Results for "Stamatios Georgoulis"

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Gated CRF Loss for Weakly Supervised Semantic Image SegmentationJun 11 2019State-of-the-art approaches for semantic segmentation rely on deep convolutional neural networks trained on fully annotated datasets, that have been shown to be notoriously expensive to collect, both in terms of time and money. To remedy this situation, ... More
Towards End-to-End Lane Detection: an Instance Segmentation ApproachFeb 15 2018Modern cars are incorporating an increasing number of driver assist features, among which automatic lane keeping. The latter allows the car to properly position itself within the road lanes, which is also crucial for any subsequent lane departure or trajectory ... More
DeLight-Net: Decomposing Reflectance Maps into Specular Materials and Natural IlluminationMar 27 2016In this paper we are extracting surface reflectance and natural environmental illumination from a reflectance map, i.e. from a single 2D image of a sphere of one material under one illumination. This is a notoriously difficult problem, yet key to various ... More
Error Correction for Dense Semantic Image LabelingDec 11 2017Pixelwise semantic image labeling is an important, yet challenging, task with many applications. Typical approaches to tackle this problem involve either the training of deep networks on vast amounts of images to directly infer the labels or the use of ... More
Non-Local Color Image Denoising with Convolutional Neural NetworksNov 21 2016We propose a novel deep network architecture for grayscale and color image denoising that is based on a non-local image model. Our motivation for the overall design of the proposed network stems from variational methods that exploit the inherent non-local ... More
Exemplar Guided Unsupervised Image-to-Image Translation with Semantic ConsistencyMay 28 2018Oct 13 2018Image-to-image translation has recently received significant attention due to advances in deep learning. Most works focus on learning either a one-to-one mapping in an unsupervised way or a many-to-many mapping in a supervised way. However, a more practical ... More
Fast Scene Understanding for Autonomous DrivingAug 08 2017Most approaches for instance-aware semantic labeling traditionally focus on accuracy. Other aspects like runtime and memory footprint are arguably as important for real-time applications such as autonomous driving. Motivated by this observation and inspired ... More
Natural Illumination from Multiple Materials Using Deep LearningNov 28 2016Recovering natural illumination from a single Low-Dynamic Range (LDR) image is a challenging task. To remedy this situation we exploit two properties often found in everyday images. First, images rarely show a single material, but rather multiple ones ... More
Exemplar Guided Unsupervised Image-to-Image Translation with Semantic ConsistencyMay 28 2018Mar 13 2019Image-to-image translation has recently received significant attention due to advances in deep learning. Most works focus on learning either a one-to-one mapping in an unsupervised way or a many-to-many mapping in a supervised way. However, a more practical ... More
What Is Around The Camera?Nov 28 2016Aug 01 2017How much does a single image reveal about the environment it was taken in? In this paper, we investigate how much of that information can be retrieved from a foreground object, combined with the background (i.e. the visible part of the environment). Assuming ... More
Disentangled Person Image GenerationDec 07 2017Jun 15 2018Generating novel, yet realistic, images of persons is a challenging task due to the complex interplay between the different image factors, such as the foreground, background and pose information. In this work, we aim at generating such images based on ... More
Analysis Of Discontinuous Galerkin Methods Using Mesh-Dependent NormsOct 17 2016We prove the inf-sup stability of a discontinuous Galerkin scheme for second order elliptic operators in (unbalanced) mesh-dependent norms for quasi-uniform meshes for all spatial dimensions. This results in a priori error bounds in these norms.
Recovered Finite Element MethodsMay 10 2017We introduce a family of Galerkin finite element methods which are constructed via recovery operators over element-wise discontinuous approximation spaces. This new family, termed collectively as recovered finite element methods (R-FEM) has a number of ... More
Hessian Schatten-Norm Regularization for Linear Inverse ProblemsSep 14 2012Feb 02 2013We introduce a novel family of invariant, convex, and non-quadratic functionals that we employ to derive regularized solutions of ill-posed linear inverse imaging problems. The proposed regularizers involve the Schatten norms of the Hessian matrix, computed ... More
Near-Sun and 1 AU magnetic field of coronal mass ejections: A parametric studySep 01 2016Aims. The magnetic field of coronal mass ejections (CMEs) determines their structure, evolution, and energetics, as well as their geoeffectiveness. However, we currently lack routine diagnostics of the near-Sun CME magnetic field, which is crucial for ... More
Analysis of Discontinuous Galerkin Methods using Mesh-Dependent Norms and Applications to Problems with Rough DataOct 17 2016Mar 28 2017We prove the inf-sup stability of a discontinuous Galerkin scheme for second order elliptic operators in (unbalanced) mesh-dependent norms for quasi-uniform meshes for all spatial dimensions. This results in a priori error bounds in these norms. As an ... More
A posteriori error bounds for discontinuous Galerkin methods for quasilinear parabolic problemsJan 17 2010We derive a posteriori error bounds for a quasilinear parabolic problem, which is approximated by the $hp$-version interior penalty discontinuous Galerkin method (IPDG). The error is measured in the energy norm. The theory is developed for the semidiscrete ... More
An a posteriori error estimator for discontinuous Galerkin methods for non-stationary convection-diffusion problemsNov 15 2012This work is concerned with the derivation of a robust a posteriori error estimator for a discontinuous Galerkin method discretisation of linear non-stationary convection-diffusion initial/boundary value problems and with the implementation of a corresponding ... More
$hp$-Version space-time discontinuous Galerkin methods for parabolic problems on prismatic meshesMay 04 2016Apr 28 2017We present a new $hp$-version space-time discontinuous Galerkin (dG) finite element method for the numerical approximation of parabolic evolution equations on general spatial meshes consisting of polygonal/polyhedral (polytopic) elements, giving rise ... More
A posteriori $L^\infty(L^2)$-error bounds in finite element approximation of the wave equationMar 18 2010Nov 28 2012We address the error control of Galerkin discretization (in space) of linear second order hyperbolic problems. More specifically, we derive a posteriori error bounds in the L\infty(L2)-norm for finite element methods for the linear wave equation, under ... More
Babuška-Osborn techniques in discontinuous Galerkin methods: $L^2$-norm error estimates for unstructured meshesApr 18 2017Oct 24 2018We prove the inf-sup stability of the interior penalty class of discontinuous Galerkin schemes in unbalanced mesh-dependent norms, under a mesh condition allowing for a general class of meshes, which includes many examples of geometrically graded element ... More
Adaptive discontinuous Galerkin approximations to fourth order parabolic problemsMar 11 2013An adaptive algorithm, based on residual type a posteriori indicators of errors measured in $L^{\infty}(L^2)$ and $L^2(L^2)$ norms, for a numerical scheme consisting of implicit Euler method in time and discontinuous Galerkin method in space for linear ... More
Multilevel Sparse Kernel-Based InterpolationApr 18 2012A multilevel kernel-based interpolation method, suitable for moderately high-dimensional function interpolation problems, is proposed. The method, termed multilevel sparse kernel-based interpolation (MLSKI, for short), uses both level-wise and direction-wise ... More
Discontinuous Galerkin Methods for Mass Transfer through Semi-Permeable MembranesSep 11 2012Apr 14 2013A discontinuous Galerkin (dG) method for the numerical solution of initial/boundary value multi-compartment partial differential equation (PDE) models, interconnected with interface conditions, is presented and analysed. The study of interface problems ... More
Nonlinear force-free reconstruction of the global solar magnetic field: methodologyNov 24 2010Nov 25 2010We present a novel numerical method that allows the calculation of nonlinear force-free magnetostatic solutions above a boundary surface on which only the distribution of the normal magnetic field component is given. The method relies on the theory of ... More
$hp$-Version space-time discontinuous Galerkin methods for parabolic problems on prismatic meshesMay 04 2016We present a new $hp$-version space-time discontinuous Galerkin (dG) finite element method for the numerical approximation of parabolic evolution equations on general spatial meshes consisting of polygonal/polyhedral (polytopic) elements, giving rise ... More
Recovered finite element methods on polygonal and polyhedral meshesApr 23 2018Recovered finite element methods (R-FEM) have been recently introduced for meshes consisting of simplicial and/or box-type meshes. Here, utilising the flexibility of R-FEM framework, we extend their definition on polygonal and polyhedral meshes in two ... More
$hp$-Version discontinuous Galerkin methods on essentially arbitrarily-shaped elementsJun 04 2019We extend the applicability of the popular interior-penalty discontinuous Galerkin (dG) method discretizing advection-diffusion-reaction problems to meshes comprising extremely general, essentially arbitrarily-shaped element shapes. In particular, our ... More
A Trefftz polynomial space-time discontinuous Galerkin method for the second order wave equationOct 06 2016A new space-time discontinuous Galerkin (dG) method utilising special Trefftz polynomial basis functions is proposed and fully analysed for the scalar wave equation in a second order formulation. The dG method considered is motivated by the class of interior ... More
Statistical properties of the energy release in emerging and evolving active regionJul 16 2002The formation and evolution of active regions is an inherently complex phenomenon. Magnetic fields generated at the base of the convection zone follow a chaotic evolution before reaching the solar surface. In this article, we use a 2-D probabilistic Cellular ... More
Virtual Element Method for Quasilinear Elliptic ProblemsJul 05 2017May 25 2018We present a Virtual Element Method (VEM) for the solution of Dirichlet problems for the quasilinear equation $-\text{div} (k(u)\text{grad} u)=f$ with essential boundary conditions. Within the VEM the nonlinear coefficient is evaluated with the piecewise ... More
Extending the range of error estimates for radial approximation in Euclidean space and on spheresMay 30 2007We adapt Schaback's error doubling trick [R. Schaback. Improved error bounds for scattered data interpolation by radial basis functions. Math. Comp., 68(225):201--216, 1999.] to give error estimates for radial interpolation of functions with smoothness ... More
Simulating Flaring Events in Complex Active Regions Driven by Observed MagnetogramsFeb 11 2011We interpret solar flares as events originating from active regions that have reached the Self Organized Critical state, by using a refined Cellular Automaton model with initial conditions derived from observations. Aims: We investigate whether the system, ... More
Implementation of the Continuous-Discontinuous Galerkin Finite Element MethodJan 13 2012For the stationary advection-diffusion problem the standard continuous Galerkin method is unstable without some additional control on the mesh or method. The interior penalty discontinuous Galerkin method is stable but at the expense of an increased number ... More
A posteriori error estimates for the virtual element methodMar 18 2016Apr 24 2017An posteriori error analysis for the virtual element method (VEM) applied to general elliptic problems is presented. The resulting error estimator is of residual-type and applies on very general polygonal/polyhedral meshes. The estimator is fully computable ... More
Fast multilevel sparse Gaussian kernels for high-dimensional approximation and integrationJan 14 2015A fast multilevel algorithm based on directionally scaled tensor-product Gaussian kernels on structured sparse grids is proposed for interpolation of high-dimensional functions and for the numerical integration of high-dimensional integrals. The algorithm ... More
A posteriori error bounds for fully-discrete hp-discontinuous Galerkin timestepping methods for parabolic problemsAug 19 2017We consider fully discrete time-space approximations of abstract linear parabolic partial differential equations (PDEs) consisting of an $hp$-version discontinuous Galerkin (DG) time stepping scheme in conjunction with standard (conforming) Galerkin discretizations ... More
Analysing the effects of apodizing windows on local correlation tracking using Nirvana simulations of convectionFeb 09 2015We employ different shapes of apodizing windows in the local correlation tracking (LCT) routine to retrieve horizontal velocities using numerical simulations of convection. LCT was applied on a time sequence of temperature maps generated by the Nirvana ... More
On the Stability of Continuous-Discontinuous Galerkin Methods for Advection-Diffusion-Reaction ProblemsNov 05 2012We consider a finite element method which couples the continuous Galerkin method away from internal and boundary layers with a discontinuous Galerkin method in the vicinity of layers. We prove that this consistent method is stable in the streamline diffusion ... More
A posteriori error control for discontinuous Galerkin methods for parabolic problemsApr 27 2008Mar 18 2010We derive energy-norm a posteriori error bounds for an Euler time-stepping method combined with various spatial discontinuous Galerkin schemes for linear parabolic problems. For accessibility, we address first the spatially semidiscrete case, and then ... More
Convergence of a discontinuous Galerkin multiscale methodNov 23 2012A convergence result for a discontinuous Galerkin multiscale method for a second order elliptic problem is presented. We consider a heterogeneous and highly varying diffusion coefficient in $L^\infty(\Omega,\mathbb{R}^{d\times d}_{sym})$ with uniform ... More
Adaptivity and blow-up detection for nonlinear evolution problemsFeb 11 2015This work is concerned with the development of a space-time adaptive numerical method, based on a rigorous a posteriori error bound, for a semilinear convection-diffusion problem which may exhibit blow-up in finite time. More specifically, a posteriori ... More
A posteriori error estimates for leap-frog and cosine methods for second order evolution problemsNov 27 2014We consider second order explicit and implicit two-step time-discrete schemes for wave-type equations. We derive optimal order aposteriori estimates controlling the time discretization error. Our analysis, has been motivated by the need to provide aposteriori ... More
On Local Super-Penalization of Interior Penalty Discontinuous Galerkin MethodsMay 25 2012We prove in an abstract setting that standard (continuous) Galerkin finite element approximations are the limit of interior penalty discontinuous Galerkin approximations as the penalty parameter tends to infinity. We apply this result to equations of ... More
Accurate fibre dispersion from multiple diffusion encodingJan 17 2019Measuring fibre dispersion in white matter with diffusion magnetic resonance imaging (MRI) remains difficult due to an inherent degeneracy between fibre dispersion and diffusion anisotropy. This means that estimates of fibre dispersion rely on strong ... More
Collective Modes in Two Dimensional Binary Yukawa SystemsNov 27 2012We analyze via theoretical approaches and molecular dynamics simulations the collective mode structure of strongly coupled two-dimensional binary Yukawa systems, for selected density, mass and charge ratios, both in the liquid and crystalline solid phases. ... More
Individual rules for trail pattern formation in Argentine ants (Linepithema humile)Jan 27 2012May 09 2012We studied the formation of trail patterns by Argentine ants exploring an empty arena. Using a novel imaging and analysis technique we estimated pheromone concentrations at all spatial positions in the experimental arena and at different times. Then we ... More
Non-neutralized Electric Current Patterns in Solar Active Regions: Origin of the Shear-Generating Lorentz ForceOct 10 2012Using solar vector magnetograms of the highest available spatial resolution and signal-to-noise ratio we perform a detailed study of electric current patterns in two solar active regions: a flaring/eruptive, and a flare-quiet one. We aim to determine ... More
A posteriori error estimates for the virtual element methodMar 18 2016An posteriori error analysis for the virtual element method (VEM) applied to general elliptic problems is presented. The resulting error estimator is of residual-type and applies on very general polygonal/polyhedral meshes. The estimator is fully computable ... More
Bayesian Image Quality Transfer with CNNs: Exploring Uncertainty in dMRI Super-ResolutionMay 01 2017May 30 2017In this work, we investigate the value of uncertainty modeling in 3D super-resolution with convolutional neural networks (CNNs). Deep learning has shown success in a plethora of medical image transformation problems, such as super-resolution (SR) and ... More
An observationally-driven kinetic approach to coronal heatingMar 23 2016Sep 27 2016Coronal heating through the explosive release of magnetic energy remains an open problem in solar physics. Recent hydrodynamical models attempt an investigation by placing swarms of 'nanoflares' at random sites and times in modeled one-dimensional coronal ... More
Combining particle acceleration and coronal heating via data-constrained calculations of nanoflares in coronal loopsMay 22 2013We model nanoflare heating of extrapolated active-region coronal loops via the acceleration of electrons and protons in Harris-type current sheets. The kinetic energy of the accelerated particles is estimated using semi-analytical and test-particle-tracing ... More
Study of the three-dimensional shape and dynamics of coronal loops observed by Hinode/EISJun 01 2012We study plasma flows along selected coronal loops in NOAA Active Region 10926, observed on 3 December 2006 with Hinode's EUV Imaging Spectrograph (EIS). From the shape of the loops traced on intensity images and the Doppler shifts measured along their ... More
Micro-Sigmoids as Progenitors of Coronal Jets - Is Eruptive Activity Self-Similarly Multi-Scaled?May 21 2010Observations from the X-ray telescope (XRT) on Hinode are used to study the nature of X-ray bright points, sources of coronal jets. Several jet events in the coronal holes are found to erupt from small-scale, S-shaped bright regions. This finding suggests ... More
Non-neutralized electric currents in solar active regions and flare productivityAug 23 2017We explore the association of non-neutralized currents with solar flare occurrence in a sizable sample of observations, aiming to show the potential of such currents in solar flare prediction. We use the regularly produced high quality vector magnetograms ... More
Micro-Sigmoids as Progenitors of Polar Coronal JetsSep 15 2010Observations from the Hinode X-ray telescope (XRT) are used to study the structure of X-ray bright points (XBPs), sources of coronal jets. Several jet events are found to erupt from S-shaped bright points, suggesting that coronal micro-sigmoids are progenitors ... More
The correlation of fractal structures in the photospheric and the coronal magnetic fieldAug 27 2009This work examines the relation between the fractal properties of the photospheric magnetic patterns and those of the coronal magnetic fields in solar active regions.Our results show no correlation between the fractal dimensions of the observed 2d photospheric ... More
Active Region Photospheric Magnetic Properties Derived from Line-of-sight and Radial FieldsDec 19 2017The effect of using two representations of the normal-to-surface magnetic field to calculate photospheric measures that are related to active region (AR) potential for flaring is presented. Several AR properties were computed using line-of-sight ($B_{\rm ... More
Testing and Improving a Set of Morphological Predictors of Flaring ActivityJul 17 2018Efficient prediction of solar flares relies on parameters that quantify the eruptive capability of solar active regions. Several such quantitative predictors have been proposed in the literature, inferred mostly from photospheric magnetograms and/or white-light ... More
25 Years of Self-Organized Criticality: Numerical Detection MethodsJun 26 2015The detection and characterization of self-organized criticality (SOC), in both real and simulated data, has undergone many significant revisions over the past 25 years. The explosive advances in the many numerical methods available for detecting, discriminating, ... More
Magnetic helicity estimations in models and observations of the solar magnetic field. Part I: Finite volume methodsOct 07 2016Magnetic helicity is a conserved quantity of ideal magneto-hydrodynamics characterized by an inverse turbulent cascade. Accordingly, it is often invoked as one of the basic physical quantities driving the generation and structuring of magnetic fields ... More
Magnetic Helicity Estimations in Models and Observations of the Solar Magnetic Field. Part III: Twist Number MethodApr 07 2017We study the writhe, twist and magnetic helicity of different magnetic flux ropes, based on models of the solar coronal magnetic field structure. These include an analytical force-free Titov--D\'emoulin equilibrium solution, non force-free magnetohydrodynamic ... More
Spherical deconvolution of multichannel diffusion MRI data with non-Gaussian noise models and spatial regularizationOct 23 2014Oct 08 2015Spherical deconvolution (SD) methods are widely used to estimate the intra-voxel white-matter fiber orientations from diffusion MRI data. However, while some of these methods assume a zero-mean Gaussian distribution for the underlying noise, its real ... More
SOLIS Vector Spectromagnetograph: status and scienceDec 28 2007The Vector Spectromagnetograph (VSM) instrument has been recording photospheric and chromospheric magnetograms daily since August 2003. Full-disk photospheric vector magnetograms are observed at least weekly and, since November 2006, area-scans of active ... More
On the evolution of pre-flare patterns of a 3-dimensional model of AR 11429Jan 01 2018We apply a novel pre-flare tracking of sunspot groups towards improving the estimation of flare onset time by focusing on the evolution of the 3D magnetic field construction of AR 11429. The 3D magnetic structure is based on potential field extrapolation ... More
ATCA-based ATLAS FTK input interface systemOct 31 2014The first stage of the ATLAS Fast TracKer (FTK) is an ATCA-based input interface system, where hits from the entire silicon tracker are clustered and organized into overlapping eta-phi trigger towers before being sent to the tracking engines. First, FTK ... More
25 Years of Self-Organized Criticality: Solar and AstrophysicsMar 25 2014Shortly after the seminal paper {\sl "Self-Organized Criticality: An explanation of 1/f noise"} by Bak, Tang, and Wiesenfeld (1987), the idea has been applied to solar physics, in {\sl "Avalanches and the Distribution of Solar Flares"} by Lu and Hamilton ... More
The Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) on the New Horizons MissionSep 27 2007The Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) comprises the hardware and accompanying science investigation on the New Horizons spacecraft to measure pick-up ions from Pluto's outgassing atmosphere. To the extent that Pluto ... More
A Comparison of Flare Forecasting Methods, I: Results from the "All-Clear" WorkshopAug 22 2016Solar flares produce radiation which can have an almost immediate effect on the near-Earth environment, making it crucial to forecast flares in order to mitigate their negative effects. The number of published approaches to flare forecasting using photospheric ... More
Forecasting Solar Flares Using Magnetogram-based Predictors and Machine LearningJan 17 2018We propose a forecasting approach for solar flares based on data from Solar Cycle 24, taken by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) mission. In particular, we use the Space-weather HMI Active Region ... More
Solar System Ice Giants: Exoplanets in our BackyardApr 10 2018Future remote sensing of exoplanets will be enhanced by a thorough investigation of our solar system Ice Giants (Neptune-size planets). What can the configuration of the magnetic field tell us (remotely) about the interior, and what implications does ... More