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<div class="chapter">
<div class="titlepage"><div><div><h2 class="title">
<a name="what-is-harfbuzz"></a>What is HarfBuzz?</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="what-is-harfbuzz.html#what-is-text-shaping">What is text shaping?</a></span></dt>
<dt><span class="section"><a href="why-do-i-need-a-shaping-engine.html">Why do I need a shaping engine?</a></span></dt>
<dt><span class="section"><a href="ch01s03.html">What does HarfBuzz do?</a></span></dt>
<dt><span class="section"><a href="what-harfbuzz-doesnt-do.html">What HarfBuzz doesn't do</a></span></dt>
<dt><span class="section"><a href="why-is-it-called-harfbuzz.html">Why is it called HarfBuzz?</a></span></dt>
</dl></div>
<p>
HarfBuzz is a <span class="emphasis"><em>text-shaping engine</em></span>. If you
give HarfBuzz a font and a string containing a sequence of Unicode
codepoints, HarfBuzz selects and positions the corresponding
glyphs from the font, applying all of the necessary layout rules
and font features. HarfBuzz then returns the string to you in the
form that is correctly arranged for the language and writing
system.
</p>
<p>
HarfBuzz can properly shape all of the world's major writing
systems. It runs on all major operating systems and software
platforms and it supports the major font formats in use
today.
</p>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="what-is-text-shaping"></a>What is text shaping?</h2></div></div></div>
<p>
Text shaping is the process of translating a string of character
codes (such as Unicode codepoints) into a properly arranged
sequence of glyphs that can be rendered onto a screen or into
final output form for inclusion in a document.
</p>
<p>
The shaping process is dependent on the input string, the active
font, the script (or writing system) that the string is in, and
the language that the string is in.
</p>
<p>
Modern software systems generally only deal with strings in the
Unicode encoding scheme (although legacy systems and documents may
involve other encodings).
</p>
<p>
There are several font formats that a program might
encounter, each of which has a set of standard text-shaping
rules.
</p>
<p>The dominant format is <a class="ulink" href="http://www.microsoft.com/typography/otspec/" target="_top">OpenType</a>. The
OpenType specification defines a series of <a class="ulink" href="https://github.com/n8willis/opentype-shaping-documents" target="_top">shaping models</a> for
various scripts from around the world. These shaping models depend on
the font incorporating certain features as
<span class="emphasis"><em>lookups</em></span> in its <code class="literal">GSUB</code>
and <code class="literal">GPOS</code> tables.
</p>
<p>
Alternatively, OpenType fonts can include shaping features for
the <a class="ulink" href="https://graphite.sil.org/" target="_top">Graphite</a> shaping model.
</p>
<p>
TrueType fonts can also include OpenType shaping
features. Alternatively, TrueType fonts can also include <a class="ulink" href="https://developer.apple.com/fonts/TrueType-Reference-Manual/RM09/AppendixF.html" target="_top">Apple
Advanced Typography</a> (AAT) tables to implement shaping
support. AAT fonts are generally only found on macOS and iOS systems.
</p>
<p>
Text strings will usually be tagged with a script and language
tag that provide the context needed to perform text shaping
correctly. The necessary <a class="ulink" href="https://docs.microsoft.com/en-us/typography/opentype/spec/scripttags" target="_top">script</a>
and <a class="ulink" href="https://docs.microsoft.com/en-us/typography/opentype/spec/languagetags" target="_top">language</a>
tags are defined by OpenType.
</p>
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