On-the-Go Reflectance Transformation Imaging with Ordinary Smartphones

Reflectance Transformation Imaging (RTI) is a popular technique that allows the recovery of per-pixel reflectance information by capturing an object under different light conditions. This can be later used to reveal surface details and interactively relight the subject. Such process, however, typically requires dedicated hardware setups to recover the light direction from multiple locations, making the process tedious when performed outside the lab. We propose a novel RTI method that can be carried out by recording videos with two ordinary smartphones. The flash led-light of one device is used to illuminate the subject while the other captures the reflectance. Since the led is mounted close to the camera lenses, we can infer the light direction for thousands of images by freely moving the illuminating device while observing a fiducial marker surrounding the subject. To deal with such amount of data, we propose a neural relighting model that reconstructs object appearance for arbitrary light directions from extremely compact reflectance distribution data compressed via Principal Components Analysis (PCA). Experiments shows that the proposed technique can be easily performed on the field with a resulting RTI model that can outperform state-of-the-art approaches involving dedicated hardware setups

A Practical Reflectance Transformation Imaging Pipeline for Surface Characterization in Cultural Heritage

We present a practical acquisition and processing pipeline to characterize the surface structure of cultural heritage objects. Using a free-form Reflectance Transformation Imaging (RTI) approach, we acquire multiple digital photographs of the studied object shot from a stationary camera. In each photograph, a light is freely positioned around the object in order to cover a wide variety of illumination directions. Multiple reflective spheres and white Lambertian surfaces are added to the scene to automatically recover light positions and to compensate for non-uniform illumination. An estimation of geometry and reflectance parameters (e.g., albedo, normals, polynomial texture maps coefficients) is then performed to locally characterize surface properties. The resulting object description is stable and representative enough of surface features to reliably provide a characterization of measured surfaces. We validate our approach by comparing RTI-acquired data with data acquired with a high-resolution microprofilometer.Terms: "European Union (EU)" & "Horizon 2020" / Action: H2020-EU.3.6.3. - Reflective societies - cultural heritage and European identity / Acronym: Scan4Reco / Grant number: 66509

Arduino-controlled Reflectance Transformation Imaging to the study of cultural heritage objects

Fundacao para a Ciencia e a Tecnologia, Portugal (Grant Nos. UIDB/04349/2020 and UID/FIS/04559/2019)- Private funds. V.C. acknowledges the support from UID/Multi/04349/2019. J.C. acknowledges NOVA.ID.FCT.This article examines the development of a low-cost and portable set-up controlled by an Arduino board to perform Reflectance Transformation Imaging technique, from the information derived from 45 digital photographs of an object acquired using a stationary camera. The set-up consists of 45 high-intensity light emitting diodes (LEDs) distributed over a hemispherical dome of 70 cm in diameter and a digital camera on the top of the dome. The LEDs are controlled by an Arduino board, and the user can individually control the LEDs state (ON or OFF) and duration of illumination. An old manuscript written with iron-gall ink and a set of 1 Euro coins mint in 2002 were photographed with the set-up. The interactive re-lighting and the mathematical enhancement of the object's surface revealed corrosion, loss of material, scratches and other details, which were not perceived in standard images. These unique features, which can be extracted using edge detection processing, have immediate application in different fields such as cultural heritage or forensic studies, where they can be used as fingerprints to identify unique objects, allowing also recognizing the use of tools to alter the surface of coins to increase the price in the market.publishersversionpublishe

Assessment of plastics in the National Trust: a case study at Mr Straw's House

The National Trust is a charity that cares for over 300 publically accessible historic buildings and their contents across England, Wales and Northern Ireland. There have been few previous studies on preservation of plastics within National Trust collections, which form a significant part of the more modern collections of objects. This paper describes the design of an assessment system which was successfully trialled at Mr Straws House, a National Trust property in Worksop, UK. This system can now be used for future plastic surveys at other National Trust properties. In addition, the survey gave valuable information about the state of the collection, demonstrating that the plastics that are deteriorating are those that are known to be vulnerable, namely cellulose nitrate/acetate, PVC and rubber. Verifying this knowledge of the most vulnerable plastics enables us to recommend to properties across National Trust that these types should be seen as a priority for correct storage and in-depth recording

Digital Techniques for Documenting and Preserving Cultural Heritage

In this unique collection the authors present a wide range of interdisciplinary methods to study, document, and conserve material cultural heritage. The methods used serve as exemplars of best practice with a wide variety of cultural heritage objects having been recorded, examined, and visualised. The objects range in date, scale, materials, and state of preservation and so pose different research questions and challenges for digitization, conservation, and ontological representation of knowledge. Heritage science and specialist digital technologies are presented in a way approachable to non-scientists, while a separate technical section provides details of methods and techniques, alongside examples of notable applications of spatial and spectral documentation of material cultural heritage, with selected literature and identification of future research. This book is an outcome of interdisciplinary research and debates conducted by the participants of the COST Action TD1201, Colour and Space in Cultural Heritage, 2012–16 and is an Open Access publication available under a CC BY-NC-ND licence.https://scholarworks.wmich.edu/mip_arc_cdh/1000/thumbnail.jp

Conversion of a digital camera into a non-contact colorimeter for use in stone cultural heritage: The application case to Spanish granites

In this study, a digital CMOS camera was calibrated for use as a non-contact colorimeter for measuring the color of granite artworks. The low chroma values of the granite, which yield similar stimulation of the three color channels of the camera, proved to be the most challenging aspect of the task. The appropriate parameters for converting the device-dependent RGB color space into a device-independent color space were established. For this purpose, the color of a large number of Munsell samples (corresponding to the previously defined color gamut of granite) was measured with a digital camera and with a spectrophotometer (reference instrument). The color data were then compared using the CIELAB color formulae. The best correlations between measurements were obtained when the camera works to 10-bits and the spectrophotometric measures in SCI mode. Finally, the calibrated instrument was used successfully to measure the color of six commercial varieties of Spanish granite.The present study was financially supported by the Xunta de Galicia (09TMT014203PR) and the European Union and Spanish Ministry of Economy and Competitiveness under grants DPI2008-06455-C02-02 and DPI2011-30090-C02-02. Dr. Patricia Sanmartín acknowledges a scholarship for postgraduate studies abroad (2012 Call) from Barrié de la Maza Foundation. She is currently supported by a postdoctoral contract within the framework of the 2011–2015 Galician Plan for Research, Innovation and Growth (Plan I2C) for the year 2012

Digital Techniques for Documenting and Preserving Cultural Heritage

This book presents interdisciplinary approaches to the examination and documentation of material cultural heritage, using non-invasive spatial and spectral optical technologies

Using the contrast ratio method and achromatic transmission densitometry as a substitute for Status A transmission densitometry with the Photographic Activity Test For Enclosure Materials

The discontinuation of conventional photographic spot-reading transmission densitometers -including the widely adopted X-Rite model 310- due to the rapid decrease in demand for analogic photographic laboratory work has had a broadly felt effect in the conservation community. In the cultural heritage conservation field, instruments like the X-Rite 310 are widely used, specifically in the performance of the Photographic Activity Test (PAT) for the preservation of photographic materials. In the present research, five possible alternate metrics were investigated as substitutes for the increasingly unavailable spot reading transmission densitometers in Status-A readings as mandated by the current PAT. The analyzed metrics were: (1) ratio in reflection using normal illumination geometry and circumferential 45° viewing (0/45:c), (2) contrast ratio in reflection using diffuse illumination and 8° viewing geometry with specular component included (d/8:i), (3) contrast ratio in reflection using diffuse illumination and 8° viewing geometry with specular component excluded (d/8:e), (4) Ortho-transmission densitometry and (5) UV- transmission densitometry. The contrast ratio metric can be obtained with commonly available reflection spectrophotometers, such as the X-Rite 939 and the X-Rite SP64. The use of contrast ratio metric could open up new possibilities for measurement in the field of art reproduction and cultural heritage preservation to analyze changes in density and opacity. The proposed work analyzed the readings obtained by three measurement instruments: (1) X-Rite 361T, (2) X-Rite 939 and (3) X-Rite SP64, in a set of three achromatic transmission step-wedges (15-Step Transmission Stouffer© Graphic Arts T1530CC step-wedge) used as a surrogate for the colloidal silver strip used in the PAT. The goal was to evaluate the performance of the five proposed metrics and geometries as a possible alternative to transmission densitometry measurements when recording data using the Photographic Activity Test. The results indicate that there exists a near-perfect linear relationship between the readings using the X-Rite 361T in Ortho-transmission densitometry channel and the readings from the Status-A transmission density using the X-Rite 310 across the entire densitometric range represented by the Stouffer wedge. The UV channel measurements also exhibit a near seamless linear regression model with the Status-A readings. Both relationships were found to be statistically significant. On the other hand, the measurements with the setups using contrast ratio measurements did not exhibit the same linear relationship when the entire measurement range is considered. However, in readings of less than .95 opacity, the contrast ratio measurements did exhibit a meaningful linear relationship when compared to the Status-A transmission readings with a density value of less than 1.8, albeit still with lower correlation than both readings with the X-Rite 361T