Publications and communications of Marc Van Droogenbroeck [u182591]
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See detailUnderstanding how people with MS get tired while walking
Pierard, Sébastien ULg; Phan-Ba, Rémy; Van Droogenbroeck, Marc ULg

in Multiple Sclerosis Journal (2015, October 09)

BACKGROUND. Walking impairment is frequent, appears early in the disease course of MS patients (MSP), and is perceived as the most disabling symptom. When walking, patients get tired more and differently ... [more ▼]

BACKGROUND. Walking impairment is frequent, appears early in the disease course of MS patients (MSP), and is perceived as the most disabling symptom. When walking, patients get tired more and differently than healthy people (HP) [Phan-Ba et al PLOS 2012]. This limits their walking perimeter. Understanding this phenomenon is thus important to suggest adequate therapies at the right time. OBJECTIVE. Our aim is to understand how MSP get tired while walking compared to HP. Two groups of MSP are considered: those with a low disability level (MSPL) and those with a high one (MSPH). We consider two criteria to measure the disability: the EDSS and the deceleration index (DI) [Phan-Ba et al PLOS 2012]. The limit between the groups is set at DI=0.8 and EDSS=3 (inclusive for MSPL). METHODS. Many gait characteristics (GC) have been measured with the system GAIMS along a 500m path walked as fast as possible. The dataset gathers 464 visits of HP and 70 of MSP. Some people have been assessed several times. There are 33 visits in the group MSPL with the EDSS criterion, and 25 with the DI criterion. Statistical tests (Welch) were performed on the differences and relative differences of the GC measured during the first and last 100m of the test to detect differences between HP and MSPL, and between MSPL and MSPH, as in [ECTRIMS 2012 P755]. RESULTS. Both criteria for defining the groups lead to similar conclusions. For many GC, the distributions of the variations are significantly different between MSPL and MSPH. The largest difference is for the relative difference of speed (p=0.000119 for EDSS and p=0.000021 for DI). In contrast, only the variation of the average lateral distance between the feet, which is related to the size of the base of support (and thus to the balance) shows a very significant difference between HP and MSPL (p=0.000116 for EDSS and p=0.000120 for DI). The balance does not seem to change much from MSPL to MSPH. Besides, we note that the variance decreases slightly from HP to MSPL and increases a lot from MSPL to MSPH. CONCLUSIONS. Statistically, from the motor fatigue point of view, it seems that the course of the MS disease is divided in two different stages. In the first one, MSP get more tired than HP because of a deterioration of the balance. Then, in the second one, their fatigue becomes related to a faster decrease of the walking speed. This suggests that physical therapy exercises focused on the balance could be given to MSP in the early stage of the disease. [less ▲]

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See detailA Generic Feature Selection Method for Background Subtraction Using Global Foreground Models
Braham, Marc ULg; Van Droogenbroeck, Marc ULg

in Advanced Concepts for Intelligent Vision Systems (2015, October)

Over the last few years, a wide variety of background subtraction algorithms have been proposed for the detection of moving objects in videos acquired with a static camera. While much effort have been ... [more ▼]

Over the last few years, a wide variety of background subtraction algorithms have been proposed for the detection of moving objects in videos acquired with a static camera. While much effort have been devoted to the development of robust background models, the automatic spatial selection of useful features for representing the background has been neglected. In this paper, we propose a generic and tractable feature selection method. Interesting contributions of this work are the proposal of a selection process coherent with the segmentation process and the exploitation of global foreground models in the selection strategy. Experiments conducted on the ViBe algorithm show that our feature selection technique improves the segmentation results. [less ▲]

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See detailTime Ordering Shuffling for Improving Background Subtraction
Laugraud, Benjamin ULg; Latour, Philippe ULg; Van Droogenbroeck, Marc ULg

in Advanced Concepts for Intelligent Vision Systems (ACIVS) (2015, October)

By construction, a video is a series of ordered frames, whose order is defined at the time of the acquisition process. Background subtraction methods then take this input video and produce a series of ... [more ▼]

By construction, a video is a series of ordered frames, whose order is defined at the time of the acquisition process. Background subtraction methods then take this input video and produce a series of segmentation maps expressed in terms of foreground objects and scene background. To our knowledge, this natural ordering of frames has never been questioned or challenged. In this paper, we propose to challenge, in a prospective view, the natural ordering of video frames in the context of background subtraction, and examine alternative time orderings. The idea consists in changing the order before background subtraction is applied, by means of shuffling strategies, and re-ordering the segmentation maps afterwards. For this purpose, we propose several shuffling strategies and show that, for some background subtraction methods, results are preserved or even improved. The practical advantage of time shuffling is that it can been applied to any existing background subtraction seamlessly. [less ▲]

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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 detailSimple Median-Based Method for Stationary Background Generation Using Background Subtraction Algorithms
Laugraud, Benjamin ULg; Pierard, Sébastien ULg; Braham, Marc ULg et al

in New Trends in Image Analysis and Processing - ICIAP 2015 Workshops (2015, September)

The estimation of the background image from a video sequence is necessary in some applications. Computing the median for each pixel over time is effective, but it fails when the background is visible for ... [more ▼]

The estimation of the background image from a video sequence is necessary in some applications. Computing the median for each pixel over time is effective, but it fails when the background is visible for less than half of the time. In this paper, we propose a new method leveraging the segmentation performed by a background subtraction algorithm, which reduces the set of color candidates, for each pixel, before the median is applied. Our method is simple and fully generic as any background subtraction algorithm can be used. While recent background subtraction algorithms are excellent in detecting moving objects, our experiments show that the frame difference algorithm is a technique that compare advantageously to more advanced ones. Finally, we present the background images obtained on the SBI dataset, which appear to be almost perfect. The source code of our method can be downloaded at http://www.ulg.ac.be/telecom/research/sbg . [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 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), 21(4), 532-533

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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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