Docs: usermanual, add Shaping Concepts chapter.
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Nathan Willis
Khaled Hosny
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@ -13,8 +13,8 @@
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<para>
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HarfBuzz is an <ulink url="http://www.microsoft.com/typography/otspec/">OpenType</ulink>
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text shaping engine. Using the HarfBuzz library allows
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programs to convert a sequence of Unicode input text into
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properly formatted and positioned output—for any writing
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programs to convert a sequence of Unicode input into
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properly formatted and positioned text output—for any writing
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system and language.
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</para>
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@ -34,6 +34,7 @@
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<xi:include href="usermanual-what-is-harfbuzz.xml"/>
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<xi:include href="usermanual-install-harfbuzz.xml"/>
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<xi:include href="usermanual-hello-harfbuzz.xml"/>
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<xi:include href="usermanual-shaping-concepts.xml"/>
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<xi:include href="usermanual-buffers-language-script-and-direction.xml"/>
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<xi:include href="usermanual-fonts-and-faces.xml"/>
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<xi:include href="usermanual-clusters.xml"/>
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<chapter id="shaping-concepts">
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<title>Shaping concepts</title>
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<section id="text-shaping-concepts">
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<title>Text shaping</title>
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<para>
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Text shaping is the process of transforming a sequence of Unicode
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codepoints that represent individual characters (letters,
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diacritics, tone marks, numbers, symbols, etc.) into the
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orthographically and linguistically correct two-dimensional layout
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of glyph shapes taken from a specified font.
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</para>
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<para>
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For some writing systems (or <emphasis>scripts</emphasis>) and
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languages, the process is simple, requiring the shaper to do
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little more than advance the horizontal position forward by the
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correct amount for each successive glyph.
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</para>
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<para>
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But, for <emphasis>complex scripts</emphasis>, any combination of
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several shaping operations may be required, and the rules for how
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and when they are applied vary from script to script. HarfBuzz and
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other shaping engines implement these rules.
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</para>
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<para>
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The exact rules and necessary operations for a particular script
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constitute a shaping <emphasis>model</emphasis>. OpenType
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specifies a set of shaping models that covers all of
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Unicode. Other shaping models are available, however, including
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Graphite and Apple Advanced Typography (AAT).
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</para>
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</section>
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<section id="complex-scripts">
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<title>Complex scripts</title>
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<para>
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In text-shaping terminology, scripts are generally classified as
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either <emphasis>complex</emphasis> or <emphasis>non-complex</emphasis>.
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</para>
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<para>
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Complex scripts are those for which transforming the input
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sequence into the final layout requires some combination of
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operations—such as context-dependent substitutions,
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context-dependent mark positioning, glyph-to-glyph joining,
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glyph reordering, or glyph stacking.
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</para>
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<para>
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In some complex scripts, the shaping rules require that a text
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run be divided into syllables before the operations can be
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applied. Other complex scripts may apply shaping operations over
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entire words or over the entire text run, with no subdivision
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required.
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</para>
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<para>
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Non-complex scripts, by definition, do not require these
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operations. However, correctly shaping a text run in a
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non-complex script may still involve Unicode normalization,
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ligature substitutions, mark positioning, kerning, and applying
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other font features. The key difference is that a text run in a
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non-complex script can be processed sequentially and in the same
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order as the input sequence of Unicode codepoints, without
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requiring an analysis stage.
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</para>
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</section>
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<section id="shaping-operations">
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<title>Shaping operations</title>
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<para>
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Shaping a complex-script text run involves transforming the
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input sequence of Unicode codepoints with some combination of
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operations that is specified in the shaping model for the
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script.
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</para>
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<para>
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The specific conditions that trigger a given operation for a
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text run varies from script to script, as do the order that the
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operations are performed in and which codepoints are
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affected. However, the same general set of shaping operations is
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common to all of the complex-script shaping models.
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</para>
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<itemizedlist>
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<listitem>
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<para>
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A <emphasis>reordering</emphasis> operation moves a glyph
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from its original ("logical") position in the sequence to
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some other ("visual") position.
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</para>
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<para>
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The shaping model for a given complex script might involve
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more than one reordering step.
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</para>
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</listitem>
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<listitem>
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<para>
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A <emphasis>joining</emphasis> operation replaces a glyph
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with an alternate form that is designed to connect with one
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or more of the adjacent glyphs in the sequence.
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</para>
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</listitem>
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<listitem>
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<para>
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A contextual <emphasis>substitution</emphasis> operation
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replaces either a single glyph or a subsequence of several
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glyphs with an alternate glyph. This substitution is
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performed when the original glyph or subsequence of glyphs
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occurs in a specified position with respect to the
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surrounding sequence. For example, one substitution might be
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performed only when the target glyph is the first glyph in
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the sequence, while another substitution is performed only
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when a different target glyph occurs immediately after a
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particular string pattern.
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</para>
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<para>
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The shaping model for a given complex script might involve
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multiple contextual-substitution operations, each applying
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to different target glyphs and patterns, and which are
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performed in separate steps.
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</para>
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</listitem>
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<listitem>
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<para>
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A contextual <emphasis>positioning</emphasis> operation
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moves the horizontal and/or vertical position of a
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glyph. This positioning move is performed when the glyph
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occurs in a specified position with respect to the
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surrounding sequence.
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</para>
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<para>
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Many contextual positioning operations are used to place
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<emphasis>mark</emphasis> glyphs (such as diacritics, vowel
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signs, and tone markers) with respect to
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<emphasis>base</emphasis> glyphs. However, some complex
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scripts may use contextual positioning operations to
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correctly place base glyphs as well, such as
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when the script uses <emphasis>stacking</emphasis> characters.
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</para>
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</listitem>
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</itemizedlist>
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</section>
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<section id="unicode-character-categories">
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<title>Unicode character categories</title>
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<para>
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Shaping models are typically specified with respect to how
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scripts are defined in the Unicode standard.
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</para>
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<para>
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Every codepoint in the Unicode Character Database (UCD) is
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assigned a <emphasis>Unicode General Category</emphasis> (UGC),
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which provides the most fundamental information about the
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codepoint: whether the codepoint represents a
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<emphasis>Letter</emphasis>, a <emphasis>Mark</emphasis>, a
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<emphasis>Number</emphasis>, <emphasis>Punctuation</emphasis>, a
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<emphasis>Symbol</emphasis>, a <emphasis>Separator</emphasis>,
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or something else (<emphasis>Other</emphasis>).
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</para>
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<para>
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These UGC properties are "Major" categories. Each codepoint is
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further assigned to a "minor" category within its Major
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category, such as "Letter, uppercase" (<literal>Lu</literal>) or
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"Letter, modifier" (<literal>Lm</literal>).
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</para>
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<para>
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Shaping models are concerned primarily with Letter and Mark
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codepoints. The minor categories of Mark codepoints are
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particularly important for shaping. Marks can be nonspacing
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(<literal>Mn</literal>), spacing combining
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(<literal>Mc</literal>), or enclosing (<literal>Me</literal>).
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</para>
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<para>
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In addition to the UGC property, codepoints in the Indic and
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Southeast Asian scripts are also assigned
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<emphasis>Unicode Indic Syllabic Category</emphasis> (UISC) and
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<emphasis>Unicode Indic Positional Category</emphasis> (UIPC)
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property that provides more detailed information needed for
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shaping.
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</para>
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<para>
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The UISC property sub-categorizes Letters and Marks according to
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common script-shaping behaviors. For example, UISC distinguishes
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between consonant letters, vowel letters, and vowel marks. The
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UIPC property sub-categorizes Mark codepoints by the visual
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position that they occupy (above, below, right, left, or in
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multiple positions).
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</para>
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<para>
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Some complex scripts require that the text run be split into
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syllables, and what constitutes a valid syllable in these
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scripts is specified in regular expressions of the Letter and
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Mark codepoints that take the UISC and UIPC properties into account.
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</para>
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</section>
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<section id="text-runs">
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<title>Text runs</title>
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<para>
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Real-world text usually contains codepoints from a mixture of
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different Unicode scripts (including punctuation, numbers, symbols,
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white-space characters, and other codepoints that do not belong
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to any script). Real-world text may also be marked up with
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formatting that changes font properties (including the font,
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font style, and font size).
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</para>
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<para>
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For shaping purposes, all real-world text streams must be first
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segmented into runs that have a uniform set of properties.
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</para>
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<para>
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In particular, shaping models always assume that every codepoint
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in a text run has the same <emphasis>direction</emphasis>,
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<emphasis>script</emphasis> tag, and
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<emphasis>language</emphasis> tag.
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</para>
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</section>
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<section id="opentype-shaping-models">
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<title>OpenType shaping models</title>
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<para>
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OpenType provides shaping models for the following scripts:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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The <emphasis>default</emphasis> shaping model handles all
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non-complex scripts, and may also be used as a fallback for
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handling unrecognized scripts.
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</para>
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</listitem>
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<listitem>
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<para>
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The <emphasis>Indic</emphasis> shaping model handles the Indic
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scripts Bengali, Devanagari, Gujarati, Gurmukhi, Kannada,
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Malayalam, Oriya, Tamil, Telugu, and Sinhala.
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</para>
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<para>
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The Indic shaping model was revised significantly in
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2005. To denote the change, a new set of <emphasis>script
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tags</emphasis> was assigned for Bengali, Devanagari,
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Gujarati, Gurmukhi, Kannada, Malayalam, Oriya, Tamil, and
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Telugu. For the sake of clarity, the term "Indic2" is
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sometimes used to refer to the current, revised shaping
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model.
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</para>
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</listitem>
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<listitem>
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<para>
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The <emphasis>Arabic</emphasis> shaping model supports
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Arabic, Mongolian, N'Ko, Syriac, and several other connected
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or cursive scripts.
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</para>
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</listitem>
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<listitem>
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<para>
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The <emphasis>Thai/Lao</emphasis> shaping model supports
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the Thai and Lao scripts.
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</para>
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</listitem>
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<listitem>
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<para>
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The <emphasis>Khmer</emphasis> shaping model supports the
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Khmer script.
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</para>
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</listitem>
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<listitem>
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<para>
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The <emphasis>Myanmar</emphasis> shaping model supports the
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Myanmar (or Burmese) script.
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</para>
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</listitem>
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<listitem>
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<para>
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The <emphasis>Tibetan</emphasis> shaping model supports the
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Tibetan script.
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</para>
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</listitem>
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<listitem>
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<para>
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The <emphasis>Hangul</emphasis> shaping model supports the
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Hangul script.
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</para>
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</listitem>
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<listitem>
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<para>
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The <emphasis>Hebrew</emphasis> shaping model supports the
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Hebrew script.
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</para>
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</listitem>
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<listitem>
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<para>
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The <emphasis>Universal Shaping Engine</emphasis> (USE)
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shaping model supports complex scripts not covered by one of
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the above, script-specific shaping models, including
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Javanese, Balinese, Buginese, Batak, Chakma, Lepcha, Modi,
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Phags-pa, Tagalog, Siddham, Sundanese, Tai Le, Tai Tham, Tai
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Viet, and many others.
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</para>
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</listitem>
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<listitem>
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<para>
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Text runs that do not fall under one of the above shaping
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models may still require processing by a shaping engine. Of
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particular note is <emphasis>Emoji</emphasis> shaping, which
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may involve variation-selector sequences and glyph
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substitution. Emoji shaping is handled by the default
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shaping model.
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</para>
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</listitem>
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</itemizedlist>
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</section>
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<section id="graphite-shaping">
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<title>Graphite shaping</title>
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<para>
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In contrast to OpenType shaping, Graphite shaping does not
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specify a predefined set of shaping models or a set of supported
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scripts.
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</para>
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<para>
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Instead, each Graphite font contains a complete set of rules that
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implement the required shaping model for the intended
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script. These rules include finite-state machines to match
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sequences of codepoints to the shaping operations to perform.
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</para>
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<para>
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Graphite shaping can perform the same shaping operations used in
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OpenType shaping, as well as other functions that have not been
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defined for OpenType shaping.
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</para>
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</section>
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<section id="aat-shaping">
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<title>AAT shaping</title>
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<para>
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In contrast to OpenType shaping, AAT shaping does not specify a
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predefined set of shaping models or a set of supported scripts.
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</para>
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<para>
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Instead, each AAT font includes a complete set of rules that
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implement the desired shaping model for the intended
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script. These rules include finite-state machines to match glyph
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sequences and the shaping operations to perform.
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</para>
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<para>
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Notably, AAT shaping rules are expressed for glyphs in the font,
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not for Unicode codepoints. AAT shaping can perform the same
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shaping operations used in OpenType shaping, as well as other
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functions that have not been defined for OpenType shaping.
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</para>
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</section>
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</chapter>
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