Fausto Rabitti

Approximate Similarity Retrieval with M-trees

Abstract. Motivated by the urgent need to improve the efficiency of similarity queries, approximate similarity retrieval is investigated in the environment of a metric tree index called the M-tree. Three different approximation techniques are proposed, which show how to forsake query precision for improved performance. Measures are defined that can quantify the improvements in performance efficiency and the quality of approximations. The proposed approximation techniques are then tested on various synthetic and real-life files. The evidence obtained from the experiments confirms our hypothesis that a high-quality approximated similarity search can be performed at a much lower cost than that needed to obtain the exact results. The proposed approximation techniques are scalable and appear to be independent of the metric used. Extensions of these techniques to the environments of other similarity search indexes are also discussed.

Dynamic partitioning of signature files

The signature file access method has proved to be a convenient indexing technique, in particular for text data Because it can deal with unformatted data, many application domains have shown interest in signature file techniques, e.g., office information systems, statistical and logic databases. We argue that multimedia databases should also take advantage of this method, provided convenient storage structures for organizing signature tiles are available, Our main concern here is the dynamic organization of signatures based on a partitioning paradigm called Quick Filter. A signature file is partitioned by a hashing function and the partitions are orgamzed by linear hashing, Thorough performance evaluation of the new scheme is provided, and it is compared with single-level and multdevel storage structures Results show that quick filter is economical in space and very convenient for applications dealing with large files of dynamic data, and where user queries result in signatures with high weights. These characteristics are particularly interesting for multimedia databases, where integrated access to attributes, text and images must be provided.

Conceptual modeling of multimedia documents

An approach to the document-retrieval problem that aims to increase the efficiency and effectiveness of document-retrieval systems by exploiting the semantic contents of the documents is presented. The document retrieval problem is delineated, and conceptual document modeling basics and requirements are discussed. An experimental system, the Multimedia Office Server (Multos), which implements some of the document-model concepts described, is presented.<<ETX>>

Digital library information-technology infrastructures

This paper charts a research agenda on systems-oriented issues in digital libraries. It focuses on the most central and generic system issues, including system architecture, user-level functionality, and the overall operational environment. With respect to user-level functionality, in particular, it abstracts the overall information lifecycle in digital libraries to five major stages and identifies key research problems that require solution in each stage. Finally, it recommends an explicit set of activities that would help achieve the research goals outlined and identifies several dimensions along which progress of the digital library field can be evaluated.

A Model of Authorization for Object-Oriented and Semantic Databases

This paper presents a formal model of authorization for use as the basis for an authorization mechanism in ORION, a prototype database system which directly supports the object-oriented paradigm and a number of semantic data modeling concepts. The model extends in two significant ways the existing models of authorization, which have been designed for database systems supporting the relational, network, or hierarchical models of data. First, it fully develops the concept of implicit authorization, introduced in an earlier paper [FERN75b], to help solve the storage requirement of representing all authorizations in a system by allowing the system to deduce authorizations from explicitly stored authorizations, and to provide a basis for detecting authorization definitions which conflict with existing authorizations. Second, it provides a formal basis for accommodating a number of modeling concepts which the existing models of authorization cannot address: the IS-PART-OF relationship between an object and its containing object, and versions of an object.

Milos: A Multimedia Content Management System for Digital Library Applications

This paper describes the MILOS Multimedia Content Management System: a general purpose software component tailored to support design and effective implementation of digital library applications. MILOS supports the storage and content based retrieval of any multimedia documents whose descriptions are provided by using arbitrary metadata models represented in XML. MILOS is flexible in the management of documents containing different types of data and content descriptions; it is efficient and scalable in the storage and content based retrieval of these documents. The paper illustrates the solutions adopted to support the management of different metadata descriptions of multimedia documents in the same repository, and it illustrates the experiments performed by using the MILOS system to archive documents belonging to four different and heterogenous collections which contain news agencies, scientific papers, and audio/video documentaries.

Tree Signatures for XML Querying and Navigation

In order to accelerate execution of various matching and navigation operations on collections of XML documents, new indexing structure, based on tree signatures, is proposed. We show that XML tree structures can be efficiently represented as ordered sequences of preorder and postorder ranks, on which extended string matching techniques can easily solve the tree matching problem. We also show how to apply tree signatures in query processing and demonstrate that a speedup of up to one order of magnitude can be achieved over the containment join strategy. Other alternatives of using the tree signatures in intelligent XML searching are outlined in the conclusions.

A metric cache for similarity search

Similarity search in metric spaces is a general paradigm that can be used in several application fields. It can also be effectively exploited in content-based image retrieval systems, which are shifting their target towards the Web-scale dimension. In this context, an important issue becomes the design of scalable solutions, which combine parallel and distributed architectures with caching at several levels. To this end, we investigate the design of a similarity cache that works in metric spaces. It is able to answer with exact and approximate results: even when an exact match is not present in cache, our cache may return an approximate result set with quality guarantees. By conducting tests on a collection of one million high-quality digital photos, we show that the proposed caching techniques can have a significant impact on performance, like caching on text queries has been proved effective for traditional Web search engines.

A dynamic signature technique for multimedia databases

A signature file acts as a filtering mechanism to reduce the amount of data that needs to be searched during query evaluation. Even though several techniques for organizing and searching signature files have been proposed in literature, they have serious limitations when applied to multimedia databases, where integrated access methods to text and image content are needed. A new signature technique, called Quick Filter, is proposed in the paper. According to this technique, signatures are divided into partitions, each of which holds signatures sharing the same characteristic key. As a result, it is possible to determine if the signatures in a partition satisfy a query by merely examining the key. Partitions not matching the key need not be searched. This method is based on dynamic hashing since signatures are hashed into partitions according to the keys and the file size, computed algorithmically from the signatures. Implementation of this technique is illustrated using an example and is verified by analytical performance evaluation. The result is a signature technique which satisfies the requirements for access methods in multimedia databases: dynamicity, with respect to insertions and updates, good query processing performance on large databases for high-weight queries.

Caching content-based queries for robust and efficient image retrieval

In order to become an effective complement to traditional Web-scale text-based image retrieval solutions, content-based image retrieval must address scalability and efficiency issues. In this paper we investigate the possibility of caching the answers to content-based image retrieval queries in metric space, with the aim of reducing the average cost of query processing, and boosting the overall system throughput. Our proposal exploits the similarity between the query object and the cache content, and allows the cache to return approximate answers with acceptable quality guarantee even if the query processed has never been encountered in the past. Moreover, since popular images that are likely to be used as query have several near-duplicate versions, we show that our caching algorithm is robust, and does not suffer of cache pollution problems due to near-duplicate query objects. We report on very promising results obtained with a collection of one million high-quality digital photos. We show that it is worth pursuing caching strategies also in similarity search systems, since the proposed caching techniques can have a significant impact on performance, like caching on text queries has been proven effective for traditional Web search engines.