Papers by Giorgos Stoilos
This deliverable presents the evaluation campaign carried out in 2005 and the improvement partici... more This deliverable presents the evaluation campaign carried out in 2005 and the improvement participants to these campaign and others have to their systems. We draw lessons from this work and proposes improvements for future campaigns.
Signal, Image and Video Processing, 2008
The effective management and exploitation of multimedia documents requires the extraction of the ... more The effective management and exploitation of multimedia documents requires the extraction of the underlying semantics. Multimedia analysis algorithms can produce fairly rich, though imprecise information about a multimedia document which most of the times remains unexploited. In this paper we propose a methodology for semantic indexing and retrieval of images, based on techniques of image segmentation and classification combined with fuzzy reasoning. In the proposed knowledge-assisted analysis architecture a segmentation algorithm firstly generates a set of over-segmented regions. After that, a region classification process is employed to assign semantic labels using a confidence degree and simultaneously merge regions based on their semantic similarity. This information comprises the assertional component of a fuzzy knowledge base which is used for the refinement of mistakenly classified regions and also for the extraction of rich implicit knowledge used for global image classification. This knowledge about images is stored in a semantic repository permitting image retrieval and ranking.
Conjunctive query answering is a key reasoning service for many ontology-based applications. In o... more Conjunctive query answering is a key reasoning service for many ontology-based applications. In order to improve scalability, many Semantic Web query answering systems give up completeness (i.e., they do not guarantee to return all query answers). It may be useful or even critical to the designers and users of such systems to understand how much and what kind of information is (potentially) being lost. We present a method for generating test data that can be used to provide at least partial answers to these questions, a purpose for which existing benchmarks are not well suited. In addition to developing a general framework that formalises the problem, we describe practical data generation algorithms for some popular ontology languages, and present some very encouraging results from our preliminary evaluation.
Ontology classification-the computation of subsumption hierarchies for classes and properties-is ... more Ontology classification-the computation of subsumption hierarchies for classes and properties-is one of the most important tasks for OWL reasoners. Based on the algorithm by Shearer and Horrocks [1], we present a new classification procedure that addresses several open issues of the original algorithm, and that uses several novel optimisations in order to achieve superior performance. We also consider the classification of (object and data) properties. We show that algorithms commonly used to implement that task are incomplete even for relatively weak ontology languages. Furthermore, we show how to reduce the property classification problem into a standard (class) classification problem, which allows reasoners to classify properties using our optimised procedure. We have implemented our algorithms in the OWL HermiT reasoner, and we present the results of a performance evaluation.
Largely motivated by Semantic Web applications, many highly scalable, but incomplete, query answe... more Largely motivated by Semantic Web applications, many highly scalable, but incomplete, query answering systems have been recently developed. Evaluating the scalability-completeness trade-off exhibited by such systems is an important requirement for many applications. In this paper, we address the problem of formally comparing complete and incomplete systems given an ontology schema (or TBox) T . We formulate precise conditions on TBoxes T expressed in the EL, QL or RL prole of OWL 2 under which an incomplete system is indistinguishable from a complete one w.r.t. T , regardless of the input query and data. Our results also allow us to quantify the degree of incompleteness of a given system w.r.t. T as well as to automatically identify concrete queries and data patterns for which the incomplete system will miss answers.
Page 1. D2.5.3 Report on Implementation and Optimisation Techniques for Ontology Query Systems Je... more Page 1. D2.5.3 Report on Implementation and Optimisation Techniques for Ontology Query Systems Jeff Z. Pan (UoM) Enrico Franconi, Sergio Tessaris (FUB), Dmitry Tsarkov, Ian Horrocks (UoM), Giorgos Stoilos, Giorgos Stamou (IT), Holger Wache (VUA), Daniele Turi, Sean Bechhofer, and Lei Li (UoM) Abstract.
Euclidian Roles in Description Logics Giorgos Stoilos and Giorgos Stamou National and Technical U... more Euclidian Roles in Description Logics Giorgos Stoilos and Giorgos Stamou National and Technical University of Athens Department of Electrical and Computer Engineering Abstract In the current paper we investigate the role of Euclidian roles in Description Logics. 1 Introduction The last years much research effort has been spend towards increasing the expressiveness of Description Logics with respect to what can be said about roles.
Foreword This volume contains the papers presented at the 5th International Workshop on Uncertain... more Foreword This volume contains the papers presented at the 5th International Workshop on Uncertainty Reasoning for the Semantic Web (URSW 2009), held as a part of the 8th International Semantic Web Conference (ISWC 2009) at the Westfields Conference Center near Washington, DC, USA, October 26, 2009. It contains 6 technical papers and 3 position papers, which were selected in a rigorous reviewing process, where each paper was reviewed by at least four program committee members.
Abstract: This deliverable describes the progress of the X-Media consortium with respect to uncer... more Abstract: This deliverable describes the progress of the X-Media consortium with respect to uncertain reasoning with very large ontologies, in particular A-Boxes. We recall the main requirements for reasoning derived in the context of deliverable D10. 6. Further, we present the results of our empirical analysis of an approach to fuzzy reasoning which reduces the fuzzy ontologies to OWL-DL ontologies, thus allowing to use standard OWL reasoners for fuzzy query answering.
A key application of Description-Logic based ontologies is Ontology-Based Data Access (OBDA)[9]. ... more A key application of Description-Logic based ontologies is Ontology-Based Data Access (OBDA)[9]. In such scenarios a TBox is used to describe the schema of the application while answers to conjunctive queries reflect both the schema and the data. Unfortunately, it is well-known that conjunctive query (CQ) answering over expressive Description Logics (DLs) is of very high computational complexity [7, 5].
Abstract. Fuzzy Description Logics (f-DLs) have been proposed as extensions of classical Descript... more Abstract. Fuzzy Description Logics (f-DLs) have been proposed as extensions of classical Description Logics able to handle imprecise and vague knowledge. Although several extensions to expressive DLs have been proposed today and many reasoning algorithms exist there is still no scalable and efficient reasoning system reported, mainly due to inherited computational complexity from crisp DLs and the lack for optimisation techniques for fuzzy DL reasoning algorithms.
Abstract Query rewriting is a prominent reasoning technique in ontology-based data access applica... more Abstract Query rewriting is a prominent reasoning technique in ontology-based data access applications. A wide variety of query rewriting algorithms have been proposed in recent years and implemented in highly optimised reasoning systems. Query rewriting systems are complex software programs; even if based on provably correct algorithms, sophisticated optimisations make the systems more complex and errors become more likely to happen.
Abstract Query rewriting over lightweight ontologies, like DL-Lite ontologies, is a prominent app... more Abstract Query rewriting over lightweight ontologies, like DL-Lite ontologies, is a prominent approach for ontology-based data access. It is often the case in realistic scenarios that users ask an initial query which they later refine, by eg, extending it with new constraints, making their initial request more precise. So far, all DL-Lite systems would need to process the new query from scratch.
According to the ISO-IEC 9126-1 standard [5], interoperability is a software functionality sub-ch... more According to the ISO-IEC 9126-1 standard [5], interoperability is a software functionality sub-characteristic defined as “the capability of the software product to interact with one or more specified systems”. In order to interact with other systems a DLBS must conform to the standard input formats and must be able to perform standard inference services. In our setting, the standard input format is the OWL 2 language. We evaluate the standard inference services:
Abstract (for dissemination) This deliverable presents the final outcome of Task 3.1 “Multimedia ... more Abstract (for dissemination) This deliverable presents the final outcome of Task 3.1 “Multimedia content and descriptor ontologies” in the BOEMIE project. Two ontologies, namely the Multimedia Content Ontology (MCO) and the Multimedia Descriptor Ontology (MDO) have been developed aiming to support the effective extraction of multimedia content semantics through the formal representation of media related knowledge. In the first year of the project, the initial versions of the ontologies were released.
Abstract The Semantic Web is about adding formal structure and explicit semantics to web content ... more Abstract The Semantic Web is about adding formal structure and explicit semantics to web content for the purpose of more efficient information management and access by both humans and computers. Through the use of machine understandable semantics, web resources become much easier and more readily accessible. Populating the Semantic Web with multimedia metadata requires for appropriate technologies to support both the analysis (metadata extraction) and the annotation (metadata creation) processes.
Abstract. EU-IST Network of Excellence (NoE) IST-2004-507482 KWEB Deliverable D2. 5.4 (WP2. 5) Ke... more Abstract. EU-IST Network of Excellence (NoE) IST-2004-507482 KWEB Deliverable D2. 5.4 (WP2. 5) Keyword list: description logics, ontology language, query language, RDF, OWL DL, OWL-E The current deliverable surveys a number of real-world use cases encountered in the course of the KnowledgeWeb project which demonstrate limitations of the Semantic Web ontology languages RDF and OWL.
Abstract. In the current paper we study the reasoning problem for fuzzy SI (f-SI) under arbitrary... more Abstract. In the current paper we study the reasoning problem for fuzzy SI (f-SI) under arbitrary continuous fuzzy operators. Our work can be seen as an extension of previous works that studied reasoning algorithms for f-SI, but focused on specific fuzzy operators, eg fKD-SI and of reasoning algorithms for less expressive fuzzy DLs, like fL-ALC and fP-ALC (fuzzy ALC under the Lukasiewicz and product fuzzy operators, respectively).
Conjunctive query answering is a key reasoning service for many ontology-based applications. In o... more Conjunctive query answering is a key reasoning service for many ontology-based applications. In order to improve scalability, many Semantic Web query answering systems give up completeness (i.e., they do not guarantee to return all query answers). It may be useful or even critical to the designers and users of such systems to understand how much and what kind of information is (potentially) being lost. We present a method for generating test data that can be used to provide at least partial answers to these questions, a purpose for which existing benchmarks are not well suited. In addition to developing a general framework that formalises the problem, we describe practical data generation algorithms for some popular ontology languages, and present some very encouraging results from our preliminary evaluation.
Scalable Fuzzy Algorithms for Data …, Jan 1, 2009
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Papers by Giorgos Stoilos