Journal articles

titre

Desertscape Simulation

auteur

Axel Paris, Adrien Peytavie, Eric Guérin, Oscar Argudo, Eric Galin

resume

We present an interactive aeolian simulation to author hot desert scenery. Wind is an important erosion agent in deserts which, despite its importance, has been neglected in computer graphics. Our framework overcomes this and allows generating a variety of sand dunes, including barchans, longitudinal and anchored dunes, and simulates abrasion which erodes bedrock and sculpts complex landforms. Given an input time varying high altitude wind field, we compute the wind field at the surface of the terrain according to the relief, and simulate the transport of sand blown by the wind. The user can interactively model complex desert landscapes, and control their evolution throughout time either by using a variety of interactive brushes or by prescribing events along a user-defined time-line.

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titre

Semi‐Procedural Textures Using Point Process Texture Basis Functions

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Pascal Guehl, Rémi Allègre, Jean-Michel Dischler, Bedrich Benes, Eric Galin

resume

We introduce a novel semi‐procedural approach that avoids drawbacks of procedural textures and leverages advantages of data‐driven texture synthesis. We split synthesis in two parts: 1) structure synthesis, based on a procedural parametric model and 2) color details synthesis, being data‐driven. The procedural model consists of a generic Point Process Texture Basis Function (PPTBF), which extends sparse convolution noises by defining rich convolution kernels. They consist of a window function multiplied with a correlated statistical mixture of Gabor functions, both designed to encapsulate a large span of common spatial stochastic structures, including cells, cracks, grains, scratches, spots, stains, and waves. Parameters can be prescribed automatically by supplying binary structure exemplars. As for noise‐based Gaussian textures, the PPTBF is used as stand‐alone function, avoiding classification tasks that occur when handling multiple procedural assets. Because the PPTBF is based on a single set of parameters it allows for continuous transitions between different visual structures and an easy control over its visual characteristics. Color is consistently synthesized from the exemplar using a multiscale parallel texture synthesis by numbers, constrained by the PPTBF. The generated textures are parametric, infinite and avoid repetition. The data‐driven part is automatic and guarantees strong visual resemblance with inputs.

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titre

Representation of Concurrent Points of View of Urban Changes for City Models

auteur

John Samuel Samuel, Sylvie Servigne, Gilles Gesquière

resume

Cities evolve over time and their evolution is often studied using material objects and historical documents. Based on available evidence, researchers propose various hypotheses explaining the probable evolution, both imaginary and factual. Furthermore, city models are used to model and visualize 3D structures and semantic information of the cities. With the help of versioning of city objects, it is possible to represent temporal changes of city structures. And with the support for representing scenarios, it is possible to represent different possible sequences of urban changes. In this article, we formalize a set of rules for representation of concurrent points of view of researchers related to urban changes based on standard city model. The goal is to understand how the cities have evolved, what were the key changes and allow exchange between various hypotheses (or processes). We developed a proof-ofconcept named UrbanCo2Fab based on these rules to demonstrate the highly potential use cases of our proposition.

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titre

Describing, Comparing and Analysing Digital Urban Heritage Tools: A Methodology Designed with a Multidisciplinary Approach

auteur

Vincent Jaillot, Manon Istasse, Sylvie Servigne, Gilles Gesquière, Michel Rautenberg, Isabelle Lefort

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Urban cultural heritage is taking full advantage of digital technologies. This can be seen in the great number of digital tools for representing and interacting with cultural heritage (digital urban heritage tools) that are intended for the public or designed in the context of research programs and dedicated to a more limited public. These tools do not only display heritage, but also permit interaction with it, even allowing users to contribute with their own data (knowledge, memories, documents, questions, etc.). In this article, we present and apply a methodology for describing, analysing and comparing these tools. After observing the lack of such a methodology, we built DHAL (Digital urban Heritage tools AnaLysis). This methodology allows for qualitative and multidisciplinary analysis of digital urban heritage tools that combine at least two of the following aspects: digital representation of the city, multimedia data documenting the city and its heritage, and participatory functionalities for adding to the digital city. Firstly, we introduce the methodology and the process established for building it. We then show its application, usefulness and full potential in the context our project (Fab-Pat) by testing it on twelve tools. This implementation - one possible among others - allows the positioning of the Fab-Pat tool among similar tools and a detailed description of them. We conclude with a discussion of the methodology’s advantages and potential avenues for future developments.

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titre

Delivering time-evolving 3D city models for web visualization

auteur

Vincent Jaillot, Sylvie Servigne, Gilles Gesquière

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Studying and planning urban evolution is essential to understand the past and to design the cities of tomorrow. It can be facilitated by providing means for the navigation and visualization of cities’ evolution on the web. Standard formats, methods and tools exist for the visualization of large-scale 3D cities on the web. In this article, we go further by integrating the temporal dimension of cities to geospatial web delivery standard formats. We represent the temporal dimension with states and transitions between these states expressing the changes in the city. This proposal permits to achieve dynamic spatio-temporal visualization of large-scale time-evolving 3D city models on the web. A key characteristic of this article lies in the four-step generic approach proposed. First, we design a generic UML conceptual model of standard formats for delivering 3D cities on the web. Next, we integrate the temporal dimension of cities to this model. Then, we apply our generic model to the 3D Tiles standard at conceptual and technical specification level, resulting in an extension of 3D Tiles for delivering time-evolving 3D city models on the web. Finally, we propose an implementation in a 4-tiers web architecture and provide tests on datasets of the city of Lyon, France.

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Journal articles

titre

Orometry-based Terrain Analysis and Synthesis

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Oscar Argudo, Eric Galin, Adrien Peytavie, Axel Paris, James Gain, Eric Guérin

resume

Mountainous digital terrains are an important element of many virtual environments and find application in games, film, simulation and training. Unfortunately, while existing synthesis methods produce locally plausible results they often fail to respect global structure. This is exacerbated by a dearth of automated metrics for assessing terrain properties at a macro level. We address these issues by building on techniques from orometry, a field that involves the measurement of mountains and other relief features. First, we construct a sparse metric computed on the peaks and saddles of a mountain range and show that, when used for classification, this is capable of robustly distinguishing between different mountain ranges. Second, we present a synthesis method that takes a coarse elevation map as input and builds a graph of peaks and saddles respecting a given orometric distribution. This is then expanded into a fully continuous elevation function by deriving a consistent river network and shaping the valley slopes. In terms of authoring, users provide various control maps and are also able to edit, reposition, insert and remove terrain features all while retaining the characteristics of a selected mountain range. The result is a terrain analysis and synthesis method that considers and incorporates orometric properties, and is, on the basis of our perceptual study, more visually plausible than existing terrain generation methods.

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titre

Procedural Riverscapes

auteur

Adrien Peytavie, Thibault Dupont, Eric Guérin, Yann Cortial, Benes Benes, James Gain, Eric Galin

resume

This paper addresses the problem of creating animated riverscapes through a novel procedural framework that generates the inscribing geometry of a river network and then synthesizes matching real-time water movement animation. Our approach takes bare-earth heightfields as input, derives hydrologically-inspired river network trajectories, carves riverbeds into the terrain, and then automatically generates a corresponding blend-flow tree for the water surface. Characteristics, such as the riverbed width, depth and shape, as well as elevation and flow of the fluid surface, are procedurally derived from the terrain and river type. The riverbed is inscribed by combining compactly supported elevation modifiers over the river course. Subsequently, the water surface is defined as a time-varying continuous function encoded as a blend-flow tree with leaves that are parameterized procedural flow primitives and internal nodes that are blend operators. While river generation is fully automated, we also incorporate intuitive interactive editing of both river trajectories and individual riverbed and flow primitives. The resulting framework enables the generation of a wide range of river forms, ranging from slow meandering rivers to rapids with churning water, including surface effects, such as foam and leaves carried downstream.

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titre

A Psychophysical Evaluation of Texture Compression Masking Effects

auteur

Guillaume Lavoué, Michael Langer, Adrien Peytavie, Pierre Poulin

resume

Lossy texture compression is increasingly used to reduce GPU memory and bandwidth consumption. However, as raised by recent studies, evaluating the quality of compressed textures is a difficult problem. Indeed using Peak Signal-to-Noise Ratio (PSNR) on texture images, like done in most applications, may not be a correct way to proceed. In particular, there is evidence that masking effects apply when the texture image is mapped on a surface and combined with other textures (e.g., affecting geometry or normal). These masking effects have to be taken into account when compressing a set of texture maps, in order to have a real understanding of the visual impact of the compression artifacts on the rendered images. In this work, we present the first psychophysical experiment investigating the perceptual impact of texture compression on rendered images. We explore the influence of compression bit rate, light direction, and diffuse and normal map content on the visual impact of artifacts. The collected data reveal huge masking effects from normal map to diffuse map artifacts and vice versa, and reveal the weakness of PSNR applied on individual textures for evaluating compression quality. The results allow us to also analyze the performance and failures of image quality metrics for predicting the visibility of these artifacts. We finally provide some recommendations for evaluating the quality of texture compression and show a practical application to approximating the distortion measured on a rendered 3D shape.

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titre

SPOT: Sliced Partial Optimal Transport

auteur

Nicolas Bonneel, David Coeurjolly

resume

Optimal transport research has surged in the last decade with wide applications in computer graphics. In most cases, however, it has focused on the special case of the so-called ``balanced'' optimal transport problem, that is, the problem of optimally matching positive measures of equal total mass. While this approach is suitable for handling probability distributions as their total mass is always equal to one, it precludes other applications manipulating disparate measures. Our paper proposes a fast approach to the optimal transport of constant distributions supported on point sets of different cardinality via one-dimensional slices. This leads to one-dimensional partial assignment problems akin to alignment problems encountered in genomics or text comparison. Contrary to one-dimensional balanced optimal transport that leads to a trivial linear-time algorithm, such partial optimal transport, even in 1-d, has not seen any closed-form solution nor very efficient algorithms to date. We provide the first efficient 1-d partial optimal transport solver. Along with a quasilinear time problem decomposition algorithm, it solves 1-d assignment problems consisting of up to millions of Dirac distributions within fractions of a second in parallel. We handle higher dimensional problems via a slicing approach, and further extend the popular iterative closest point algorithm using optimal transport -- an algorithm we call Fast Iterative Sliced Transport. We illustrate our method on computer graphics applications such a color transfer and point cloud registration.

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titre

Terrain Amplification with Implicit 3D Features

auteur

Axel Paris, Eric Galin, Adrien Peytavie, Eric Guérin, James Gain

resume

While three-dimensional landforms, such as arches and overhangs, occupy a relatively small proportion of most computer generated landscapes, they are distinctive and dramatic and have an outsize visual impact. Unfortunately, the dominant heightfield representation of terrain precludes such features, and existing in-memory volumetric structures are too memory intensive to handle larger scenes. In this paper, we present a novel memory-optimized paradigm for representing and generating volumetric terrain based on implicit surfaces. We encode feature shapes and terrain geology using construction trees that arrange and combine implicit primitives. The landform primitives themselves are positioned using Poisson sampling, built using open shape grammars guided by stratified erosion and invasion percolation processes, and, finally, queried during polygonization. Users can also interactively author landforms using high-level modeling tools to create or edit the underlying construction trees, with support for iterative cycles of editing and simulation. We demonstrate that our framework is capable of importing existing large-scale heightfield terrains and amplifying them with such diverse structures as slot canyons, sea arches, stratified cliffs, fields of hoodoos, and complex karst cave networks.

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titre

A Review of Digital Terrain Modeling

auteur

Eric Galin, Eric Guérin, Adrien Peytavie, Guillaume Cordonnier, Marie-Paule Cani, Bedrich Benes, James Gain

resume

Terrains are a crucial component of three-dimensional scenes and are present in many Computer Graphics applications. Terrain modeling methods focus on capturing landforms in all their intricate detail, including eroded valleys arising from the interplay of varied phenomena, dendritic mountain ranges, and complex river networks. Set against this visual complexity is the need for user control over terrain features, without which designers are unable to adequately express their artistic intent. This article provides an overview of current terrain modeling and authoring techniques, organized according to three categories: procedural modeling, physically-based simulation of erosion and land formation processes, and example-based methods driven by scanned terrain data. We compare and contrast these techniques according to several criteria, specifically: the variety of achievable landforms; realism from both a perceptual and geomorphological perspective; issues of scale in terms of terrain extent and sampling precision; the different interaction metaphors and attendant forms of user-control, and computation and memory performance. We conclude with an in-depth discussion of possible research directions and outstanding technical and scientific challenges.

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titre

Procedural Tectonic Planets

auteur

Yann Cortial, Adrien Peytavie, Eric Galin, Eric Guérin

resume

We present a procedural method for authoring synthetic tectonic planets. Instead of relying on computationally demanding physically-based simulations, we capture the fundamental phenomena into a procedural method that faithfully reproduces large-scale planetary features generated by the movement and collision of the tectonic plates. We approximate complex phenomena such as plate subduction or collisions to deform the lithosphere, including the continental and oceanic crusts. The user can control the movement of the plates, which dynamically evolve and generate a variety of landforms such as continents, oceanic ridges, large scale mountain ranges or island arcs. Finally, we amplify the large-scale planet model with either procedurally-defined or real-world elevation data to synthesize coherent detailed reliefs. Our method allows the user to control the evolution of an entire planet interactively, and to trigger specific events such as catastrophic plate rifting.

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Journal articles

titre

Visualisation and personalisation of multi-representations city models

auteur

Jérémy Gaillard, Adrien Peytavie, Gilles Gesquière

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titre

Mumford-Shah Mesh Processing using the Ambrosio-Tortorelli Functional

auteur

Nicolas Bonneel, David Coeurjolly, Pierre Gueth, Jacques-Olivier Lachaud

resume

The Mumford-Shah functional approximates a function by a piecewise smooth function. Its versatility makes it ideal for tasks such as image segmentation or restoration, and it is now a widespread tool of image processing. Recent work has started to investigate its use for mesh segmentation and feature lines detection, but we take the stance that the power of this functional could reach far beyond these tasks and integrate the everyday mesh processing toolbox. In this paper, we discretize an Ambrosio-Tortorelli approximation via a Discrete Exterior Calculus formulation. We show that, combined with a new shape optimization routine, several mesh processing problems can be readily tackled within the same framework. In particular, we illustrate applications in mesh denoising, normal map embossing, mesh inpainting and mesh segmentation.

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titre

UrbanCo2Fab: Comprehension of Concurrent Viewpoints of Urban Fabric Based on Git

auteur

John Samuel Samuel, Sylvie Servigne, Gilles Gesquière

resume

The study of urban evolution requires representation and management of concurrent points of view using interoperable standards. An extension to CityGML has been recently proposed (Chaturvedi et al., 2017) to represent urban evolution. In this article, we further extend this work to represent different possible scenarios . The key characteristic of our approach is that it makes the most of existing technologies like version control system so that users can easily work with our proposed extension. Our approach also lets the users add and even modify information to versions and version transitions of scenarios in a given workspace.

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titre

Sparse Geometric Representation Through Local Shape Probing

auteur

Julie Digne, Sébastien Valette, Raphaëlle Chaine

resume

We propose a new shape analysis approach based on the non-local analysis of local shape variations. Our method relies on a novel description of shape variations, called Local Probing Field (LPF), which describes how a local probing operator transforms a pattern onto the shape. By carefully optimizing the position and orientation of each descriptor, we are able to capture shape similarities and gather them into a geometrically relevant dictionary over which the shape decomposes sparsely. This new representation permits to handle shapes with mixed intrinsic dimensionality (e.g. shapes containing both surfaces and curves) and to encode various shape features such as boundaries. Our shape representation has several potential applications; here we demonstrate its efficiency for shape resampling and point set denoising for both synthetic and real data. Index Terms—Shape similarity-Local shape descriptor-Point set denoising and resampling.

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titre

Sculpting Mountains: Interactive Terrain Modeling Based on Subsurface Geology

auteur

Guillaume Cordonnier, Marie-Paule Cani, Bedrich Benes, Jean Braun, Eric Galin

resume

Most mountain ranges are formed by the compression and folding of colliding tectonic plates. Subduction of one plate causes large-scale asymmetry while their layered composition (or stratigraphy) explains the multi-scale folded strata observed on real terrains. We introduce a novel interactive modeling technique to generate visually plausible, large scale terrains that capture these phenomena. Our method draws on both geological knowledge for consistency and on sculpting systems for user interaction. The user is provided hands-on control on the shape and motion of tectonic plates, represented using a new geologically-inspired model for the Earth crust. The model captures their volume preserving and complex folding behaviors under collision, causing mountains to grow. It generates a volumetric uplift map representing the growth rate of subsurface layers. Erosion and uplift movement are jointly simulated to generate the terrain. The stratigraphy allows us to render folded strata on eroded cliffs. We validated the usability of our sculpting interface through a user study, and compare the visual consistency of the earth crust model with geological simulation results and real terrains.

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titre

Wavejets: A Local Frequency Framework for Shape Details Amplification

auteur

Yohann Béarzi, Julie Digne, Raphaëlle Chaine

resume

Detail enhancement is a well-studied area of 3D rendering and image processing, which has few equivalents for 3D shape processing. To enhance details, one needs an efficient analysis tool to express the local surface dynamics. We introduce Wavejets, a new function basis for locally decomposing a shape expressed over the local tangent plane, by considering both angular oscillations of the surface around each point and a radial polynomial. We link the Wavejets coefficients to surface derivatives and give theoretical guarantees for their precision and stability with respect to an approximate tangent plane. The coefficients can be used for shape details amplification, to enhance, invert or distort them, by operating either on the surface point positions or on the normals. From a practical point of view, we derive an efficient way of estimating Wavejets on point sets and demonstrate experimentally the amplification results with respect to noise or basis truncation.

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titre

Procedural Cloudscapes

auteur

Antoine Webanck, Yann Cortial, Eric Guérin, Eric Galin

resume

We present a phenomenological approach for modeling and animating cloudscapes. We propose a compact procedural model for representing the different types of cloud over a range of altitudes. We define primitive-based field functions that allow the user to control and author the cloud cover over large distances easily. Our approach allows us to animate cloudscapes by morphing: instead of simulating the evolution of clouds using a physically-based simulation, we compute the movement of clouds using key-frame interpolation and tackle the morphing problem as an Optimal Transport problem. The trajectories of the cloud cover primitives are generated by solving an Anisotropic Shortest Path problem with a cost function that takes into account the elevation of the terrain and the parameters of the wind field.

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titre

Interactive Generation of Time-evolving, Snow-Covered Landscapes with Avalanches

auteur

Guillaume Cordonnier, Pierre Ecormier, Eric Galin, James Gain, Bedrich Benes, Marie-Paule Cani

resume

We introduce a novel method for interactive generation of visually consistent, snow-covered landscapes and provide control of their dynamic evolution over time. Our main contribution is the real-time phenomenological simulation of avalanches and other user-guided events, such as tracks left by Nordic skiing, which can be applied to interactively sculpt the landscape. The terrain is modeled as a height field with additional layers for stable, compacted, unstable, and powdery snow, which behave in combination as a semi-viscous fluid. We incorporate the impact of several phenomena, including sunlight, temperature, prevailing wind direction, and skiing activities. The snow evolution includes snow-melt and snowdrift, which affect stability of the snow mass and the probability of avalanches. A user can shape landscapes and their evolution either with a variety of interactive brushes, or by prescribing events along a winter season time-line. Our optimized GPU-implementation allows interactive updates of snow type and depth across a large (10 × 10 km) terrain, including real-time avalanches, making this suitable for visual assets in computer games. We evaluate our method through perceptual comparison against exiting methods and real snow-depth data.

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titre

Super-resolution of Point Set Surfaces using Local Similarities

auteur

Azzouz Hamdi-Cherif, Julie Digne, Raphaëlle Chaine

resume

3D scanners provide a virtual representation of object surfaces at some given precision that depends on many factors such as the object material, the quality of the laser-ray or the resolution of the camera. This precision may even vary over the surface, depending for example on the distance to the scanner which results in uneven and unstructured point sets, with an uncertainty on the coordinates. To enhance the quality of the scanner output, one usually resorts to local surface interpolation between measured points. However, object surfaces often exhibit interesting statistical features such as repetitive geometric textures. Building on this property, we propose a new approach for surface super-resolution that detects repetitive patterns or self-similarities and exploits them to improve the scan resolution by aggregating scattered measures. In contrast with other surface super-resolution methods, our algorithm has two important advantages. First, when handling multiple scans, it does not rely on surface registration. Second, it is able to produce super-resolution from even a single scan. These features are made possible by a new local shape description able to capture differential properties of order above 2. By comparing those descriptors, similarities are detected and used to generate a high-resolution surface. Our results show a clear resolution gain over state-of-the-art interpolation methods.

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Journal articles

titre

Authoring Landscapes by Combining Ecosystem and Terrain Erosion Simulation

auteur

Guillaume Cordonnier, Eric Galin, James Gain, Bedrich Benes, Eric Guérin, Adrien Peytavie, Marie-Paule Cani

resume

We introduce a novel framework for interactive landscape authoring that supports bi-directional feedback between erosion and vegetation simulation. Vegetation and terrain erosion have strong mutual impact and their interplay innuences the overall realism of virtual scenes. Despite their importance, these complex interactions have been neglected in computer graphics. Our framework overcomes this by simulating the eeect of a variety of geomor-phological agents and the mutual interaction between diierent material and vegetation layers, including rock, sand, humus, grass, shrubs, and trees. Users are able to exploit these interactions with an authoring interface that consistently shapes the terrain and populates it with details. Our method, validated through side-by-side comparison with real terrains, can be used not only to generate realistic static landscapes, but also to follow the temporal evolution of a landscape over a few centuries.

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titre

Le transport optimal pour des applications en informatique graphique

auteur

Nicolas Bonneel

resume

Le transport optimal est une théorie mathématique qui permet de comparer deux histogrammes. Ceci dit, pourquoi s'intéresser aux histogrammes ?

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titre

Consistent Video Filtering for Camera Arrays

auteur

Nicolas Bonneel, James Tompkin, Deqing Sun, Oliver Wang, Kalyan Sunkavalli, Sylvain Paris, Hanspeter Pfister

resume

Visual formats have advanced beyond single-view images and videos: 3D movies are commonplace, researchers have developed multi-view navigation systems, and VR is helping to push light field cameras to mass market. However, editing tools for these media are still nascent, and even simple filtering operations like color correction or stylization are problematic: naively applying image filters per frame or per view rarely produces satisfying results due to time and space inconsistencies. Our method preserves and stabilizes filter effects while being agnostic to the inner working of the filter. It captures filter effects in the gradient domain, then uses \emph{input} frame gradients as a reference to impose temporal and spatial consistency. Our least-squares formulation adds minimal overhead compared to naive data processing. Further, when filter cost is high, we introduce a filter transfer strategy that reduces the number of per-frame filtering computations by an order of magnitude, with only a small reduction in visual quality. We demonstrate our algorithm on several camera array formats including stereo videos, light fields, and wide baselines.

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titre

Intrinsic Decompositions for Image Editing

auteur

Nicolas Bonneel, Balazs Kovacs, Sylvain Paris, Kavita Bala

resume

Intrinsic images are a mid-level representation of an image that decompose the image into reflectance and illumination layers. The reflectance layer captures the color/texture of surfaces in the scene, while the illumination layer captures shading effects caused by interactions between scene illumination and surface geometry. Intrinsic images have a long history in computer vision and recently in computer graphics, and have been shown to be a useful representation for tasks ranging from scene understanding and reconstruction to image editing. In this report, we review and evaluate past work on this problem. Specifically, we discuss each work in terms of the priors they impose on the intrinsic image problem. We introduce a new synthetic ground-truth dataset that we use to evaluate the validity of these priors and the performance of the methods. Finally, we evaluate the performance of the different methods in the context of image-editing applications.

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titre

Fine scale image registration in large-scale urban LIDAR point sets

auteur

Maximilien Guislain, Julie Digne, Raphaëlle Chaine, Gilles Monnier

resume

Urban scenes acquisition is very often performed using laser scanners onboard a vehicle. In parallel, color information is also acquired through a set of coarsely aligned camera pictures. The question of combining both measures naturally arises for adding color to the 3D points or enhancing the geometry, but it faces important challenges. Indeed, 3D geometry acquisition is highly accurate while the images suffer from distortion and are only coarsely registered to the geometry. In this paper, we introduce a two-step method to register images to large-scale complex point clouds. Our method performs the image-to-geometry registration by iteratively registering the real image to a synthetic image obtained from the estimated camera pose and the point cloud, using either reflectance or normal information. First a coarse registration is performed by generating a wide-angle synthetic image and considering that small pitch and yaw rotations can be estimated as translations in the image plane. Then a fine registration is performed using a new image metric which is adapted to the difference of modality between the real and synthetic images. This new image metric is more resilient to missing data and large transformations than standard Mutual Information. In the process, we also introduce a method to generate synthetic images from a 3D point cloud that is adapted to large-scale urban scenes with occlusions and sparse areas. The efficiency of our algorithm is demonstrated both qualitatively and quantitatively on datasets of urban scans and associated images.

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titre

Interactive Example-Based Terrain Authoring with Conditional Generative Adversarial Networks

auteur

Eric Guérin, Julie Digne, Eric Galin, Adrien Peytavie, Christian Wolf, Bedrich Benes, Benoît Martinez

resume

Authoring virtual terrains presents a challenge and there is a strong need for authoring tools able to create realistic terrains with simple user-inputs and with high user control. We propose an example-based authoring pipeline that uses a set of terrain synthesizers dedicated to specific tasks. Each terrain synthesizer is a Conditional Generative Adversarial Network trained by using real-world terrains and their sketched counterparts. The training sets are built automatically with a view that the terrain synthesizers learn the generation from features that are easy to sketch. During the authoring process, the artist first creates a rough sketch of the main terrain features, such as rivers, valleys and ridges, and the algorithm automatically synthesizes a terrain corresponding to the sketch using the learned features of the training samples. Moreover, an erosion synthesizer can also generate terrain evolution by erosion at a very low computational cost. Our framework allows for an easy terrain authoring and provides a high level of realism for a minimum sketch cost. We show various examples of terrain synthesis created by experienced as well as inexperienced users who are able to design a vast variety of complex terrains in a very short time.

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titre

Reconstructing 3D Building Models with the 2D Cadastre for Semantic Enhancement

auteur

Frédéric Pedrinis, Gilles Gesquière

resume

Virtual city models are increasingly used in urban land management processes, which involve the use of different sources of spatial information. This heterogeneous data is, however, often complementary and it may be necessary to give the possibility to join information provided by different sources. This paper presents a method to enhance 3D buildings by using usual 2D vectorial polygon database. These polygons may represent districts, building footprints, or any seg-mentation of the urban area that adds information to the city model. The enhancement consists in using this polygon database to split the 3D buildings into a set of city objects where each element possesses a 3D geometry and the semantic information of the polygon it is linked to. In this paper, for an illustration purpose, we will present how to create this link between 3D buildings and the cadastre map, in order to create a set of semantically rich 3D building models.

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titre

The Bilateral Filter for Point Clouds

auteur

Julie Digne, Carlo de Franchis

resume

Point sets obtained by 3D scanners are often corrupted with noise, that can have several causes, such as a tangential acquisition direction, changing environmental lights or a reflective object material. It is thus crucial to design efficient tools to remove noise from the acquired data without removing important information such as sharp edges or shape details. To do so, Fleish-man et al. introduced a bilateral filter for meshes adapted from the bilateral filter for gray level images. This anisotropic filter denoises a point with respect to its neighbors by considering not only the distance from the neighbors to the point but also the distance along a normal direction. This simple fact allows for a much better preservation of sharp edges. In this paper, we analyze a parallel implementation of the bilateral filter adapted for point clouds. Source Code The ANSI C++ source code permitting to reproduce results from the on-line demo is available on the web page of the article 1 .

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titre

Coherent multi-layer landscape synthesis

auteur

Oscar Argudo, Carlos Andujar, Antonio Chica, Eric Guérin, Julie Digne, Adrien Peytavie, Eric Galin

resume

We present an efficient method for generating coherent multi-layer landscapes. We use a dictionary built from exemplars to synthesize high-resolution fully featured terrains from input low-resolution elevation data. Our example-based method consists in analyzing real-world terrain examples and learning the procedural rules directly from these inputs. We take into account not only the elevation of the terrain, but also additional layers such as the slope, orientation, drainage area, the density and distribution of vegetation, and the soil type. By increasing the variety of terrain exemplars, our method allows the user to synthesize and control different types of landscapes and biomes, such as temperate or rain forests, arid deserts and mountains.

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Journal articles

titre

Integration of Multiple Heterogeneous and Autonomous Web Services using Mediation Approach: Open Challenges

auteur

John Samuel Samuel, Christophe Rey

resume

Regular users and enterprises are now increasingly dependent on web services. This growing dependence on one hand has simplified routine tasks, but on the other hand it has resulted in loss of direct control over the data. Nevertheless, both users and enterprises require simplified and generic solutions to access their data. The classical mediation approach from the data integration field provides a uniform query interface to diverse data sources hiding the underlying heterogeneity. But using this approach over multiple heterogeneous and autonomous web services has several open challenges. In this article, we will take a look at some of these challenges that need to be addressed for achieving a fully automated solution.

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titre

Efficient modeling of entangled details for natural scenes

auteur

Eric Guérin, Eric Galin, François Grosbellet, Adrien Peytavie, Jean-David Genevaux

resume

Digital landscape realism often comes from the multitude of details that are hard to model such as fallen leaves, rock piles or entangled fallen branches. In this article, we present a method for augmenting natural scenes with a huge amount of details such as grass tufts, stones, leaves or twigs. Our approach takes advantage of the observation that those details can be approximated by replications of a few similar objects and therefore relies on mass-instancing. We propose an original structure, the Ghost Tile, that stores a huge number of overlapping candidate objects in a tile, along with a pre-computed collision graph. Details are created by traversing the scene with the Ghost Tile and generating instances according to user-defined density fields that allow to sculpt layers and piles of entangled objects while providing control over their density and distribution.

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titre

Heterogeneous Sensor Data Exploration and Sustainable Declarative Monitoring Architecture: Application to Smart Building

auteur

Sylvie Servigne, Yann Gripay, Ozgun Pinarer, John Samuel Samuel, Atay Ozgovde, Jacques Jay

resume

Concerning energy consumption and monitoring architectures, our goal is to develop a sustainable declarative monitoring architecture for lower energy consumption taking into account the monitoring system itself. Our second is to develop theoretical and practical tools to model, explore and exploit heterogeneous data from various sources in order to understand a phenomenon like energy consumption of smart building vs inhabitants' social behaviours. We focus on a generic model for data acquisition campaigns based on the concept of generic sensor. The concept of generic sensor is centered on acquired data and on their inherent multi- dimensional structure, to support complex domain-specific or field-oriented analysis processes. We consider that a methodological breakthrough may pave the way to deep understanding of voluminous and heterogeneous scientific data sets. Our use case concerns energy efficiency of buildings to understand relationship between physical phenomena and user behaviors. The aim of this paper is to give a presentation of our methodology and results concerning architecture and user-centric tools.

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titre

Volumetric Spot Noise for Procedural 3D Shell Texture Synthesis

auteur

Nicolas Pavie, Guillaume Gilet, Jean-Michel Dischler, Eric Galin, Djamchid Ghazanfarpour

resume

In this paper, we present an extension of the Locally Controlled Spot Noise and a visualization pipeline for volumetric fuzzy details synthesis. We extend the noise model to author volumetric fuzzy details using filtered 3D quadratic kernel functions convolved with a projective non-uniform 2D distribution of impulses. We propose a new method based on order independent splatting to compute a fast view dependent approximation of shell noise at interactive rates. Our method outperforms ray marching techniques and avoids aliasing artifacts, thus improving interactive content authoring feedback. Moreover, generated surface details share the same properties as procedural noise: they extend on potentially infinite surfaces, are defined in an extremely compact way, are non-repetitive, continuous (no discrete voxel-artifacts when zooming) and independent of the definition of the underlying surface (no surface parameterization is required).

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titre

Large Scale Terrain Generation from Tectonic Uplift and Fluvial Erosion

auteur

Guillaume Cordonnier, Jean Braun, Marie-Paule Cani, Bedrich Benes, Eric Galin, Adrien Peytavie, Eric Guérin

resume

At large scale, landscapes result from the combination of two major processes: tectonics which generate the main relief through crust uplift, and weather which accounts for erosion. This paper presents the first method in computer graphics that combines uplift and hydraulic erosion to generate visually plausible terrains. Given a user-painted uplift map, we generate a stream graph over the entire domain embedding elevation information and stream flow. Our approach relies on the stream power equation introduced in geology for hydraulic erosion. By combining crust uplift and stream power erosion we generate large realistic terrains at a low computational cost. Finally, we convert this graph into a digital elevation model by blending landform feature kernels whose parameters are derived from the information in the graph. Our method gives high-level control over the large scale dendritic structures of the resulting river networks, watersheds, and mountains ridges.

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titre

Sparse representation of terrains for procedural modeling

auteur

Eric Guérin, Julie Digne, Eric Galin, Adrien Peytavie

resume

In this paper, we present a simple and efficient method to represent terrains as elevation functions built from linear combinations of landform features (atoms). These features can be extracted either from real world data-sets or procedural primitives, or from any combination of multiple terrain models. Our approach consists in representing the elevation function as a sparse combination of primitives, a concept which we call Sparse Construction Tree, which blends the different landform features stored in a dictionary. The sparse representation allows us to represent complex terrains using combinations of atoms from a small dictionary, yielding a powerful and compact terrain representation and synthesis tool. Moreover, we present a method for automatically learning the dictionary and generating the Sparse Construction Tree model. We demonstrate the efficiency of our method in several applications: inverse procedural modeling of terrains, terrain amplification and synthesis from a coarse sketch.

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titre

Wasserstein Barycentric Coordinates: Histogram Regression Using Optimal Transport

auteur

Nicolas Bonneel, Gabriel Peyré, Marco Cuturi

resume

This article defines a new way to perform intuitive and geometrically faithful regressions on histogram-valued data. It leverages the theory of optimal transport, and in particular the definition of Wasserstein barycenters, to introduce for the first time the notion of barycentric coordinates for histograms. These coordinates take into account the underlying geometry of the ground space on which the histograms are defined, and are thus particularly meaningful for applications in graphics to shapes, color or material modification. Beside this abstract construction, we propose a fast numerical optimization scheme to solve this backward problem (finding the barycentric coordinates of a given histogram) with a low computational overhead with respect to the forward problem (computing the barycenter). This scheme relies on a backward algorithmic differentiation of the Sinkhorn algorithm which is used to optimize the entropic regularization of Wasserstein barycenters. We showcase an illustrative set of applications of these Wasserstein coordinates to various problems in computer graphics: shape approximation, BRDF acquisition and color editing.

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titre

Environmental Objects for Authoring Procedural Scenes

auteur

François Grosbellet, Adrien Peytavie, Eric Guérin, Eric Galin, Stéphane Mérillou, Bedrich Benes

resume

We propose a novel approach for authoring large scenes with automatic enhancement of objects to create geometric decoration details such as snow cover, icicles, fallen leaves, grass tufts or even trash. We introduce environmental objects that extend an input object geometry with a set of procedural effects that defines how the object reacts to the environment, and by a set of scalar fields that defines the influence of the object over of the environment. The user controls the scene by modifying environmental variables, such as temperature or humidity fields. The scene definition is hierarchical: objects can be grouped and their behaviours can be set at each level of the hierarchy. Our per object definition allows us to optimize and accelerate the effects computation, which also enables us to generate large scenes with many geometric details at a very high level of detail. In our implementation, a complex urban scene of 10 000 m², represented with details of less than 1 cm, can be locally modified and entirely regenerated in a few seconds.

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titre

Continuous semantic description of 3D meshes

auteur

Vincent Léon, Nicolas Bonneel, Guillaume Lavoué, Jean-Philippe Vandeborre

resume

We propose a novel high-level signature for continuous semantic description of 3D shapes. Given an approximately segmented and labeled 3D mesh, our descriptor consists of a set of geodesic distances to the different semantic labels. This local multidimensional signature effectively captures both the semantic information (and relationships between labels) and the underlying geometry and topology of the shape. We illustrate its benefits on two applications: automatic semantic labeling, seen as an inverse problem along with supervised-learning, and semantic-aware shape editing for which the isocurves of our harmonic description are particularly relevant .

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titre

Temporally Coherent Sculpture of Composite Objects

auteur

Artur P. Sampaio, Raphaëlle Chaine, Creto A.Vidal, Joaquim B. Cavalcante-Neto

resume

We address the problem of virtual sculpting and deformation of shapes composed of small, randomly placed objects. Objects may be tightly packed-such as pebbles, pills, seeds and grains, or be sparsely distributed with an overarching shape-such as flocks of birds or schools of fish. Virtual sculpture has rapidly become a standard in the entertainment industry. Composites, though, are still usually created in a static way by individually placing each object or by sculpting a support surface and procedurally populating the final shape. That raises problems for the generalisation to evolving shapes with visual continuity of the components. Large amounts of geometrical data are generated, and must be maintained and processed, both by the CPU and by the GPU. Whenever the shape is deformed, one has to define how these compositing objects should turn, displace or disappear inside the volume, as well as how new instances should become visible to the outside. It is difficult to rely on a physical system to perform that task in real time. The system we suggest can be constructed upon any uniform mesh-based representation that can be deformed and whose connectivity can be updated by operations such as edge splits, collapses, and flips. The mesh remains populated with an aperiodic distribution of composing elements that are automatically updated under deformation. The idea is to sculpt the shape as if it were filled with little objects, without handling the complexity of manipulating volumetric shapes. For this purpose, we suggest exploiting the properties of the uniform sampling of the surface. We show that we are able to properly handle virtual sculpting of composites in real-time and maintaining temporal continuity. This system also uses GPU optimisations to render individual elements efficiently. To our knowledge, no previous sculpting system allows the user to simultaneously see and sculpt agglomerates in such a fast and reliable fashion.

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Journal articles

titre

An Implementation and Parallelization of the Scale-Space Meshing Algorithm

auteur

Julie J. Digne

resume

Creating an interpolating mesh from an unorganized set of oriented points is a difficult problem which is often overlooked. Most methods focus indeed on building a watertight smoothed mesh by defining some function whose zero level set is the surface of the object. However in some cases it is crucial to build a mesh that interpolates the points and does not fill the acquisition holes: either because the data are sparse and trying to fill the holes would create spurious artifacts or because the goal is to explore visually the data exactly as they were acquired without any smoothing process. In this paper we detail a parallel implementation of the Scale-Space Meshing algorithm, which builds on the scale-space framework for reconstructing a high precision mesh from an input oriented point set. This algorithm first smoothes the point set, producing a singularity free shape. It then uses a standard mesh reconstruction technique, the Ball Pivoting Algorithm, to build a mesh from the smoothed point set. The final step consists in back-projecting the mesh built on the smoothed positions onto the original point set. The result of this process is an interpolating, hole-preserving surface mesh reconstruction. Source Code The ANSI C++ source code permitting to reproduce results from the on-line demo is available at the IPOL web page of this article 1. The scale-space meshing algorithm uses the Ball Pivoting Algorithm which is linked to patent US6968299B1. It is made available for the exclusive aim of serving as a scientific tool to verify the soundness and completeness of the algorithm description.

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titre

Blind video temporal consistency

auteur

Nicolas Bonneel, James Tompkin, Kalyan Sunkavalli, Deqing Sun, Sylvain Paris, Hanspeter Pfister

resume

Extending image processing techniques to videos is a non-trivial task; applying processing independently to each video frame often leads to temporal inconsistencies, and explicitly encoding temporal consistency requires algorithmic changes. We describe a more general approach to temporal consistency. We propose a gradient-domain technique that is blind to the particular image processing algorithm. Our technique takes a series of processed frames that suffers from flickering and generates a temporally-consistent video sequence. The core of our solution is to infer the temporal regularity from the original unprocessed video, and use it as a temporal consistency guide to stabilize the processed sequence. We formally characterize the frequency properties of our technique, and demonstrate , in practice, its ability to stabilize a wide range of popular image processing techniques including enhancement and stylization of color and tone, intrinsic images, and depth estimation.

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titre

CGALmesh: a Generic Framework for Delaunay Mesh Generation

auteur

Clément Jamin, Pierre Alliez, Mariette Yvinec, Jean-Daniel Boissonnat

resume

CGALmesh is the mesh generation software package of the Computational Geometry Algorithm Library (CGAL). It generates isotropic simplicial meshes -- surface triangular meshes or volume tetrahedral meshes -- from input surfaces, 3D domains as well as 3D multi-domains, with or without sharp features. The underlying meshing algorithm relies on restricted Delaunay triangulations to approximate domains and surfaces, and on Delaunay refinement to ensure both approximation accuracy and mesh quality. CGALmesh provides guarantees on approximation quality as well as on the size and shape of the mesh elements. It provides four optional mesh optimization algorithms to further improve the mesh quality. A distinctive property of CGALmesh is its high flexibility with respect to the input domain representation. Such a flexibility is achieved through a careful software design, gathering into a single abstract concept, denoted by the oracle, all required interface features between the meshing engine and the input domain. We already provide oracles for domains defined by polyhedral and implicit surfaces.

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titre

Terrain Modelling from Feature Primitives

auteur

Jean-David Génevaux, Eric Galin, Adrien Peytavie, Eric Guérin, Cyril Briquet, François Grosbellet, Bedrich Benes

resume

We introduce a compact hierarchical procedural model that combines feature-based primitives to describe complex terrains with varying level of detail. Our model is inspired by skeletal implicit surfaces and defines the terrain elevation function by using a construction tree. Leaves represent terrain features and they are generic parametrized skeletal primitives, such as mountains, ridges, valleys, rivers, lakes or roads. Inner nodes combine the leaves and subtrees by carving, blending or warping operators. The elevation of the terrain at a given point is evaluated by traversing the tree and by combining the contributions of the primitives. The definition of the tree leaves and operators guarantees that the resulting elevation function is Lipschitz, which speeds up the sphere tracing used to render the terrain. Our model is compact and allows for the creation of large terrains with a high level o detail using a reduced set of primitives. We show the creation of different kinds of landscapes and demonstrate that our model allows to efficiently control the shape and distribution of landform features.

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titre

Sliced and Radon Wasserstein Barycenters of Measures

auteur

Nicolas Bonneel, Julien Rabin, Gabriel Peyré, Hanspeter Pfister

resume

This article details two approaches to compute barycenters of measures using 1-D Wasserstein distances along radial projections of the input measures. The first me- thod makes use of the Radon transform of the measures, and the second is the solution of a convex optimization problem over the space of measures. We show several properties of these barycenters and explain their relationship. We show numerical approximation schemes based on a discrete Radon transform and on the resolution of a non-convex optimization problem. We explore the respective merits and drawbacks of each approach on applications to two image processing problems: color transfer and texture mixing.

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titre

Change Detection of Cities

auteur

Frédéric Pedrinis, Maxime Morel, Gilles Gesquière

resume

Today, many cities have at their disposal a digital model useful in many applications such as decision making in urban planning. 3D data representing objects in the city such as land and buildings often comes from successive acquisition campaigns. Unfortunately, digital models of cities can have many versions of the same area. Having tools to detect changes becomes a necessity. It is then possible to highlight any differences between multiple versions of the same area in 3D. A second application can be related to the possibility of making a temporal representation by taking into account the detected changes. In this paper, we propose a set of tools to detect changes. The use case is done on buildings. Our method is based on CityGML and cadastre files. The output is a CityGML file containing a representation of the evolution over time of the objects in the city.

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Journal articles

titre

Interactive intrinsic video editing

auteur

Nicolas Bonneel, Kalyan Sunkavalli, James Tompkin, Deqing Sun, Sylvain Paris, Hanspeter Pfister

resume

Separating a photograph into its reflectance and illumination intrinsic images is a fundamentally ambiguous problem, and state-of-the-art algorithms combine sophisticated reflectance and illumination priors with user annotations to create plausible results. However, these algorithms cannot be easily extended to videos for two reasons: first, naively applying algorithms designed for single images to videos produce results that are temporally incoherent; second, effectively specifying user annotations for a video requires interactive feedback , and current approaches are orders of magnitudes too slow to support this. We introduce a fast and temporally consistent algorithm to decompose video sequences into their reflectance and illumination components. Our algorithm uses a hybrid formulation that separates image gradients into smooth illumination and sparse reflectance gradients using look-up tables. We use a multi-scale parallelized solver to reconstruct the reflectance and illumination from these gradients while enforcing spatial and temporal reflectance constraints and user annotations. We demonstrate that our algorithm automatically produces reasonable results, that can be interactively refined by users, at rates that are two orders of magnitude faster than existing tools, to produce high-quality decompositions for challenging real-world video sequences. We also show how these decompositions can be used for a number of video editing applications including recoloring, retexturing, illumination editing, and lighting-aware compositing.

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titre

Semantic Interoperability of Knowledge in Feature-based CAD Models

auteur

Samer Abdul Ghafour, Parisa Ghodous, Behzad Shariat, Eliane Perna, Farzad Khosrowshahi

resume

A major issue in product development is the exchange and sharing of product knowledge among many actors. This knowledge includes many concepts such as design history, component structure, features, parameters, constraints, and more. Regarding CAD models, most of the current CAD systems provide feature-based design for the construction of solid models and to carry, semantically, product information throughout its life cycle. Unfortunately, existing solutions and standards, such as STEP, for exchanging product information, are limited to the process of geometrical data, where semantics assigned to product model are completely lost during the translation process. Moreover, STEP does not provide a sound basis to reason with knowledge. The work described in this paper is part of our approach based on the development of OWL ontologies to preserve semantics associated with product data. In this work, we will focus on the semantic integration of these ontologies by defining axioms and rules. The integration process relies basically on reasoning capabilities provided by description logics in order to recognize automatically additional mappings among ontologies entities. Furthermore, the mapping process is enhanced with a semantic similarity measure to detect similar design features. Similarity measure integrates all aspects of OWL DL language. Thus, similarity functions are defined for each type of entity to involve all the features that make its definition. However, this will enable data analysis, as well as manage and discover implicit relationships among product data based on semantic modeling and reasoning.

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titre

Time-Lapse Photometric Stereo and Applications

auteur

Fangyang Shen, Kalyan Sunkavalli, Nicolas Bonneel, Szymon Rusinkiewicz, Hanspeter Pfister, Xin Tong

resume

This paper presents a technique to recover geometry from time-lapse sequences of outdoor scenes. We build upon photometric stereo techniques to recover approximate shadowing, shading and normal components allowing us to alter the material and normals of the scene. Previous work in analyzing such images has faced two fundamental difficulties: 1. the illumination in outdoor images consists of time-varying sunlight and skylight, and 2. the motion of the sun is restricted to a near-planar arc through the sky, making surface normal recovery unstable. We develop methods to estimate the reflection component due to skylight illumination. We also show that sunlight directions are usually non-planar, thus making surface normal recovery possible. This allows us to estimate approximate surface normals for outdoor scenes using a single day of data. We demonstrate the use of these surface normals for a number of image editing applications including reflectance, lighting, and normal editing.

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titre

An analysis and implementation of a parallel ball pivoting algorithm

auteur

Julie Digne

resume

The problem of surface reconstruction from a set of 3D points given by their coordinates and oriented normals is a difficult problem, which has been tackled with many different approaches. In 1999, Bernardini and colleagues [BMR+ 99] introduced a very elegant and efficient reconstruction method that uses a ball pivoting around triangle edges and adds new triangles if the ball is incident to three points and contains no other points. This paper details an implementation and parallelization of this algorithm.

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titre

Numerical analysis of differential operators on raw point clouds

auteur

Julie J. Digne, Jean-Michel Morel

resume

3D acquisition devices acquire object surfaces with growing accuracy by obtaining 3D point samples of the surface. This sampling depends on the geometry of the device and of the scanned object and is therefore very irregular. Many numerical schemes have been proposed for applying PDEs to regularly meshed 3D data. Nevertheless, for high precision applications it remains necessary to compute differential operators on raw point clouds prior to any meshing. Indeed differential operators such as the mean curvature or the principal curvatures provide crucial information for the orientation and meshing process itself. This paper reviews a half dozen local schemes which have been proposed to compute discrete curvature-like shape indicators on raw point clouds. All of them will be analyzed mathematically in a unified framework by computing their asymptotic form when the size of the neighborhood tends to zero. They are given in terms of the principal curvatures or of higher order intrinsic differential operators which, in return, characterize the discrete operators. All considered local schemes are of two kinds: either they perform a polynomial local regression, or they compute directly local moments. But the polynomial regression of order 1 is demonstrated to play a special role, because its iterations yield a scale space. This analysis is completed with numerical experiments comparing the accuracies of these schemes. We demonstrate that this accuracy is enhanced for all operators by applying previously the scale space.

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titre

Auto-similarité pour la compression haute précision des surfaces échantillonnées.

auteur

Julie Digne, Raphaëlle Chaine, Sébastien Valette

resume

La plupart des objets qui nous entourent sont caractérisés par une forte auto-similarité de leur surface, qui diffère suivant qu’il s’agisse d’une œuvre d’art en pierre ou en bronze ou bien d’un objet mécanique. Cette propriété est liée à l’essence même des formes naturelles dont l’homme s’inspire, mais aussi à leur processus de fabrication, comme la régularité du geste du sculpteur ou de la machine-outil, ou bien les mécanismes d’érosion. Dans cet article, nous nous proposons d’exploiter l’auto-similarité sous-jacente à chaque forme, pour comprimer les surfaces échantillonnées avec une haute précision, pouvant produire des nuages de plusieurs millions de points. Notre approche rééchantillonne la forme de manière à mettre en évidence l’auto-similarité de sa surface, mais demeure respectueuse de la forme originale et de la précision du scanner. L’auto-similarité peut alors être utilisée pour créer un dictionnaire adéquat, sur lequel les voisinages locaux seront représentés de manière parcimonieuse, offrant ainsi la perspective d’une représentation économique de la surface globale. Nous illustrons la validité de notre approche sur plusieurs nuages de points issus de la numérisation de statues ou d’objets mécaniques, mais aussi de scènes urbaines. De plus, nous montrons que notre approche permet d’obtenir un filtrage du bruit dont l’amplitude est plus petite que la précision du scanner. Cette communication est une traduction de [Digne, Chaine, Valette, Eurographics 2014]

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titre

Self-similarity for accurate compression of point sampled surfaces

auteur

Julie Digne, Raphaëlle Chaine, Sébastien Valette

resume

Most surfaces, be it from a fine-art artifact or a mechanical object, are characterized by a strong self-similarity. This property finds its source in the natural structures of objects but also in the fabrication processes: regularity of the sculpting technique, or machine tool. In this paper, we propose to exploit the self-similarity of the underlying shapes for compressing point cloud surfaces which can contain millions of points at a very high precision. Our approach locally resamples the point cloud in order to highlight the self-similarity of the shape, while remaining consistent with the original shape and the scanner precision. It then uses this self-similarity to create an ad hoc dictionary on which the local neighborhoods will be sparsely represented, thus allowing for a light-weight representation of the total surface. We demonstrate the validity of our approach on several point clouds from fine-arts and mechanical objects, as well as a urban scene. In addition, we show that our approach also achieves a filtering of noise whose magnitude is smaller than the scanner precision.

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Journal articles

titre

A comprehensive process of reverse engineering from 3D meshes to CAD models

auteur

Roseline Bénière, Gilles Gesquière, François Le Breton, William Puech, Gérard Subsol

resume

In an industrial context, most manufactured objects are designed using CAD (Computer-Aided Design) software. For visualization, data exchange or manufacturing applications, the geometric model has to be discretized into a 3D mesh composed of a finite number of vertices and edges. However, the initial model may sometimes be lost or unavailable. In other cases, the 3D discrete representation may be modified, e.g. after numerical simulation, and no longer corresponds to the initial model. A retro-engineering method is then required to reconstruct a 3D continuous representation from the discrete one. In this paper, we present an automatic and comprehensive retro-engineering process dedicated mainly to 3D meshes obtained initially by mechanical object discretization. First, several improvements in automatic detection of geometric primitives from a 3D mesh are presented. Then a new formalism is introduced to define the topology of the object and compute the intersections between primitives. The proposed method is validated on 3D industrial meshes.

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titre

Sculpting multi-dimensional nested structures

auteur

Lucian Stanculescu, Raphaëlle Chaine, Marie-Paule Cani, Karan Singh

resume

Solid shape is typically segmented into surface regions to define the appearance and function of parts of the shape; these regions in turn use curve networks to represent boundaries and creases, and feature points to mark corners and other shape landmarks. Conceptual modeling requires these multi-dimensional nested structures to persist throughout the modeling process, an aspect not supported, up to now, in free-form sculpting systems. We present the first shape sculpting framework that preserves and controls the evolution of such nested shape features. We propose a range of geometric and topological behaviors (such as rigidity or mutability) applied hierarchically to points, curves or surfaces in response to a set of typical free-form sculpting operations, such as stretch, shrink, split or merge. Our method is illustrated within a free-form sculpting system for self-adaptive quasi-uniform polygon meshes, where geometric and topology changes resulting from sculpting operations are applied to points, edges and triangular facets. We thus facilitate, for example, the persistence of sharp features that automatically split or merge with variable rigidity, even when the shape changes genus. Sculpting nested structures expands the capabilities of most conceptual design workflows, as exhibited by a suite of models created by our system.

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titre

Terrain Generation Using Procedural Models Based on Hydrology

auteur

Jean-David Genevaux, Eric Galin, Eric Guérin, Adrien Peytavie, Bedrich Benes

resume

We present a framework that allows quick and intuitive modeling of terrains using concepts inspired by hydrology. The terrain is generated from a simple initial sketch, controlled by a few parameters, and can be considered hydrographically realistic and visually plausible. Our terrain representation is both analytic and continuous, and can be rendered using varying level of detail. The terrain data is stored in a novel data structure: a construction tree whose internal nodes define a combination of operations, and whose leaves represent terrain features. The framework uses rivers as modeling elements and it first creates a hierarchical drainage network that is represented as a geometric graph over a given input domain. The network is then analyzed to construct watersheds and to characterize the different types and trajectories of rivers. The terrain is finally generated by combining procedural terrain and river patches with blending and carving operators.

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titre

Three-dimensional reconstruction using multiresolution photoclinometry by deformation

auteur

Claire Capanna, Gilles Gesquière, Laurent Jorda, Philippe Lamy, Didier Vibert

resume

We present a new photoclinometric reconstruction method based on the deformation of a 3D mesh. The optimization process of our method relies on a maximum likelihood estimation with a density function measuring discrepancies between observed images and the correspondingsynthetic images calculated from the progressively deformed 3D mesh. An input mesh is necessary and can be obtained from other methods or created by implementing a multiresolution scheme. We present a 3D shape model of an asteroid obtained by this method and compare it with the models obtained with two high-resolution 3D reconstruction techniques, stereophotogrammetry, and stereophotoclinometry.

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titre

Feature-Preserving Surface Reconstruction and Simplification from Defect-Laden Point Sets

auteur

Julie Digne, David Cohen-Steiner, Pierre Alliez, Fernando de Goes, Mathieu Desbrun

resume

We introduce a robust and feature-capturing surface reconstruction and simpli cation method that turns an input point set into a low triangle-count simplicial complex. Our approach starts with a (possibly non-manifold) simplicial complex ltered from a 3D Delaunay triangulation of the input points. This initial approximation is iteratively simpli ed based on an error metric that measures, through optimal transport, the distance between the input points and the current simplicial complex|both seen as mass distributions. Our approach is shown to exhibit both robustness to noise and outliers, as well as preservation of sharp features and boundaries. Our new feature-sensitive metric between point sets and triangle meshes can also be used as a post-processing tool that, from the smooth output of a reconstruction method, recovers sharp features and boundaries present in the initial point set.

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Journal articles

titre

3D Visualization of Urban Data Based on CityGML with WebGL

auteur

Gilles Gesquière, Alexis Manin

resume

Due to the advances in computer graphics and improved network speed it is now possible to navigate in 3D virtual world in real time. Until now, technologies employed require to install standalone application or plugins on navigators. The arrival of HTML 5 brings news solutions to visualize 3D data in a browser with WebGL. Several globe projects have proven that such technologies can be employed. Unfortunately, demonstrations are often based on proprietary formats to exchange or to store data. In this work, we propose to use CityGML: a standard provided by the Open Geospatial Consortium. CityGML files are imported in our Environment Editor. With several tools that we present in this paper, data are processed and stored. A client server application is also presented to permit the visualization of geometry and semantic in a navigator

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titre

Procedural Generation of Villages on Arbitrary Terrains

auteur

Arnaud Emilien, Adrien Bernhardt, Adrien Peytavie, Marie-Paule Cani, Eric Galin

resume

Although procedural modeling of cities has attracted a lot of attention for the past decade, populating arbitrary landscapes with non-urban settlements remains an open problem. In this work, we focus on the modeling of small, European villages that took benefit of terrain features to settle in safe, sunny or simply convenient places. We introduce a three step procedural generation method. First, an iterative process based on interest maps is used to progressively generate settlement seeds and the roads that connect them. The fact that a new road attracts settlers while a new house often leads to some extension of the road network is taken into account. Then, an anisotropic conquest method is introduced to segment the land into parcels around settlement seeds. Finally, we introduce open shape grammar to generate 3D geometry that adapts to the local slope. We demonstrate the effectiveness of our method by generating different kinds of villages on arbitrary terrains, from a mountain hamlet to a fisherman village, and validate through comparison with real data.

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titre

Reconstruction of small solar system bodies using photoclinometry by deformation

auteur

Claire Capanna, Philippe Lamy, Laurent Jorda, Gilles Gesquière

resume

This article tackles the problem of reconstructing the 3D shape model of asteroids and cometary nuclei from images obtained with a visible imaging system aboard a planetary spacecraft. We describe a photoclinometry method based on the optimization of the chi-square difference between observed and synthetic images of the object by deformations of its initial shape, described here as a mesh of triangular facets. The non-linear optimization is performed using the so-called “limited-memory Broyden-Fletcher-Golbfarb-Shanno” algorithm. The deformations can be applied: (i) by modifying the coefficients of a spherical harmonics expansion in order to extract the global shape of the object, and/or (ii) by moving the height of the vertices of a triangular mesh in order to increase the accuracy of the global shape model and/or to derive local topographic maps of the surface. This method has been tested on images of the asteroids Steins and Lutetia obtained by the imaging system aboard the Rosetta spacecraft of the European Space Agency.

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titre

La modélisation de la ville : Interopérabilité et intégration des données et modèles urbains : standards, normes et tendances

auteur

Emmanuel Devys, Gilles Gesquière

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La convergence des besoins relatifs à la modélisation urbaine entre les besoins utilisateurs d’une part, et ceux de l’ingénierie, de la maîtrise d’ouvrage et de la maîtrise d’oeuvre, ainsi que du monde de la recherche, est un gage de succès de projets urbains durables et pérennes. Ce besoin de convergence conduit en particulier à traiter de manière adaptée la problématique d’interopérabilité et d’intégration des données et modèles urbains. Cet état de l’art a pour objectif de donner un aperçu des standards et normes utilisables pour de tels modèles urbains (2D et 3D), notamment à l’ISO, l’Open Geospatial Consortium (OGC) et au Web3D Consortium, des tendances et initiatives en cours, notamment dans le cadre d’INSPIRE. Il illustrera également l’intérêt et la mise en oeuvre d’un standard tel que CityGML pour les diverses thématiques envisagées pour les modèles urbains, et quelques problématiques d’intérêt, comme par exemple l’aspect multi-échelles, la gestion de la sémantique, l’intégration de données terrain et l’imagerie. Un exemple simple extrait de la problématique de la mise à disposition de modèles numériques urbains pour la réalisation de simulation de phénomènes physiques sera ensuite abordé. L’objectif est de montrer que le partage d’un modèle et le fait de s'appuyer sur des échanges standardisés permet de faciliter les interactions entre les modèles physiques, et ainsi de coupler plusieurs modèles entre eux.

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Simulating How Salt Decay Ages Buildings

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Nicolas Mérillou, Stéphane Mérillou, Eric Galin, Djamchid Ghazanfarpour

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Salt-based aging plays an important part in the realism of scenes featuring stone structures and monuments. A proposed model, based on physical behaviors and principles, replaces the simulation of complex physical formulations with ad hoc algorithms. It leads to plausible results, ultimately helping designers create aging patterns on affected objects.

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Journal articles

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Authoring Hierarchical Road Networks

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Eric Galin, Adrien Peytavie, Eric Guérin, Bedřich Beneš

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We present a procedural method for generating hierarchical road networks connecting cities, towns and villages over large terrains. Our approach relies on an original geometric graph generation algorithm based on a non-Euclidean metric combined with a path merging algorithm that creates junctions between the different types of roads. Unlike previous work, our method allows high level user control by manipulating the density and the pattern of the network. The geometry of the highways, primary and secondary roads as well as the interchanges and intersections are automatically created from the graph structure by instantiating generic parameterized models.

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Détection de cylindres dans les nuages de points de scènes industrielles basée sur l'utilisation de modèles 3D a priori

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Aurélien Bey, Raphaëlle Chaine, Raphaël Marc, Guillaume Thibault

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Nous nous intéressons dans cet article au problème de la reconstruction de modèles 3D CAO à partir de nuages de points 3D issus d’acquisition laser en environnement industriel. Nous proposons pour cela d’utiliser des modèles 3D existants approximant les nuages de points. Ces modèles peuvent en effet être considérés comme des estimations initiales des scènes traitées, et constituent à ce titre une source de connaissance a priori importante. Nous nous intéressons ici plus particulièrement au traitement des cylindres. La solution que nous proposons permet de générer aléatoirement des ensembles de cylindres candidats potentiellement présents dans le nuage de points, et s’appuie pour cela sur l’information a priori. Le recours à l’a priori permet de diminuer la quantité de candidats non pertinents par rapport aux méthode stochastiques existantes (RANSAC), et de produire des cylindres plus fiables. Nous présentons notamment les résultats obtenus sur quelques jeux de données. Pour finir, nous discutons des possibilités d’intégration de cette solution dans une démarche plus globale visant à produire un modèle 3D consistant à partir des cylindres candidats ainsi générés.

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Freestyle: Sculpting meshes with self-adaptive topology

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Lucian Stanculescu, Raphaëlle Chaine, Marie-Paule Cani

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We present a real-time method for sculpting triangular manifold meshes while enabling arbitrary surface deformation with seamless topological changes. Our insight is that the use of quasi-uniform mesh sampling, an interesting option now that very large meshes can be edited and displayed in real-time, provides the right framework for expressing and efficiently processing arbitrary changes of topological genus. The user controls deformation by gesture: he sweeps tools that apply a variety of deformation fields, from smoothing and trimming ones to local inflation and constant volume deformation tools. Meanwhile, the quasi-regular mesh seamlessly splits or locally blends when and where needed, while still following the user-specified deformation. Our method guarantees a closed, self-intersection-free mesh, whatever the user action. We demonstrate the practical usability of the resulting, interactive sculpting system through the sculpture of models that would have been extremely difficult to achieve with both current research methods and state of the art professional software.

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Journal articles

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Détection de cylindres dans les nuages de points de scènes industrielles basée sur l’utilisation de modèles 3D a priori

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Aurélien Bey, Raphaëlle Chaine, Raphaël Marc, Guillaume Thibault

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Nous nous intéressons dans cet article au problème de la reconstruction de modèles 3D CAO à partir de nuages de points 3D issus d’acquisition laser en environnement industriel. Nous proposons pour cela d’utiliser des modèles 3D existants approximant les nuages de points. Ces modèles peuvent en effet être considérés comme des estimations initiales des scènes traitées, et constituent à ce titre une source de connaissance a priori importante. Nous nous intéressons ici plus particulièrement au traitement des cylindres. La solution que nous proposons permet de générer aléatoirement des ensembles de cylindres candidats potentiellement présents dans le nuage de points, et s’appuie pour cela sur l’information a priori. Le recours à l’a priori permet de diminuer la quantité de candidats non pertinents par rapport aux approches aléatoires existantes (RANSAC), et de produire des cylindres plus fiables. Nous présentons notamment les résultats obtenus sur quelques jeux de données. Pour finir, nous discutons des possibilités d’intégration de cette solution dans une approche plus globale visant à produire un modèle 3D consistant à partir des cylindres candidats ainsi produits.

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Feature based terrain generation using diffusion equation

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Houssam Hnaidi, Eric Guérin, Samir Akkouche, Adrien Peytavie, Eric Galin

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This paper presents a diffusion method for generating terrains from a set of parameterized curves that characterize the landform features such as ridge lines, riverbeds or cliffs. Our approach provides the user with an intuitive vector-based feature-oriented control over the terrain. Different types of constraints (such as elevation, slope angle and roughness) can be attached to the curves so as to define the shape of the terrain. The terrain is generated from the curve representation by using an efficient multigrid diffusion algorithm. The algorithm can be efficiently implemented on the GPU, which allows the user to interactively create a vast variety of landscapes.

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Multiresolution control of curves and surfaces with a self-similar model

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Houssam Hnaidi, Eric Guérin, Samir Akkouche

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This paper presents two self-similar models that allow the control of curves and surfaces. The first model is based on IFS (Iterated Function Systems) theory and the second on subdivision curve and surface theory. Both of these methods employ the detail concept as in the wavelet transform and allow the multiresolution control of objects with control points at any resolution level. In the first model, the detail is inserted independently of control points, requiring it to be rotated when applying deformations. On the contrary, the second method describes details relative to control points, allowing free control point deformations. Modelling examples of curves and surfaces are presented, showing manipulation facilities of the models.

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Heat Transfer Simulation for Modeling Realistic Winter Sceneries

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Nicolas Maréchal, Eric Guérin, Eric Galin, Stéphane Mérillou, Nicolas Mérillou

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This paper presents a physically based method for simulating the heat transfers between the different environmental elements to synthesize realistic winter sceneries. We simulate the snow fall over the ground, as well as the conductive, convective and radiative thermal transfers using a finite volume method according to the variations of air and dew point temperatures, the amount of snow, cloud cover and day-night cycles. Our approach takes into account phase changes such as snow melting into water or water freezing into ice.

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Procedural Generation of Roads

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Nicolas Marechal, Eric Galin, Adrien Peytavie, N Maréchal, Eric Guérin

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In this paper, we propose an automatic method for generating roads based on a weighted anisotropic shortest path algorithm. Given an input scene, we automatically create a path connecting an initial and a final point. The trajectory of the road minimizes a cost function that takes into account the different parameters of the scene including the slope of the terrain, natural obstacles such as rivers, lakes, mountains and forests. The road is generated by excavating the terrain along the path and instantiating generic parameterized models.

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Journal articles

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MIXED-ASPECT FRACTAL CURVES

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Christian Gentil, Eric Tosan, Marc Neveu

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The aim of our work is to elaborate a method to build parametric shapes (curves, surfaces,\dots) with a non uniform local aspect : every point is assigned a ''geometric texture'' that evolves continuously from a smooth aspect to a rough aspect.\\ We rely on previous work that enables us to represent free form curves that are either smooth or fractal. We propose a formalism based on finite families of iterated function systems that generalizes this previous approach. The principle is to blend shapes with uniform aspects to define a shape with a variable aspect.

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CHuMI Viewer: Compressive Huge Mesh Interactive Viewer

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Clément Jamin, Pierre-Marie Gandoin, Samir Akkouche

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The preprocessing of large meshes to provide and optimize interactive visualization implies a complete reorganization that often introduces significant data growth. This is detrimental to storage and network transmission, but in the near future could also affect the efficiency of the visualization process itself, because of the increasing gap between computing times and external access times. In this article, we attempt to reconcile lossless compression and visualization by proposing a data structure that radically reduces the size of the object while supporting a fast interactive navigation based on a viewing distance criterion. In addition to this double capability, this method works out-of-core and can handle meshes containing several hundred million vertices. Furthermore, it presents the advantage of dealing with any n-dimensional simplicial complex, including triangle soups or volumetric meshes, and provides a significant rate-distortion improvement. The performance attained is near state-of-the-art in terms of the compression ratio as well as the visualization frame rates, offering a unique combination that can be useful in numerous applications.

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Reconstruction Algorithms as a Suitable Basis for Mesh Connectivity Compression

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Raphaëlle Chaine, Pierre-Marie Gandoin, Céline Roudet

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During a highly productive period running from 1995 to about 2002, the research in lossless compression of surface meshes mainly consisted in a hard battle for the best bitrates. But for a few years, compression rates seem stabilized around 1.5 bit per vertex for the connectivity coding of usual triangular meshes, and more and more work is dedicated to remeshing, lossy compression, or gigantic mesh compression, where memory access and CPU optimizations are the new priority. However, the size of 3D models keeps growing, and many application fields keep requiring lossless compression. In this paper, we present a new contribution for single-rate lossless connectivity compression, which first brings improvement over current state of the art bitrates, and secondly, does not constraint the coding of the vertex positions, offering therefore a good complementarity with the best performing geometric compression methods. The initial observation having motivated this work is that very often, most of the connectivity part of a mesh can be automatically deduced from its geometric part using reconstruction algorithms. This has already been used within the limited framework of projectable objects (essentially terrain models and GIS), but finds here its first generalization to arbitrary triangular meshes, without any limitation regarding the topological genus, the number of connected components, the manifoldness or the regularity. This can be obtained by constraining and guiding a Delaunay-based reconstruction algorithm so that it outputs the initial mesh to be coded. The resulting rates seem extremely competitive when the meshes are fully included in Delaunay, and are still good compared to the state of the art in the case of scanned models.

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Progressive Lossless Mesh Compression Via Incremental Parametric Refinement

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Sébastien Valette, Raphaëlle Chaine, Rémy Prost

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In this paper, we propose a novel progressive lossless mesh compression algorithm based on Incremental Parametric Refinement, where the connectivity is uncontrolled in a first step, yielding visually pleasing meshes at each resolution level while saving connectivity information compared to previous approaches. The algorithm starts with a coarse version of the original mesh, which is further refined by means of a novel refinement scheme. The mesh refinement is driven by a geometric criterion, in spirit with surface reconstruction algorithms, aiming at generating uniform meshes. The vertices coordinates are also quantized and transmitted in a progressive way, following a geometric criterion, efficiently allocating the bit budget. With this assumption, the generated intermediate meshes tend to exhibit a uniform sampling. The potential discrepancy between the resulting connectivity and the original one is corrected at the end of the algorithm. We provide a proof-of-concept implementation, yielding very competitive results compared to previous works in terms of rate/distortion trade-off.

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Un modèle itératif de surfaces pour la construction en bois

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Gilles Gouaty, Eric Tosan, Ivo Stotz, Yves Weinand

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Dans le cadre de la construction en bois, nous nous intéressons à la modélisation de maillages surfaciques 3D basés sur un modèle itératif inspiré du modèle IFS (Iterated Function System). Les formes modélisées doivent satisfaire certaines propriétés afin d’assurer leur constructibilité physique. Nous nous intéressons ici aux contraintes géométriques de planéité des faces d’un maillage quadrangulaire, permettant la construction d’assemblages de panneaux de bois découpés. Nous décrivons dans un premier temps un modèle permettant de représenter certains types de maillages à faces planes. Ce modèle est défini comme une certaine somme de deux courbes. Nous présentons ensuite quelques propriétés élémentaires du modèle itératif que nous utilisons, inspiré du modèle IFS. Nous définissons enfin un modèle de surfaces générées par un opérateur de combinaison entre deux IFS décrivant chacun une certaine courbe. Nous montrons qu’il peut également être considéré comme un carreau à pôles généré par un produit d’IFS.

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Compression out-of-core pour la visualisation interactive de maillages volumineux

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Clément Jamin, Pierre-Marie Gandoin, Samir Akkouche

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Le prétraitement des maillages à des fins de visualisation interactive implique une réorganisation complète des données qui induit souvent une augmentation significative de la taille des fichiers. Cela nuit non seulement au stockage et à la transmission réseau, mais pourrait aussi, dans un futur proche, affecter l'efficacité de la visualisation elle-même, au vu de l'écart croissant entre la vitesse des processeurs et la lenteur relative des mémoires externes. Dans cet article, nous tentons de réconcilier compression sans perte et visualisation, grâce à une structure de donnée qui réduit la taille des objets tout en permettant une navigation interactive. En supplément de cette double capacité, cette méthode est out-of-core et peut ainsi traiter des maillages de plusieurs centaines de millions de triangles. En outre, tout complexe simplicial de dimension n est accepté en entrée, que ce soit une soupe de triangles ou un maillage volumétrique, et la compression distribue les données de façon à optimiser le ratio débit-distorsion. Les performances sont proches de l'état de l'art, aussi bien en terme de taux de compression qu'en terme de vitesse d'affichage, ce qui offre un compromis utile à de nombreuses applications.

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Arches : a Framework for Modelling Complex Terrains

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Adrien Peytavie, Eric Galin, Jérôme Grosjean, Stéphane Mérillou

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In this paper, we present a framework for representing complex terrains with such features as overhangs, arches and caves and including different materials such as sand and rocks. Our hybrid model combines a volumetric discrete data structure that stores the different materials and an implicit representation for sculpting and reconstructing the surface of the terrain. Complex scenes can be edited and sculpted interactively with high level tools. We also propose an original rock generation technique that enables us to automatically generate complex rocky sceneries with piles of rocks without any computationally demanding physically-based simulation.

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Procedural Generation of Rock Piles Using Aperiodic Tiling

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Adrien Peytavie, Eric Galin, Jérôme Grosjean, Stéphane Mérillou

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In this paper, we present a tiling method for generating piles of rocks without any computationally demanding physically-based simulation. Previous techniques rely on a periodic tiling of rocks and generate unrealistic repetitive patterns. In contrast, our approach relies on a modified corner cube algorithm to generate a set of aperiodic tiles. We generalize the construction method so that the geometry of rocks should straddle corner cubes with a view to avoiding unrealistic gaps in the arrangement of rocks. Moreover, we propose an original technique to control the shape of rocks into contact by computing the Voronoï cells using a parameterized anisotropic distance. Our method has been successfully used to generate landscapes and stone huts and walls with thousands of rocks piled together.

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Journal articles

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A theoretical framework mapping Grammar based systems and Fractal description

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Ikbal Zammouri, Eric Tosan, Béchir Ayeb

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In this work, we propose a new method to describe fractal shapes using parametric L-systems. This method consists of introducing scaling factors in the production rules of the parametric L-system grammars. We present a turtle monoid on which we base our calculations to show the exact mathematical relation between L-systems and iterated function systems (IFS); we then establish the conditions for the scaling factors to produce plants' and curves' fractal shapes from parametric L-systems. We demonstrate that with specific values of the scaling factors, we find the exact relationship established by Prusinkiewicz and Hammel between L-systems and IFS. Finaly, we present some examples of fractal plant forms ans curves created using parametric L-systems with scaling factors.

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Toward an efficient triangle-based spherical harmonics representation of 3D objects

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Mohamed Mousa, Raphaëlle Chaine, Samir Akkouche, Eric Galin

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Version étendue et améliorée de l'article publié à Pacific Graphics l'année précédente

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Generating Vast Varieties of Realistic Leaves with Parametric 2Gmaps L-Systems

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Alexandre Peyrat, Olivier Terraz, Stephane Merillou, Eric Galin

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Creating realistic plants and trees require the ability to generate thousands of leaves with different shapes and textures for different given species. This paper presents an original method to generate large atlases of leaves with many details from a single formal grammar. Leaves are described by a parameterized 2Gmap L-system that describes their evolution in shape and texture through out their entire life cycle. Our approach automatically synthesizes the deformations as well as the color and texture changes as the leaves age, as well as defects such as holes or cracks produced by insect attacks or accidents.

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Journal articles

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Reconstruction dynamique de surface par convection géométrique

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Rémi Allègre, Raphaëlle Chaine, Samir Akkouche

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In this paper, we present a selective and evolutive method for reconstructing a surface from a point cloud that makes it possible to produce a simplified triangulated surface and to update it dynamically in an efficient manner. This method is based on a geometric convection algorithm and extends recent work on dynamic surface reconstruction introduced in (Allègre et al., 2005). We present a new data structure that significantly speeds up the original selective reconstruction algorithm and makes it possible to handle point set models with millions of sample points efficiently. We also show how to control the quality of the resulting meshes. The proposed method can also take into account some attributes like color in the selective reconstruction process.

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Dynamic Delaunay Tetrahedralisation of a Deforming Surface

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Jean-Baptiste Debard, Romain Balp, Raphaëlle Chaine

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Reconstruction algorithms make it possible to retrieve a surface from the Delaunay tetrahedralisation (DT) of a point sampling, whose density reflects the surface local geometry and thickness. Most of these algorithms are static and some work remains to be done to handle deforming surfaces. In such case, we defend the idea that each point of the sampling should move with the surface using the information given by the motion to allow fast reconstruction. In this article, we tackle the problem of producing a good evolving sampling of a deforming surface S, and maintaining its DT along the motion. The surface is known only through a projection operator (O 1), and a normal operator (O 2) that returns the oriented normal at a point on the surface. On that basis, we offer some perspectives on how reconstruction algorithms can be extended to the tracking of deforming surfaces.

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CAD/CAPP Integration using Feature Ontology

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Christel Dartigues, Parisa Ghodous, Michael Gruninger, Denis Pallez, Ram Sriram

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In a collaborative computer-supported engineering environment, the interoperation of various applications will need a representation that goes beyond the current geometry-based representation, which is inadequate for capturing semantic information. The primary purpose of this study is to discuss a semantically based information exchange protocol that will facilitate seamless interoperability among current and next generation computer-aided design systems (CAD) and between CAD and other systems that use product data. An ontological approach is described to integrating computer-aided design (CAD) and computer-aided process planning (CAPP). Two commercial software applications are used to demonstrate the approach. This involves the development of a shared ontology and domain specific ontologies in the Knowledge Interchange Format (KIF) language. Domain specific ontologies — which are feature-based — are developed after a detailed analysis of the CAD and the CAPP software. Mapping between the domain ontologies and the shared ontology is achieved by several mapping rules. The approach is validated by using a variety of parts.

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A flexible framework for surface reconstruction from large point sets

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Rémi Allègre, Raphaëlle Chaine, Samir Akkouche

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This paper presents a flexible method to reconstruct simplified mesh surfaces from large unstructured point sets, extending recent work on dynamic surface reconstruction. The method consists of two core components: an efficient selective reconstruction algorithm, based on geometric convection, that simplifies the input point set while reconstructing a surface, and a local update algorithm that dynamically refines or coarsens the reconstructed surface according to specific local sampling constraints. A new data structure is introduced that significantly accelerates the original selective reconstruction algorithm and makes it possible to handle point set models with millions of sample points. This data structure mixes a kd-tree with the Delaunay triangulation of the selected points enriched with a sparse subset of landmark sample points. This design efficiently responds to the specific spatial location issues of the geometric convection algorithm. It also permits the development of an out-of-core implementation of the method, so that simplified mesh surfaces can be seamlessly reconstructed and interactively updated from point sets that do not fit into main memory.

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Journal articles

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Direct Spherical Harmonics Transform of a Triangulated Mesh

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Mohamed Mousa, Raphaëlle Chaine, Samir Akkouche

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Spherical harmonics transform plays an important role in research in shape description. Current methods to compute the spherical harmonics decomposition of the characteristic function of the intersection of a polyhedral solid with a sphere involve expensive voxelization, and are prone to numerical errors associated with the size of the voxels. This paper describes a fast and accurate technique for computing spherical harmonics coefficients directly from the description of the mesh. The algorithm runs in linear time O(pn), where n is the number of triangles of the mesh and p is the number of terms calculated, which is roughly linear.

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The HybridTree: Mixing Skeletal Implicit Surfaces, Triangle Meshes and Point Sets in a Free-form Modeling System

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Rémi Allègre, Eric Galin, Raphaëlle Chaine, Samir Akkouche

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In this paper, we present a hybrid modeling framework for creating complex 3D objects incrementally. Our system relies on an extended CSG tree that assembles skeletal implicit primitives, triangle meshes and point set models in a coherent fashion: we call this structure the HybridTree. Editing operations are performed by exploiting the complementary abilities of implicit and polygonal mesh surface representations in a complete transparent way for the user. Implicit surfaces are powerful for combining shapes with Boolean and blending operations, while triangle meshes are well-suited for local deformations such as FFD and fast visualization. Our system can handle point sampled geometry through a mesh surface reconstruction algorithm. The HybridTree may be evaluated through four kinds of queries, depending on the implicit or explicit formulation is required: field function and gradient at a given point in space, point membership classification, and polygonization. Every kind of query is achieved automatically in a specific and optimized fashion for every node of the HybridTree.

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Journal articles

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A framework for modeling, animating and morphing textured implicit models

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Aurélien Barbier Accary, Eric Galin, Samir Akkouche

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This paper presents a framework for modeling, animating and morphing textured implicit models. Our hierarchical skeletal implicit surface model incorporates key-frame animation and procedural solid texturing in a unified and coherent way. Our system enables the designer to create complex special effects by synchronizing shape, animation and texture transformations.

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Journal articles

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Fast distance Computation between a Point and Cylinders, Cones, Line-swept-Spheres and Cone-Spheres

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Aurélien Barbier Accary, Eric Galin

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This paper presents algorithms for computing the distance between a point and a cylinder, a cone, a cylinder-sphere and a cone-sphere. Some optimizations are provided when queries are performed along a line which may be useful for ray tracing applications.

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