Photometric stereo with only two images: A theoretical study and numerical resolution

This work tackles the problem of two-image photometric stereo. This problem constitutes the intermediate case between conventional photometric stereo with at least three images, which is well-posed, and shape-from-shading, which is ill-posed. We first provide a theoretical study of ambiguities arising in this intermediate case. Based on this study, we show that when the albedo is known, disambiguation can be formulated as a binary labeling problem, using integrability and a nonstationary Ising model. The resulting optimization problem is solved efficiently by resorting to the graph cut algorithm. These theoretical and numerical contributions are eventually validated in an application to three-image photometric stereo with shadows.Roberto Mecca was a Marie Curie Fellow of the Instituto Nazionale di Alta Matematic

3D DIGITIZATION OF HERITAGE: PHOTOMETRIC STEREO CAN HELP

There are mainly two families of photographic 3D reconstruction. Photogrammetry techniques work according to the principle of triangulation, from the matching of different views, while photometric techniques link the appearance of a 3D point to the orientation of its normal, relative to the direction of the incident light. While photogrammetry allows to find the global shape of a 3D scene, if it is sufficiently textured, photometric techniques highlight the details of the relief, as long as the model linking the lighting to the shape and reflectance of the scene is sufficiently realistic. In order to avoid errors in the 3D models obtained, all the photographic techniques of 3D reconstruction have benefited, over the years, from algorithmic improvements that make them more and more robust to outliers or unreliable data. Moreover, the complementarity between these two types of approaches having been identified for a long time, many solutions have been proposed to merge them. Our work aims at providing the free and open-source photogrammetry software Meshroom with the benefits it could get from photometric stereo, particularly in the context of the 3D digitization of heritage, knowing that it is the only photometric technique for 3D reconstruction that has really proven itself

Solving the Uncalibrated Photometric Stereo Problem using Total Variation

International audienceIn this paper we propose a new method to solve the problem of uncalibrated photometric stereo, making very weak assumptions on the properties of the scene to be reconstructed. Our goal is to solve the generalized bas-relief ambiguity (GBR) by performing a total variation regularization of both the estimated normal field and albedo. Unlike most of the previous attempts to solve this ambiguity, our approach does not rely on any prior information about the shape or the albedo, apart from its piecewise smoothness. We test our method on real images and obtain results comparable to the state-of-the-art algorithms

Ultrahigh-power micrometre-sized supercapacitors based on onion-like carbon

Electrochemical capacitors, also called supercapacitors, store energy in two closely spaced layers with opposing charges, and are used to power hybrid electric vehicles, portable electronic equipment and other devices¹. By offering fast charging and discharging rates, and the ability to sustain millions of ²⁻⁵, electrochemical capacitors bridge the gap between batteries, which offer high energy densities but are slow, and conventional electrolytic capacitors, which are fast but have low energy densities. Here, we demonstrate microsupercapacitors with powers per volume that are comparable to electrolytic capacitors, capacitances that are four orders of magnitude higher, and energies per volume that are an order of magnitude higher. We also measured discharge rates of up to 200 V s⁻¹, which is three orders of magnitude higher than conventional supercapacitors. The microsupercapacitors are produced by the electrophoretic deposition of a several micrometre-thick layer of nanostructured carbon onions⁶‚⁷ with diameters of 6-7 nm. Integration of these nanoparticles in a microdevice with a high surface-to-volume ratio, without the use of organic binders and polymer separators, improves performance because of the ease with which ions can access the active material. Increasing the energy density and discharge rates of supercapacitors will enable them to compete with batteries and conventional electrolytic capacitors in a number of applications

Projet Archaeoroom : numérisation du patrimoine archéologique et développement de la suite logicielle open-source AliceVision Meshroom

International audienc

Numérisation 3D du patrimoine par stéréophotométrie

International audienceThe two most common techniques used by archaeologists to digitise heritage objects in 3D are photogrammetry, which is a photographic technique, and lasergrammetry. Numerous software programs allow people working in the field to collect data under a wide variety of operating conditions and then produce 3D models afterwards. A recent technique, called RTI (Reflectance Transformation Imaging), meets the needs of archaeologists when the scene to be analysed includes fine engravings, by allowing them to simulate new lighting, for example grazing light. Our ambition is to develop these practices, which are certainly operational and meet the main needs of heritage professionals, by illustrating the benefits that a 3D photographic reconstruction technique called stereophotometry (SP) could bring them. In particular, SP has three major advantages over the techniques already mentioned:- Firstly, the resolution of the 3D models obtained by SP is limited only by the size of the pixels, i.e. by the dimensions of the portion of surface corresponding to a pixel.- Secondly, SP is the only 3D digitisation technique that allows to estimate not only the relief of the scene, but also its intrinsic colour (as opposed to the apparent colour).- Thirdly, the production of a complete 3D model (relief + colour) is extremely fast, compared to other techniques, notably photogrammetry. In this presentation, we will show on several examples that SP can outperform the 3D digitisation techniques currently used by heritage actors. The comparison between photogrammetry and SP will be illustrated with the free software Meshroom, in a very recent version, which allows the manipulation of stereophotometric data.Les deux techniques les plus utilisées par les archéologues pour numériser en 3D les objets du patrimoine sont la photogrammétrie, qui est une technique photographique, et la lasergrammétrie. De nombreux logiciels permettent aux personnes travaillant sur le terrain de recueillir des données dans des conditions opératoires très variées, puis de produire des modèles 3D a posteriori. Une technique récente, appelée RTI (Reflectance Transformation Imaging), répond aux besoins des archéologues lorsque la scène à analyser comporte de fines gravures, en leur permettant de simuler de nouveaux éclairages, par exemple un éclairage rasant. Notre ambition est de faire évoluer ces pratiques, certes opérationnelles et répondant aux principaux besoins des acteurs du patrimoine, en illustrant les bénéfices que pourrait leur apporter une technique photographique de reconstruction 3D appelée stéréophotométrie (SP). En particulier, la SP présente trois avantages majeurs sur les techniques déjà citées :- Primo, la résolution des modèles 3D obtenus par SP n’est limitée que par la taille des pixels, c’est-`a-dire par les dimensions de la portion de surface correspondant `a un pixel.- Secundo, la SP est la seule technique de numérisation 3D qui permette d’estimer non seulement le relief de la scène, mais également sa couleur intrinsèque (par opposition à la couleur apparente).- Tertio, la production d’un modèle 3D complet (relief + couleur) est extrêmement rapide, par rapport aux autres techniques, notamment la photogrammétrie. Dans cet exposé, nous montrerons sur plusieurs exemples que la SP peut surpasser les techniques de numérisation 3D actuellement utilisées par les acteurs du patrimoine. La comparaison entre photogrammétrie et SP sera illustrée avec le logiciel libre Meshroom, dans une version très récente, qui autorise la manipulation de données de stéréophotométrie

A comprehensive introduction to photometric 3D-reconstruction

Photometric 3D-reconstruction techniques aim at inferring the geometry of a scene from one or several images, by inverting a physical model describing the image formation. This chapter presents an introductory overview of the main photometric 3D-reconstruction techniques which are shape-from-shading, photometric stereo and shape-from-polarisation