91 research outputs found

    IBISA: Making Image-Based Identification of Ancient Coins Robust to Lighting Conditions

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    International audienceThe IBISA (Image-Based Identification/Search for Archaeology) system manages databases of digital images of archaeological objects, e.g. ancient coins, and allows the user to perform searches by examples. IBISA was designed to help the user decide, from their images, if two objects (coins) are either the same, come from the same matrix (die), share resemblance in style, or are completely different. The system searches for similarities in the databases using a registration method that must be resilient to the viewing conditions. Based on the Fourier transform, it cancels rigid transforms among images. Sub-pixel accuracy can be achieved with a very simple technique. However lighting conditions remain an issue. Fortunately, it is possible to extend this registration method to a light-independent model, considering the elevation or normal maps instead of intensity. The model is also useful for interactive visualization and museography. Although this model registration is now resilient to all viewing conditions, it is not practical in real scenarios where the target is a single image, from which a model can hardly be derived. Finally, a hybrid approach is investigated, with a target image but a model of the reference. It is more realistic, resilient to light conditions, gives excellent results with translations, but shows limitations for rotations

    A Novel Framework for Highlight Reflectance Transformation Imaging

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    We propose a novel pipeline and related software tools for processing the multi-light image collections (MLICs) acquired in different application contexts to obtain shape and appearance information of captured surfaces, as well as to derive compact relightable representations of them. Our pipeline extends the popular Highlight Reflectance Transformation Imaging (H-RTI) framework, which is widely used in the Cultural Heritage domain. We support, in particular, perspective camera modeling, per-pixel interpolated light direction estimation, as well as light normalization correcting vignetting and uneven non-directional illumination. Furthermore, we propose two novel easy-to-use software tools to simplify all processing steps. The tools, in addition to support easy processing and encoding of pixel data, implement a variety of visualizations, as well as multiple reflectance-model-fitting options. Experimental tests on synthetic and real-world MLICs demonstrate the usefulness of the novel algorithmic framework and the potential benefits of the proposed tools for end-user applications.Terms: "European Union (EU)" & "Horizon 2020" / Action: H2020-EU.3.6.3. - Reflective societies - cultural heritage and European identity / Acronym: Scan4Reco / Grant number: 665091DSURF project (PRIN 2015) funded by the Italian Ministry of University and ResearchSardinian Regional Authorities under projects VIGEC and Vis&VideoLa

    Metrology Enabled Reflection Transformation Imaging to Reconstruct Local Detail in Manufactured Surfaces

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    Understanding the performance of large high performance manufactured structures can require highly accurate dimensional measurement across large volumes with the often conflicting capability to record critical parts of the structure in fine detail. Examples include turbine blades, aircraft wings and off-site manufactured modular structures assembled on-site for city, energy and transport infrastructure. Established large-volume industrial metrology systems such as laser trackers and photogrammetry partially meet the need through the measurement of targets and reflectors, but are limited in capability to record high density local detail needed to capture the finest manufactured features. Whilst large-volume surface sensing is possible with laser radar, photogrammetric pattern projection and contact probing for example, the detail required at a local level typically demands local sensing which generally takes the form of a tracked sensor such as a triangulation laser scanner or hand held touch probe. Local sensing systems face challenges where surfaces have fine detail of similar magnitude to the local sensing system sampling capability and particularly for optical sensors where the light reflected back to the sensor by the surface includes specular reflections compounded by local geometry. This paper investigates how Reflection Transformation Imaging (RTI) with a dome camera and lighting system might be calibrated, characterised and tracked as an alternative technology that is more robust to material surface properties and capable of very fine surface detail capture. Laboratory results demonstrate the capability to characterise and locate the dome to sub-millimetric accuracy within a large-volume tracked space to achieve local surface sampling at the 30 μm × 30 μm level. A method utilising sparse touch probe points to seed conversion of low and high frequency normal maps into a common 3D surface is explored with local agreement with laser tracker surface probe check points to the order of 30 μm

    Three-dimensional reconstruction of Roman coins from photometric image sets

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    A method is presented for increasing the spatial resolution of the three-dimensional (3-D) digital representation of coins by combining fine photometric detail derived from a set of photographic images with accurate geometric data from a 3-D laser scanner. 3-D reconstructions were made of the obverse and reverse sides of two ancient Roman denarii by processing sets of images captured under directional lighting in an illumination dome. Surface normal vectors were calculated by a “bounded regression” technique, excluding both shadow and specular components of reflection from the metallic surface. Because of the known difficulty in achieving geometric accuracy when integrating photometric normals to produce a digital elevation model, the low spatial frequencies were replaced by those derived from the point cloud produced by a 3-D laser scanner. The two datasets were scaled and registered by matching the outlines and correlating the surface gradients. The final result was a realistic rendering of the coins at a spatial resolution of 75  pixels/mm (13-μm spacing), in which the fine detail modulated the underlying geometric form of the surface relief. The method opens the way to obtain high quality 3-D representations of coins in collections to enable interactive online viewing

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

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    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

    APPLICATIONS OF REFLECTANCE TRANSFORMATION IMAGING FOR DOCUMENTATION AND SURFACE ANALYSIS IN CONSERVATION

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    Conservators at Universitat Politécnica de Valencia, Spain, explore the advantages and potential applications of Reflectance Transformation Imaging (RTI) as an innovative instrument for the documentation and analysis of texture of cultural objects undergoing conservation treatments. A computational photography technique, RTI creates highly accurate and interactive images, where objects can be illuminated from different directions and through a variety of filters to emphasize their surface texture and color. RTI was implemented to try the technique effectiveness for performing detailed examination and diagnostics on a variety of materials, including paintings, works on paper, wooden sculptures, metals, and stone. Resulting images yielded new and valuable information in the identification of manufacturing techniques and assessments of surface condition that were not previously recognized through direct examination or any other photographic techniques. RTI also proved highly effective for detailed documentation of painting and paper objects before and after treatments, helping to assess many subtle changes on pictorial layers and paper supports caused by conservation processes. RTI is an affordable and accessible resource which would be beneficial for to both cultural institutions and individual conservators for the interpretation and evaluation of cultural heritage.Manrique Tamayo, SN.; Valcarcel Andrés, JC.; Osca Pons, MJ. (2013). APPLICATIONS OF REFLECTANCE TRANSFORMATION IMAGING FOR DOCUMENTATION AND SURFACE ANALYSIS IN CONSERVATION. International Journal of Conservation Science. 4:535-548. http://hdl.handle.net/10251/56622S535548

    Application of multi-modal 2D and 3D imaging and analytical techniques to document and examine coins on the example of two Roman silver denarii

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    This case study is applying imaging and analytical techniques from multiple scientific disciplines to digitise coins and evaluate 3D multi-modal visualisation. Two ancient Roman silver denarii were selected as test objects to establish whether the proposed digital recording methods can support professional numismatic comparison of features and properties. The coins raise questions concerning their provenance, authenticity, design, purpose of issue and historic usage, but they also pose considerable recording challenges due to their material and surface properties, which are the main focus in this paper. The coins have been examined by the following techniques: dome photography for image sets for PTM/RTI visualisation and photometric stereo; X-ray microtomography for detection of cracks or impurities; Scanning Electron Microscopy for detailed surface investigation; Energy-Dispersive X-ray Spectroscopy for elemental analysis; micro X-ray fluorescence spectrometry mapping; 3D laser and structured light scanning for 3D spatial capture; photogrammetry/structure from motion, focus-stacking. The results indicate the feasibility of such techniques for museum documentation and as contribution to scientific examination of coins in general
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