Apache™ FOP: Complex Scripts

This page describes the complex scripts features of Apache™ FOP, which include:

• Support for languages written with right-to-left scripts, such as Arabic and Hebrew scripts.
• Support for languages written with South Asian and Southeast Asian scripts, such as Devanagari, Khmer, Tamil, Thai, and others.
• Support for advanced substitution, reordering, and positioning of glyphs according to language and script sensitive rules.
• Support for advanced number to string formatting.

Complex script features are enabled by default. If some application of FOP does not require this support, then it can be disabled in three ways:

1. Command line: The command line option -nocs turns off complex script features: fop -nocs -fo mydocument.fo -pdf mydocument.pdf
2. Embedding: userAgent.setComplexScriptFeaturesEnabled(false);
3. Optional setting in fop.xconf file:

   <fop version="1.0">
     <complex-scripts disabled="true"/>
     ...
   </fop>
             

When complex scripts features are enabled, additional information related to bidirectional level resolution, the association between characters and glyphs, and glyph position adjustments are added to the internal, parsed representation of the XSL-FO tree and its corresponding formatted area tree. This additional information will somewhat increase the memory requirements for processing documents that use these features.

In most circumstances, XSL-FO content does not need to change in order to make use of complex scripts features; however, in certain contexts, fully automatic processing is not sufficient. In these cases, an author may make use of the following XSL-FO constructs:

• The script property.
• The language property.
• The writing-mode property.
• The number to string conversion properties: format, grouping-separator, grouping-size, letter-value, and fox:number-conversion-features.
• The fo:bidi-override element.
• Explicit bidirectional control characters: U+200E LRM, U+200F RLM, U+202A LRE, U+202B RLE, U+202C PDF, U+202D LRO, U+202E RLO.
• Explicit join control characters: U+200C ZWNJ and U+200D ZWJ.

The complex scripts related effects of the above enumerated XSL-FO constructs are more fully described in the following sub-sections.

Script Property

In order to apply font specific complex script features, it is necessary to know the script that applies to the text undergoing layout processing. This script is determined using the following algorithm:

1. If the FO element that governs the text specifies a script property and its value is not the empty string or "auto", then that script is used.
2. Otherwise, the dominant script of the text is determined automatically by finding the script whose constituent characters appear most frequently in the text.

In case the automatic algorithm does not produce the desired results, an author may explicitly specify a script property with the desired script. If specified, it must be one of the four-letter script code specified in ISO 15924 Code List or in the Extended Script Codes table. Comparison of script codes is performed in a case-insensitive manner, so it does not matter what case is used when specifying these codes in an XSL-FO document.

Standard Script Codes

The following table enumerates the standard ISO 15924 4-letter codes recognized by FOP.

Extended Script Codes

The following table enumerates a number of non-standard extended script codes recognized by FOP.

Language Property

Certain fonts that support complex script features can make use of language information in order for language specific processing rules to be applied. For example, a font designed for the Arabic script may support typographic variations according to whether the written language is Arabic, Farsi (Persian), Sindhi, Urdu, or another language written with the Arabic script. In order to apply these language specific features, the author may explicitly mark the text with a language property.

When specifying the language property, the value of the property must be either an ISO639-2 3-letter code or an ISO639-1 2-letter code. Comparison of language codes is performed in a case-insensitive manner, so it does not matter what case is used when specifying these codes in an XSL-FO document.

Writing Mode Property

The writing-mode property is used to determine the axes and direction of the inline progression direction, the block progression direction, the column progression direction (in tables and flows), the shift direction, region placement, the resolution of writing-mode relative property values (such as start, end, before, after), and the default block (paragraph) bidirectionality level.

The writing-mode property is inherited, so it can appear on any XSL-FO element type; however, it applies (semantically) only to the following element types:

• fo:page-sequence
• fo:simple-page-master
• fo:region-*
• fo:block-container
• fo:inline-container
• fo:table

If it is not specified on one of these element types, but is specified on an ancestor element, then the value specified on that ancestor element (the inherited value) is used; otherwise, the initial value lr-tb is used.

At present, only the following values of the writing-mode property are supported:

• lr-tb
• rl-tb
• lr
• rl

Writing modes that employ a vertical inline progression direction are not yet supported.

Bidi Override Element

The fo:bidi-override element may be used to override default bidirectional processing behavior, including default embedding levels and default character directionality. In the absence of either this element or use of explicit Bidi Control Characters, the default behavior prescribed by the Unicode Bidirectional Algorithm applies.

Bidi Control Characters

In addition to the use of the Bidi Override Element, an author may make use of the following explicit Unicode Bidi Control Characters:

• U+200E - LEFT-TO-RIGHT MARK (LRM)
• U+200F - RIGHT-TO-LEFT MARK (RLM)
• U+202A - LEFT-TO-RIGHT EMBEDDING (LRE)
• U+202B - RIGHT-TO-LEFT EMBEDDING (RLE)
• U+202C - POP DIRECTIONAL FORMATTING (PDF)
• U+202D - LEFT-TO-RIGHT OVERRIDE (LRO)
• U+202E - RIGHT-TO-LEFT OVERRIDE (RLO)

If an embedding or override is not terminated (using U+202C PDF) prior to the end of a delimited text range, then it is automatically terminated by FOP.

Join Control Characters

In order to prevent joining behavior in contexts where joining occurs by default, for example, between U+0628 ARABIC LETTER BEH and U+0646 ARABIC LETTER NOON, an author may used a U+200C ZERO WIDTH NON-JOINER (ZWNJ).

Conversely, in order to force joining behavior in contexts where joining does not occur by default, for example, between U+0628 ARABIC LETTER BEH and U+0020 SPACE, an author may used a U+200D ZERO WIDTH JOINER (ZWJ).

The behavior of ZWNJ and ZWJ is script specific. See The Unicode Standard, Chapter 8, Middle Eastern Scripts for information on the use of these control characters with the Arabic script. See The Unicode Standard, Chapter 9, South Asian Scripts - I for information on the use of these control characters with common Indic scripts.

Support for specific complex scripts is enumerated in the following table. Support for those marked as not being supported is expected to be added in future revisions.

Support for specific fonts is enumerated in the following sub-sections. If a given font is not listed, then it has not been tested with these complex scripts features.

Arabic Fonts

Devanagari Fonts

Complex scripts support in Apache FOP is relatively new, so there are certain limitations. Please help us identify and close any gaps.

• Only the PDF output format fully supports complex scripts features at the present time.
• Shaping context does not extend across an element boundary. This limitation prevents the use of fo:character, fo:inline or fo:wrapper in order to colorize individual Arabic letters without affecting shaping behavior across the element boundary.

In addition to the XSL-FO specification, a number of external resources provide guidance about authoring documents that employ complex scripts and the features described above:

• The Unicode Standard
• Unicode Bidirectional Algorithm
• OpenType Advanced Typographic Extensions
• Examples of Complex Rendering