Dept. of Computer Science

Portland State University

Overview of Superimposed Information

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Superimposed information

Simply put, superimposed information is new information (or new interpretations) laid over existing information. Bookmarks to web pages, tables of contents, annotations, and concordances are all examples of superimposed information. Information exists in two layers in such settings: a base layer, where the original information resides, and a superimposed layer, where only the new interpretations reside. Superimposed information management (managing layered information) preserves these layers of information. It facilitates superimposed applications using which people can interact with heterogeneous information sources.

Addressing base information

A Mark is an abstraction of an address of information inside base layer. A superimposed application can use a mark to navigate back to the source of information, regardless of the base type and access protocol. Typically, an implementation of the abstraction exists for each base type and the implementation hides the addressing details for that type. For example, an implementation for XML documents might use an XLink whereas an implementation for MS Word documents might use character indexes to denote the span of a selection. However, both implementations support the same programmatic interface so that a superimposed application can interact with them seamlessly.

Figure 1: Layers of information and marks in a superimposed information system.

Need for architectural support

Superimposed application developers like to interact with base layers using a common interface regardless of base-information types or access protocols. Also, users like to perform operations such as navigation and querying with any base layer. Unfortunately, base applications differ in their support for these operations. Appropriate architectural abstractions placed between these layers can ease communication between the two layers and make up for some of the deficiencies of base applications. They can also allow independent evolution of components in these layers. SLIM (Superimposed Layer Information Management architecture), and its successor SPARCE (the Superimposed Pluggable Architecture for Contexts and Excerpts SPARCE) are two such architectures.

SPARCE Applications

SPARCE may be used to build any superimposed application. A superimposed application designer may choose an information and user interface model appropriate to his/her goals. For example, Figure 2 shows superimposed information elements that were created and organized in a superimposed application called RIDPad. It shows four items labeled ‘Statement’, ‘FONSI’, ‘Details’ and ‘Issues,’ each linked to a selection in base documents such as spreadsheets and word processor documents. For example, the item labeled ‘Issues’ is linked to a selection in an MS Excel spreadsheet; the item labeled ‘FONSI’ is linked to a selection in a MS Word document shown in Figure 3 (document from USDA Forest Service).

Figure 2: A superimposed document in RIDPad.                           Figure 3: A selection in a base MS Word document.

Contexts and Excerpts

A superimposed application is able to use a mark to "navigate" back to a base selection. Because all mark implementations support the same interface, a superimposed application is able to navigate to any type of base selection without any special effort.

In addition to navigating to base selections, some superimposed applications might need to do more. They might also need to access content and context information from base selections. SPARCE defines the abstractions excerpt and context for this purpose. An excerpt is the content of a marked region. The content type varies among marks (text, image, and so on) and it might be possible to transform content of one type to another. Context is information related to a marked region, gleaned from the source. Presentation information such as font name, placement information such as line number, and topological information such as next sentence are examples of context elements. For example, consider the MS Word selection in Figure 3. The string "Finding of No Significant....not be necessary." is the text excerpt from this selection, where as "Times New Roman" is the font name for this selection. Figure 4 shows the context for this selection in a Context Browser. The browser is currently displaying the HTML markup required to display the base selection.

4: Context of a selection in a MS Word document

Context can vary across base types, across marks of the same base type, and even for the same mark, over time. For example, the context for a selection in an MS Excel spreadsheet includes the context elements "Row number", whereas the context for an MS Word selection does not include this context element (unless the selection is situated inside a table).



Page modified: 13 Sep 2006 10:05 AM