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XSL Formatting Objects

XSL Formatting Objects. XSL-FO. Why XSL-FO? Some History. The history of HTML shows that, in the early days, both its designers and its users were confused about its purpose Initially, it was intended for specifying the structure of documents for the web

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XSL Formatting Objects

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  1. XSL Formatting Objects XSL-FO

  2. Why XSL-FO? Some History • The history of HTML shows that, in the early days, both its designers and its users were confused about its purpose • Initially, it was intended for specifying the structure of documents for the web • Its tags included <h1> <p> <ol> <ul> <li> (See http://www.w3.org/History/19921103-hypertext/hypertext/WWW/MarkUp/Tags.html) • These documents were then displayed using whatever rendering choices were made by the displaying program

  3. Why XSL-FO? Some History (contd.) • In the early days, there was some confusion • People did not make the distinction between structure and style • Even Tim Berners-Lee made the mistake of writing “Most of the tags are just style tags: this goes for the headings H1 to H6, the lists UL and OL with list elements LI, the glossary DL with elements DT and DD. ” (See http://lists.w3.org/Archives/Public/www-talk/1991SepOct/0003.html)

  4. Why XSL-FO? Some History (contd.) • This confusion caused the mistaken introduction of rendering tags (and rendering attributes) into the language • For example: <b> <i> <tt> <font> • Elimination of this confusion led to the introduction of Cascading Style Sheets and the deprecation of rendering tags and attributes • It is now recognized that there should be a sharp distinction between the specification of structure and of rendering • This distinction was already recognized by the time that XML was developed

  5. Why XSL-FO? Some History (contd.) • The structure/rendering distinction is reflected in the separation between XML and XSL • XML is used for specifying document structure • XSL is used for specifying document rendering • The W3 specification for XSL can be found here http://www.w3.org/TR/xsl/

  6. Why XSL-FO? Some History (contd.) • XSLT is able to do only part of the job of rendering XML documents • XSLT is for specifying how to analyse the structure of an XML document in order to compute the appropriate rendering of the document • But XSLT must be complemented by a second notation in which we can specify the desired rendering • We have seen that HTML is sometimes used as the second notation • However, XSL Formatting Objects (XSL-FO) is the more general notation for specifying the desired rendering

  7. Why XSL-FO? Some History (contd.) • It is common for XSLT to map XML documents into HTML documents • This is because HTML has a rendering semantics • However, HTML allows us to render XML documents only as web pages • using browsers as the rendering engines • Frequently, we need to render XML as other kinds of documents, including • PDF files, • Image files • We need another notation which can be used to specify the rendering of PDF files and images

  8. Why XSL-FO? Some History (contd.) • XSL has two parts • XSLT can be used to analyse the structure of an XML document, in order to generate a document in some other notation • XSL-FO is one such other notation • XSL-FO is designed specifically for the purpose of describing the layout of documents • In this sense, its purpose is a little like that of CSS, • However, XSL-FO is quite different from CSS • A layout description given in XSL-FO can be used to generate a growing variety of documents, including • PDF files, • Image files

  9. XSL-FO is an XML application-specific language • Every XSL-FO specification must be well-formed XML • For technical reasons, it is not possible to have a comprehensive DTD for XSL-FO • However, an unofficial DTD is here http://www.cs.ucc.ie/j.bowen/usefulResources/fo2000.dtd.html • Warning: this unofficial DTD was written by somebody from a company which produces a commercial FO processor program called XEP • The unofficial DTD contains some commercial extensions which are associated with the namespace http://www.renderx.com/XSL/Extensions • These extensions are identified in the DTD by use of the prefix rx:

  10. XSL-FO is an XML application-specific language • The namespace for XSL-FO is http://www.w3.org/1999/XSL/Format • The prefix used is fo • The root element in XSL-FO has the tag name root • The declaration for this element in the unofficial DTD is <!ELEMENT fo:root (fo:layout-master-set, fo:declarations?,fo:page-sequence+)> <!ATTLIST fo:root xmlns:fo CDATA #REQUIRED> • Thus, the root element is of the form <fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format"> … </fo:root>

  11. How do we use XSL-FO?

  12. First, let's revisit how we used XSLT to view XML files in browsers

  13. A reminder: viewing XML in a web browser • The browser requests an XML document • The XML document refers to an XSLT stylesheet • The browser then requests the stylesheet • The browser's XSLT Engine generates HTML from XML+XSLT • The browser's HTML engine renders the HTML in the browser window

  14. Using a browser's XSLT engine to generate HTML from XML ... <?xml version="1.0"?> <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"> <xsl:template match="/"> <html><head><title>Greetings</title></head> <body><xsl:apply-templates select="./names/name"/></body></html> </xsl:template> <xsl:template match="name"> <p>Hello, <xsl:value-of select="."/>!</p> </xsl:template> </xsl:stylesheet>

  15. ... Then rendering the HTML with the browser's HTML engine <?xml version="1.0"?> <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"> <xsl:template match="/"> <html><head><title>Greetings</title></head> <body><xsl:apply-templates select="./names/name"/></body></html> </xsl:template> <xsl:template match="name"> <p>Hello, <xsl:value-of select="."/>!</p> </xsl:template> </xsl:stylesheet>

  16. What happens if we try the same thing with XSL-FO as we just did with HTML?

  17. Generating XSL-FO from XML <?xml version="1.0"?> <xsl:stylesheet version="1.0"xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:fo="http://www.w3.org/1999/XSL/Format"> <xsl:output method="xml" indent="yes"/> <xsl:template match="/"> <fo:root><fo:layout-master-set><fo:simple-page-master master-name="A4-portrait" page-height="29.7cm" page-width="21.0cm" margin="2cm"><fo:region-body/></fo:simple-page-master></fo:layout-master-set><fo:page-sequence master-reference="A4-portrait"><fo:flow flow-name="xsl-region-body"><xsl:apply-templates select="./names/name"/></fo:flow></fo:page-sequence></fo:root> </xsl:template> <xsl:template match="name"><fo:block>Hello, <xsl:value-of select="."/>!</fo:block> </xsl:template> </xsl:stylesheet>

  18. How is the resultant XSL-FO rendered in the browser? <?xml version="1.0" encoding="UTF-8"?> <fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format"> <fo:layout-master-set><fo:simple-page-master margin="2cm" page-width="21.0cm" page-height="29.7cm" master-name="A4-portrait"><fo:region-body/></fo:simple-page-master></fo:layout-master-set><fo:page-sequence master-reference="A4-portrait"> <fo:flow flow-name="xsl-region-body"> <fo:block>Hello, Tom! </fo:block> <fo:block>Hello, Dick! </fo:block> <fo:block>Hello, Harry! </fo:block> </fo:flow></fo:page-sequence></fo:root>

  19. How is the resultant XSL-FO rendered in the browser? • The browser cannot render XSL-FO <?xml version="1.0" encoding="UTF-8"?> <fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format"> <fo:layout-master-set><fo:simple-page-master margin="2cm" page-width="21.0cm" page-height="29.7cm" master-name="A4-portrait"><fo:region-body/></fo:simple-page-master></fo:layout-master-set><fo:page-sequence master-reference="A4-portrait"> <fo:flow flow-name="xsl-region-body"> <fo:block>Hello, Tom! </fo:block> <fo:block>Hello, Dick! </fo:block> <fo:block>Hello, Harry! </fo:block> </fo:flow></fo:page-sequence></fo:root>

  20. How is the resultant XSL-FO rendered in the browser? • The browser cannot render XSL-FO • The fo: tags are simply ignored <?xml version="1.0" encoding="UTF-8"?> <fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format"> <fo:layout-master-set><fo:simple-page-master margin="2cm" page-width="21.0cm" page-height="29.7cm" master-name="A4-portrait"><fo:region-body/></fo:simple-page-master></fo:layout-master-set><fo:page-sequence master-reference="A4-portrait"> <fo:flow flow-name="xsl-region-body"> <fo:block>Hello, Tom! </fo:block> <fo:block>Hello, Dick! </fo:block> <fo:block>Hello, Harry! </fo:block> </fo:flow></fo:page-sequence></fo:root>

  21. How is the resultant XSL-FO rendered in the browser? • The browser cannot render XSL-FO • The fo: tags are simply ignored • We just see the included text <?xml version="1.0" encoding="UTF-8"?> <fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format"> <fo:layout-master-set><fo:simple-page-master margin="2cm" page-width="21.0cm" page-height="29.7cm" master-name="A4-portrait"><fo:region-body/></fo:simple-page-master></fo:layout-master-set><fo:page-sequence master-reference="A4-portrait"> <fo:flow flow-name="xsl-region-body"> <fo:block>Hello, Tom! </fo:block> <fo:block>Hello, Dick! </fo:block> <fo:block>Hello, Harry! </fo:block> </fo:flow></fo:page-sequence></fo:root>

  22. What went wrong? • The problem is that web browsers do not have engines for rendering XSL-FO • Instead, web browsers have engines for rendering HTML

  23. So how do we render XSL-FO?

  24. So how do we render XSL-FO? • We need a different kind of program • We need a program which contains an FO engine • These programs are called FO processors • Usually they also contain an XSLT engine • They can generate various kinds of output, including PDF and image files

  25. Generating XSL-FO from XML and then rendering the XSL-FO • Apache FOP is a commonly-used FO processor • fop contains both an XSLT engine and an FO engine • Example usage on the cs1 server: cs1> fop-xml names2.xml-xsl names2.xsl-pdf names2.pdf • This example command tells fop to apply an XSL stylesheet to an XML document and output a PDF file • The XML file need/should not contain an xml-stylesheet instruction

  26. Starting to write XSL-FO

  27. First stage of an XSLT stylesheet for generating XSL-FO from XML <?xml version="1.0"?> <xsl:stylesheet version="1.0"xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:fo="http://www.w3.org/1999/XSL/Format"> <xsl:output method="xml" indent="yes"/> <xsl:template match="/"> <fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format"> … </fo:root> </xsl:template> </xsl:stylesheet>

  28. Contents of the fo:root element • Element declaration: <!ELEMENT fo:root ( fo:layout-master-set, fo:declarations?, fo:page-sequence+ )> • Thus, the minimum contents of an fo:root element are: • fo:layout-master-set a description of the page layouts that are used in the document being specified • fo:page-sequence+ a set of specifications for one or more sequences of actual pages in the document

  29. Minimum fo:root element • Example XSL-FO document <?xml version="1.0"?> <fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format"> <fo:layout-master-set> … </fo:layout-master-set> <fo:page-sequence … > … </fo:page-sequence> </fo:root>

  30. The layout-master-set • Declaration in the unofficial DTD is <!ELEMENT fo:layout-master-set ( fo:page-sequence-master*, fo:simple-page-master, ( fo:simple-page-master | fo:page-sequence-master )* ) > • Thus, the layout-master-set must contain at least one element of type fo:simple-page-master • Example XSL-FO document <?xml version="1.0"?> <fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format"> <fo:layout-master-set> <fo:simple-page-master master-name=“…"> … </fo:simple-page-master> </fo:layout-master-set> <fo:page-sequence … > … </fo:page-sequence> </fo:root>

  31. Page sequences refer to page masters • Each page sequence must refer to a page master <?xml version="1.0"?> <fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format"> <fo:layout-master-set> <fo:simple-page-master master-name="someName"> … </fo:simple-page-master> </fo:layout-master-set> <fo:page-sequence master-reference="someName"> … </fo:page-sequence> </fo:root>

  32. How pages are modelled in XSL-FO • In the XSL-FO page model, a page in the document being specified contains several areas • The part of the page where the content will be placed is called the page-reference-area • There are four margins around the page-reference-area • These margins can be zero

  33. How pages are modelled in XSL-FO (contd.) • The page-reference-area can be divided into five regions • Only the region-body is required • Most content will be placed in the region-body • The other regions may or may not be present • If the other regions are not present, the regions-body will be the same size as the page-reference-area

  34. How pages are modelled in XSL-FO (contd.) • Unofficial DTD declaration for fo:simple-page-master is <!ELEMENT fo:simple-page-master (fo:region-body,fo:region-before?, fo:region-after?,fo:region-start?,fo:region-end?)>

  35. How pages are modelled in XSL-FO (contd.) • Unofficial DTD declaration for fo:simple-page-master is <!ELEMENT fo:simple-page-master (fo:region-body,fo:region-before?, fo:region-after?,fo:region-start?,fo:region-end?)> <!ATTLIST fo:simple-page-master %margin-properties-CSS; master-name CDATA #REQUIRED page-height CDATA #IMPLIED page-width CDATA #IMPLIED media-usage CDATA #IMPLIED size CDATA #IMPLIED reference-orientation ( 0 | 90 | 180 | 270 | -90 | -180 | -270 | 0deg | 90deg | 180deg | 270deg | -90deg| - 180deg | -270deg | inherit ) #IMPLIED writing-mode ( lr-tb | rl-tb | tb-rl | lr | rl | tb | inherit ) #IMPLIED >

  36. How pages are modelled in XSL-FO (contd.) • The definition, in the unofficial DTD, of the CSS-type margin properties is

  37. How pages are modelled in XSL-FO (contd.) • The definition, in the unofficial DTD, of the CSS-type margin properties is <!ENTITY % margin-properties-CSS " margin CDATA #IMPLIED margin-bottom CDATA #IMPLIED margin-left CDATA #IMPLIED margin-right CDATA #IMPLIED margin-top CDATA #IMPLIED margin-after CDATA #IMPLIED margin-start CDATA #IMPLIED margin-end CDATA #IMPLIED margin-before CDATA #IMPLIED ">

  38. Example XSL-FO document <?xml version="1.0"?> <fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format"> <fo:layout-master-set> <fo:simple-page-master master-name="myPageType" page-height="29.7cm" page-width="21.0cm" margin="2cm"> <fo:region-body/> </fo:simple-page-master> </fo:layout-master-set> <fo:page-sequence master-reference="myPageType"> ...... </fo:page-sequence> </fo:root>

  39. The region-body element • Declaration in the unofficial DTD is <!ELEMENT fo:region-body EMPTY> <!ATTLIST fo:region-body %region-properties; %margin-properties-CSS; column-count CDATA #IMPLIED column-gap CDATA #IMPLIED >

  40. The region-body element • Declaration in the unofficial DTD is <!ELEMENT fo:region-body EMPTY> <!ATTLIST fo:region-body %region-properties; %margin-properties-CSS; column-count CDATA #IMPLIED column-gap CDATA #IMPLIED > <!ENTITY % region-properties " %border-padding-background-properties; %area-properties; region-name CDATA #IMPLIED"> <!ENTITY % area-properties " clip CDATA #IMPLIED overflow ( visible | hidden | scroll | auto | error-if-overflow | inherit ) #IMPLIED display-align ( auto | before | center | after | inherit ) #IMPLIED reference-orientation ( 0 | 90 | 180 | 270 | -90 | -180 | -270 | 0deg | 90deg | 180deg | 270deg | -90deg | -180deg | -270deg | inherit ) #IMPLIED writing-mode ( lr-tb | rl-tb | tb-rl | lr | rl | tb | inherit ) #IMPLIED ">

  41. The fo:page-sequence element • Declaration in the unofficial DTD <!ELEMENT fo:page-sequence (fo:title?, fo:static-content*, fo:flow)> <!ATTLIST fo:page-sequence id CDATA #IMPLIED master-name CDATA #REQUIRED initial-page-number CDATA #IMPLIED force-page-count ( auto | even | odd | end-on-even | end-on-odd | no-force | inherit) #IMPLIED format CDATA #IMPLIED letter-value ( auto | alphabetic | traditional ) #IMPLIED grouping-separator CDATA #IMPLIED grouping-size CDATA #IMPLIED %inheritable-properties; > • Thus a minimal fo:page-sequence must have a master-name and must contain an fo:flow

  42. The fo:flow element • Declaration in the unofficial DTD <!ELEMENT fo:flow ( %blocks; )+ > <!ATTLIST fo:flow %flow-properties; > <!ENTITY % blocks " %basic-blocks; | %out-of-lines; | %wrappers; "> <!ENTITY % flow-properties " id CDATA #IMPLIED flow-name CDATA #REQUIRED %inheritable-properties; "> <!ENTITY % basic-blocks " fo:block | fo:block-container | fo:table-and-caption | fo:table | fo:list-block "> • Thus a minimal fo:flow must have a flow-name (which specifies the region in which its content will be placed) and must contain at least one blocks entity • The simplest instance of blocks is an fo:block element

  43. The fo:block element • Declaration in the unofficial DTD <!ELEMENT fo:block ( #PCDATA | fo:initial-property-set | %basic-inlines; | %basic-blocks; | %out-of-lines; | %wrappers; )* > <!ATTLIST fo:block %block-properties;> • The simplest content for an fo:block element is just some text

  44. Example XSL-FO document <?xml version="1.0"?> <fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format"> <fo:layout-master-set> <fo:simple-page-master master-name="myPageType" page-height="29.7cm" page-width="21.0cm" margin="2.0cm"> <fo:region-body/> </fo:simple-page-master> </fo:layout-master-set> <fo:page-sequence master-reference="myPageType"> <fo:flow flow-name="xsl-region-body"> <fo:block>Hello world</fo:block> </fo:flow> </fo:page-sequence> </fo:root>

  45. Specifying lengths in XSL-FO • A length in XSL-FO is written as a real number followed by a unit qualification • Section 5.9.13 of the XSL specification http://www.w3.org/TR/xsl/#d0e5752 lists the allowable units as • cm • mm • in • pt (1/72 in) • pc (12 pt) • px • em (size of capital M in current font size)

  46. The fop processor accepts XSL-FO files as input • Place the text on the previous slide in a file called demo.fo • Then run the command fop -fo demo.fo -pdf demo.pdf • A pdf file will be generated

  47. The fop processor can output several types of file • Assume that demo.fo contains the source code shown on the previous slide • The command fop -fo demo.fo -png demo.png will output the document as png image • To see the full range of file types that can be generated, run the command fop -out list

  48. Using XSLT to generate XSL-FO • Suppose we have the XML file shown • Suppose we want to generate the XSL-FO for the PDF document shown • Our XSLT program must • generate the "boiler-plate" XSL-FO when it finds the root node • and generate a sentence when it finds each country node • We will need two templates

  49. The "boiler-plate" template <xsl:template match="/"> <fo:root> <fo:layout-master-set> <fo:simple-page-master master-name="A4-portrait" page-height="29.7cm" page-width="21.0cm" margin="2cm"> <fo:region-body/> </fo:simple-page-master> </fo:layout-master-set> <fo:page-sequence master-reference="A4-portrait"> <fo:flow flow-name="xsl-region-body"> <xsl:apply-templates select="./countries/country"/> </fo:flow> </fo:page-sequence> </fo:root> </xsl:template>

  50. The country template <xsl:template match="country"> <fo:block> <xsl:value-of select="."/> is a country. </fo:block> </xsl:template>

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