20 research outputs found
Simplified Langevin approach to the Peyrard-Bishop-Dauxois model of DNA
A simple Langevin approach is used to study stationary properties of the
Peyrard-Bishop-Dauxois model for DNA, allowing known properties to be recovered
in an easy way. Results are shown for the denaturation transition in
homogeneous samples, for which some implications, so far overlooked, of an
analogy with equilibrium wetting transitions are highlighted. This analogy
implies that the order-parameter, asymptotically, exhibits a second order
transition even if it may be very abrupt for non-zero values of the stiffness
parameter. Not surprisingly, we also find that for heterogeneous DNA, within
this model the largest bubbles in the pre-melting stage appear in
adenine-thymine rich regions, while we suggest the possibility of some sort of
not strictly local effects owing to the merging of bubbles.Comment: 4 pages, 2 figure
Extraction des traits caractéristiques du visage à l'aide de modèles paramétriques adaptés
- Dans cet article, nous nous intéressons à l'extraction automatique des contours des traits permanents du visage à savoir : les yeux, les sourcils et les lèvres. Pour chacun des traits considérés, un modèle paramétrique spécifique capable de rendre compte de toutes les déformations possibles est défini. Lors de la phase d'initialisation, des points caractéristiques du visage sont extraits (coins des yeux et de la bouche par exemple) et servent de points d'ancrage initiaux pour chacun des modèles. Dans la phase d'évolution, chaque modèle est déformé afin de plaquer au mieux sur les contours des traits présents dans le visage analysé. Cette déformation se fait par maximisation d'un flux de gradient (de luminance et/ou de chrominance) le long des contours définis par chaque courbe du modèle. La définition de modèles permet d'introduire naturellement une contrainte de régularisation sur les contours recherchés. Mais, les modèles choisis restent suffisamment flexibles pour permettre une extraction réaliste des contours des yeux, des sourcils et de la bouche. L'extraction précise des contours des principaux traits du visage constitue la première étape d'un système de reconnaissance des dynamiques émotionnelles
Parametric models for facial features segmentation
In this paper, we are dealing with the problem of facial features segmentation (mouth, eyes and eyebrows). A specific
parametric model is defined for each feature, each model being able to take into account all the possible deformations.
In order to initialize each model, some characteristic points are extracted on each image to be processed (for example,
the corners of the eyes, mouth and eyebrows). In order to fit the model with the contours to be extracted, a gradient
flow (of luminance or chrominance ) through the estimated contour is maximized because at each point of the searched
contour, the gradient (of luminance or chrominance) is normal. The advantage of the definition of a model associated to
each feature is to be able to introduce a regularisation constraint. However, the chosen models are flexible enough in
order to produce realistic contours for the mouth, the eyes and eyebrows. This facial features segmentation is the first
step of a set of multi-media applications.Dans cet article, nous nous intéressons à l’extraction automatique des contours des traits permanents
du visage à savoir : les yeux, les sourcils et les lèvres. Pour chacun des traits considérés, un modèle
paramétrique spécifique capable de rendre compte de toutes les déformations possibles est défini. Lors de la
phase d’initialisation, des points caractéristiques du visage sont extraits (coins des yeux et de la bouche par
exemple) et servent de points d’ancrage initiaux pour chacun des modèles. Dans la phase d’évolution, chaque
modèle est déformé afin de coïncider au mieux avec les contours des traits présents sur le visage analysé.
Cette déformation se fait par maximisation d’un flux de gradient (de luminance et/ou de chrominance) le long
des contours définis par chaque courbe du modèle. La définition de modèles permet d’introduire
naturellement une contrainte de régularisation sur les contours recherchés. Néanmoins, les modèles choisis
restent suffisamment flexibles pour permettre une extraction réaliste des contours des yeux, des sourcils et
de la bouche. L’extraction précise des contours des principaux traits du visage constitue la première étape
d’un ensemble d’applications multimédia
Nonequilibrium wetting
When a nonequilibrium growing interface in the presence of a wall is
considered a nonequilibrium wetting transition may take place. This transition
can be studied trough Langevin equations or discrete growth models. In the
first case, the Kardar-Parisi-Zhang equation, which defines a very robust
universality class for nonequilibrium moving interfaces, with a soft-wall
potential is considered. While in the second, microscopic models, in the
corresponding universality class, with evaporation and deposition of particles
in the presence of hard-wall are studied. Equilibrium wetting is related to a
particular case of the problem, it corresponds to the Edwards-Wilkinson
equation with a potential in the continuum approach or to the fulfillment of
detailed balance in the microscopic models. In this review we present the
analytical and numerical methods used to investigate the problem and the very
rich behavior that is observed with them.Comment: Review, 36 pages, 16 figure
Applications of Field-Theoretic Renormalization Group Methods to Reaction-Diffusion Problems
We review the application of field-theoretic renormalization group (RG)
methods to the study of fluctuations in reaction-diffusion problems. We first
investigate the physical origin of universality in these systems, before
comparing RG methods to other available analytic techniques, including exact
solutions and Smoluchowski-type approximations. Starting from the microscopic
reaction-diffusion master equation, we then pedagogically detail the mapping to
a field theory for the single-species reaction k A -> l A (l < k). We employ
this particularly simple but non-trivial system to introduce the
field-theoretic RG tools, including the diagrammatic perturbation expansion,
renormalization, and Callan-Symanzik RG flow equation. We demonstrate how these
techniques permit the calculation of universal quantities such as density decay
exponents and amplitudes via perturbative eps = d_c - d expansions with respect
to the upper critical dimension d_c. With these basics established, we then
provide an overview of more sophisticated applications to multiple species
reactions, disorder effects, L'evy flights, persistence problems, and the
influence of spatial boundaries. We also analyze field-theoretic approaches to
nonequilibrium phase transitions separating active from absorbing states. We
focus particularly on the generic directed percolation universality class, as
well as on the most prominent exception to this class: even-offspring branching
and annihilating random walks. Finally, we summarize the state of the field and
present our perspective on outstanding problems for the future.Comment: 10 figures include
A Human Body Analysis System
<p/> <p>This paper describes a system for human body analysis (segmentation, tracking, face/hands localisation, posture recognition) from a single view that is fast and completely automatic. The system first extracts low-level data and uses part of the data for high-level interpretation. It can detect and track several persons even if they merge or are completely occluded by another person from the camera's point of view. For the high-level interpretation step, static posture recognition is performed using a belief theory-based classifier. The belief theory is considered here as a new approach for performing posture recognition and classification using imprecise and/or conflicting data. Four different static postures are considered: standing, sitting, squatting, and lying. The aim of this paper is to give a global view and an evaluation of the performances of the entire system and to describe in detail each of its processing steps, whereas our previous publications focused on a single part of the system. The efficiency and the limits of the system have been highlighted on a database of more than fifty video sequences where a dozen different individuals appear. This system allows real-time processing and aims at monitoring elderly people in video surveillance applications or at the mixing of real and virtual worlds in ambient intelligence systems.</p