Densification during hot-pressing of carbon nanotube–metal–magnesium aluminate spinel nanocomposites

The densification by hot-pressing of ceramic–matrix composites containing a dispersion of carbon nanotubes (CNT), mostly single-walled, is studied for the first time. Fifteen different CNT–Co/Mo–MgAl2O4 composite powders containing between 1.2 and 16.7 vol.% CNT were prepared by catalytic chemical vapour deposition. The in situ growth of CNT within the oxide powder made it possible to obtain a highly homogeneous distribution of CNT. Low contents of CNT (up to 5 vol.%) are beneficial for the first shrinkage step (up to 1100 ◦C), dominated by the rearrangement process, while higher contents are detrimental. At higher temperatures (1100–1300 ◦C), CNT clearly inhibit the shrinkage, and this detrimental effect regularly increases with the CNT content. Several explanations are proposed, in relation with the particular mechanical properties of CNT and their highly connected web-like distribution within the material

Percolation of single-walled carbon nanotubes in ceramic matrix nanocomposites

The percolation of carbon nanotubes (CNT) in an electrical insulating ceramic is studied for the first time. The in situ synthesis of the CNT (0.2–25 vol%) by a CCVD route allows to achieve their homogeneous distribution in the spinel matrix. Up to 11 vol% CNT, the DC electrical conductivity (σ) is well fitted by the scaling law of the percolation theory σ=k(p−pc)t with a low percolation threshold pc=0.64 vol%. At the threshold, σ jumps over seven order of magnitude (from 10−10 to 0.0040 S cm−1) and then reaches a maximum at 8.5 S cm−1. The results are discussed in relation with the characteristics of the CNT, their damaging during the hot-pressing at 1300 °C and the microstructure of the composites. CNT-ceramic composites become attractive materials not only for their enhanced mechanical properties, but also for the possibility to tailor the electrical conductivity through the CNT content

Triangulation of uniform particle systems: its application to the implicit surface texturing

Particle systems, as originally presented by Witkin and Heckbert [32], offer an elegant solution to sample implicit surfaces of arbitrary genus, while providing an extremely regular distribution of samples over the surface. In this paper, we present an ef cient technique that uses particle systems to rapidly generate a triangular mesh over an implicit surface, where each triangle is almost equilateral. The major advantage of such a triangulation is that it minimizes the deformations between the mesh and the underlying implicit surface. We exploit this property by using few triangular texture samples mapped in a non-periodic fashion as presented by Neyret and Cani [16]. The result is a pattern-based texturing method that maps homogeneous non-periodic textures to arbitrary implicit surfaces, with almost no deformation

Gestion de la complexité géométrique dans le calcul d'éclairement pour la présentation publique de scènes archéologiques complexes

International audienceFor cultural heritage, more and more 3D objects are acquired using 3D scanners [Levoy 2000]. The resulting objects are very detailed with a large visual richness but their geometric complexity requires specific methods to render them. We first show how to simplify those objects using a low-resolution mesh with its associated normal maps [Boubekeur 2005] which encode details. Using this representation, we show how to add global illumination with a grid-based and vector-based representation [Pacanowski 2005]. This grid captures efficiently low-frequency indirect illumination. We use 3D textures (for large objects) and 2D textures (for quasi-planar objects) for storing a fixed set of irradiance vectors. These grids are built during a preprocessing step by using almost any existing stochastic global illumination approach. During the rendering step, the indirect illumination within a grid cell is interpolated from its associated irradiance vectors, resulting in a smooth everywhere representation. Furthermore, the vector-based representation offers additional robustness against local variations of geometric properties of the scene.Pour l’étude du patrimoine, de plus en plus d’objets 3D sont acquis par le biais de scanners 3D [Levoy 2000]. Les objets ainsi acquis contiennent de nombreux détails et fournissent une très grande richesse visuelle. Mais pour les afficher, leur très grande complexité géométrique nécessite l’utilisation d’algorithmes spécifiques. Nous montrons ici comment simplifier ces objets par un maillage de faible résolution et une collection de cartes de normales [Boubekeur 2005] pour préserver les détails. Avec cette représentation, nous montrons comment il est possible de calculer un éclairement réaliste à l’aide d’une grille et de données vectorielles [Pacanowski 2005]. Cette grille permet de capturer efficacement les basses fréquences d’un éclairement indirect. Nous utilisons des textures 3D (pour des gros objets) et potentiellement des textures 2D (pour les objets quasi-plans) afin de stocker un nombre prédéterminé de vecteurs d’irradiance. Ces grilles sont calculées au cours d’un pré-calcul à l’aide de n’importe quelle méthode stochastique de calcul d’éclairement global. Pour l’affichage, l’éclairement indirect dû à la grille est interpolé au sein de la cellule associée à la position courante, fournissant ainsi une représentation continue. De plus, cette approche vectorielle permet une plus grande robustesse aux variations locales des propriétés géométriques de la scène

Approximation of Subdivision Surfaces for Interactive Applications

International audienceIn this sketch, we propose a visually plausible approximation subdivision surfaces for interactive applications. The complete idea is discussed in the full paper "QAS: Realtime Quadratic Approximation of Subdivision Surfaces", published in the proceedings of Pacific Graphics 2007 and available online (http://iparla.labri.fr/publications/2007/BS07c/)

Interactive Out-Of-Core Texturing

International audienceInteractive rendering of huge objects becomes available on common workstations thanks to highly optimized data-structures and out-of-core frameworks for rendering. However, interactive editing, and in particular interactive texturing of such objects, is still a challenging task, since the dynamic information added during this editing step would break any highly-optimized data-structures, such as GPU vertex buffers or specific out-of-core representations of huge objects. We propose Point-Sampled Textures (PST) for interactive texturing of large models at various scales without requiring 2D parameterization (complex and expensive for large models). This framework allows the user to interactively set any appearance property of the original object, from per-sample color to complex BRDFs

A Flexible Kernel for Adaptive Mesh Refinement on GPU

International audienceWe present a flexible GPU kernel for adaptive on-the-fly refinement of meshes with arbitrary topology. By simply reserving a small amount of GPU memory to store a set of adaptive refinement patterns, on-the-fly refinement is performed by the GPU, without any preprocessing nor additional topology data structure. The level of adaptive refinement can be controlled by specifying a per-vertex depth-tag, in addition to usual position, normal, color and texture coordinates. This depth-tag is used by the kernel to instanciate the correct refinement pattern. Finally, the refined patch produced for each triangle can be displaced by the vertex shader, using any kind of geometric refinement, such as Bezier patch smoothing, scalar valued displacement, procedural geometry synthesis or subdivision surfaces. This refinement engine does neither require multi-pass rendering nor any use of fragment processing nor special preprocess of the input mesh structure. It can be implemented on any GPU with vertex shading capabilities

Generic Mesh Refinement On GPU

International audienceMany recent publications have shown that a large variety of computation involved in computer graphics can be moved from the CPU to the GPU, by a clever use of vertex or fragment shaders. Nonetheless there is still one kind of algorithms that is hard to translate from CPU to GPU: mesh refinement techniques. The main reason for this, is that vertex shaders available on current graphics hardware do not allow the generation of additional vertices on a mesh stored in graphics hardware. In this paper, we propose a general solution to generate mesh refinement on GPU. The main idea is to define a generic refinement pattern that will be used to virtually create additional inner vertices for a given polygon. These vertices are then translated according to some procedural displacement map defining the underlying geometry (similarly, the normal vectors may be transformed according to some procedural normal map). For illustration purpose, we use a tesselated triangular pattern, but many other refinement patterns may be employed. To show its flexibility, the technique has been applied on a large variety of refinement techniques: procedural displacement mapping, as well as more complex techniques such as curved PN-triangles or ST-meshes

Improving Shape Depiction under Arbitrary Rendering

International audienceBased on the observation that shading conveys shape information through intensity gradients, we present a new technique called Radiance Scaling that modifies the classical shading equations to offer versatile shape depiction functionalities. It works by scaling reflected light intensities depending on both surface curvature and material characteristics. As a result, diffuse shading or highlight variations become correlated to surface feature variations, enhancing concavities and convexities. The first advantage of such an approach is that it produces satisfying results with any kind of material for direct and global illumination: we demonstrate results obtained with Phong and Ashikmin-Shirley BRDFs, Cartoon shading, sub-Lambertian materials, perfectly reflective or refractive objects. Another advantage is that there is no restriction to the choice of lighting environment: it works with a single light, area lights, and inter-reflections. Third, it may be adapted to enhance surface shape through the use of precomputed radiance data such as Ambient Occlusion, Prefiltered Environment Maps or Lit Spheres. Finally, our approach works in real-time on modern graphics hardware making it suitable for any interactive 3D visualization

QAS: Real-time Quadratic Approximation of Subdivision Surfaces.

International audienceWe introduce QAS, an efficient quadratic approximation of subdivision surfaces which offers a very close appearance compared to the true subdivision surface but avoids recursion, providing at least one order of magnitude faster rendering. QAS uses enriched polygons, equipped with edge vertices, and replaces them on-the-fly with low degree polynomials for interpolating positions and normals. By systematically projecting the vertices of the input coarse mesh at their limit position on the subdivision surface, the visual quality of the approximation is good enough for imposing only a single subdivision step, followed by our patch fitting, which allows real-time performances for million polygons output. Additionally, the parametric nature of the approximation offers an efficient adaptive sampling for rendering and displacement mapping. Last, the hexagonal support associated to each coarse triangle is adapted to geometry processors