A Framework for n-dimension Visibility Calculation

4 pagesVisibility computation is a fundamental task in computer graphics, as in many other scientific domains. While it is well understood in two dimensions, this does not remain true in high dimensional spaces. Using Grassmann Algebra, we propose a framework for solving visibility problems in any n-dimensional spaces, for n ≥ 2. Our presentation recalls the problem statement, in two and three dimensions. Then, we formalize the space of n-dimensional lines. Finally, we show how this leads to a global framework for visibility computations, giving an example of use with exact soft shadows

Etude et application des algèbres géométriques pour le calcul de la visibilité globale dans un espace projectif de dimension n >= 2

Cette thèse propose une étude des algèbres de Grassmann et de Clifford, du point de vue de leur définition mathématique, puis de leur application à l informatique graphique, afin de définir un cadre théorique et pratique pour le calcul de la visibilité globale dans un espace projectif de dimension n >= 2. En effet, le calcul d une information de visibilité est un des problèmes majeurs depuis les débuts de la synthèse d images. L émergence de problèmes plus complexes dans diverses disciplines nécessite une information de visibilité globale, c est-à-dire depuis tout point de l espace. Dans un premier temps, l étude des algèbres apporte des connaissances essentielles sur leurs structures mathématiques, ainsi qu un meilleur recul vis-à-vis de leurs méthodes d application à la géométrie. Dans un second temps, la définition d une théorie algébrique de la visibilité procure une meilleure connaissance du problème et une meilleure façon de raisonner, de formuler les opérations géométriques et de démontrer leur consistance. Elle permet ensuite de proposer une méthode algorithmique très efficace et très robuste, pour calculer une représentation de la visibilité globale dans l espace tridimensionnel, et la première solution à ce calcul dans des espaces de dimensions supérieures à trois.This thesis proposes a study of the Grassmann and Clifford algebras, from the mathematical definition and the computer graphics applications points of view, in order to define a theoretical and applicative framework to compute a global visibility information in a projective space of dimension n >= 2. Indeed, the computation of a visibility information is one of the major problems since the beginnings of the image synthesis. The emergence of more complex problems in various fields require a global visibility information, that is to say an information from any point in space. Firstly, the study of the algebras contributes to essential knowledge on their mathematical structures and an original way to explain their meaning and their applications to geometry. Secondly, the definition of an algebraic theory of visibility offers a better knowledge of the problem and a better way to reason, to express geometric operations and to prove their consistency. It then allows to propose a robust and efficient method to compute a representation of the global visibility in three dimensional space and the first solution to this calculation in spaces of dimensions greater than three.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Modélisation nD à base d'algèbres géométriques

3ème prix du meilleur article jeunes chercheursDans cet article nous nous intéressons au plongement d'une structure topologique. Pour modéliser un objet, l'approche usuelle en modélisation à base topologique est de séparer la structure de l'objet, sa topologie, de sa forme, son plongement géométrique. Par conséquent, le plus souvent la forme des objets est traitée soit de façon très minimale ; des points d'une certaine dimension sont associés aux sommets de la structure topologique, soit de façon très spécifique ; la forme des objets est définie pour une dimension donnée (généralement un espace à 3 dimensions) et pour un paradigme géométrique donné (continu euclidien, discret). Nous proposons ici une façon de plonger les G-cartes de façon générique ; indépendante de la dimension et du paradigme géométrique. Le résultat est une structure de données qui permet de modéliser et manipuler des objets géométriques facettisés de toutes dimensions

Overweight is associated to a better prognosis in metastatic colorectal cancer: A pooled analysis of FFCD trials

IF 7.191 (2017)International audienceBACKGROUND:Previous studies showed that high and low body mass index (BMI) was associated with worse prognosis in early-stage colorectal cancer (CRC), and low BMI was associated with worse prognosis in metastatic CRC (mCRC). We aimed to assess efficacy outcomes according to BMI.PATIENTS AND METHODS:A pooled analysis of individual data from 2085 patients enrolled in eight FFCD first-line mCRC trials from 1991 to 2013 was performed. Comparisons were made according to the BMI cut-off: Obese (BMI ≥30), overweight patients (BMI ≥ 25), normal BMI patients (BMI: 18.5-24) and thin patients (BMI <18.5). Interaction tests were performed between BMI effect and sex, age and the addition of antiangiogenics to chemotherapy.RESULTS:The rate of BMI ≥25 patients was 41.5%, ranging from 37.6% (1991-1999 period) to 41.5% (2000-2006 period) and 44.8% (2007-2013 period). Comparison of overweight patients versus normal BMI range patients revealed a significant improvement of median overall survival (OS) (18.5 versus 16.3 months, HR = 0.88 [0.80-0.98] p = 0.02) and objective response rate (ORR) (42% versus 36% OR = 1.23 [1.01-1.50] p = 0.04) but a comparable median progression-free survival (PFS) (7.8 versus 7.2 months, HR = 0.96 [0.87-1.05] p = 0.35). Subgroup analyses revealed that overweight was significantly associated with better OS in men. OS and PFS were significantly shorter in thin patients.CONCLUSION:Overweight patients had a prolonged OS compared with normal weight patients with mCRC. The association of overweight with better OS was only observed in men. The pejorative prognosis of BMI <18.5 was confirmed.Copyright © 2018 The Authors. Published by Elsevier Ltd.. All rights reserve

High Risk of Anal and Rectal Cancer in Patients With Anal and/or Perianal Crohn’s Disease

International audienceBackground & AimsLittle is known about the magnitude of the risk of anal and rectal cancer in patients with anal and/or perineal Crohn’s disease. We aimed to assess the risk of anal and rectal cancer in patients with Crohn’s perianal disease followed up in the Cancers Et Surrisque Associé aux Maladies Inflammatoires Intestinales En France (CESAME) cohort.MethodsWe collected data from 19,486 patients with inflammatory bowel disease (IBD) enrolled in the observational CESAME study in France, from May 2004 through June 2005; 14.9% of participants had past or current anal and/or perianal Crohn’s disease. Subjects were followed up for a median time of 35 months (interquartile range, 29–40 mo). To identify risk factors for anal cancer in the total CESAME population, we performed a case-control study in which participants were matched for age and sex.ResultsAmong the total IBD population, 8 patients developed anal cancer and 14 patients developed rectal cancer. In the subgroup of 2911 patients with past or current anal and/or perianal Crohn’s lesions at cohort entry, 2 developed anal squamous-cell carcinoma, 3 developed perianal fistula–related adenocarcinoma, and 6 developed rectal cancer. The corresponding incidence rates were 0.26 per 1000 patient-years for anal squamous-cell carcinoma, 0.38 per 1000 patient-years for perianal fistula–related adenocarcinoma, and 0.77 per 1000 patient-years for rectal cancer. Among the 16,575 patients with ulcerative colitis or Crohn’s disease without anal or perianal lesions, the incidence rate of anal cancer was 0.08 per 1000 patient-years and of rectal cancer was 0.21 per 1000 patient-years. Among factors tested by univariate conditional regression (IBD subtype, disease duration, exposure to immune-suppressive therapy, presence of past or current anal and/or perianal lesions), the presence of past or current anal and/or perianal lesions at cohort entry was the only factor significantly associated with development of anal cancer (odds ratio, 11.2; 95% CI, 1.18-551.51; P = .03).ConclusionsIn an analysis of data from the CESAME cohort in France, patients with anal and/or perianal Crohn’s disease have a high risk of anal cancer, including perianal fistula–related cancer, and a high risk of rectal cancer