While there have been dramatic increases in the use of digital technologies for information storage, processing and delivery over the last twenty years, the affordances of paper have ensured its retention as a key information medium. Despite predictions of the paperless office, paper is ever more present in our daily work as reflected by the continuously increasing worldwide paper consumption.

Many researchers have argued for the retention of paper as an information resource and its integration into cross-media environments as opposed to its replacement. This has resulted in a wide variety of projects and technological developments for digitally augmented paper documents over the past decade. However, the majority of the realised projects focus on technical advances in terms of hardware but pay less attention to the very fundamental information integration and cross-media information management issues.

Our information-centric approach for a tight integration of paper and digital information is based on extending an object-oriented database management system with functionality for cross-media information management. The resulting iServer platform introduces fundamental link concepts at an abstract level. The iServer's core link management functionality is available across different multimedia resources. Only the media-specific portion of these general concepts, for example the specification of a link's source anchor, has to be implemented in the form of a plug-in to support new resource types.
This resource plug-in mechanism results in a flexible and extensible system where new types of digital as well as physical resources can easily be integrated and, more importantly, cross-linked to the growing set of supported multimedia resources. In addition to the associative linking of information, our solution allows for the integration of semantic metadata and supports multiple classification of information units. iServer can, not only link between various static information entities, but also link to active content and this has proven to be very effective in enabling more complex interaction design.

As part of the European project Paper++, under the Disappearing Computer Programme, an iServer plug-in for interactive paper has been implemented to fully integrate paper and digital media, thereby gaining the best of the physical and the digital worlds. It not only supports linking from physical paper to digital information, but also enables links from digital content to physical paper or even paper to paper links. This multi-mode user interface results in highly interactive systems where users can easily switch back and forth between paper and digital information. The definition of
an abstract input device interface further provides flexibility for supporting emerging technologies for paper link definition in addition to the hardware solutions for paper link definition and activation that were developed within the Paper++ project.

We introduce different approaches for cross-media information authoring where information is either compiled by established publishers with an expertise in a specific domain or by individuals who produce their own cross-media information environments. Preauthored information can be combined with personally aggregated information. A distributed peer-to-peer version of the iServer platform supports collaborative authoring and the sharing of link knowledge within a community of users.

The associations between different types of resources as well as other application-specific information can be visualised on different output channels. Universal access to the iServer's information space is granted using the eXtensible Information Management Architecture (XIMA), our publishing platform for multi-channel access.

Our fundamental concepts for interactive paper and cross-media information management have been designed independently of particular hardware solutions and modes of interaction which enables the iServer platform to easily adapt to both new technologies and applications. Finally, the information infrastructure that we have developed has great potential as an experimental platform for the investigation of emerging multimedia resources in general and interactive paper with its possible applications in particular.