3 H<W DIREZIONE STUDI E RICERCHE Relaz. n° UTI/RM 11-1 CENTRO RICERCA Dl AUTOMATICA Rapp. n° 472/96 Sacielri per nzioni Units tecnologie dell’lnformazione ENEL-ZRfii-UTX/m-'}\-l Dario Lucarella - Antonella Zanzi A VISUAL RETRIEVAL ENVIRONMENT FOR HYPERM ED IA INFORMATION SYSTEM ACM TRANSACTION ON INFORMATION SYSTEMS January, 1996 WASTER Marzo 1996 3N OF THIS DOCUMENT IS UNLIMITED FOREIGN SALES PROHIBITED OQ DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. A Visual Retrieval Environment for Hypermedia Information Systems DARIO LUCARELLA and ANTONELLA ZANZI Centro Ricerca di Automatica, ENEL We present a graph-based object model that may be used as a uniform framework for direct manipulation of multimedia information. After an introduction motivating the need for abstrac­ tion and structuring mechanisms in hypermedia systems, we introduce the data model and the notion of perspective, a form of data abstraction that acts as a user interface to the system, providing control over the visibility of the objects and their properties. A perspective is defined to include an intension and an extension. The intension is defined in terms of a pattern, a subgraph of the schema graph, and the extension is the set of pattern-matching instances. Perspectives, as well as database schema and instances, are graph structures that can be manipulated in various ways. The resulting uniform approach is well suited to a visual interface. A visual interface for complex information systems provides high semantic power, thus exploiting the semantic expressibility of the underlying data model, while maintaining ease of interaction with the system. In this way, we reach the goal of decreasing cognitive load on the user, with the additional advantage of always maintaining the same interaction style. We present a visual retrieval environment that effectively combines filtering, browsing, and navigation to provide an integrated view of the retrieval problem. Design and implementation issues are outlined for MORE (Multimedia Object Retrieval Environment), a prototype system relying on the proposed model. The focus is on the main user interface functionalities, and actual interaction sessions are presented including schema creation, information loading, and information retrieval. Categories and Subject Descriptors: H.2.1 [Database Management]: Logical Design—data models', H.3.3 [Information Storage and Retrieval]: Information Search and Retrieval—query formulation-, selection process; H.5.1 [Information Interfaces and Presentation] : Multimedia Information Systems —hypertext navigation and maps; H.5.2 [Information Interfaces and Presentation] : User Interfaces —interaction styles General Terms: Design, Human Factors, Management Additional Key Words and Phrases: Browsing, complex objects, direct object manipulation, graph-oriented models, hypermedia applications, information filtering, visual interface This work was supported by the Italian Electrical Energy Company under the research project O.L.240 Multimedia Systems. Authors’ addresses. D. Lucarella, Centro Ricerca di Automatica, ENEL, Via Volta 1, 1-20093 Cologne Monzese, Milano, Italy and Dipartimento di Scienze dellTnformazione, Universita degli Studi di Milano, 1-20135 Milano, Italy; email: [email protected] ; A. Zanzi, Centro Ricerca di Automatica, ENEL, Via Volta 1, 1-20093 Cologne Monzese, Milano, Italy; email: [email protected] . Permission to make digital/hard copy of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage, the copyright notice, the title of the publication, and its date appear, and notice is given that copying is by permission of ACM, Inc. To copy otherwise, to republish, to post on servers, or to redistribute to lists, requires prior specific permission and/or a fee. © 1996 ACM 0734-2047/96/0100-0003 $03.50 ACM Transactions on Information Systems, Vol. 14, No. 1, January 1996, Pages 3-29. 4 D. Lucarella and A. Zanzi 1. INTRODUCTION Hypermedia has been simply defined as a system to manage a collection of information that can be accessed nonsequentially. It consists of units of information that are arbitrarily diverse in form and content. Such units may contain texts, graphics, images, sound, video, and animation and are con­ nected by logical links to form an information network. The variety of nodes and links that can be defined makes hypermedia a very flexible environment in which information is provided both by what is stored in each node and by the way the information nodes are linked to each other. In addition, current hypermedia systems provide sophisticated user interface tools that enable the reader to inspect the node content and to navigate through the network by selecting paths to follow on the basis of interests emerging along the way [Nielsen 1990]. There is a growing interest today in such technologies for the implementa­ tion of massive multimedia information systems, but unfortunately, several well-recognized problems continue to be open research issues [Halasz 1988]. Among these, central points to be addressed are information modeling and information retrieval. 1.1 Information Modeling The simplicity of the basic hypermedia model does not appropriately repre­ sent the structure of the information. There is an inadequate separation between a node in the hypermedia network and the content associated with the node. Conversely, a strong separation of the structure from the content would allow many structures to be superimposed over the same set of information units or a unit to be shared among many nodes within a single structure. In addition, a node is a storage unit for a collection of data items without any structural information, and each node and link are assumed to be of the same type. As a result, modeling is more or less a bottom-up process in which we have to analyze how information can be broken down into different elements and then to recognize these individual elements by adding links among them. The problem here is that such an analysis is only useful for that particularinstance, and we cannot use this same structure for other instances [Tompa 1989]. The key point is that the basic hypermedia data model is too simplistic. It is not suitable for modeling the real world or capturing its semantics as required in most applications [Furuta and Stotts 1990; Garzotto et al. 1993; Schnase et al. 1993a]. As a consequence, the user has difficulty in perceiv­ ing the conceptual model of the application, resulting in cognitive overhead [Conklin 1987]. In authoring mode, extra mental effort is needed to establish the links required to connect the created nodes. In reading mode, extra mental effort is needed for choosing the path to follow through the network, with the risk of becoming lost or disoriented. One of the main ideas proposed by Garg [1988] is that information embed­ ded into the hypertext network should be described by a set of predefined ACM Transactions on Information Systems, Vol. 14, No. 1, January 1996. Visual Retrieval Environment for Information Systems 5 domain objects. In this way, the actual content of the hypertext would be represented by a set of information objects, each of which is an instance of a domain object, inheriting by default all of the properties of the domain object. The idea can be compared to the notion of database schema, as opposed to a specific instance of the database. According to this trend, many hypermedia systems have been proposed with the support of underlying databases [Campbell and Goodman 1988; Christodoulakis et al. 1986; Lange 1990; Schnase et al. 1993b; Schutt and Streitz 1990]. Recently, requirements for representing the structurally complex interrela­ tionships that arise in hypermedia have generated a renewed interest in semantic data models [Hull and King 1987]. Semantic models attempt to provide more powerful abstraction and structuring mechanisms for specifying database schemas in order to overcome the limited modeling capabilities of traditional database systems [Beeri 1990; Lieberherr and Xiao 1993]. Schnase et al. [1993a] presented a comparative analysis of semantic models, concluding that a structural object-oriented paradigm appears to be superior for hypermedia modeling. Of particular interest are graph-based data models since they provide a natural way of handling data that appear in applications such as hypermedia or multimedia information systems. Gyssens et al. [1990] proposed a graph-oriented object database model in which the database schema as well as the database instances can be seen as graphs with the data manipulation language expressed in terms of graph transfor­ mations. Amann and Scholl [1992] presented a graph data model with an associated algebraic language based on regular expressions over the data types and showed how such a language can be exploited for hypertext querying. In the same direction, in this article we propose a graph-based object model which provides high semantic expressibility, and we use it as a uniform framework both for conceptual modeling and for direct manipulation of the stored objects. 1.2 Information Retrieval In hypermedia information systems, interaction is mainly devoted to informa­ tion retrieval. A canonical approach is based on formal querying [Bertino et al. 1992; Straube and Ozsu 1990]. Conversely, browsing techniques consist of exhaustively