94 research outputs found
A note on the computation of the fraction of smallest denominator in between two irreducible fractions
International audienceGiven two irreducible fractions f and g, with f < g, we characterize the fraction h such that f < h < g and the denominator of h is as small as possible. An output-sensitive algorithm of time complexity O(d), where d is the depth of h is derived from this characterization
Representation of Imprecise Digital Objects
International audienceIn this paper, we investigate a new framework to handle noisy digital objects. We consider digital closed simple 4-connected curves that are the result of an imperfect digital conversion (scan, picture, etc), and call digital imprecise contours such curves for which an imprecision value is known at each point. This imprecision value stands for the radius of a ball around each point, such that the result of a perfect digitization lies in the union of all the balls. In the first part, we show how to define an imprecise digital object from such an imprecise digital contour. To do so, we define three classes of pixels : inside, outside and uncertain pixels. In the second part of the paper, we build on this definition for a volumetric analysis (as opposed to contour analysis) of imprecise digital objects. From so-called toleranced balls, a filtration of objects, called λ-objects is defined. We show how to define a set of sites to encode this filtration of objects
Petit manuel de survie en milieu digital
National audienceSaviez-vous que les leÌgos ou Minecraft posseÌdent leur propre geÌomeÌtrie ? Bien suÌr, la deÌnomination est diffeÌrente mais il sâagit bien dâeÌtudier les formes que lâon peut construire avec des briques eÌleÌmentaires aÌ faces carreÌes. En imagerie, les carreÌs et les cubes sont respectivement appeleÌs pixels et voxels et trouvent une repreÌsentation naturelle dans la grille des entiers Z2 et Z3. Bizarrement, le but premier de cette theÌorie nâest pas de construire des vaisseaux spatiaux, des chaÌteaux remplis de ninjas ou des villes titanesques mais des droites, des cercles, des spheÌres ou tout objet matheÌmatique qui ressemble un tant soit peu aux figures de la geÌomeÌtrie eÌleÌmentaire.En dehors de ses applications ludiques, la geÌomeÌtrie digitale se deÌfinit comme la geÌomeÌtrie de Z2, Z3 ou plus geÌneÌralement Zn, autant dire des espaces peu favorables aÌ la geÌomeÌtrie. Si vous vous aventurez sur le chemin qui meÌne dans ces contreÌes hostiles aÌ la penseÌe matheÌmatique et informatique, vous risquez de croiser le membre de lâune de ses tribus archaiÌques. Au cas fort improbable ouÌ vous arriveriez aÌ communiquer avec cet eÌtre primitif, vous en apprendrez peut-eÌtre un peu plus sur les raisons eÌtranges qui leur font deÌvelopper cette geÌomeÌtrie rudimentaire en milieu si hostile :â dâabord sans doute une certaine nostalgie pour les jeux de construction, â pour les estheÌtes, la beauteÌ de la theÌorie,â et pour dâautres, lâambition de jouer aux Mac Gyver de la geÌomeÌtrie mais derrieÌre ces fantaisies extravagantes qui peuvent les rendre sympathiques, voir naiÌfs ou inoffensifs, se terre un argument de fond qui ne releÌve pas de la simple lubie mais de lâemprise du numeÌrique sur les sciences et technologies actuelles.De tous temps, les sciences physiques aÌ moyennes et grandes eÌchelles ont guideÌ le deÌveloppement dâune partie des matheÌmatiques et en particulier de theÌories geÌomeÌtriques continues telles que la geÌomeÌtrie diffeÌrentielle avec en soubassement le corps des nombres reÌels ou complexes. Ce paradigme (R) a fait ses preuves mais sa nature continue le rend fondamentalement inadapteÌ au traitement des donneÌes recueillies par les millions de peÌripheÌriques qui alimentent les bases de donneÌes du monde entier. Les capteurs enregistrent des donneÌes sous forme digitale, soit aÌ une reÌsolution fixeÌe des tableaux dâentiers câest-aÌ-dire des fonctions de Zd aÌ valeur dans Z. Peut-on les traiter comme si câeÌtaient des fonctions de Rd dans R ? Probablement pas sans preÌcautions mais câest pourtant la voie la plus courante : lâusager pioche lâoutil dont il a besoin dans les matheÌmatiques continues, puis il recherche le moyen de lâappliquer aÌ des structures entieÌres, parfois graÌce aÌ un bricolage dont il garde le secret tant il existe dâinnombrables façons de faire. MeÌme si le reÌsultat peut sâaveÌrer significatif, passer par les nombres reÌels, câest-aÌ-dire une theÌorie baseÌe sur des suites rationnelles de Cauchy convergentes, pour ensuite lâappliquer dans un cadre entier via des probabiliteÌs, une autre theÌorie ou un subterfuge est un deÌtour consideÌrable. Puisque de treÌs nombreuses donneÌes aÌ traiter se preÌsentent sous la forme dâobjets composeÌs dâentiers -le b.a.-ba des nombres pourquoi ne pas deÌvelopper une theÌorie geÌomeÌtrique qui soit directement adapteÌe aÌ ce format de donneÌes ? Câest le chemin que nous vous proposons dâexplorer. Il parcourt un territoire primitif encore largement vierge et donc propice aÌ la recherche. Les agiteÌs du bocal dont je vous ai deÌjaÌ parleÌ -on pourrait aussi les appeler des pionniers ont bien suÌr commenceÌ aÌ le deÌfricher mais en comparaison de lâampleur de la taÌche, on en peut pas dire quâils soient treÌs nombreux. Câest un travail en cours, un chantier aÌ ciel ouvert et un terrain de jeu sur lequel il est vivement recommandeÌ de sâaventurer en dehors des chemins baliseÌs. Mais avant de vous laÌcher en pleine jungle, nous vous proposons un itineÌraire baliseÌ. Alors, remontez vos chaussettes, aspergez-vous de citronnelle, empoignez vos coupe-coupes et suivez le guide..
Efficient Distance Transformation for Path-based Metrics
In many applications, separable algorithms have demonstrated their efficiency to perform high performance volumetric processing of shape, such as distance transformation or medial axis extraction. In the literature, several authors have discussed about conditions on the metric to be considered in a separable approach. In this article, we present generic separable algorithms to efficiently compute Voronoi maps and distance transformations for a large class of metrics. Focusing on path-based norms (chamfer masks, neighborhood sequences...), we propose efficient algorithms to compute such volumetric transformation in dimension . We describe a new algorithm for shapes in a domain for chamfer norms with a rational ball of facets (compared to with previous approaches). Last we further investigate an even more elaborate algorithm with the same worst-case complexity, but reaching a complexity of experimentally, under assumption of regularity distribution of the mask vectors
An update on the coin-moving game on the square grid
This paper extends the work started in 2002 by Demaine, Demaine and Verill
(DDV) on coin-moving puzzles. These puzzles have a long history in the
recreational literature, but were first systematically analyzed by DDV, who
gave a full characterization of the solvable puzzles on the triangular grid and
a partial characterization of the solvable puzzles on the square grid. This
article specifically extends the study of the game on the square grid. Notably,
DDV's result on puzzles with two "extra coins" is shown to be overly broad:
this paper provides counterexamples as well as a revised version of this
theorem. A new method for solving puzzles with two extra coins is then
presented, which covers some cases where the aforementioned theorem does not
apply. Puzzles with just one extra coin seem even more complicated, and are
only touched upon by DDV. This paper delves deeper, studying a class of such
puzzles that may be considered equivalent to a game of "poking" coins. Within
this class, some cases are considered that are amenable to analysis
Epsilon-covering is NP-complete
International audienceConsider the dilation and erosion of a shape S by a ball of radius Δ. We call Δ-covering of S any collection of balls whose union lies between the dilation and erosion of S. We prove that finding an Δ-covering of minimum cardinality is NP-complete, using a reduction from vertex cover
High toxicity and specificity of the saponin 3-GlcA-28-AraRhaxyl-medicagenate, from Medicago truncatula seeds, for Sitophilus oryzae
<p>Abstract</p> <p>Background</p> <p>Because of the increasingly concern of consumers and public policy about problems for environment and for public health due to chemical pesticides, the search for molecules more safe is currently of great importance. Particularly, plants are able to fight the pathogens as insects, bacteria or fungi; so that plants could represent a valuable source of new molecules.</p> <p>Results</p> <p>It was observed that <it>Medicago truncatul</it>a seed flour displayed a strong toxic activity towards the adults of the rice weevil <it>Sitophilus oryzae</it> (Coleoptera), a major pest of stored cereals. The molecule responsible for toxicity was purified, by solvent extraction and HPLC, and identified as a saponin, namely 3-GlcA-28-AraRhaxyl-medicagenate. Saponins are detergents, and the CMC of this molecule was found to be 0.65âmg per mL. Neither the worm <it>Caenorhabditis elegans</it> nor the bacteria <it>E. coli</it> were found to be sensitive to this saponin, but growth of the yeast <it>Saccharomyces cerevisiae</it> was inhibited at concentrations higher than 100âÎŒg per mL. The purified molecule is toxic for the adults of the rice weevils at concentrations down to 100âÎŒg per g of food, but this does not apply to the others insects tested, including the coleopteran <it>Tribolium castaneum</it> and the Sf9 insect cultured cells.</p> <p>Conclusions</p> <p>This specificity for the weevil led us to investigate this saponin potential for pest control and to propose the hypothesis that this saponin has a specific mode of action, rather than acting <it>via</it> its non-specific detergent properties.</p
Fast recognition of a Digital Straight Line subsegment: Two algorithms of logarithmic time complexity
International audienceGiven a Digital Straight Line (DSL) of known characteristics (a, b, ”), we address the problem of computing the characteristics of any of its subsegments. We propose two new algorithms that use the fact that a digital straight segment (DSS) can be defined by its set of separating lines. The representation of this set in the Z 2 space leads to a first algorithm of logarithmic time complexity. This algorithm precises and extends existing results for DSS recognition algorithms. The other algorithm uses the dual representation of the set of separating lines. It consists of a smart walk in the so called Farey Fan, which can be seen as the representation of all the possible sets of separating lines for DSSs. Indeed, we take profit of the fact that the Farey Fan of order n represents in a certain way all the digital segments of length n. The computation of the characteristics of a DSL subsegment is then equivalent to the localization of a point in the Farey Fan. Using fine arithmetical properties of the fan, we design a fast algorithm of theoretical complexity O(log(n)) where n is the length of the subsegment. Experiments show that our algorithms are also efficient in practice, with a comparison to the ones previously proposed by Lachaud and Said [1]: in particular, the second one is faster in the case of " small " segments
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