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3.2 Limitations of ODA and possible modifications

ODA, a standard for the storage and interchange of multimedia documents, deals with both logical structure and layout structure or presentation (unlike SGML). ODA currently includes graphics and images and extensions are being considered to handle audio, video, and hypertext [Cole & Brown, 1990].

a. Separation of logical structure and layout structure

Though ODA supports both logical structure and layout structure, they are not completely separated. In order to change the style of a document the logical structure must be edited since the layout process uses the logical structure, the generic structures and the content architectures to create the specific layout. This limitation can be eliminated by carrying over the SGML mechanism of applying different set of layout and presentation styles (or style sheets) for different views of the same logical document.

b. Comprehensive attribute inheritance

The ODA mechanism for inheriting layout attributes (such as placement of blocks of contents within pages and rectangular areas called frames) and presentation attributes (such as character sets and the placement of items within blocks) is not sufficient. If an attribute value is not specified for the object or its class, then the value can only be inherited according to the object’s position in the tree and not according to its class (chapter, list etc.).

Attribute inheritance can be achieved by adding a facility called “style tables” which will enable the style inherited by an object (and hence its format) to depend both on its class and is position in the document. This will be very valuable for hypertext in order to distinguish between objects of the same type that have different status (such as open and close buttons). It can also be extended to specify changes of state (for example, when selecting a hotspot) by changing the style table.

c. Links

ODA does not have the ability to specify the purpose of a link and also how the layout process can express that purpose. This can be accomplished by having classes for links (just as there are classes for logical objects). The class of the link will determine how and where in the document the link can be used. Thus, the representation of the link will depend on both the class and its position in the document.

d. Selective and multiple presentation

ODA does not have the ability to suppress the appearance of a logical object (or contents) during the layout process nor the ability to present the object many times. Such a feature will be of great help in a hypertext document where a reviewer’s comments can be suppressed from appearing in a printout or different versions of the same basic document can be produced for various purposes. This can also be accomplished by the usage of style tables suggested earlier.

e. Complete interactivity

The ODA layout process is sequential and page based and hence does not provide complete interactivity. It does not support online editing capabilities such as the ability to scroll through a document, the ability to display selected items (outlining facility), the ability to popup additional information on demand (such as footnotes, glossaries etc.), the ability to “fold” documents revealing hidden sections only on request, the ability to follow links automatically.

Complete interactivity would require extensions. Outlining can be done by having style tables that select objects by class and required level. Popup displays can be arranged by changing to a different style table and returning to the original table after the popup information has been displayed. Similarly, folding can be achieved indirectly through popups and popdowns. Link traversal can be done by replacing the current object with the target object or displaying the target object as a temporary popup item. A style table can be used to specify whether or not to display the linked object.