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Search Results: 1 - 10 of 117942 matches for " T. Vlachos "
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A Method for Single-Stimulus Quality Assessment of Segmented Video
Piroddi R,Vlachos T
EURASIP Journal on Advances in Signal Processing , 2006,
Abstract: We present a unified method for single-stimulus quality assessment of segmented video. This method takes into consideration colour and motion features of a moving sequence and monitors their changes across segment boundaries. Features are estimated using a local neighbourhood which preserves the topological integrity of segment boundaries. Furthermore the proposed method addresses the problem of unreliable and/or unavailable feature estimates by applying normalized differential convolution (NDC). Our experimental results suggest that the proposed method outperforms competing methods in terms of sensitivity as well as noise immunity for a variety of standard test sequences.
Isometric immersions of warped products
M. Dajczer,T. Vlachos
Mathematics , 2011,
Abstract: We provide conditions under which an isometric immersion of a (warped) product of manifolds into a space form must be a (warped) product of isometric immersions.
Local Path Fitting: A New Approach to Variational Integrators
D S Vlachos,O T Kosmas
Physics , 2009,
Abstract: In this work, we present a new approach to the construction of variational integrators. In the general case, the estimation of the action integral in a time interval $[q_k,q_{k+1}]$ is used to construct a symplectic map $(q_k,q_{k+1})\to (q_{k+1},q_{k+2})$. The basic idea here, is that only the partial derivatives of the estimation of the action integral of the Lagrangian are needed in the general theory. The analytic calculation of these derivatives, give raise to a new integral which depends not on the Lagrangian but on the Euler--Lagrange vector, which in the continuous and exact case vanishes. Since this new integral can only be computed through a numerical method based on some internal grid points, we can locally fit the exact curve by demanding the Euler--Lagrange vector to vanish at these grid points. Thus the integral vanishes, and the process dramatically simplifies the calculation of high order approximations. The new technique is tested for high order solutions in the two-body problem with high eccentricity (up to 0.99) and in the outer solar system.
High Order Phase-fitted Discrete Lagrangian Integrators for Orbital Problems
O. T. Kosmas,D. S. Vlachos
Physics , 2009,
Abstract: In this work, the benefits of the phase fitting technique are embedded in high order discrete Lagrangian integrators. The proposed methodology creates integrators with zero phase lag in a test Lagrangian in a similar way used in phase fitted numerical methods for ordinary differential equations. Moreover, an efficient method for frequency evaluation is proposed based on the eccentricities of the moving objects. The results show that the new method dramatically improves the accuracy and total energy behaviour in Hamiltonian systems. Numerical tests for the 2-body problem with ultra high eccentricity up to 0.99 for 1000000 periods and to the Henon-Heiles Hamiltonian system with chaotic behaviour, show the efficiency of the proposed approach.
Operational Optimal Ship Routing Using a Hybrid Parallel Genetic Algorithm
O. T. Kosmas,D. S. Vlachos
Physics , 2008,
Abstract: Optimization of ship routing depends on several parameters, like ship and cargo characteristics, environmental factors, topography, international navigation rules, crew comfort etc. The complex nature of the problem leads to oversimplifications in analytical techniques, while stochastic methods like simulated annealing can be both time consuming and sensitive to local minima. In this work, a hybrid parallel genetic algorithm - estimation of distribution algorithm is developed in the island model, to operationally calculate the optimal ship routing. The technique, which is applicable not only to clusters but to grids as well, is very fast and has been applied to very difficult environments, like the Greek seas with thousands of islands and extreme micro-climate conditions.
Simulated Annealing for Optimal Ship Routing
O. T. Kosmas,D. S. Vlachos
Physics , 2008,
Abstract: A simulated annealing based algorithm is presented for the determination of optimal ship routes through the minimization of a cost function. This cost function is a weighted sum of the time of voyage and the voyage comfort (safety is taken into account too). The latter is dependent on both the wind speed and direction and the wave height and direction. The algorithm first discretizes an initial route and optimizes it by considering small deviations which are accepted by utilizing the simulated annealing technique. Using calculus of variations we prove a key theorem which dramatically accelerates the convergence of the algorithm. Finally both simulated and real experiments are presented.
Gamma Ray Spectrum Unfolding Using Derivative Kernels
D. S. Vlachos,O. T. Kosmas
Physics , 2008,
Abstract: The unfolding of a gamma ray spectrum experience many difficulties due to noise in the recorded data, that is based mainly on the change of photon energy due to scattering mechanisms (either in the detector or the medium), the accumulation of recorded counts in a fixed energy interval (the channel width of the detector) and finally the statistical fluctuation inside the detector. In order to deal with these problems, a new method is developed which interpolates the ideal spectrum with the use of special designed derivative kernels. Preliminary simulation results are presented and show that this approach is very effective even in spectra with low statistics.
Phase-fitted Discrete Lagrangian Integrators
O. T. Kosmas,D. S. Vlachos
Physics , 2009, DOI: 10.1016/j.cpc.2009.11.005
Abstract: Phase fitting has been extensively used during the last years to improve the behaviour of numerical integrators on oscillatory problems. In this work, the benefits of the phase fitting technique are embedded in discrete Lagrangian integrators. The results show improved accuracy and total energy behaviour in Hamiltonian systems. Numerical tests on the long term integration (100000 periods) of the 2-body problem with eccentricity even up to 0.95 show the efficiency of the proposed approach. Finally, based on a geometrical evaluation of the frequency of the problem, a new technique for adaptive error control is presented.
Minimal hypersurfaces with zero Gauss-Kronecker curvature
T. Hasanis,A. Savas-Halilaj,T. Vlachos
Mathematics , 2004,
Abstract: We investigate complete minimal hypersurfaces in the Euclidean space $% \ {R}^{4}$, with Gauss-Kronecker curvature identically zero. We prove that, if $f:M^{3}\to {R}^{4}$ is a complete minimal hypersurface with Gauss-Kronecker curvature identically zero, nowhere vanishing second fundamental form and scalar curvature bounded from below, then $f(M^{3})$ splits as a Euclidean product $L^{2}\times {R}$, where $L^{2}$ is a complete minimal surface in $ {R}^{3}$ with Gaussian curvature bounded from below.
Complete minimal hypersurfaces of $S^4$ with zero Gauss-Kronecker curvature
T. Hasanis,A. Savas-Halilaj,T. Vlachos
Mathematics , 2004,
Abstract: We investigate the structure of 3-dimensional complete minimal hypersurfaces in the unit sphere with Gauss-Kronecker curvature identically zero.
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