5 . 5 ODA
"Never eat more than you can lift." - Miss Piggy
And in this corner, off to a slow start, weighing in with both an architecture and interchange format, is the Office Document Architecture (ODA) standard. ODA was the other major player in the electronic document standards game, and European users will argue that it is still relevant, so some background is useful. Not limited to the structure of a document, it addresses the complete range of visual presentation issueshow a document looks. It is important to recognize the word "architecture" in the ODA name. It is an entire framework for representing, in a complete manner, both the structure and the visual presentation of the various elements that make up the structure.
If a document is encoded using ODA, and you give it to another ODA site, you can expect it to look exactly the same when printed at the other site. Clearly, ODA is a much more ambitious standard than SGML. Just as clearly, that is the reason why the standard itself ,and the implementations of ODA, have been slower to come into existence.
ODA describes a document as a hierarchical collection of objectsa tree-structured relationshipjust like SGML. But the terminology ODA uses is, of course, different from the terminology used by SGML.
ODA refers to document structure in two fundamental ways. Documents have a logical structure and a layout structure. The logical structure of a document is very similar to the SGML document structure defined in a DTD. The layout structure, however, refers to the positioning of elements as objects to be placed on paper. This layout structure divides a document into page sets, pages, frames, and blocks to describe the way information is to be placed on paper. The connection between the logical and layout structures is made via a layout directive.
Next the relationship of the logical and layout structures for this simple business letter are illustrated. Note one important property: each content object is directly associated with the lowest, most basic logical and layout objects. This makes a good deal of intuitive sense. Some small (logically indivisible) piece of content must be associated with a layout object to be visible. The content itself forms a sort of interface between the logical and layout trees of an ODA document.
It is important to clearly understand that ODA is an interchange standard. To ensure, for example, that my page will appear correctly on your printer, ODA addresses the appearance of the document's many component parts. These components can include such things as fonts, raster, and geometric graphics. (Raster graphics are bitmapped objects. See Section 6 . 1 Bitmaps and Objects in Chapter 6 Media and Document Integration for a discussion of graphics issues.) If one systems's idea of a curved line is not the same as another's, interchange will suffer. ODA attacks this difficult problem by standardizing the representation for the component parts.
We can trace the processing of an image for a particular content object as follows. Let's consider a line. The line is represented according to the Geometric Content Architecture portion of ODA as a CGM (Computer Graphics Metafile) object. It is imaged according to a presentation attribute. The presentation attribute (from the layout structure of the overall representation) is associated with a semantic logical object (from the logical structure) via a layout directive.
ODA and SGML take fundamentally different approaches to electronic document standardization. OneSGMLis a robust extensible language that focuses on document structure and does not address document layout. The otherODAprovides an architectural framework that addresses both document structure and layout.
