Maintaining temporal consistency of multimedia documents using constraint networks

The recent advances in multimedia systems, together with the advent of high speed networks, paved the way to a new generation of applications. In particular, the authoring environments have found in multimedia the means of increasing the richness of information contained in electronic documents. With the evolution of new computer systems that can handle multimedia information, time-based data can be integrated in electronic documents taking into account their temporal dimension. In such documents, temporal dependencies between the different media objects define a temporal structure within the document. This structure is the basic support for the representation of dependencies between data such as audio, video and virtual images. Furthermore, it allows the scheduling of presentation actions during the document presentation. The presentation of multimedia documents is dynamic and the positioning of objects in time together with their duration have to be specified. To achieve this operation efficiently, a high level temporal representation is needed which allows the author to specify all the temporal dependencies between multimedia objects. In this paper, we propose an interval-based temporal model and constraints which provide a basis for the management of the consistency of multimedia documents. We propose an efficient algorithm allowing the detection of a wide range of inconsistencies. The emphasis in the design of these algorithms is put on the handling of both the flexibility of temporal specifications and the indeterministic behaviour of some media objects. Furthermore, we use the logical organization of the document in nested entities to enhance the performance of the methods used for detecting inconsistencies. The aim of our approach is to fulfill the following requirements . Structured modelling: The document content is defined by a hierarchy of nested components where leaves are basic media objects and nodes composite objects. Incremental manipulation of the document: This means that the author adds one constraint at a time, retaining all previously introduced relations as the current state of the document. • Consistency checking: For every specification introduced by the author, the system checks if there exists a solution to the set of constraints, otherwise an error is reported to the author. • Integration of the different dimensions of multimedia documents: In our model, we focus on four dimensions of the document (logical, temporal, spatial and hyperlink). These dimensions are closely related and have to be considered simultaneously as they may interfere. In this paper, we will focus on the temporal and logical dimensions of the document but we will describe briefly how this fits with the spatial one. Automatic production of temporal layout: The author specifies relations using a high-level language to describe the document temporal layout, and does not have to explicitly set the low-level synchronization dates. In section 2, we present our model of document composition showing the logical, spatial, and temporal dimensions. In section 3, we describe how we can create and manipulate the internal representation of the multimedia document in the form of constraint networks. In section 4, a consistency checking algorithm is proposed, followed by a discussion. Finally, in section 5, we conclude our work and give some future directions.