References of "Van Droogenbroeck, Marc"
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See detailA perfect estimation of a background image does not lead to a perfect background subtraction: analysis of the upper bound on the performance
Pierard, Sébastien ULg; Van Droogenbroeck, Marc ULg

in International Conference on Image Analysis and Processing (ICIAP) (2015, September)

The quest for the “best” background subtraction technique is ongoing. Despite that a considerable effort has been undertaken to develop flexible and efficient methods, some elementary questions are still ... [more ▼]

The quest for the “best” background subtraction technique is ongoing. Despite that a considerable effort has been undertaken to develop flexible and efficient methods, some elementary questions are still unanswered. One of them is the existence of an intrinsic upper bound to the performance. In fact, data are affected by noise, and therefore it is illusory to believe that it is possible to achieve a perfect segmentation. This paper aims at exploring some intrinsic limitations of the principle of background subtraction. The purpose consists in studying the impact of several limiting factors separately. One of our conclusions is that even if an algorithm would be able to calculate a perfect background image, it is not sufficient to achieve a perfect segmentation with background subtraction, due to other intrinsic limitations. [less ▲]

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See detailDirect exoplanet imaging with small-angle Vortex coronagraphs
Defrere, Denis ULg; Absil, Olivier ULg; Mawet, D. et al

Conference (2015, July 16)

Vortex coronagraphs are among the most promising solutions to perform high contrast imaging at small angular separations from bright stars. They enhance the dynamic range at very small inner working angle ... [more ▼]

Vortex coronagraphs are among the most promising solutions to perform high contrast imaging at small angular separations from bright stars. They enhance the dynamic range at very small inner working angle (down to the diffraction limit of the telescope) and provide a clear 360 degree discovery space for high-contrast direct imaging of exoplanets. In this talk, we will report on the first scientific results obtained with Vortex coronagraphs installed on 10-m class telescopes (i.e., the VLT and the LBT) and on the recent installation of one Vortex at Keck. We will describe the in-lab and on-sky performance of the Vortex, and describe the lessons learned after a few years of operation. Finally, we will discuss the prospects of our vortices for future extremely large telescopes and space missions. [less ▲]

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See detailIndividual variation of gait characteristics along a 500 meter walk in people with multiple sclerosis and healthy volunteers
Phan-Ba, Rémy; Pierard, Sébastien ULg; Giet, Amaury et al

in Multiple Sclerosis Journal (2015, April 10), 21(4), 532-533

Background: we previously demonstrated the usefulness of the Deceleration Index (DI, the ratio between the last 100m of the Timed 500-Meter Walk test –T500MW – and the walking speed - WS – of the Timed 25 ... [more ▼]

Background: we previously demonstrated the usefulness of the Deceleration Index (DI, the ratio between the last 100m of the Timed 500-Meter Walk test –T500MW – and the walking speed - WS – of the Timed 25-Foot Walk Test with a propelled start – T25FW+) to evaluate motor fatigue over a long walking distance in people with multiple sclerosis (pwMS). We also recently designed and internally validated a new gait analysis tool for pwMS (GAIMS) that can measure other relevant gait characteristics than the sole WS, such as ataxia, asymmetry and perhaps spasticity. Aims: (i) To compare various gait characteristics between the last and the first 100m of the T500MW in a population of pwMS and healthy volunteers (HV), (ii) to compare the ratio between the last and the first 100m of the T500MW with the DI, and (iii) their relationship with the EDSS. Methods : Subjects were asked to perform the T25FW+ and the T500MW as part of a multimodal evaluation at the MS Clinic of the CHU of Liège. Their gait characteristics were measured using GAIMS. (i) Paired Student’s t-tests were performed on various gait characteristics extracted during the last and first 100m of the T500MW with .05 as a level of significance, (ii) Spearman correlation coefficient (ρ) was calculated (ii) between these ratio and (iii) subject’s EDSS. Results: Seventy-one pwMS and 129 were enrolled in our study. (i) Significant differences were observed for speed related gait characteristics between the last and first 100m of the T500MW, but also for gait characteristics related to ataxia and precision of foot placement. (ii) A moderate positive correlation was observed between the WS ratio of the last and first 100m of the T500MW and the DI. (iii) The correlation between the DI and the EDSS was weakly negative, while the one between the last and first 100m of the T500MW ratio and the EDSS was moderatly negative. Conclusion: (i) As previously demonstrated, we here confirm that alongside to WS, there are other gait features affected by locomotor fatigue over a long walking distance, (ii) the moderate positive correlation between the DI and the last/first 100m of the T500MW indicates that these measures are not the same and that next to a long distance walking test such as the T500MW, a short one such as the T25FW+ remains useful. (iii) The last/first 100m of the T500MW is better correlated to the EDSS and might be a better predictive tool of pwMS’ neurologic state than the DI. [less ▲]

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See detailIndividual variation of gait parameters along a 500 meter walk in people with multiple sclerosis and healthy volonteers
Phan-Ba, Rémy; Giet, Amaury; Pierard, Sébastien ULg et al

Poster (2015, April 10)

Background: we previously demonstrated the usefulness of the Deceleration Index (DI, the ratio between the last 100m of the Timed 500-Meter Walk test –T500MW – and the walking speed - WS – of the Timed 25 ... [more ▼]

Background: we previously demonstrated the usefulness of the Deceleration Index (DI, the ratio between the last 100m of the Timed 500-Meter Walk test –T500MW – and the walking speed - WS – of the Timed 25-Foot Walk Test with a propelled start – T25FW+) to evaluate motor fatigue over a long walking distance in people with multiple sclerosis (pwMS). We also recently designed and internally validated a new gait analysis tool for pwMS (GAIMS) that can measure other relevant gait characteristics than the sole WS, such as ataxia, asymmetry and perhaps spasticity. Aims: (i) To compare various gait characteristics between the last and the first 100m of the T500MW in a population of pwMS and healthy volunteers (HV), (ii) to compare the ratio between the last and the first 100m of the T500MW with the DI, and (iii) their relationship with the EDSS. Methods : Subjects were asked to perform the T25FW+ and the T500MW as part of a multimodal evaluation at the MS Clinic of the CHU of Liège. Their gait characteristics were measured using GAIMS. (i) Paired Student’s t-tests were performed on various gait characteristics extracted during the last and first 100m of the T500MW with .05 as a level of significance, (ii) Spearman correlation coefficient (ρ) was calculated (ii) between these ratio and (iii) subject’s EDSS. Results: Seventy-one pwMS and 129 were enrolled in our study. (i) Significant differences were observed for speed related gait characteristics between the last and first 100m of the T500MW, but also for gait characteristics related to ataxia and precision of foot placement. (ii) A moderate positive correlation was observed between the WS ratio of the last and first 100m of the T500MW and the DI. (iii) The correlation between the DI and the EDSS was weakly negative, while the one between the last and first 100m of the T500MW ratio and the EDSS was moderatly negative. Conclusion: (i) As previously demonstrated, we here confirm that alongside to WS, there are other gait features affected by locomotor fatigue over a long walking distance, (ii) the moderate positive correlation between the DI and the last/first 100m of the T500MW indicates that these measures are not the same and that next to a long distance walking test such as the T500MW, a short one such as the T25FW+ remains useful. (iii) The last/first 100m of the T500MW is better correlated to the EDSS and might be a better predictive tool of pwMS’ neurologic state than the DI. [less ▲]

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See detailGAIMS: a tool specifically developed for the clinical gait analysis of patients with multiple sclerosis
Pierard, Sébastien ULg; Phan-Ba, Rémy; Giet, Amaury et al

in Multiple Sclerosis Journal (2015, April), 21(4), 498-499

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See detailComputer Vision
Van Droogenbroeck, Marc ULg

Learning material (2015)

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See detailA physically motivated pixel-based model for background subtraction in 3D images
Braham, Marc ULg; Lejeune, Antoine ULg; Van Droogenbroeck, Marc ULg

in International Conference on 3D Imaging (IC3D) (2014, December)

This paper proposes a new pixel-based background subtraction technique, applicable to range images, to detect motion. Our method exploits the physical meaning of depth information, which leads to an ... [more ▼]

This paper proposes a new pixel-based background subtraction technique, applicable to range images, to detect motion. Our method exploits the physical meaning of depth information, which leads to an improved background/foreground segmentation and the instantaneous suppression of ghosts that would appear on color images. In particular, our technique considers certain characteristics of depth measurements, such as failures for certain pixels or the non-uniformity of the spatial distribution of noise in range images, to build an improved pixel-based background model. Experiments show that incorporating specificities related to depth measurements allows us to propose a method whose performance is increased with respect to other state-of-the-art methods. [less ▲]

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See detailEvaluation of pairwise calibration techniques for range cameras and their ability to detect a misalignment
Lejeune, Antoine ULg; Grogna, David ULg; Van Droogenbroeck, Marc ULg et al

in International Conference on 3D Imaging (IC3D) (2014, December)

Many applications require the use of multiple cameras to cover a larger volume. In this paper, we evaluate several pair-wise calibration techniques dedicated to range cameras. We compare the precision of ... [more ▼]

Many applications require the use of multiple cameras to cover a larger volume. In this paper, we evaluate several pair-wise calibration techniques dedicated to range cameras. We compare the precision of a self-calibration technique based on the movement in front of the cameras to object based calibration. While the self-calibration method is less precise than its counterparts, it yields a first estimation of the transformation between the cameras and permits to detect when the cameras become mis-aligned. Therefore it is useful in a practical situations. [less ▲]

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See detailDiagnosing multiple sclerosis with a gait measuring system, an analysis of the motor fatigue, and machine learning
Pierard, Sébastien ULg; Azrour, Samir ULg; Phan-Ba, Rémy et al

in Multiple Sclerosis Journal (2014, September 11), 20(S1), 171

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See detailThe VORTEX project: first results and perspectives
Absil, Olivier ULg; Mawet, Dimitri; Delacroix, Christian ULg et al

in Marchetti, Enrico; Close, Laird; Véran, Jean-Pierre (Eds.) Adaptive Optics Systems IV (2014, July 21)

Vortex coronagraphs are among the most promising solutions to perform high contrast imaging at small angular separations from bright stars. They feature a very small inner working angle (down to the ... [more ▼]

Vortex coronagraphs are among the most promising solutions to perform high contrast imaging at small angular separations from bright stars. They feature a very small inner working angle (down to the diffraction limit of the telescope), a clear 360 degree discovery space, have demonstrated very high contrast capabilities, are easy to implement on high-contrast imaging instruments, and have already been extensively tested on the sky. Since 2005, we have been designing, developing and testing an implementation of the charge-2 vector vortex phase mask based on concentric sub-wavelength gratings, referred to as the Annular Groove Phase Mask (AGPM). Science-grade mid-infrared AGPMs were produced in 2012 for the first time, using plasma etching on synthetic diamond substrates. They have been validated on a coronagraphic test bench, showing broadband peak rejection up to 500:1 in the L band, which translates into a raw contrast of about 6e-5 at 2λ/D. Three of them have now been installed on world-leading diffraction-limited infrared cameras, namely VLT/NACO, VLT/VISIR and LBT/LMIRCam. During the science verification observations with our L-band AGPM on NACO, we observed the beta Pictoris system and obtained unprecedented sensitivity limits to planetary companions down to the diffraction limit (0.1"). More recently, we obtained new images of the HR 8799 system at L band during the AGPM first light on LMIRCam. After reviewing these first results obtained with mid-infrared AGPMs, we will discuss the short- and mid-term goals of the on-going VORTEX project, which aims to improve the performance of our vortex phase masks for future applications on second-generation high-contrast imager and on future extremely large telescopes (ELTs). In particular, we will briefly describe our current efforts to improve the manufacturing of mid-infrared AGPMs, to push their operation to shorter wavelengths, and to provide deeper starlight extinction by creating new designs for higher topological charge vortices. Within the VORTEX project, we also plan to develop new image processing techniques tailored to coronagraphic images, and to study some pre- and post-coronagraphic concepts adapted to the vortex coronagraph in order to reduce scattered starlight in the final images. [less ▲]

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See detailViBe: A Disruptive Method for Background Subtraction
Van Droogenbroeck, Marc ULg; Barnich, Olivier

in Bouwans; Porikli; Hoferlin (Eds.) et al Background Modeling and Foreground Detection for Video Surveillance (2014)

This chapter presents ViBe and the underlying ideas of the algorithm. ViBe is an algorithm for the dection of motion by background subtraction. It is a very fast algorithm, based on samples and several ... [more ▼]

This chapter presents ViBe and the underlying ideas of the algorithm. ViBe is an algorithm for the dection of motion by background subtraction. It is a very fast algorithm, based on samples and several innovative processes (time subsampling, random substitution, spatial diffusion, etc). In addtion, we propose a way to measure the computation time by introducing the notion of complexity factor. [less ▲]

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See detailOverview and Benchmarking of Motion Detection Methods
Jodoin, Pierre-Marc; Pierard, Sébastien ULg; Wang, Yi et al

in Bouwmans; Porikli; Hoferlin (Eds.) et al Background Modeling and Foreground Detection for Video Surveillance (2014)

In this chapter, we provide an overview of the most highly cited motion detection meth- ods. We identify the most commonly used background models together with their features, the kind of updating scheme ... [more ▼]

In this chapter, we provide an overview of the most highly cited motion detection meth- ods. We identify the most commonly used background models together with their features, the kind of updating scheme they use, some spatial aggregation models as well as the most widely used post-processing operations. We also provide an overview of datasets used to validate motion detection methods. Please note that this literature review is by no means exhaustive and thus we provide a list of surveys that the reader can rely on for further details. [less ▲]

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See detailOn-the-fly domain adaptation of binary classifiers
Pierard, Sébastien ULg; Marcos Alvarez, Alejandro ULg; Lejeune, Antoine ULg et al

in 23rd Belgian-Dutch Conference on Machine Learning (BENELEARN) (2014, June 06)

This work considers the on-the-fly domain adaptation of supervised binary classifiers, learned off-line, in order to adapt them to a target context. The probability density functions associated to ... [more ▼]

This work considers the on-the-fly domain adaptation of supervised binary classifiers, learned off-line, in order to adapt them to a target context. The probability density functions associated to negative and positive classes are supposed to be mixtures of the source distributions. Moreover, the mixture weights and the priors are only available at runtime. We present a theoretical solution to this problem, and demonstrate the effectiveness of the proposed approach on a real computer vision application. Our theoretical solution is applicable to any classifier approximating Bayes' classifier. [less ▲]

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See detailA New Three Object Triangulation Algorithm for Mobile Robot Positioning
Pierlot, Vincent ULg; Van Droogenbroeck, Marc ULg

in IEEE Transactions on Robotics (2014), 30(3), 566-577

Positioning is a fundamental issue in mobile robot applications. It can be achieved in many ways. Among them, triangulation based on angles measured with the help of beacons is a proven technique. Most of ... [more ▼]

Positioning is a fundamental issue in mobile robot applications. It can be achieved in many ways. Among them, triangulation based on angles measured with the help of beacons is a proven technique. Most of the many triangulation algorithms proposed so far have major limitations. For example, some of them need a particular beacon ordering, have blind spots, or only work within the triangle defined by the three beacons. More reliable methods exist; however, they have an increasing complexity or they require to handle certain spatial arrangements separately. In this paper, we present a simple and new three object triangulation algorithm, named ToTal, that natively works in the whole plane, and for any beacon ordering. We also provide a comprehensive comparison between many algorithms, and show that our algorithm is faster and simpler than comparable algorithms. In addition to its inherent efficiency, our algorithm provides a very useful and unique reliability measure, assessable anywhere in the plane, which can be used to identify pathological cases, or as a validation gate in Kalman filters. [less ▲]

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See detailBeAMS: a Beacon based Angle Measurement Sensor for mobile robot positioning
Pierlot, Vincent ULg; Van Droogenbroeck, Marc ULg

in IEEE Transactions on Robotics (2014), 30(30), 533-549

Positioning is a fundamental issue in mobile robot applications, and it can be achieved in multiple ways. Among these methods, triangulation based on angle measurements is widely used, robust, accurate ... [more ▼]

Positioning is a fundamental issue in mobile robot applications, and it can be achieved in multiple ways. Among these methods, triangulation based on angle measurements is widely used, robust, accurate, and flexible. This paper presents BeAMS, a new active beacon-based angle measurement system used for mobile robot positioning. BeAMS introduces several major innovations. One innovation is the use of a unique unsynchronized channel with On-Off Keying modulated infrared signals to measure angles and to identify the beacons. We also introduce a new mechanism to measure angles: our system detects a beacon when it enters and leaves an angular window. We show that the estimator resulting from the center of this angular window provides an unbiased estimate of the beacon angle. A theoretical framework for a thorough performance analysis of BeAMS is provided. We establish the upper bound of the variance and validate this bound through experiments and simulations; the overall error measure of BeAMS is lower than 0.24 deg for an acquisition rate of 10 Hz. In conclusion, BeAMS is a low power, flexible, and robust solution for angle measurement, and a reliable component for robot positioning. [less ▲]

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See detailPerformances of low-level audio classifiers for large-scale music similarity
Osmalsky, Julien ULg; Van Droogenbroeck, Marc ULg; Embrechts, Jean-Jacques ULg

in International Conference on Systems, Signals and Image Processing (2014, May)

This paper proposes a survey of the performances of binary classifiers based on low-level audio features, for music similarity in large-scale databases. Various low-level descriptors are used individually ... [more ▼]

This paper proposes a survey of the performances of binary classifiers based on low-level audio features, for music similarity in large-scale databases. Various low-level descriptors are used individually and then combined using several fusion schemes in a content-based audio retrieval system. We show the performances of the classifiers in terms of pruning and loss and we demonstrate that some combination schemes achieve a better performance at a minimum computational cost. [less ▲]

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See detailDesign of a reliable processing pipeline for the non-intrusive measurement of feet trajectories with lasers
Pierard, Sébastien ULg; Azrour, Samir ULg; Van Droogenbroeck, Marc ULg

in International Conference on Acoustics, Speech, and Signal Processing (ICASSP) (2014, May)

Reliable measurements of feet trajectories are needed in some applications, such as biomedical applications. This paper describes the data processing pipeline used in GAIMS, which is a non-intrusive ... [more ▼]

Reliable measurements of feet trajectories are needed in some applications, such as biomedical applications. This paper describes the data processing pipeline used in GAIMS, which is a non-intrusive system that measures feet trajectories based on multiple range laser scanners. Our processing pipeline relies on a new tracking paradigm, and it is based on two innovative algorithms: the first algorithm localizes the feet directly from the observed point cloud without any clustering, and the other algorithm identifies the feet. After reviewing the various types of noise affecting the point cloud, this paper explains the limitations of the classical processing approach and gives an overview of our new pipeline. The effectiveness of the proposed approach is established by discussing the results that have been obtained in several studies based on GAIMS. [less ▲]

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See detailMachine learning techniques to assess the performance of a gait analysis system
Pierard, Sébastien ULg; Phan-Ba, Rémy; Van Droogenbroeck, Marc ULg

in European Symposium on Artificial Neural Networks, Computational Intelligence and Machine Learning (ESANN) (2014, April 24)

This paper presents a methodology based on machine learning techniques to assess the performance of a system measuring the trajectories of the lower limbs extremities for the follow-up of patients with ... [more ▼]

This paper presents a methodology based on machine learning techniques to assess the performance of a system measuring the trajectories of the lower limbs extremities for the follow-up of patients with multiple sclerosis. We show how we have established, with the help of machine learning, four important properties about this system: (1) an automated analysis of gait characteristics provides an improved analysis with respect to that of a human expert, (2) after learning, the gait characteristics provided by this system are valuable compared to measures taken by stopwatches, as used in the standardized tests, (3) the motion of the lower limbs extremities contains a lot of useful information about the gait, even if it is only a small part of the body motion, (4) a measurement system combined with a machine learning tool is sensitive to intra-subject modifications of the walking pattern. [less ▲]

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