RLG DigiNews August 15, 2003, Volume 7, Number 4

I've heard of XML being used for file formats such as SVG (Scalable Vector Graphics) and OpenOffice and to encode metadata. Can XML also be used to transform one file format into another?

This issue's FAQ is answered by Christopher Hamilton, Programmer/Analyst in the IRIS Research Department, Cornell University Library.

Extensible Markup Language (XML) allows for the markup of information in a standard vocabulary free from control of proprietary software vendors. It has been said, “XML is shifting the balance of power from software vendors to software users” (Tidwell) because XML and its related family of technologies are open, standards-based, and platform-neutral. Metadata embedded in platform-neutral XML documents could outlive the original digital object or storage medium. However, to preserve metadata in XML documents is only one way to harness the power of XML for digital preservation. The XML standard for transforming XML documents into different formats is Extensible Stylesheet Language (XSL).

XSL consists of a family of three World Wide Web Consortium (W3C) recommendations. These three technologies together form a set of tools for writing style sheets that transform XML documents and format them for display. XSL Transformations (XSLT) is the general-purpose tag-based language that defines rules for transforming an XML document into another XML document or another format such as XHTML, JPG, PDF, or SVG. XML Path Language (XPath) is the path expression language that is used with multiple XML standards for locating nodes within, and traversing the tree structure of, an XML document. XPath is embedded within an XSLT style sheet, and together they instruct the XSLT processor to transform an XML document. The other part of XSL is XSL Formatting Objects (XSL-FO), another tag-based language that adds advanced styling features and is optimized for describing the page layout of print and Web documents.

With the ability to transform XML documents into other formats, it becomes possible to use XSLT to migrate digital objects to new formats. A typical example is using XSLT to transform an XML document into a PDF or an XHTML document. However, current research is working to extend XSLT to allow for the transformation of multimedia files (images, audio, video) to other formats. For example, XSLT could transform a TIFF file from an archive of multimedia objects into JPEG2000 format for display on the Web.

However, a standard TIFF file is not encoded in an XML-compliant format. Therefore, something new is needed for parsing and generating XML descriptions of bitstreams, the most basic form of a digital object. For this purpose two languages are currently in development: Bitstream Syntax Description Language (BSDL) and Formal Language for Audio-Visual Object Representation (XFlavor). To continue with the above example, BSDL or XFlavor would be used to generate an XML-based bitstream-level description of a TIFF file, which is then transformable via XSLT into the desired JPEG2000 format.

Moreover, using BSDL or XFlavor would not alter or incrementally corrupt the original object over time, therefore preserving the longevity of the original object.

Once it’s possible to generate an XML-based description of a multimedia object’s bitstream, an XSLT style sheet could be applied to transform the file into another format. Entire digital collections could be migrated to the latest file format or platform. Tools could also be developed that automate XML/XSLT-based migration of files (i.e., with one XSLT style sheet an entire digital collection could be migrated into a derivative format). Users would no longer need to rely on software that reads obsolete file formats, since obsolete formats would be transformable into more-stable formats with XSLT. While much work remains before such scenarios become real, successful implementation would represent a substantial improvement over file format migration using current tools and techniques.

Eleftheriadis, A. and Hong, D. “Flavor: A Language for Media Representation”. To appear as chapter four in Handbook of Video Databases: Design and Applications, Furht, B., and Marques, O., Ed. CRC Press, September, 2003.

RLG DigiNews (ISSN 1093-5371) is a Web-based newsletter conceived by the RLG preservation community and developed to serve a broad readership around the world. It is produced by staff in the Department of Research, Cornell University Library, in consultation with RLG and is published six times a year at www.rlg.org.

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Please send comments and questions about this or other issues to the RLG DigiNews editors.

Co-Editors: Anne R. Kenney and Nancy Y. McGovern; Associate Editor: Robin Dale (RLG); Technical Researcher: Richard Entlich; Contributor: Erica Olsen; Copy Editor: Martha Crowe; Production Coordinator: Carla DeMello; Assistant: Valerie Jacoski.

All links in this issue were confirmed accurate as of August 15, 2003.


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