XSL Formatting Objects
From Wikipedia, the free encyclopedia
XSL Formatting Objects, or XSL-FO, is an XML markup language for document formatting. XSL-FO is part of XSL, a set of W3C technologies designed for the transformation and formatting of XML data. The other parts of XSL are XSLT and XPath.
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[edit] XSL-FO Basics
Unlike the combination of HTML and CSS, XSL-FO is a unified presentational language. It has no semantic markup in the way it is meant in HTML. And, unlike CSS which modifies the default presentation of an external XML or HTML document, it stores all of the document's data within itself.
The general idea behind XSL-FO's use is that the user writes a document, not in FO, but in an XML language. XHTML, DocBook, and TEI are all possibilities, but it could literally be any XML language. Then, the user obtains an XSLT transform, either by writing one themselves or by finding one for the document type in question. This XSLT transform converts the XML into XSL-FO.
Once the XSL-FO document is generated, it is then passed to an application called a FO processor. FO processors convert the XSL-FO document into something that is readable, printable or both. The most common output of XSL-FO is a PDF file or as PS, but some FO processors can output to other formats like RTF files or even just a window in the user's GUI displaying the sequence of pages and their contents.
The XSLT language itself was originally conceived only for this purpose; it has obviously now been supplanted for more general XML transformations. This transformation step is taken so much for granted in XSL-FO that it is not uncommon for people to call the XSLT that turns XML into XSL-FO the actual XSL-FO document itself. Even tutorials on XSL-FO tend to be written with XSLT commands around the FO processing instructions.
The XSLT transformation step is exceptionally powerful. It allows for the automatic generation of a table of contents, linked references, an index, and various other possibilities.
An XSL-FO document is not like a PDF or a PostScript document. It does not fully describe the layout of the text on various pages. Instead, it describes what the pages look like and where the various content goes. From there, a FO processor determines how to position the text within the boundaries described by the FO document. The XSL-FO specification even allows different FO processors to have varying responses with regard to the resultant generated pages.
For example, some FO processors can hyphenate words to minimize space when breaking a line, while others choose not to. Different processors may even use different hyphenation algorithms, from very simple ones to more complex ones that take into account whether the previous or next line also is hyphenated. These will change, in some borderline cases quite substantially, the layout of the various pages. There are other cases where the XSL-FO specification explicitly allows FO processors some degree of choice with regard to layout.
This differentiation between FO processors, creating inconsistent results between processors. is often not a concern. This is because the general purpose behind XSL-FO is to generate paged, printed media. XSL-FO documents themselves are usually used as intermediaries, mostly to generate either PDF files or a printed document as the final form to be distributed. This is as opposed to how HTML is generated and distributed as a final form directly to the user. As such, a person wanting to generate a printed document only has to select the FO processor that fullfills their needs, usually in the realm of layout quality and reduction of unnecessary whitespace, rather than having to test their XSL-FO document on multiple processors.
[edit] XSL-FO Language Concepts
The XSL-FO language was designed for paged media, in much the same way that HTML and CSS were designed for unpaged (or screen-based) media. As such, the concept of pages is an integral part of XSL-FO's structure, and FO gives the user significant power in dealing with how information is displayed on a page.
FO works best for what could be called "content-driven" design. This is the standard method of layout for books, articles, legal documents, and so forth. It involves a single flowing span of fairly contiguous text, with various repeating information built into the margins of a page. This is as opposed to "layout-driven" design, which is used in newspapers or magazines. If content in those documents does not fit in the required space, some of it is trimmed away until it does fit. XSL-FO does not easily handle the tight restrictions of magazine layout; indeed, in many cases, it lacks the ability to express some forms of said layout.
Despite the basic nature of the language's design, it is capable of a great deal of expressiveness. Tables, lists, side floats, and a variety of other features are available. These features are comparable to CSS's layout features, though some of those features are expected to be built by the XSLT.
[edit] XSL-FO Document Structure
XSL-FO documents are XML documents, but they do not have to conform to any DTD or schema. Instead, they conform to a syntax defined in the XSL-FO specification.
XSL-FO documents contain two required sections. The first section details a list of named page layouts. The second section is a list of document data, with markup, that uses the various page layouts to determine how the content fills the various pages.
Page layouts define the properties of the page. They can define the directions for the flow of text, so as to match the conventions for the language in question. They define the size of a page as well as the margins of that page. More importantly, they can define sequences of pages that allow for effects where the odd and even pages look different. For example, one can define a page layout sequence that gives extra space to the inner margins for printing purposes; this allows more space to be given to the margin where the book will be bound.
The document data portion is broken up into a sequence of flows, where each flow is attached to a page layout. The flows contain a list of blocks which, in turn, each contain a list of text data, inline markup elements, or a combination of the two. Content may also be added to the margins of the document, for page numbers, chapter headings and the like.
Blocks and inline elements function in much the same way as for CSS, though some of the rules for padding and margins differ between FO and CSS. The direction, relative to the page orientation, for the progression of blocks and inlines can be fully specified, thus allowing FO documents to function under languages that are read different from English. The language of the FO specification, unlike that of CSS 2.1, uses direction-neutral terms like start and end rather than left and right when describing these directions.
XSL-FO's basic content markup is derived from CSS and its cascading rules. As such, many attributes in XSL-FO propagate into the child elements unless explicitly overridden.
[edit] Capabilities of XSL-FO
XSL-FO is capable of a great deal of textual layout functionality. In addition to the information as specified above, XSL-FO's language allows for the specification of the following.
[edit] Multiple Columns
A page can be defined to have multiple columns. When this is the case, blocks flow from one column into the next by default. Individual blocks can be set to span all columns, creating a textual break in the page. The columns above this break will flow into each other, as will the columns below the break. But no text is allowed to flow from the above section to the below section.
Because of the nature of XSL-FO's page specification, multiple pages may actually have different numbers and widths of columns. As such, text can flow from a 3 column page to a 5 column page to a 1 column page quite easily.
All FO features work within the restrictions of a multi-column page.
[edit] Lists
An XSL-FO list is, essentially, two sets of blocks stacked side by side. An entry consists of a block on the "left", or start inline direction, and a block sequence on the "right", or end inline direction. The block on the left is conceptually what would be the number or bullet in a list. However, it could just as easily be a string of text, as one might see in a glossary entry. The block on the right works as expected. Both of these blocks can be block containers, or have multiple blocks in a single list entry.
Numbering of XSL-FO lists, when they are numbered, is expected to be done by the XSLT, or whatever other process, that generated the XSL-FO document. As such, number lists are to be explicitly numbered in XSL-FO.
[edit] Pagination Controls
The user can specify Widow and Orphan for blocks or for the flow itself, and allow the attributes to cascade into child blocks. Additionally, blocks can be specified to be kept together on a single page. For example, an image block and the description of that image can be set to never be separated. The FO processor will do its best to adhere to these commands, even if it requires creating a great deal of empty space on a page.
[edit] Footnotes
The user can create footnotes that appear at the bottom of a page. The footnote is written, in the FO document, in the regular flow of text at the point where it is referenced. The reference is represented as an inline definition, though it is not required. The body is one or more blocks that are placed by the FO processor to the bottom of the page. The FO processor guarantees that wherever the reference is, the footnote cited by that reference will appear on the same page. This will be so even if it means creating extra empty space on a page.
[edit] Tables
A FO table functions much like an HTML/CSS table. The user specifies rows of data for each individual cell. The user can, also, specify some styling information for each column, such as background color. Additionally, the user can specify the first row as a table header row, with its own separate styling information.
The FO processor can be told exactly how much space to give each column, or it can be told to auto-fit the text in the table.
[edit] Text Orientation Controls
FO has extensive controls for orienting blocks of text. One can, in the middle of a page, designate a block of text to be oriented in a different orientation. These oriented blocks can be used for languages in a different orientation from the rest of the document, or simply if one needs to orient the text for layout purposes. These blocks can contain virtually any kind of content, from tables to lists or even other blocks of reoriented text.
[edit] Miscellaneous
- Page number citations. A page that contains a special tag can be cited in text, and the FO processor will fill in the actual page number where this tag appears.
- Block borders, in a number of styles.
- Background colors and images.
- Font controls and weighting, as in CSS.
- Side floats.
- Miscellaneous Inline Elements
[edit] Advantages of XSL-FO
Because it is an XML language, only an XSLT transform (and an XSLT processor) are required to generate XSL-FO code from any XML language. One can easily write a document in TEI or DocBook, and transform it into HTML for web viewing or PDF (through a FO processor) for printing. In fact, there are many pre-existing TEI and DocBook XSLTs for both of these purposes.
Also, because it is an XML language, and specifically because it is without an explicit schema or DTD, it can store any kind of XML data. The most popular extra data would be SVG images, which a number of FO processors can read and add into the finished result.
Another advantage of XSL-FO is the relative ease of use. Much of the functionality of the language is based on work from CSS, so a CSS user will be familiar with the basics of the markup attributes. Understanding what a specific section of a FO document will look like is usually quite easy. This is compared to layout and typesetting packages like TeX, which are generally considered arcane.
[edit] Drawbacks of XSL-FO
The principal drawback of FO is the lack of support. FO processors are not easy to come by, and virtually none of them offer 100% compatibility with the formal specification. This despite the fact that the specification has been in existence for years. This is, in general, brought about by a lack of demand for XSL-FO. TeX and its like have long held the position as being a typesetting language, and most users of TeX see little need to switch to XSL-FO.
A less important disadvantage of XSL-FO is the fact that its ability to put content into the margins is not all that it could be. The ability to control such data is, while functional, not nearly as complete as one might prefer.
Another problem with FO is that, while it is easy to understand, it is difficult and incredibly verbose to write by hand. XSLT was designed to make writing XSL-FO documents easy, so there was no impetus in the language for making the FO documents themselves be more concise. The problem that this creates is that a neophyte must learn not only the details of writing FO documents, but how to write an XSLT transform as well. That is, of course, only a valid concern if one is not using a pre-existing XSLT transform written for one of the standard document formats.
[edit] Current and Future Versions
The current version of XSL-FO is v1.0. V1.1 is currently a W3C Proposed Recommendation. New functionality in 1.1 includes bookmarking, multiple flows, and index support.
[edit] See also
[edit] External links
- XSL-FO 1.0 Specification
- XSL-FO 1.1 Specification In PR phase, as of 06 October 2006
- What is XSL-FO? on XML.com
- Apache FOP Open source and royalty free implementation of XSL-FO
- W3 Schools A good source for beginners and dabblers
- FO examples and techniques Excellent reference site setup by Dave Pawson, for those who want to use FOP, but were afraid to ask.
- XSL-FO: Ready for Prime Time? on the Gilbane Report
- XSL-FO Tutorial and Samples on Antenna House
- Data2Type (german) XSL-FO information
- XSL Formatting Objects Tutorial on RenderX
- XSL-FO Tutorial and Samples HTML or PDF (PDF, 476 KB) on Ecrion.com
