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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="id-1.2.4.3"></a>Terminology</h2></div></div></div>
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<td><p><span class="term">script</span></p></td>
<td>
<p>
In text shaping, a <span class="emphasis"><em>script</em></span> is a
writing system: a set of symbols, rules, and conventions
that is used to represent a language or multiple
languages.
</p>
<p>
In general computing lingo, the word "script" can also
be used to mean an executable program (usually one
written in a human-readable programming language). For
the sake of clarity, HarfBuzz documents will always use
more specific terminology when referring to this
meaning, such as "Python script" or "shell script." In
all other instances, "script" refers to a writing system.
</p>
<p>
For developers using HarfBuzz, it is important to note
the distinction between a script and a language. Most
scripts are used to write a variety of different
languages, and many languages may be written in more
than one script.
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<td><p><span class="term">shaper</span></p></td>
<td><p>
In HarfBuzz, a <span class="emphasis"><em>shaper</em></span> is a
handler for a specific script-shaping model. HarfBuzz
implements separate shapers for Indic, Arabic, Thai and
Lao, Khmer, Myanmar, Tibetan, Hangul, Hebrew, the
Universal Shaping Engine (USE), and a default shaper for
non-complex scripts.
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<td><p><span class="term">cluster</span></p></td>
<td>
<p>
In text shaping, a <span class="emphasis"><em>cluster</em></span> is a
sequence of codepoints that must be treated as an
indivisible unit. Clusters can include code-point
sequences that form a ligature or base-and-mark
sequences. Tracking and preserving clusters is important
when shaping operations might separate or reorder
code points.
</p>
<p>
HarfBuzz provides three cluster
<span class="emphasis"><em>levels</em></span> that implement different
approaches to the problem of preserving clusters during
shaping operations.
</p>
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<td><p><span class="term">grapheme</span></p></td>
<td>
<p>
In linguistics, a <span class="emphasis"><em>grapheme</em></span> is one
of the indivisible units that make up a writing system or
script. Often, graphemes are individual symbols (letters,
numbers, punctuation marks, logograms, etc.) but,
depending on the writing system, a particular grapheme
might correspond to a sequence of several Unicode code
points.
</p>
<p>
In practice, HarfBuzz and other text-shaping engines
are not generally concerned with graphemes. However, it
is important for developers using HarfBuzz to recognize
that there is a difference between graphemes and shaping
clusters (see above). The two concepts may overlap
frequently, but there is no guarantee that they will be
identical.
</p>
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<td><p><span class="term">syllable</span></p></td>
<td>
<p>
In linguistics, a <span class="emphasis"><em>syllable</em></span> is an
a sequence of sounds that makes up a building block of a
particular language. Every language has its own set of
rules describing what constitutes a valid syllable.
</p>
<p>
For text-shaping purposes, the various definitions of
"syllable" are important because script-specific shaping
operations may be applied at the syllable level. For
example, a reordering rule might specify that a vowel
mark be reordered to the beginning of the syllable.
</p>
<p>
Syllables will consist of one or more Unicode code
points. The definition of a syllable for a particular
writing system might correspond to how HarfBuzz
identifies clusters (see above) for the same writing
system. However, it is important for developers using
HarfBuzz to recognize that there is a difference between
syllables and shaping clusters. The two concepts may
overlap frequently, but there is no guarantee that they
will be identical.
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