Modeling and acquisition of plant architecture

Virtual plant models are powerful tools to better understand growth and functioning of plants and their interaction with their environment (light, pest and disease propagation, etc). However, numerous problems are raised for their acquisition, representation, analysis and simulation. We have been developing for several years a modeling methodology of the structure and geometry of plants that makes it possible to take into account their multi?scale aspect and exploit it for acquisition. In this talk, I will present our recent works on the acquisition of complex plant architectures. In the first part, I will introduce sketching based methods and present the approach we developed based on the structure from silhouettes paradigm. Indeed, we claim and show with our prototype that sketching the silhouettes of foliage at multiple scales is quicker and more intuitive for a user than having to sketch each branch of a tree. This choice allows us to incorporate botanical knowledge, enabling us to infer branches that connect in a plausible way to their parent branch and have a correct distribution in 3D. Users have to give the outlines of the different parts of the tree and an example of structures at each scale to make it possible for the program to generate a complete structure that the user can finally correct easily. While this makes it possible to define intuitively plausible virtual models in few minutes, we also address the problem of reconstructing faithfully actual plant architectures. Using laser scanners is a natural way to capture such complex, real geometry; however, due to the multi-resolution nature of trees, it produces data at different levels of precision. Points set are usually dense on the surface of the trunk and of the main branches, but only sparsely cover thin branches and small twigs. While previous methods typically loose accuracy by populating sparse?points regions with plausible structures, we propose a faithful reconstruction method of the tree skeletal structure. This is achieved thanks to a method locally adaptive to the different levels of precision of the data that combines a contraction phase and a local point tracking algorithm. In addition, we developed a quantitative evaluation procedure enabling us to compare our reconstructions against expertised structures of real plants. We used it to assess the accuracy of our reconstruction algorithm. (Texte intégral

A survey of computer representations of trees for realistic and efficient rendering

This paper gives an overview of computer graphics representations of trees commonly used for the rendering of complex scene of vegetation. Looking for the right compromise between realism and efficiency has lead researchers to consider various types of geometrical plant models with different types of complexity. To achieve realist plant model, a complex structure of plant with full details is generally considered. In contrast, to promote efficiency, other approaches summarize plant geometry with few primitives allowing rapid rendering. Finally, to find a good compromise, structures with adaptive complexity are defined. Theses different types of representations and the ways to use them are presented, classified and discussed. The proposed classification principles rely on the type of structural details used in the plants representations. Characterization of all these methods is completed with various additional criteria including rendering primitive type, distance validity, interactive possibilities, animation ability and lighting properties. (Résumé d'auteur

Coupling Avicennia tree architecture to radar backscatter models for estimating mangrove forest biomass

The understanding of mangrove ecosystems functioning requires techniques allowing their forest structure and forest biomass to be assessed. Among them, radar measurements have demonstrated powerful capabilities that, however, must be validated extensively, both spatially and physically. Beyond their intrinsic potential to provide basic information on tree growth and forest functioning, 3D tree architecture studies could help the interpretation of radar signal scattering within forest canopies to be performed and thus, improve the accuracy of radar inversion algorithms for estimating forest parameters such as total above-ground biomass and component biomass. Topological measurements have been conducted at different growth stages on Avicennia germinans trees with height less than 5 meters. For each tree measured, leaf size, nodes length and diameter are coded using the AMAPmod terminology. Thanks to this software and assuming stands composed of identical trees, input parameters for the radar backscatter models of forests are then derived from architectural analysis. The simulated radar responses are compared to radar measurements when available. The effect of tree architecture and the structure of associated stands are discussed with the objectives to estimate forest parameters using radar data. This work comes within the international project ALOS Kyoto and Carbon Initiative aiming to monitor forest dynamics, particularly mangrove forests. (Texte intégral

La sociología que realmente importa

La diversidad de la sociología puede ser capturada con la ayuda de cuatro tipos ideales. El primero es la sociología informativa o «de consultoría»: aquella que produce datos y análisis orientados hacia la toma de decisiones. El segundo es la sociología crítica, que identifica los defectos de la sociedad y propone remedios para los mismos. El tercero es la sociología que intenta despertar emociones describiendo los fenómenos sociales de una manera vívida. Un cuarto tipo, el cognitivo, tiene por objetivo la explicación de fenómenos sociales enigmáticos. Las cuatro orientaciones se encuentran tanto en la sociología contemporánea como en la clásica, pero no son igualmente valiosas. Tocqueville, Weber o Durkheim ejemplifican el cuarto tipo, pero varios factores han causado que actualmente los otros tres se expandan a sus expensas. Como consecuencia de esta diversidad en la sociología, y de su alejamiento respecto del tipo cognitivo, el escepticismo sobre la disciplina ha crecido en los últimos años.The diversity of sociology can be captured with the help of four ideal types. First is cameral or informative sociology: that which produces data and analyses oriented towards decision- makers. Second is critical sociology, which identifies the defects of society and proposes remedies for them. Third is sociology which aims to arouse emotions by describing social phenomena in a vivid fashion: it can be characterized as expressive. A fourth type, the cognitive type, sees the explanation of puzzling social phenomena as its objective. The four orientations characterize contemporary as well as classical sociology but are not equally valuable. Tocqueville, Weber, or Durkheim illustrate the fourth type, but various factors today have caused the other three to expand at its expense. As a consequence of this diversity within sociology and the trend away from the cognitive type, scepticism about the discipline has grown in recent years

Les volcans actifs de l'Outre-mer français, des édifices à haut risque

National audienceCertains volcans de l'outre-mer français font partie des volcans les plus actifs et les plus instables de la planète : les plus violents se sont déjà tristement illustrés pendant la période historique. Afin d'obtenir une bonne évaluation de l'aléa éruptif, il est indispensable de reconstituer l'histoire de ces volcans afin de définir les scénarios éruptifs les plus probables. Les risques encourus par les populations étant avérés, ces volcans sont placés sous haute surveillance et font l'objet de plans de prévention et de gestion de crise

Multiscale geometric representation of heterogeneous stands

The aim of this talk is to present a general geometric model for plant representation that considers different plant perceptions at different levels of organization (or scales). This model is based on a representation of plant topology that integrates several scales within a single framework. This model, called Multiscale Tree Graph (MTG), can be seen as a set of recursively nested graph, representing different levels of topological organisation. Each entity of a MTG, at any scale, can be augmented with a geometric model. Several geometric representations of the plant can thus be computed, depending on the scale considered to represent geometry. Since these models are geometric representations of the same real object at different scales, they must be consistent with one another. To ensure model definition consistency, within-scale and between-scale constraints are introduced. This constraints enable flexible management of "multiscale geometric models". Multiscale geometric models can be used to compute geometric representations of plant architecture that are best adapted to some particular criteria. For example, in tree simulation, computer applications are often limited by the tremendous amount of topological and geometric data resulting from the simulation engine. Multiscale geometric models can be used to control the amount of memory used by the representations. It allows us to obtain, for example, compressed plants representations. Different possible applications of this multiscale geometric model to the representation of stands are finally discussed. (Texte intégral

L’entrelacs architectural : (ou le « jeu des passages »)

Le rapport entre enveloppement et cheminement définit une articulation fondamentale dans la composition architecturale contemporaine. Partant de cette considération, nous développons un art de la composition dont les principes sont au-delà de la manifestation sensible, sous forme de volumes habitables ; de par la continuité des parcours, cet art n’est pas sans faire penser aux « jeux de ficelle » des enfants dont la signification n’est pas seulement ludique mais cosmique. Les figures souples que ceux-ci dessinent peuvent être comparées à des espaces architecturaux en évolution, traduits en termes de matériaux abstraits.Connections between covering and path are now very important in the definition of an architectural composition ; according to this consideration, we develop a semiotics of this art of composition in which principles are beyond the empirical expressions of architecture in terms of dwelling spaces. In this notion of continuity of paths, this art of composition is not without reminding us of the children’s “ string games ” in which the meaning is not only in the expressions of the game but also in its cosmic representations. The flexible shapes drawn by the game may look like the architectural spaces in evolution turned into abstract materials

Penser contre les mots : le discours néo-zapatiste ou une rébellion littéraire

Le corpus des textes produits par l'EZLN (ensemble des communiqués livrés à la presse depuis 1994) s'inscrit dans une problématique des confins : entre discours de contre-pouvoir et pouvoirs d'un certain discours, c'est à la limite des deux champs de pensée et d'action que ces textes sont lus. Dans ce discours apparaît une rupture des genres et des modes de fonctionnement du texte comme miroir idéologique et comme objet littéraire. Le corpus n'est pas homogène ni au plan formel ni au plan sémantique. Il se construit peu à peu. Cette hétérogénéité n'empêche pas de le considérer comme un tout : occurrences, récurrences, stratégies narratives identifiables organisent l'ensemble de manière cohérente. Le corpus puise à de multiples sources idéologiques et narratives mêlant à un positionnement révolutionnaire de type marxiste, l'héritage intellectuel de la révolution de 1910, une parole indigène, des axes de réflexion émanant de la théologie de la libération, la perspective altermondialiste contre le néolibéralisme. Dans ce cadre kaléidoscopique, il perdure une valeur relevant du champ politique : la question du pouvoir. Question revisitée dans l'espace narratif du discours : il émane de ceux qui n'ont pas le pouvoir, et il s'inscrit dans le champ discursif des tenants du pouvoir. La réflexion s'articule autour de deux axes : l'emploi des items lexicaux democracia/libertad/justicia et du syntagme verbal de la parole vraie. C'est par une « rébellion littéraire » que l'Ezln conquiert cet espace de la parole « désintéressée » comme pratique du pouvoir. Ce discours constitue l'emblème d'un éclatement du genre politique. Il met en œuvre une esthétique de la rupture

Vingt ans de recherches sur la sculpture française des XVIe et XVIIe siècles

National audienceAu milieu des années 1980, F. Souchal estimait que " le manque d'intérêt pour l'histoire de la sculpture se traduit d'abord par l'absence de mises au point des problèmes et des travaux " (F. Souchal, " La sculpture française des XVIIe et XVIIIe siècles. Etat des travaux ", Formes, n° 19, pp. 21-26). Quelques vingt années plus tard, un état des lieux est nécessaire pour comprendre où en est la recherche sur la sculpture française des XVIe et XVIIe siècles et quelles sont les perspectives qui s'offrent à elle. Force est de constater que la sculpture occupe en effet depuis longtemps une place réduite dans les travaux des historiens de l'art français. La recension des articles et ouvrages parus dans les vingt dernières années, confirme cette tendance, d'autant plus sensible que ceux sur la peinture et l'architecture ont connu, à l'inverse, un développement croissant. Le plus inquiétant est que le petit nombre des publications semble être le reflet d'un désintérêt plus général pour la matière, qui touche tant le monde de la recherche que celui du marché de l'art, et qui conduit à distinguer la France de ses voisins européens et des Etats-Unis