Graph Mining for Object Tracking in Videos

International audienceThis paper shows a concrete example of the use of graph mining for tracking objects in videos with moving cameras and without any contextual information on the objects to track. To make the mining algorithm efficient, we benefit from a video representation based on dy- namic (evolving through time) planar graphs. We then define a number of constraints to efficiently find our so-called spatio-temporal graph pat- terns. Those patterns are linked through an occurrences graph to allow us to tackle occlusion or graph features instability problems in the video. Experiments on synthetic and real videos show that our method is effec- tive and allows us to find relevant patterns for our tracking application

Towards effective visual analytics on multiplex and multilayer networks

In this article we discuss visualisation strategies for multiplex networks. Since Moreno's early works on network analysis, visualisation has been one of the main ways to understand networks thanks to its ability to summarise a complex structure into a single representation highlighting multiple properties of the data. However, despite the large renewed interest in the analysis of multiplex networks, no study has proposed specialised visualisation approaches for this context and traditional methods are typically applied instead. In this paper we initiate a critical and structured discussion of this topic, and claim that the development of specific visualisation methods for multiplex networks will be one of the main drivers pushing current research results into daily practice

Flexible constrained sampling with guarantees for pattern mining

Pattern sampling has been proposed as a potential solution to the infamous pattern explosion. Instead of enumerating all patterns that satisfy the constraints, individual patterns are sampled proportional to a given quality measure. Several sampling algorithms have been proposed, but each of them has its limitations when it comes to 1) flexibility in terms of quality measures and constraints that can be used, and/or 2) guarantees with respect to sampling accuracy. We therefore present Flexics, the first flexible pattern sampler that supports a broad class of quality measures and constraints, while providing strong guarantees regarding sampling accuracy. To achieve this, we leverage the perspective on pattern mining as a constraint satisfaction problem and build upon the latest advances in sampling solutions in SAT as well as existing pattern mining algorithms. Furthermore, the proposed algorithm is applicable to a variety of pattern languages, which allows us to introduce and tackle the novel task of sampling sets of patterns. We introduce and empirically evaluate two variants of Flexics: 1) a generic variant that addresses the well-known itemset sampling task and the novel pattern set sampling task as well as a wide range of expressive constraints within these tasks, and 2) a specialized variant that exploits existing frequent itemset techniques to achieve substantial speed-ups. Experiments show that Flexics is both accurate and efficient, making it a useful tool for pattern-based data exploration.Comment: Accepted for publication in Data Mining & Knowledge Discovery journal (ECML/PKDD 2017 journal track

The topology of a discussion: the #occupy case

We analyse a large sample of the Twitter activity developed around the social movement 'Occupy Wall Street' to study the complex interactions between the human communication activity and the semantic content of a discussion. We use a network approach based on the analysis of the bipartite graph @Users-#Hashtags and of its projections: the 'semantic network', whose nodes are hashtags, and the 'users interest network', whose nodes are users In the first instance, we find out that discussion topics (#hashtags) present a high heterogeneity, with the distinct role of the communication hubs where most the 'opinion traffic' passes through. In the second case, the self-organization process of users activity leads to the emergence of two classes of communicators: the 'professionals' and the 'amateurs'. Moreover the network presents a strong community structure, based on the differentiation of the semantic topics, and a high level of structural robustness when a certain set of topics are censored and/or accounts are removed. Analysing the characteristics the @Users-#Hashtags network we can distinguish three phases of the discussion about the movement. Each phase corresponds to specific moment of the movement: from declaration of intent, organisation and development and the final phase of political reactions. Each phase is characterised by the presence of specific #hashtags in the discussion. Keywords: Twitter, Network analysisComment: 13 pages, 9 figure

Improving the Accessibility of Urban Transportation Networks for People with Disabilities

How accessible is a public urban transportation system for people with re- duced mobility? Are there ways of quantifying this aspect of the system so that we can prioritise investment in stations that, once rendered accessible, would deliver the greatest benefits to travellers? Focussing on travel time and interchange differences, we present a method that uses network science and spatio-temporal analysis to rank un-accessible stations in order to minimise the divergence between accessible and non-accessible routes.Taking London as case study, and Origin/Destination data derived from the ‘Oyster Card’ system, we find that 50% of trips become 50% longer for people with reduced mobility. Our approach to ranking the accessibility of stations yields solutions that are 30% more effective than a solution that seeks solely to minimise the number of interchanges. These results highlight the potential for the analysis of ‘smart card’ data to enable network operators to obtain maximum value from their infrastructure investments in support of expanded access for all users

Morire di fame oggi. Funzionamento (e fallimento) dei sistemi di protezione e tutela per l’infanzia

The case of a 17-month baby starved to death has made the whole town questioning about the existing prevention systems and about the real functioning of a service net defined, by different documents, as one of the principal instruments to face with social problems. How is it possible to let people starve to death today? Starting from this question, the case of this young victim has been constantly studied: a baby who has been exposed to physical and psychological mistreatments for months and months, losing her dignity as a “person” and living an existence on the borderline between “visible and invisible”. The authors have delved deeper into the matter, analysing what has been done and what should have been done. An analysis of the context in which facts occurred, paying attention to the institutional contradictions that were the real background in the baby’s life events. After an introduction to the international, national and regional laws about childhood protection, the authors focused their attention on the activities carried out by local services, analysing the importance of Pediatricians’ role, Social Welfare Services and people. The analysis about interventions and strategies aimed to protect the childhood world has pointed out how difficult it is to switch from an initial planning to the definitive realization of practical interventions about health and social services. These difficulties are partially due to a series of legislative and bureaucratic steps that strongly slow down the projects realization. This young baby’s death was a strong emotional shock for the entire city that started to call into question health and social services systems already existing over the territory. Nevertheless this case was also a way to reflect about people’s attitude to feel as part of a community but without doing anything to play their roles as active citizens. Through the analysis of the actual situation in Bari, the authors have found important changes in policy towards children and families.il caso di una bambina di 17 mesi morta di fame ha fatto interrogare l’intera città di Bari sui sistemi di prevenzione esistenti, sul reale funzionamento di quella rete di servizi che numerosi documenti definiscono come uno degli strumenti principali per affrontare le problematiche sociali. Come è possibile che oggi si muoia ancora di fame? Partendo da questo interrogativo è stato approfondito il caso di questa piccola vittima, la quale per mesi è diventata oggetto di maltrattamenti fisici e psicologici, perdendo la dignità di “persona” e vivendo un’esistenza al confine tra il “visibile e l’invisibile”. Gli autori hanno svolto una’analisi di quello che è stato fatto e di tutto ciò che si sarebbe potuto e dovuto fare, prestando attenzione al contesto all’interno del quale i fatti si sono svolti e alle contraddizioni istituzionali che hanno fatto da sfondo alle vicende di vita della bambina. Dopo una presentazione della normativa (internazionale, nazionale e regionale) in materia di tutela dell’infanzia, gli autori hanno focalizzato l’attenzione sull’attività svolta dai servizi territoriali, analizzando nello specifico il ruolo dei Pediatri di libera scelta, dei Servizi Socio-Assistenziali e della cittadinanza. La ricognizione sugli interventi e le strategie finalizzate a tutelare il mondo dell’infanzia ha evidenziato la difficoltà di passare dalla progettazione alla realizzazione degli interventi nell’area socio-sanitaria; tale difficoltà sembrerebbe determinata, in parte, da passaggi burocratici e legislativi che rallentano l’attuazione dei progetti. La morte di questa piccola bambina è stato uno shock emotivo per la città che ha messo in discussione non soltanto il sistema dei servizi già presenti sul territorio, ma anche il modo di sentirsi parte di una collettività troppo spesso abituata a non svolgere il proprio ruolo di cittadini attivi. Attraverso l’analisi dell’attuale realtà barese, gli autori hanno rilevato segni di importanti cambiamenti nell’ambito delle politiche per i minori e le famiglie

Foundations of Multidimensional Network Analysis

Abstract—Complex networks have been receiving increasing attention by the scientific community, thanks also to the increas-ing availability of real-world network data. In the last years, the multidimensional nature of many real world networks has been pointed out, i.e. many networks containing multiple connections between any pair of nodes have been analyzed. Despite the importance of analyzing this kind of networks was recognized by previous works, a complete framework for multidimensional network analysis is still missing. Such a framework would enable the analysts to study different phenomena, that can be either the generalization to the multidimensional setting of what happens in monodimensional network, or a new class of phenomena induced by the additional degree of complexity that multidimensionality provides in real networks. The aim of this paper is then to give the basis for multidimensional network analysis: we develop a solid repertoire of basic concepts and analytical measures, which takes into account the general structure of multidimensional networks. We tested our framework on a real world multidimensional network, showing the validity and the meaningfulness of the measures introduced, that are able to extract important, non-random, information about complex phenomena. I