Showing papers by "French Institute for Research in Computer Science and Automation published in 1991"

On active contour models and balloons

Abstract: The use of energy-minimizing curves, known as “snakes,” to extract features of interest in images has been introduced by Kass, Witkin & Terzopoulos (Int. J. Comput. Vision 1, 1987, 321–331). We present a model of deformation which solves some of the problems encountered with the original method. The external forces that push the curve to the edges are modified to give more stable results. The original snake, when it is not close enough to contours, is not attracted by them and straightens to a line. Our model makes the curve behave like a balloon which is inflated by an additional force. The initial curve need no longer be close to the solution to converge. The curve passes over weak edges and is stopped only if the edge is strong. We give examples of extracting a ventricle in medical images. We have also made a first step toward 3D object reconstruction, by tracking the extracted contour on a series of successive cross sections.

The synchronous approach to reactive and real-time systems

Abstract: The state of the art in real-time programming is briefly reviewed. The synchronous approach is then introduced informally and its possible impact on the design of real-time and reactive systems is discussed. The authors present and discuss the application fields and the principles of synchronous programming. The major concern of the synchronous approach is to base synchronous programming languages on mathematical models. This makes it possible to handle compilation, logical correctness proofs, and verification of real-time programs in a formal way, leading to a clean and precise methodology for design and programming. >

Objects and views

Abstract: Object-oriented databases have been introduced primarily to ease the development of database applications. However, the difficulties encountered when, for instance, trying to restructure data or integrate databases demonstrate that the models being used still lack flexibility. We claim that the natural way to overcome these shortcomings is to introduce a sophisticated view mechanism. This paper presents such a mechanism, one which allows a programmer to restructure the class hierarchy and modify the behavior and structure of objects. The mechanism allows a programmer to specify attribute values implicitly, rather than storing them. It also allows him to introduce new classes into the class hierarchy. These virtual classes are populated by selecting existing objects from other classes and by creating new objects. Fixing the identify of new objects during database updates introduces subtle issues into view design. Our presentation, mostly informal, leans on a number of illustrative examples meant to emphasize the simplicity of our mechanism.

From Timed to Hybrid Systems

Abstract: We propose a framework for the formal specification and verification of timed and hybrid systems. For timed systems we propose a specification language that refers to time only through age functions which measure the length of the most recent time interval in which a given formula has been continuously true.

Velocity and torque feedback control of a nonholonomic cart

Abstract: A framework for designing and analysing velocity and torque feedback controls for a nonholonomic wheeled-cart is presented. A stability analysis of a set of nonlinear systems, the equations of which encompass all stable linear invariant systems, is first proposed. This analysis is then applied to the design and analysis of feedback controls for the wheeled-cart. The control inputs are either the cart's motorized wheels angular velocities (velocity control) or the torques applied to those wheels (torque control). Various control objectives are discussed and sufficient conditions for asymptotic convergence of the proposed controls are given. Among other results, the existence of smooth feedback controls that stabilize the cart at a desired position and orientation is established. This result does not contradict earlier non-existence results because the stabilizing controls depend not only on the robot's configuration variables but also on the exogeneous time variable.

Automatic mesh generator with specified boundary

Abstract: For the purpose of finite element computation for 2D or 3D geometry, one needs an appropriate mesh of the considered domain. A class of full automatic methods, derived from Voronoi's theory, is suitable to generate the mesh of any shape via a set of points which describes the geometry. Such methods providing triangles in 2D and tetrahedra in 3D can be seen, after an adequate initialization, as the merger of each given point in an existing mesh using an updating process. Unfortunately, this mesh which contains all the given points does not contain, in general, the edges (or the faces) of the boundary which are the natural data to be satisfied. The aim of this paper is, after a brief survey of the different steps of the above method, to point out the problem of the exact fitting of the given boundary and to present a method which guarantees this crucial property.

Categories, Types, and Structures: An Introduction to Category Theory for the Working Computer Scientist

Abstract: Category theory is a mathematical subject whose importance in several areas of computer science, most notably the semantics of programming languages and the design of programmes using abstract data types, is widely acknowledged. This book introduces category theory at a level appropriate for computer scientists and provides practical examples in the context of programming language design. "Categories, Types and structures" provides a self-contained introduction to general category theory and explains the mathematical structures that have been the foundation of language design for the past two decades. The authors observe that the language of categories could provide a powerful means of standardizing of methods and language, and offer examples ranging from the early dialects of LISP, to Edinburgh ML, to work in polymorphisms and modularity. The book familiarizes readers with categorical concepts through examples based on elementary mathematical notions such as monoids, groups and toplogical spaces, as well as elementary notions from programming-language semantics such as partial orders and categories of domains in denotational semantics. It then pursues the more complex mathematical semantics of data types and programs as objects and morphisms of categories.

Intrusion tolerance in distributed computing systems

Abstract: An intrusion-tolerant distributed system is a system which is designed so that any intrusion into a part of the system will not endanger confidentiality, integrity and availability. This approach is suitable for distributed systems, because distribution enables isolation of elements so that an intrusion gives physical access to only a part of the system. In particular, the intrusion-tolerant authentication and authorization servers enable a consistent security policy to be implemented on a set of heterogeneous, untrusted sites, administered by untrusted (but nonconspiring) people. The authors describe how some functions of distributed systems can be designed to tolerate intrusions. A prototype of the persistent file server presented has been successfully developed and implemented as part of the Delta-4 project of the European ESPRIT program. >

Piecewise surface flattening for non-distorted texture mapping

Abstract: This paper introduces new techniques for interactive piecewise flattening of parametric 3-D surfaces, leading to a non-distorted, hence realistic, texture mapping. Cuts are allowed on the mapped texture and we make a compromise between discontinuities and distortions. These techniques are based on results from differential geometry, more precisely on the notion of "geodesic curvature": isoparametric curves of the surface are mapped, in a constructive way, onto curves in the texture plane with preservation of geodesic curvature at each point. As an application, we give a concrete example which is a first step towards an efficient and robust CAD tool for shoe modeling.

On Correlation-Immune Functions

Abstract: We establish the link between correlation-immune functions and orthogonal arrays We give a recursive definition of any correlation-immune function of maximal degree We describe the set of quadratic balanced correlation-immune functions of maximal order Some constructions are then deduced

Positioning of a robot with respect to an object, tracking it and estimating its velocity by visual servoing

Abstract: An application is described of the so-called visual servoing approach to robot positioning with respect to an object and to target tracking. After briefly discussing how the task function approach can be applied to tasks which include the use of visual features, the authors give a simplified control expression which explicitly takes into account the case of moving objects. Estimating the target velocity while performing the tracking control leads to a kind of adaptive control scheme. The authors then consider the specific case of a square target and derive all the components of the control scheme. They present some experimental results obtained at the video rate in an experimental system composed of a camera mounted on the end effector of a six-DOF robot. >

Inheritance as implicit coercion

Abstract: We present a method for providing semantic interpretations for languages with a type system featuring inheritance polymorphism. Our approach is illustrated on an extension of the language Fun of Cardelli and Wegner, which we interpret via a translation into an extended polymorphic lambda calculus. Our goal is to interpret inheritances in Fun via coercion functions which are definable in the target of the translation. Existing techniques in the theory of semantic domains can be then used to interpret the extended polymorphic lambda calculus, thus providing many models for the original language. This technique makes it possible to model a rich type discipline which includes parametric polymorphism and recursive types as well as inheritance. A central difficulty in providing interpretations for explicit type disciplines featuring inheritance in the sense discussed in this paper arises from the fact that programs can type-check in more than one way. Since interpretations follow the type-checking derivations, coherence theorems are required: that is, one must prove that the meaning of a program does not depend on the way it was type-checked. Proofs of such theorems for our proposed interpretation are the basic technical results of this paper. Interestingly, proving coherence in the presence of recursive types, variants, and abstract types forced us to reexamine fundamental equational properties that arise in proof theory (in the form of commutative reductions) and domain theory (in the form of strict vs. non-strict functions).

Topological segmentation of discrete surfaces

Abstract: An approach to the segmentation of a discrete 3-D object into a structure of characteristic topological primitives with attached qualitative features is proposed. This structure can be seen as a qualitative description of the object. The approach concentrates on topological properties of discrete surfaces. These surfaces may correspond to the external surface of the objects, or to the skeleton surface. The labeling algorithm is based on very local computations, allowing massively parallel and real-time computations. >

Trinocular stereo vision for robotics

Abstract: An approach to building a three-dimensional description of the environment of a robot using three cameras is presented. The main advantages of trinocular versus binocular stereo are simplicity, reliability, and accuracy. It is believed that these advantages make trinocular stereo vision of practical use for many robotics applications. The technique has been successfully applied to several indoor and industrial scenes. Experimental results are presented and discussed. >

3D edge detection using recursive filtering: application to scanner images

Abstract: This paper proposes a new algorithm for three-dimensional edge detection. This method is an extension to the 3D case of the optimal 2D edge detector recently introduced by R. Deriche ( Int. J. Comput. Vison 1, 1987). It presents better theoretical and experimental performances than some classical approachs used at this date. Experimental results obtained on magnetic resonance images and on echographic images are shown. We stress that this approach can be used to detect edges in other multidimensional data, for instance 2D − t or 3D − t images.

Upstream differencing for multiphase flow in reservoir simulation

Abstract: Upstream weighting for multiphase flow in reservoir simulation is analyzed. The associated numerical flux is shown to be well defined, monotone, Lipschitz- -continuous and consistent. In the case of a two-phase flow the corresponding numerical flux is compared to that of Godunov and Engquist and Osher. Finally simple way to obtain a higher order scheme is outlined.

Animated free-form deformation: an interactive animation technique

Abstract: Current research efforts focus on providing interactive techniques that make 3D concepts easy to use and accessible to large numbers of people. In this paper, a new interactive technique for animating deformable objects is presented. The technique allows easy specification and control of a class of deformations that cannot be produced by existing techniques without considerable human intervention.The methodology proposed relies on the Free-Form Deformation (FFD) technique. It makes use of a deformation tool that is animated like any other object.This approach provides a representation of the deformations independent of the surface geometry, and can be easily integrated into traditional hierarchical animation systems.

Clustering criteria for discrete data and latent class models

Abstract: We show that a well-known clustering criterion for discrete data, the information criterion, is closely related to the classification maximum likelihood criterion for the latent class model. This relation can be derived from the Bryant-Windham construction. Emphasis is placed on binary clustering criteria which are analyzed under the maximum likelihood approach for different multivariate Bernoulli mixtures. This alternative form of criterion reveals non-apparent aspects of clustering techniques. All the criteria discussed can be optimized with the alternating optimization algorithm. Some illustrative applications are included.

On computing two-finger force-closure grasps of curved 2D objects

Abstract: An approach to the computation of stable grasps of curved two-dimensional objects is presented. The authors consider the case of a hand equipped with two hard fingers and assume point contact with friction. Objects are modeled by parametric curves, and force-closure grasps are characterized by systems of polynomial constraints in the parameters of these curves. All configuration space regions satisfying these constraints are found by a numerical cell decomposition algorithm based on curve tracing and continuation techniques. Maximal object segments where fingers can be positioned independently are found by optimization within the grasp regions. The approach has been implemented and examples are presented. >

New Results on Pseudorandom Permutation Generators Based on the DES Scheme

Abstract: We denote by ?k the permutation generator based on the DES Scheme with k rounds where the S boxes are replaced by random independant functions. We denote by |P1 - P1*|, (respectively |P1 - P1**|), the probability of distinguishing such a permutation from a random function (respectively from a random permutation) by means of a distinguishing circuit that has m oracle gates.