Patrick Callet

Centre Français de la Couleur

A statistical multi-lobe approach was recently introduced in order to efficiently handle layered materials rendering as an alternative to expensive general-purpose approaches. However, this approach poorly supports scattering volumes as the method does not account for back-scattering and resorts to single scattering approximations. In this paper, w...

Proceedings of the International Colour Association (AIC) Conference 2020. Natural colours -Digital colours Proceedings editors: P. Callet and J.-M. Vallet

In the context of a virtual reconstitution of the destroyed Royaumont abbey church, this paper investigates computer sciences issues intrinsic to the physically-based image rendering. First, a virtual model was designed from historical sources and archaeological descriptions. Then some materials physical properties were measured on remains of the c...

In this paper, we use an original ray-tracing domain decomposition method to address image rendering of naturally lighted scenes. This new method allows to particularly analyze rendering problems on parallel architectures, in the case of interactions between light-rays and glass material. Numerical experiments, for medieval glass rendering within t...

This paper covers the time consuming issues intrinsic to physically-based image rendering algorithms. First, glass materials optical properties were measured on samples of real glasses and other objects materials inside an hotel room were characterized by deducing spectral data from multiple trichromatic images. We then present the rendering model...

On a tous vu la tranche d'une table, d'une porte ou d'une vitrine en verre et noté son aspect vert sombre, parfois opaque, qui semble en contradiction avec l'idée de transparence associée au matériau. C'est une histoire de réflexions internes totales qui permettent de guider la lumière d'un bord au bord parallèle opposé. C'est aussi une histoire d'...

The reflection properties of polished metallic materials are responsible for gloss or glare effects. Unpolished metallic surfaces never cause such uncomfortable sensations. It appears that the physical process of reflection of light by metallic surfaces, in diffuse or specular mode, leads to a fast brain analysis allowing us to understand what kind...

Among optical properties required for characterizing visual appearance in any lighting and viewing conditions, the most fundamental ones play an important role in predictive rendering. Spectroscopic ellipsometry is useful to acquire the complex indices of refraction of any homogeneous material. A multiscale approach using fundamental optical proper...

We describe how and why it is necessary to use spectral data and light polarization for having pre-dictive images of natural or artificial environment. For application to architectural design of spaces or building where several kind of materials are involved (glasses, metal, stone, concrete, wood, etc.) the 3D scenes must be produced with a high le...

After a short but fundamental description of metals, semi-metals and alloys optical properties, we show, using an example, how to explain some visual effects and how to link their origin to the fundamental optical properties described by the complex indices of refractions of metallic materials. Thus the great Obelisk was transported from Louxor and...

The optical properties of a hematite-epoxy coating are predicted numerically and compared with el-lipsometry measurements. The highly-heterogeneous dispersion of nanocubic hematite particles in the epoxy resin is simulated using a previously developed two-scales random model. The local anisotropic permittivity tensor of hematite particles, and that...

This paper describes the use of domain decomposition methods for accelerating wave physics simulation. Numerical wave-based methods provide more accurate simulation than geometrical methods, but at a higher computation cost as well. In the context of virtual reality, the quality of the results is estimated according to human perception, what makes...

Predictive rendering of material appearance means going deep into the understanding of the physical interaction between light and matter and how these interactions are perceived by the human brain. In this paper we describe our approach to predict the appearance of composite materials by relying on the multi-scale nature of the involved phenomena....

In the field of image synthesis, the simulation of material's appearance requires a rigorous resolution of the light transport equation. This implies taking into account all the elements that may have an influence on the spectral radiance, and that are perceived by the human eye. Obviously, the reflectance properties of the materials have a major i...

As buildings from ancient times wither away or are transformed or destroyed, it becomes increasingly difficult to figure what it looked like back then. Virtual reality is a great tool to handle this role, although it often implies a three-dimensional model of a building disconnected from the remains. In this article, we present an on-site device de...

The present work studies the role of stereoscopy on perceived surface aspect of computer generated complex materials. The objective is to investigate if, and how, the additional information conveyed by the binocular vision affects the observer judgment on the evaluation of flake density in an effect paint simulation. We have set up a heuristic flak...

3D data acquisition and 3D virtual and physical reproductions are shortly described within the framework of our experiments with museum artefacts. Our experience in the cultural heritage field illustrates a lot of problems encountered and the solutions we brought. From this short analysis, a perspective for the next 10 or 20 years is sketched. Ther...

In this paper, an original beam-tracing domain decomposition method is proposed for church acoustics. This new method allows to analyze large-scale acoustics problems in a reasonnable time on parallel architectures. Numerical experiments, for sound holography within the church of the Royaumont abbey, illustrate the performance of the proposed beam-...

Colour and the optical properties of material are studied for many years in the cultural heritage field. Here is presented a set of results of researches leading to the expression of the visual appearance of medieval glasses (XIIIth century). A multidisciplinary approach was successful for having the main parameters of materials, shapes and symboli...

Fundamental optical properties of materials are described by intrinsic parameters that can be concentrated in only one notion: the components of the complex dielectric tensor. Such a notion is relevant in any cases and we shall give examples of applications in the cultural heritage field. Modern glasses characterization and ancient or medieval glas...

Fundamental optical properties of materials are described by intrinsic parameters that can be concentrated in only one notion: the components of the complex dieclectric tensor. Such a notion is relevant in any cases and we shall give examples of applications in the cultural heritage field and in the design of modern products based upon their matera...

optical simulations, samples realization of gilded surfaces were made for studying the visual appearance and the substrate color influence of gilded surfaces mainly encountered in sculpture. The presented study concerns the characterization of the extrinsic and intrinsic parameters useful for the optical simulation of the visual properties of the g...

The Idalion Tablet, an antique bronze (480-470 BC) found around 1850 in the antique city of Idalion in Cyprus is the main purpose of the presented interdiciplinary work. The tablet is the property of the Bibliothèque Nationale de France (BNF) since 1862, and is absent from collections of the Idalion Museum (Cyprus). Our final goal is to realise an...

After a short description of the modelling of the optical behaviour of metals and alloys, we shall present the method of optical data acquisition. These pertinent data used for the rendering and the optical simulation of metallic objects exclusively are the complex dielectric functions and more generally the complex dielectric tensor. The influence...

We present the main results of a complete study driven in cooperation with the "Musée National des Arts Asiatiques-Guimet" in Paris concerning a chinese horse bronze statuette, 20 centuries old. The visual appearance of bronze is studied, depending on their metallurgical composition and state of surface in normalized illumination and visualisation...

The elytra of the longhorn Calothyrza margaritifera exhibit bright white zones which appear, under SEM, to be composed of structures looking like long white hairs: flat cone-shaped rods of 100–200μm in length and a base of 10–20μm. Each hair is composed of an envelope of chitin or chitin and associated proteins, filled with small agglomerated spher...

Lighting either natural or not in the european medieval architecture (roman period) is studied and simulated around a special and famous example. "Notre-Dame de Paris" cathedral and its polychromy are studied for many years and, today the remains of polychromy are very reduced. This prestigious monument is then a unique opportunity to study the com...

Accounting for the main optical effects that are involved in the interaction of light with natural materials such as pearls, a multilayer model is presented and applied in the context of computer graphics. A physical description of the mutilayer medium is given first, leading to a matrix formulation of the reflectance factor. An iterative computati...

The aim of this work was the identification of the pigments of the Philippe Dagobert monument (1222–1232) by spectrophotometry (Visible (VIS) region). Based on the bibliographic information about the pigments used in the Middle Ages, reference coloured samples were prepared in the laboratory. The study was based on the comparative analysis of VIS r...

The restitution of the visual appearance of the recumbent of Philippe Dagobert (1222­1235) situated in the royal necropolis of Saint­ Denis, near Paris, is presented. The presented works gather the first results where pigments identification was made using non­ contact optical methods (spectrophotometry) and some micro­analyses for a complete discr...

The presented work is led in the framework of a general collaboration between three academic labs, industrial partners and Cultural institutions (Centre des Monuments Nationaux, Louvre museum). Such a pluridisciplinary work always in progress at Ecole Centrale Paris deals with 3D digitization, simulation, rapid prototyping, virtual restoration appl...

Pertinent data used for the rendering and the optical simulation of metallic objects are the complex dielectric functions and more generally the complex dielectric tensor. Here is described a generalization of a previous model of optical properties dedicated to homogeneous materials. Heterogeneous materials lead to a local extension of that model s...

We present the main results of a complete study concerning a small metallic statuette. A collaboration involving metallurgy, optics, computer graphics, archaeology and engineering specialists was needed. The virtual optical aspect computations obtained by spectral simulation is founded on optical measurements of metallic samples wether smooth or ro...

We present the latest results of a pluridisciplinary work already in progress at Ecole Centrale Paris involving 3D digitization, simulation, rapid prototyping of several museum artifacts. This work is led in the framework of a general collaboration between three academic labs, The Musée National des Arts Asiatiques Guimet, the Centre des Monuments...

The visual appearance of bronze is studied, depending on their metallurgical composition and state of surface for some normalized illumination conditions. The results are presented on an example where a pluridisciplinary work was accomplished involving, engineering, art history, computer science, metallurgy, optics, material sciences, etc. speciali...

La numérisation 3D sans contact d’une tête de Bouddha afghane en stuc datant du IIIe siècle de notre ère a permis de développer deux axes d’études et réalisations : La reconstitution de l’apparence chromatique des matériaux de l’œuvre, et la réalisation de reproductions en utilisant des techniques de prototypage rapide et de moulage. Ces reproducti...

We present the main results of a complete study of a small metallic statuette. A collaboration involving metallurgy, optics, computer graphics, archaeology and engineering specialists were needed. The complex processes for this study are not described in their two main phases : replication of the original object using rapid prototyping methods and...

Lorsqu’en 1998, le département des Antiquités orientales du musée du Louvre avait confié au Centre de recherche et de restauration des musées de France une statuette de taureau de style séleucide ou parthe, rien ne laissait supposer que cette étude de routine déboucherait sur la mise en évidence d’un procédé métallurgique totalement inédit. Le cadr...

Après une présentation des particularités de la réflexion métallique, nous illustrons notre démarche de simulation sur l'exemple en cours d'étude avec un groupe d'élèves ingénieurs de l'Ecole centrale de Paris. La couleur et l'apparence visuelle des matériaux métalliques sont ainsi présentées dans le cadre général de l'optique électromagnétique. No...

We shortly present the results of a complete study driven in cooperation with the Louvre Museum in Paris concerning a small metallic work of art. Our object of study offered the opportunity of a collaboration involving metallurgy, optics, computer graphics, archaeology and engineering specialists. The complex processes for this study are then descr...

Color and visual appearance of materials are presented within the general framework of their optical properties, described with the unique notion of complex index of refraction. We limit our investigation to materials described as homogeneous and forming a continuum at a macroscopic scale of observation, though there exist some methods for extracti...

This article discusses the important problem of determining pertinent data to describe a pigmented medium. Thefirst part deals with optical characteristics of pigments and the general properties of binders and fillers. It also exposes the limits and conditions of applicability of single scattering. In the second part the connection between individu...

This work was carried out in the framework of a complete effort of maintenance and restoration of the colours of the monument of Philippe Dagobert (1222-1232) exhibited in the cathedral of Saint Denis in Paris and aimed at the identification of the colouring substances through colorimetry - spectrophotometry (visible spectrum). Based on the bibliog...