Rendu interactif de modèles B-Rep sur GPU

Les logiciels de modélisation géométrique utilisés dans la Conception Assistée par Ordinateur (CAO) servent aujourd'hui à concevoir des objets de toutes sortes, allant de simples lampes de bureau à des avions de lignes entiers. Les modèles de données B-Rep utilisés permettent de définir des formes de manière précise, et de créer des entités géométriques répondant à des critères divers et variés, qu'ils soient esthétiques, qu'ils reposent sur la résistance mécanique, ou encore sur des coûts de production. Au fur et à mesure des opérations de modélisation dans le logiciel de conception (i.e. CATIA, par exemple), les modèles doivent être affichés de la manière la plus fidèle possible à la représentation analytique, telle qu'elle a été explicitement définie par l'opérateur CAO. Les logiciels de modélisation existant discrétisent les formes définies au rythme des opérations de modélisation, et affichent des polygones approximant face par face les objects 3D résultant de cette discrétisation. Des jours ou "cracks" dûs à la tessellation apparaissent avec cette méthode de rendu et sont déplaisant voire gênant pour les opérateurs. Nous proposons dans nos travaux une manière de faire un rendu haute qualité et sans cracks des modèles, sans toucher à leur définition, et en conservant de bonnes performances à l'affichage. Notre méthode peut s'intégrer aux moteurs de rendu existant. Les sessions de modélisation manipulent des objets de petite ou moyenne envergure. Une fois assemblés, les objets définissant de grosses structures comme par exemple des bateaux ou des avions doivent parfois être visualisés dans leur totalité, et toujours avec une grande précision visuelle. C'est le cas par exemple pour les applications de tests statiques où des techniciens déposent sur une maquette virtuelle des capteurs servant à évaluer le comportement mécanique de centaines voire de milliers de parties localisées sur la structure. Ces capteurs, de la taille d'un ongle, sont physiquement placés sur un produit entièrement usiné et assemblé, tel qu'une section entière de fuselage, dans un hangar. La maquette virtuelle doit permettre de visualiser la structure exactement telle qu'elle est, dans ses moindres détails. Pour réaliser le rendu d'une telle quantité de données de manière interactive et ce, sans sacrifier les performances ou la qualité de rendu à fort niveau de zoom, nous nous appuyons sur les méthodes de rendu dites basées découpe. Nous proposons une structure de découpe rapide, avec laquelle le rendu de très grands modèles peut être effectué. Cette structure est associée à des routines de tessellation dédiées pour chaque type de face B-Rep, ce qui nous permet d'afficher en temps-réel une section entière d'un avion de ligne gros porteur, constituté de centaines de milliers de surfaces B-Rep, tessellées puis découpées à la volée.Modeling software applications dedicated to Computer Aided Design (CAD) are used to design objects of all sorts, ranging from small, simple desktop lamps to entire, complex aircrafts. The B-Rep models used during this design phase allow CAD operators to define shapes in a very precise manner, and create geometric entities meeting various design criteria, whether they are related to aesthetics, mechanical behavior, or production costs. As modeling operations are performed in the CAD software application (eg. CATIA, for instance), models must be rendered with a fidelity and accuracy as high as possible with regards to their analytical definition as it has specifically been defined by the CAD operator. Existing modeling software applications discretize shapes as they are being edited and render polygons approximating 3D objects resulting from this discretization, usually done on a face by face basis. Gaps or "cracks" caused by this tesselation show up with this rendering method and are very annoying for the operators. We propose a method to perform crack-free, high fidelity rendering of B-Rep models, with no model preprocess and with good performance, allowing interaction even for large-scale objects. Our method can easily be integrated into existing engines based on dynamic tesselation. Modeling sessions deal with small or medium scale object parts. Once assembled, these parts form large structures such as boats or aircrafts. These large assemblies may sometimes be interactively visualized as a whole, and once again high precision is strongly desired. Specifically, this is the case for static test applications where technicians place sensors on a virtual model called a Digital Mockup (or DMU) that is used to assess the mechanical strength of thousands of selected locations scattered throughout the model. These nail-sized sensors are then physically placed on a product entirely built and assembled in a hangar, such as an entire aircraft fuselage section. The DMU should represent as faithfully as possible the actual, physical model. For interactive rendering to take place with good frame rates and with high image quality, we rely on a trim-based rendering method with a custom, efficient, multi-resolution trim structure suitable for the rendering of large-scale models. This structure is used in conjunction with dedicated dynamic tesselation GPU routines for common types of B-Rep faces. This combination allows us to render in real-time and on current consumer hardware an entire wide-body aircraft fuselage section composed of hundreds of thousands of B-Rep faces that are dynamically tesselated and trimmed on the fly every time a rendering takes place

An Efficient Trim Structure for Rendering Large B-Rep Models

International audienceWe present a multiresolution trim structure for fast and accurate B-Rep model visualization. To get a good tradeoff between performance and visual accuracy, we propose to use a vectorial but approximated representation of the model that allows efficient, real-time GPU exploitation. Our structure, based on a quadtree, enables us to do shallow lookups for distant fragments. For closeups, we leverage hardware tessellation. We get interactive frame rates for models that consists of hundreds of thousands of B-Rep faces, regardless of the zoom level

Une structure de découpe efficace pour l'affichage de grands modèles B-Rep

National audienceNous présentons une structure de découpe multirésolution pour l'affichage rapide et précis de modèles B-Rep. Nous proposons d'utiliser une représentation de la découpe de face basée sur un quadtree, autorisant une gestion efficace par le GPU. Ce quadtree contient des références à des courbes de découpe qui sont utilisées dans un fragment shader pour effectuer la classification de point d'une manière implicite. La façon dont sont stockées les informations multirésolution dans le quadtree nous permet de réduire les accès à notre structure lorsque les fragments sont distants de la caméra. Pour les objets en avant plan, nous nous appuyons sur la tessellation matérielle pour améliorer les performances, en réduisant la quantité de calcul à effectuer pour chaque fragment. Nous obtenons un affichage intéractif pour de très gros modèles comprenant des centaines de milliers de faces B-Rep, quel que soit le niveau de zoom

Chained segment offsetting for ray-based solid representations

International audienceWe present a novel approach to offset solids in the context of fabrication. Our input solids can be given under any representation: boundary meshes, voxels, indicator functions or CSG expressions. The result is a ray-based representation of the offset solid directly used for visualization and fabrication: We never need to recover a boundary mesh in our context. We define the offset solid as a sequence of morphological operations along line segments. This is equivalent to offsetting the surface by a solid defined as a Minkowski sum of segments, also known as a zonotope. A zonotope may be used to approximate the Euclidean ball with precise error bounds. We propose two complementary implementations. The first is dedicated to solids represented by boundary meshes. It performs offsetting by modifying the mesh in sequence. The result is a mesh improper for direct display, but that can be resolved into the correct offset solid through a ray representation. The major advantage of this first approach is that no loss of information – re-sampling – occurs during the offsetting sequence. However, it applies only to boundary meshes and cannot mix sequences of dilations and erosions. Our second implementation is more general as it applies directly to a ray-based representation of any solid and supports any sequence of erosion and dilation along segments. We discuss its fast implementation on modern graphics hardware. Together, the two approaches result in a versatile tool box for the efficient offsetting of solids in the context of fabrication

Ribbed support vaults for 3D printing of hollowed objects

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A microfacet-based BRDF for the accurate and efficient rendering of high-definition specular normal maps

International audienceComplex specular microstructures found in glittery, scratched or brushed metal materials exhibit high frequency variations in reflected light intensity. These variations are important for the human eye and give materials their uniqueness and personality. To model such microsurfaces, high definition normal maps are very effective. The works of Yan et al. [21,22] enable the rendering of such material representations by evaluating a microfacet based BRDF related to a whole ray footprint. Still, in specific configurations and especially at grazing angles, their method does not fully capture the expected material appearance. We propose to build upon their work and tackle the problem of accuracy using a more physically based reflection model. To do so, the normal map is approximated with a mixture of anisotropic, noncentered Beckmann normal distribution functions from which a closed form for the masking-shadowing term can be derived. Based on our formal definition, we provide a fast approximation leading to a performance overhead varying from 5% to 20% compared to the method of Yan et al. [22]. Our results show that we more closely match ground truth renderings than their methods

HCSG: Hashing for real-time CSG modeling

International audienceConstructive Solid Geometry models solids as boolean combinations of base primitives. It is one of the classical modeling approaches in Computer Graphics. With the advent of 3D printing, it has received a renewed interest: CSG affords for the robust definition of solids, and fits well with parametric modeling, affording for easy customization of existing designs. However, the interactive display and manipulation of CSG models is challenging: Ideally, CSG has to be performed between a variety of solid representations (meshes, implicit solids, voxels) and the renderer has to provide immediate feedback during parameter exploration. The end result has to be prepared for fabrication, which involves robustly extracting cross-sections of the model. In this work we propose a novel screen space technique for the rendering, interactive modeling and direct fabrication of parametric CSG models. It builds upon spatial hashing techniques to efficiently evaluate CSG expressions, checking whether each interval along a view ray is solid in constant time, using constant local shader memory. In addition, the scene is rendered progressively, from front to back, bounding memory usage. We describe how the hash encoding the CSG is constructed on the fly during visualization, and analyze performance on a variety of 3d models

Glint Rendering based on a Multiple‐Scattering Patch BRDF

International audienceMultiple-scattering glint integrator Chermain et al. [CCM18] Yan et al. [YHMR16] Figure 1: Glittery orb illuminated by an environment map using 1,024 spp. The micro-surface is modeled by a specular normal map with high RMS roughness (σ = 1). Previous methods based on classic normal mapping and single-scattering BRDF evaluations darken the appearance and create black holes (insets and white furnace tests). Importance sampling is suboptimal and causes fireflies. Our new multiple-scattering glint integrator tackles these issues. It produces renderings with no artifacts, and almost passes the white furnace test, for an extra 36% rendering time for this scene. Abstract Rendering materials such as metallic paints, scratched metals and rough plastics requires glint integrators that can capture all micro-specular highlights falling into a pixel footprint, faithfully replicating surface appearance. Specular normal maps can be used to represent a wide range of arbitrary micro-structures. The use of normal maps comes with important drawbacks though: the appearance is dark overall due to back-facing normals and importance sampling is suboptimal, especially when the micro-surface is very rough. We propose a new glint integrator relying on a multiple-scattering patch-based BRDF addressing these issues. To do so, our method uses a modified version of microfacet-based normal mapping [SHHD17] designed for glint rendering, leveraging symmetric microfacets. To model multiple-scattering, we reintroduce the lost energy caused by a perfectly specular, single-scattering formulation instead of using expensive random walks. This reflectance model is the basis of our patch-based BRDF, enabling robust sampling and artifact-free rendering with a natural appearance. Additional calculation costs amount to about 40% in the worst cases compared to previous methods [YHMR16, CCM18]

Raster2Mesh: Rasterization based CVT meshing

In this paper, we propose to extend high quality Centroidal Voronoi Tessellation (CVT) remeshing techniques to the case of surfaces which are not defined by triangle meshes, such as implicit surfaces. Our key observation is that rasterization routines are usually available to visualize these alternative representations, most often as OpenGL shaders efficiently producing surface samples (fragments) from the surface representation. Our technique has the ability to mesh any surface for which rasterization routines are available, and runs entirely within the OpenGL rasterization pipeline. There is no intermediate representation: the triangle mesh is computed directly from the surface fragments. Our method produces high quality meshes, as it inherits the properties of CVT meshing. Contrary to existing GPU techniques for CVT computation, it does not require a surface parameterization, and it extracts the mesh topology directly from the surface fragments. Optionally, our algorithm can produce two-manifold, consistently oriented meshes.We describe our complete implementation and show a variety of applications: direct meshing of implicit surfaces, meshing of operations between solids, mesh repair, and solid sculpting. We analyze performance, correctness and mesh quality

Enceintes ajustées pour la fabrication additive

Additive manufacturing is a process by which a three dimensional object is created layer after layer, through selective deposition of material.It often requires the automated generation of auxiliary shapes, to temporarily support the object, to protect its surface, or to carve inner cavities and reduce material usage.In this context, we define a ``printable enclosure'' as a minimal volumeenclosing a given shape and whose boundary can be printed at the smallest possible thickness while ensuring proper bonding between layers.Such an enclosure is well suited to serve as auxiliary structure for additive manufacturing: it is easy to print and require little material. In thispaper, we demonstrate its use on three different applications: enclosing aprint within protective walls that are close to the surface; generating largeinner cavities whose walls are printable, and finally modeling support structures that provide a dense support to the downward facing surfaces while vanishing as quickly as possible below the supported object.We obtain the shape of an enclosure by considering constraints on its set of slices along horizontal planes. In practice, the set of slices is discrete and the constraints afford for an efficient sweep-like construction algorithm using morphological operations on the slices. We discuss the printability and optimality of the enclosures and their boundary walls.La fabrication additive produit des objets en déposant un matériau en couchessuccessives.Cela requiert souvent la synthèse ou l'impression de formes additionnelles poursupport l'objet en cours de fabrication, protéger sa surface ou bien y creuserdes cavités afin de limiter la quantité de matériel utilisé.Dans ce contexte, nous définissons une "enceinte ajustée" comme le pluspetit volume contenant une forme donnée et dont le bord peut être fabriqué à laplus petit épaisseur possible tout en garatissant le bon maintient des couches dematière successives.Ces enceintes ajustées sont utiles pour la fabrication additive. Elles sontrapide à fabriquer et nécessitent peu de matériau. Cet article montre leurutilité dans trois applications~: la génération d'une paroi protectrice restantproche de l'objet à fabriquer, la génération de cavités aussi grandes quepossible à l'intérieur d'un objet et dont les parois restent fabriquables;enfin, la modélisation de structures de support denses dans le voisinageimmédiat des parties basses de l'objets et diminuant le plus rapidementpossible en dessous.Ces enceintes sont modélisées en considérant les contraintes s'opérant surleurs coupes transversales, par un plan horizontal. Nous décrivons unalgorithme par balayage permettant de modéliser rapidement ces enceintes àl'aide d'opérations morphologiques sur les coupes