Docs: usermanual, add Shaping Concepts chapter.

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Nathan Willis 2018-10-12 18:23:26 -05:00 committed by Khaled Hosny
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<para>
HarfBuzz is an <ulink url="http://www.microsoft.com/typography/otspec/">OpenType</ulink>
text shaping engine. Using the HarfBuzz library allows
programs to convert a sequence of Unicode input text into
properly formatted and positioned output&mdash;for any writing
programs to convert a sequence of Unicode input into
properly formatted and positioned text output&mdash;for any writing
system and language.
</para>
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<xi:include href="usermanual-what-is-harfbuzz.xml"/>
<xi:include href="usermanual-install-harfbuzz.xml"/>
<xi:include href="usermanual-hello-harfbuzz.xml"/>
<xi:include href="usermanual-shaping-concepts.xml"/>
<xi:include href="usermanual-buffers-language-script-and-direction.xml"/>
<xi:include href="usermanual-fonts-and-faces.xml"/>
<xi:include href="usermanual-clusters.xml"/>

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<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 <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="complex-scripts">
<title>Complex scripts</title>
<para>
In text-shaping terminology, scripts are generally classified as
either <emphasis>complex</emphasis> or <emphasis>non-complex</emphasis>.
</para>
<para>
Complex scripts are those for which transforming the input
sequence into the final layout 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 complex scripts, the shaping rules require that a text
run be divided into syllables before the operations can be
applied. Other complex scripts may apply shaping operations over
entire words or over the entire text run, with no subdivision
required.
</para>
<para>
Non-complex scripts, by definition, do not require these
operations. However, correctly shaping a text run in a
non-complex script may still involve Unicode normalization,
ligature substitutions, mark positioning, kerning, and applying
other font features. The key difference is that a text run in a
non-complex script can be processed sequentially and in the same
order as the input sequence of Unicode codepoints, without
requiring an analysis stage.
</para>
</section>
<section id="shaping-operations">
<title>Shaping operations</title>
<para>
Shaping a complex-script 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 complex-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 complex 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 complex 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 complex
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)
property that provides 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 visual
position that they occupy (above, below, right, left, or in
multiple positions).
</para>
<para>
Some complex scripts require that the text run be split into
syllables, and what constitutes a valid syllable in these
scripts is specified in regular expressions of 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
non-complex scripts, 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, Telugu, and Sinhala.
</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 complex 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>