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2.1.2 Formatting vs. Rendering
When creating XML we should be designing the structures around our business processes responsible for maintaining the information, instead of the structures used for presentation. An XSLFO instance describes the intent of how that stream of information is to be formatted in the target medium in a paginated fashion. This instance is typically generated by a stylesheet acting on the instance of XML information, rearranging and restructuring the information into the order and presentation desired.
This reordering takes the #PCDATA content and attribute content of the instance, repackaging it according to our intent based on our understanding of the semantics of the XSLFO vocabulary. We can reify this reordering as an intermediate file of syntax we can use for diagnostic purposes. We could also take the opportunity to store this reordering as an XML instance for "store and forward" strategies where the formatting takes place later or remotely from where the transformation takes place.
Unlike interactive formatting tools such as desktop publishing products or interactive formatting tools, there is no feedback loop from the XSLFO formatter to the stylesheet creating the XSLFO vocabulary. Therefore, the XSLFO information must be complete with respect to all desired behaviors of the formatter. Any special formatting cases or conditions can be accommodated through contingencies expressed in the XSLFO semantics.
The information arranged in the elements and attributes of our source vocabularies is repackaged into the elements and attributes of the XSLFO formatting vocabulary that express the formatting objects and their properties of the XSLFO semantics. Each formatting object specifies an aspect of either layout, appearance and impartation, or the pagination and flow.
The layout semantics express the intent of locating information positioned on the target medium. Areas of content are specified as located and nested within other areas, in a hierarchical tree of rectangles on each page.
The appearance and impartation semantics express the intent of how the information is to be conveyed to the reader. For visual media, this conveyance includes font, size, color, weight, etc. For aural synthesis, this conveyance includes voice, volume, azimuth, pitch, etc.
The pagination and flow semantics express the intent of how the stream of information being presented is to be parceled within the layout areas. The final pagination is the result of accommodating the amount of flow being presented within the areas that have been defined.
Each of the formatting objects is expressed in an XSLFO instance as an element. It is not necessary to know all formatting objects to get effective formatted results.
An XSLFO formatter is responsible for interpreting the intent to be rendered, as expressed in the XSLFO semantics corresponding to the elements and attributes in the instance created by the stylesheet. Following the Recommendation, the formatter determines what is to be rendered where by interpreting the interaction between formatting objects. How the formatter does this interpretation is defined in excruciating detail in the W3C Recommendation, as this document is written more for implementers than for stylesheet writers.
The properties expressed for each of the objects influence or are included in the structure of the resulting areas. Some of these properties are specifically targeted for certain media and are otherwise ignored by media for which they do not apply.
The Recommendation does not describe in detail the semantics of rendering. Any device-specific rendition is interpreted based on the semantics of the formatting objects that create the trees of areas and the traits found in those areas that are derived from the properties. How the rendering agent actually accomplishes the task of effecting the result of formatting to the target medium is entirely up to the agent, as long as it produces the same result as the intent described by the Recommendation.
The rendering, itself, may be a multiple-step process, producing the final form through a staged expression of rendering through interpretation on a given medium. For example, the rendering may require production of another intermediate formatting language such as TeX. Rendering may directly produce a final-form page description language such as the Portable Document Format (PDF), or the Standard Page Description Language (International Standard ISO/IEC 10180). The physical final form would then be produced from the intermediate form or final page representation. Indeed, there could be many steps to obtain a final result, e.g.: XML to XSLFO to TeX to PDF to paper.
This is a prose version of an excerpt from an edited version of the book "Practical Formatting Using XSLFO" (First Edition ISBN 1-894049-07-1 at the time of this writing) published by Crane Softwrights Ltd., written by G. Ken Holman. |
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