docs/usermanual-shaping-concepts.xml - third_party/harfbuzz - Git at Google

 <?xml version="1.0"?>
 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
                "http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd" [
   <!ENTITY % local.common.attrib "xmlns:xi  CDATA  #FIXED 'http://www.w3.org/2003/XInclude'">
   <!ENTITY version SYSTEM "version.xml">
 ]>
 <chapter id="shaping-concepts">
   <title>Shaping concepts</title>
   <section id="text-shaping-concepts">
     <title>Text shaping</title>
     <para>
       Text shaping is the process of transforming a sequence of Unicode
       codepoints that represent individual characters (letters,
       diacritics, tone marks, numbers, symbols, etc.) into the
       orthographically and linguistically correct two-dimensional layout
       of glyph shapes taken from a specified font.
     </para>
     <para>
       For some writing systems (or <emphasis>scripts</emphasis>) and
       languages, the process is simple, requiring the shaper to do
       little more than advance the horizontal position forward by the
       correct amount for each successive glyph.
     </para>
     <para>
       But, for other scripts (often unceremoniously called <emphasis>complex scripts</emphasis>), any combination of
       several shaping operations may be required, and the rules for how
       and when they are applied vary from script to script. HarfBuzz and
       other shaping engines implement these rules.
     </para>
     <para>
       The exact rules and necessary operations for a particular script
       constitute a shaping <emphasis>model</emphasis>. OpenType
       specifies a set of shaping models that covers all of
       Unicode. Other shaping models are available, however, including
       Graphite and Apple Advanced Typography (AAT).
     </para>
   </section>

   <section id="script-specific-shaping">
     <title>Script-specific shaping</title>
     <para>
       In many scripts, transforming the input
       sequence into the final layout often requires some combination of
       operations&mdash;such as context-dependent substitutions,
       context-dependent mark positioning, glyph-to-glyph joining,
       glyph reordering, or glyph stacking.
     </para>
     <para>
       In some scripts, the shaping rules require that a text
       run be divided into syllables before the operations can be
       applied. Other scripts may apply shaping operations over
       entire words or over the entire text run, with no subdivision
       required.
     </para>
     <para>
       Other scripts, do not require these
       operations. However, correctly shaping a text run in
       any script may still involve Unicode normalization,
       ligature substitutions, mark positioning, kerning, and applying
       other font features.
     </para>
   </section>

   <section id="shaping-operations">
     <title>Shaping operations</title>
     <para>
       Shaping a text run involves transforming the
       input sequence of Unicode codepoints with some combination of
       operations that is specified in the shaping model for the
       script.
     </para>
     <para>
       The specific conditions that trigger a given operation for a
       text run varies from script to script, as do the order that the
       operations are performed in and which codepoints are
       affected. However, the same general set of shaping operations is
       common to all of the script shaping models.
     </para>

     <itemizedlist>
       <listitem>
 	<para>
 	  A <emphasis>reordering</emphasis> operation moves a glyph
 	  from its original ("logical") position in the sequence to
 	  some other ("visual") position.
 	</para>
 	<para>
 	  The shaping model for a given script might involve
 	  more than one reordering step.
 	</para>
       </listitem>

       <listitem>
 	<para>
 	  A <emphasis>joining</emphasis> operation replaces a glyph
 	  with an alternate form that is designed to connect with one
 	  or more of the adjacent glyphs in the sequence.
 	</para>
       </listitem>

       <listitem>
 	<para>
 	  A contextual <emphasis>substitution</emphasis> operation
 	  replaces either a single glyph or a subsequence of several
 	  glyphs with an alternate glyph. This substitution is
 	  performed when the original glyph or subsequence of glyphs
 	  occurs in a specified position with respect to the
 	  surrounding sequence. For example, one substitution might be
 	  performed only when the target glyph is the first glyph in
 	  the sequence, while another substitution is performed only
 	  when a different target glyph occurs immediately after a
 	  particular string pattern.
 	</para>
 	<para>
 	  The shaping model for a given script might involve
 	  multiple contextual-substitution operations, each applying
 	  to different target glyphs and patterns, and which are
 	  performed in separate steps.
 	</para>
       </listitem>

       <listitem>
 	<para>
 	  A contextual <emphasis>positioning</emphasis> operation
 	  moves the horizontal and/or vertical position of a
 	  glyph. This positioning move is performed when the glyph
 	  occurs in a specified position with respect to the
 	  surrounding sequence.
 	</para>
 	<para>
 	  Many contextual positioning operations are used to place
 	  <emphasis>mark</emphasis> glyphs (such as diacritics, vowel
 	  signs, and tone markers) with respect to
 	  <emphasis>base</emphasis> glyphs. However, some
 	  scripts may use contextual positioning operations to
 	  correctly place base glyphs as well, such as
 	  when the script uses <emphasis>stacking</emphasis> characters.
 	</para>
       </listitem>

     </itemizedlist>
   </section>

   <section id="unicode-character-categories">
     <title>Unicode character categories</title>
     <para>
       Shaping models are typically specified with respect to how
       scripts are defined in the Unicode standard.
     </para>
     <para>
       Every codepoint in the Unicode Character Database (UCD) is
       assigned a <emphasis>Unicode General Category</emphasis> (UGC),
       which provides the most fundamental information about the
       codepoint: whether the codepoint represents a
       <emphasis>Letter</emphasis>, a <emphasis>Mark</emphasis>, a
       <emphasis>Number</emphasis>, <emphasis>Punctuation</emphasis>, a
       <emphasis>Symbol</emphasis>, a <emphasis>Separator</emphasis>,
       or something else (<emphasis>Other</emphasis>).
     </para>
     <para>
       These UGC properties are "Major" categories. Each codepoint is
       further assigned to a "minor" category within its Major
       category, such as "Letter, uppercase" (<literal>Lu</literal>) or
       "Letter, modifier" (<literal>Lm</literal>).
     </para>
     <para>
       Shaping models are concerned primarily with Letter and Mark
       codepoints. The minor categories of Mark codepoints are
       particularly important for shaping. Marks can be nonspacing
       (<literal>Mn</literal>), spacing combining
       (<literal>Mc</literal>), or enclosing (<literal>Me</literal>).
     </para>
     <para>
       In addition to the UGC property, codepoints in the Indic and
       Southeast Asian scripts are also assigned
       <emphasis>Unicode Indic Syllabic Category</emphasis> (UISC) and
       <emphasis>Unicode Indic Positional Category</emphasis> (UIPC)
       properties that provide more detailed information needed for
       shaping.
     </para>
     <para>
       The UISC property sub-categorizes Letters and Marks according to
       common script-shaping behaviors. For example, UISC distinguishes
       between consonant letters, vowel letters, and vowel marks. The
       UIPC property sub-categorizes Mark codepoints by the relative visual
       position that they occupy (above, below, right, left, or in
       multiple positions).
     </para>
     <para>
       Some scripts require that the text run be split into
       syllables. What constitutes a valid syllable in these
       scripts is specified in regular expressions, formed from the
       Letter and Mark codepoints, that take the UISC and UIPC
       properties into account.
     </para>

   </section>

   <section id="text-runs">
     <title>Text runs</title>
     <para>
       Real-world text usually contains codepoints from a mixture of
       different Unicode scripts (including punctuation, numbers, symbols,
       white-space characters, and other codepoints that do not belong
       to any script). Real-world text may also be marked up with
       formatting that changes font properties (including the font,
       font style, and font size).
     </para>
     <para>
       For shaping purposes, all real-world text streams must be first
       segmented into runs that have a uniform set of properties.
     </para>
     <para>
       In particular, shaping models always assume that every codepoint
       in a text run has the same <emphasis>direction</emphasis>,
       <emphasis>script</emphasis> tag, and
       <emphasis>language</emphasis> tag.
     </para>
   </section>

   <section id="opentype-shaping-models">
     <title>OpenType shaping models</title>
     <para>
       OpenType provides shaping models for the following scripts:
     </para>

     <itemizedlist>
       <listitem>
 	<para>
 	  The <emphasis>default</emphasis> shaping model handles all
 	  scripts with no script-specific shaping model, and may also be used as a fallback for
 	  handling unrecognized scripts.
 	</para>
       </listitem>

       <listitem>
 	<para>
 	  The <emphasis>Indic</emphasis> shaping model handles the Indic
 	  scripts Bengali, Devanagari, Gujarati, Gurmukhi, Kannada,
 	  Malayalam, Oriya, Tamil, and Telugu.
 	</para>
 	<para>
 	  The Indic shaping model was revised significantly in
 	  2005. To denote the change, a new set of <emphasis>script
 	  tags</emphasis> was assigned for Bengali, Devanagari,
 	  Gujarati, Gurmukhi, Kannada, Malayalam, Oriya, Tamil, and
 	  Telugu. For the sake of clarity, the term "Indic2" is
 	  sometimes used to refer to the current, revised shaping
 	  model.
 	</para>
       </listitem>

       <listitem>
 	<para>
 	  The <emphasis>Arabic</emphasis> shaping model supports
 	  Arabic, Mongolian, N'Ko, Syriac, and several other connected
 	  or cursive scripts.
 	</para>
       </listitem>

       <listitem>
 	<para>
 	  The <emphasis>Thai/Lao</emphasis> shaping model supports
 	  the Thai and Lao scripts.
 	</para>
       </listitem>

       <listitem>
 	<para>
 	  The <emphasis>Khmer</emphasis> shaping model supports the
 	  Khmer script.
 	</para>
       </listitem>

       <listitem>
 	<para>
 	  The <emphasis>Myanmar</emphasis> shaping model supports the
 	  Myanmar (or Burmese) script.
 	</para>
       </listitem>

       <listitem>
 	<para>
 	  The <emphasis>Tibetan</emphasis> shaping model supports the
 	  Tibetan script.
 	</para>
       </listitem>

       <listitem>
 	<para>
 	  The <emphasis>Hangul</emphasis> shaping model supports the
 	  Hangul script.
 	</para>
       </listitem>

       <listitem>
 	<para>
 	  The <emphasis>Hebrew</emphasis> shaping model supports the
 	  Hebrew script.
 	</para>
       </listitem>

       <listitem>
 	<para>
 	  The <emphasis>Universal Shaping Engine</emphasis> (USE)
 	  shaping model supports scripts not covered by one of
 	  the above, script-specific shaping models, including
 	  Javanese, Balinese, Buginese, Batak, Chakma, Lepcha, Modi,
 	  Phags-pa, Tagalog, Siddham, Sundanese, Tai Le, Tai Tham, Tai
 	  Viet, and many others.
 	</para>
       </listitem>

       <listitem>
 	<para>
 	  Text runs that do not fall under one of the above shaping
 	  models may still require processing by a shaping engine. Of
 	  particular note is <emphasis>Emoji</emphasis> shaping, which
 	  may involve variation-selector sequences and glyph
 	  substitution. Emoji shaping is handled by the default
 	  shaping model.
 	</para>
       </listitem>

     </itemizedlist>

   </section>

   <section id="graphite-shaping">
     <title>Graphite shaping</title>
     <para>
       In contrast to OpenType shaping, Graphite shaping does not
       specify a predefined set of shaping models or a set of supported
       scripts.
     </para>
     <para>
       Instead, each Graphite font contains a complete set of rules that
       implement the required shaping model for the intended
       script. These rules include finite-state machines to match
       sequences of codepoints to the shaping operations to perform.
     </para>
     <para>
       Graphite shaping can perform the same shaping operations used in
       OpenType shaping, as well as other functions that have not been
       defined for OpenType shaping.
     </para>
   </section>

   <section id="aat-shaping">
     <title>AAT shaping</title>
     <para>
       In contrast to OpenType shaping, AAT shaping does not specify a
       predefined set of shaping models or a set of supported scripts.
     </para>
     <para>
       Instead, each AAT font includes a complete set of rules that
       implement the desired shaping model for the intended
       script. These rules include finite-state machines to match glyph
       sequences and the shaping operations to perform.
     </para>
     <para>
       Notably, AAT shaping rules are expressed for glyphs in the font,
       not for Unicode codepoints. AAT shaping can perform the same
       shaping operations used in OpenType shaping, as well as other
       functions that have not been defined for OpenType shaping.
     </para>
   </section>
 </chapter>
	<?xml version="1.0"?>
	<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
	"http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd" [
	<!ENTITY % local.common.attrib "xmlns:xi CDATA #FIXED 'http://www.w3.org/2003/XInclude'">
	<!ENTITY version SYSTEM "version.xml">
	]>
	<chapter id="shaping-concepts">
	<title>Shaping concepts</title>
	<section id="text-shaping-concepts">
	<title>Text shaping</title>
	<para>
	Text shaping is the process of transforming a sequence of Unicode
	codepoints that represent individual characters (letters,
	diacritics, tone marks, numbers, symbols, etc.) into the
	orthographically and linguistically correct two-dimensional layout
	of glyph shapes taken from a specified font.
	</para>
	<para>
	For some writing systems (or <emphasis>scripts</emphasis>) and
	languages, the process is simple, requiring the shaper to do
	little more than advance the horizontal position forward by the
	correct amount for each successive glyph.
	</para>
	<para>
	But, for other scripts (often unceremoniously called <emphasis>complex scripts</emphasis>), any combination of
	several shaping operations may be required, and the rules for how
	and when they are applied vary from script to script. HarfBuzz and
	other shaping engines implement these rules.
	</para>
	<para>
	The exact rules and necessary operations for a particular script
	constitute a shaping <emphasis>model</emphasis>. OpenType
	specifies a set of shaping models that covers all of
	Unicode. Other shaping models are available, however, including
	Graphite and Apple Advanced Typography (AAT).
	</para>
	</section>

	<section id="script-specific-shaping">
	<title>Script-specific shaping</title>
	<para>
	In many scripts, transforming the input
	sequence into the final layout often requires some combination of
	operations—such as context-dependent substitutions,
	context-dependent mark positioning, glyph-to-glyph joining,
	glyph reordering, or glyph stacking.
	</para>
	<para>
	In some scripts, the shaping rules require that a text
	run be divided into syllables before the operations can be
	applied. Other scripts may apply shaping operations over
	entire words or over the entire text run, with no subdivision
	required.
	</para>
	<para>
	Other scripts, do not require these
	operations. However, correctly shaping a text run in
	any script may still involve Unicode normalization,
	ligature substitutions, mark positioning, kerning, and applying
	other font features.
	</para>
	</section>

	<section id="shaping-operations">
	<title>Shaping operations</title>
	<para>
	Shaping a text run involves transforming the
	input sequence of Unicode codepoints with some combination of
	operations that is specified in the shaping model for the
	script.
	</para>
	<para>
	The specific conditions that trigger a given operation for a
	text run varies from script to script, as do the order that the
	operations are performed in and which codepoints are
	affected. However, the same general set of shaping operations is
	common to all of the script shaping models.
	</para>

	<itemizedlist>
	<listitem>
	<para>
	A <emphasis>reordering</emphasis> operation moves a glyph
	from its original ("logical") position in the sequence to
	some other ("visual") position.
	</para>
	<para>
	The shaping model for a given script might involve
	more than one reordering step.
	</para>
	</listitem>

	<listitem>
	<para>
	A <emphasis>joining</emphasis> operation replaces a glyph
	with an alternate form that is designed to connect with one
	or more of the adjacent glyphs in the sequence.
	</para>
	</listitem>

	<listitem>
	<para>
	A contextual <emphasis>substitution</emphasis> operation
	replaces either a single glyph or a subsequence of several
	glyphs with an alternate glyph. This substitution is
	performed when the original glyph or subsequence of glyphs
	occurs in a specified position with respect to the
	surrounding sequence. For example, one substitution might be
	performed only when the target glyph is the first glyph in
	the sequence, while another substitution is performed only
	when a different target glyph occurs immediately after a
	particular string pattern.
	</para>
	<para>
	The shaping model for a given script might involve
	multiple contextual-substitution operations, each applying
	to different target glyphs and patterns, and which are
	performed in separate steps.
	</para>
	</listitem>

	<listitem>
	<para>
	A contextual <emphasis>positioning</emphasis> operation
	moves the horizontal and/or vertical position of a
	glyph. This positioning move is performed when the glyph
	occurs in a specified position with respect to the
	surrounding sequence.
	</para>
	<para>
	Many contextual positioning operations are used to place
	<emphasis>mark</emphasis> glyphs (such as diacritics, vowel
	signs, and tone markers) with respect to
	<emphasis>base</emphasis> glyphs. However, some
	scripts may use contextual positioning operations to
	correctly place base glyphs as well, such as
	when the script uses <emphasis>stacking</emphasis> characters.
	</para>
	</listitem>

	</itemizedlist>
	</section>

	<section id="unicode-character-categories">
	<title>Unicode character categories</title>
	<para>
	Shaping models are typically specified with respect to how
	scripts are defined in the Unicode standard.
	</para>
	<para>
	Every codepoint in the Unicode Character Database (UCD) is
	assigned a <emphasis>Unicode General Category</emphasis> (UGC),
	which provides the most fundamental information about the
	codepoint: whether the codepoint represents a
	<emphasis>Letter</emphasis>, a <emphasis>Mark</emphasis>, a
	<emphasis>Number</emphasis>, <emphasis>Punctuation</emphasis>, a
	<emphasis>Symbol</emphasis>, a <emphasis>Separator</emphasis>,
	or something else (<emphasis>Other</emphasis>).
	</para>
	<para>
	These UGC properties are "Major" categories. Each codepoint is
	further assigned to a "minor" category within its Major
	category, such as "Letter, uppercase" (<literal>Lu</literal>) or
	"Letter, modifier" (<literal>Lm</literal>).
	</para>
	<para>
	Shaping models are concerned primarily with Letter and Mark
	codepoints. The minor categories of Mark codepoints are
	particularly important for shaping. Marks can be nonspacing
	(<literal>Mn</literal>), spacing combining
	(<literal>Mc</literal>), or enclosing (<literal>Me</literal>).
	</para>
	<para>
	In addition to the UGC property, codepoints in the Indic and
	Southeast Asian scripts are also assigned
	<emphasis>Unicode Indic Syllabic Category</emphasis> (UISC) and
	<emphasis>Unicode Indic Positional Category</emphasis> (UIPC)
	properties that provide more detailed information needed for
	shaping.
	</para>
	<para>
	The UISC property sub-categorizes Letters and Marks according to
	common script-shaping behaviors. For example, UISC distinguishes
	between consonant letters, vowel letters, and vowel marks. The
	UIPC property sub-categorizes Mark codepoints by the relative visual
	position that they occupy (above, below, right, left, or in
	multiple positions).
	</para>
	<para>
	Some scripts require that the text run be split into
	syllables. What constitutes a valid syllable in these
	scripts is specified in regular expressions, formed from the
	Letter and Mark codepoints, that take the UISC and UIPC
	properties into account.
	</para>

	</section>

	<section id="text-runs">
	<title>Text runs</title>
	<para>
	Real-world text usually contains codepoints from a mixture of
	different Unicode scripts (including punctuation, numbers, symbols,
	white-space characters, and other codepoints that do not belong
	to any script). Real-world text may also be marked up with
	formatting that changes font properties (including the font,
	font style, and font size).
	</para>
	<para>
	For shaping purposes, all real-world text streams must be first
	segmented into runs that have a uniform set of properties.
	</para>
	<para>
	In particular, shaping models always assume that every codepoint
	in a text run has the same <emphasis>direction</emphasis>,
	<emphasis>script</emphasis> tag, and
	<emphasis>language</emphasis> tag.
	</para>
	</section>

	<section id="opentype-shaping-models">
	<title>OpenType shaping models</title>
	<para>
	OpenType provides shaping models for the following scripts:
	</para>

	<itemizedlist>
	<listitem>
	<para>
	The <emphasis>default</emphasis> shaping model handles all
	scripts with no script-specific shaping model, and may also be used as a fallback for
	handling unrecognized scripts.
	</para>
	</listitem>

	<listitem>
	<para>
	The <emphasis>Indic</emphasis> shaping model handles the Indic
	scripts Bengali, Devanagari, Gujarati, Gurmukhi, Kannada,
	Malayalam, Oriya, Tamil, and Telugu.
	</para>
	<para>
	The Indic shaping model was revised significantly in
	2005. To denote the change, a new set of <emphasis>script
	tags</emphasis> was assigned for Bengali, Devanagari,
	Gujarati, Gurmukhi, Kannada, Malayalam, Oriya, Tamil, and
	Telugu. For the sake of clarity, the term "Indic2" is
	sometimes used to refer to the current, revised shaping
	model.
	</para>
	</listitem>

	<listitem>
	<para>
	The <emphasis>Arabic</emphasis> shaping model supports
	Arabic, Mongolian, N'Ko, Syriac, and several other connected
	or cursive scripts.
	</para>
	</listitem>

	<listitem>
	<para>
	The <emphasis>Thai/Lao</emphasis> shaping model supports
	the Thai and Lao scripts.
	</para>
	</listitem>

	<listitem>
	<para>
	The <emphasis>Khmer</emphasis> shaping model supports the
	Khmer script.
	</para>
	</listitem>

	<listitem>
	<para>
	The <emphasis>Myanmar</emphasis> shaping model supports the
	Myanmar (or Burmese) script.
	</para>
	</listitem>

	<listitem>
	<para>
	The <emphasis>Tibetan</emphasis> shaping model supports the
	Tibetan script.
	</para>
	</listitem>

	<listitem>
	<para>
	The <emphasis>Hangul</emphasis> shaping model supports the
	Hangul script.
	</para>
	</listitem>

	<listitem>
	<para>
	The <emphasis>Hebrew</emphasis> shaping model supports the
	Hebrew script.
	</para>
	</listitem>

	<listitem>
	<para>
	The <emphasis>Universal Shaping Engine</emphasis> (USE)
	shaping model supports scripts not covered by one of
	the above, script-specific shaping models, including
	Javanese, Balinese, Buginese, Batak, Chakma, Lepcha, Modi,
	Phags-pa, Tagalog, Siddham, Sundanese, Tai Le, Tai Tham, Tai
	Viet, and many others.
	</para>
	</listitem>

	<listitem>
	<para>
	Text runs that do not fall under one of the above shaping
	models may still require processing by a shaping engine. Of
	particular note is <emphasis>Emoji</emphasis> shaping, which
	may involve variation-selector sequences and glyph
	substitution. Emoji shaping is handled by the default
	shaping model.
	</para>
	</listitem>

	</itemizedlist>

	</section>

	<section id="graphite-shaping">
	<title>Graphite shaping</title>
	<para>
	In contrast to OpenType shaping, Graphite shaping does not
	specify a predefined set of shaping models or a set of supported
	scripts.
	</para>
	<para>
	Instead, each Graphite font contains a complete set of rules that
	implement the required shaping model for the intended
	script. These rules include finite-state machines to match
	sequences of codepoints to the shaping operations to perform.
	</para>
	<para>
	Graphite shaping can perform the same shaping operations used in
	OpenType shaping, as well as other functions that have not been
	defined for OpenType shaping.
	</para>
	</section>

	<section id="aat-shaping">
	<title>AAT shaping</title>
	<para>
	In contrast to OpenType shaping, AAT shaping does not specify a
	predefined set of shaping models or a set of supported scripts.
	</para>
	<para>
	Instead, each AAT font includes a complete set of rules that
	implement the desired shaping model for the intended
	script. These rules include finite-state machines to match glyph
	sequences and the shaping operations to perform.
	</para>
	<para>
	Notably, AAT shaping rules are expressed for glyphs in the font,
	not for Unicode codepoints. AAT shaping can perform the same
	shaping operations used in OpenType shaping, as well as other
	functions that have not been defined for OpenType shaping.
	</para>
	</section>
	</chapter>