and (3) with external image editing tools. This makes it easier for the user to seamlessly draw annotations over the region of interest. At runtime, clients can access an annotated multi-layered dataset by a standard web server. Users can explore these datasets on a variety of devices; they range from small mobile devices to large-scale displays used in museum installations. On all these aforementioned platforms, JavaScript/WebGL2 clients running in browsers are fully capable of performing layer selection, interactive relighting, enhanced visualization, and annotation display. We address the problem of clutter by embedding interactive lenses. This focus-and-context-aware (multiple-layer) exploration tool supports exploration of more than one representation in a single view. That allows mixing and matching of presentation modes and annotation display. The capabilities of our approach are demonstrated on a variety of cultural heritage use-cases. That involves different kinds of annotated surface and material models.

Application of Reflectance Transformation Imaging (RTI) to surface bone changes in paleopathology.

The analysis of surface bone changes is an essential aspect of the paleopathological examination of skeletal remains from archaeological and forensic contexts, and proper digital imaging is an indispensable aspect of the documentation of bone modifications. This paper evaluates the applicability of Reflectance Transformation Imaging (RTI) to the study of pathological bone surface changes in human remains. Forty-five samples of human bones and teeth from medieval and early modern Estonian cemeteries were photographed and subjected to RTI imaging to document the pathological conditions that typically result in subtle surface modifications. Subperiosteal Bone Production (SBP), abnormal porosity, cribra orbitalia, endocranial lesions and lytic lesions in bone, as well as enamel hypoplasia and dental calculus in teeth were successfully represented with this technique. The results indicate that RTI allows visualization of shallow and discrete bone changes that are otherwise unnoticed. In some cases, this technique provides a better understanding of the nature and development of particular pathological processes than possible with conventional imaging. Although it cannot entirely replace microscopic and radiological techniques, RTI can be successfully performed in a reasonably short time by non-specialist operators with limited funding and resources, thus enabling identification of specimens that should be subject to more expensive or time-consuming analyses. Hence, RTI constitutes a valuable tool for the representation of subtle surface details in pathological bone, and can successfully support standard photography in paleopathological studies, museum display and scientific communication.