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 <div class="chapter">
 <div class="titlepage"><div><div><h2 class="title">
 <a name="clusters"></a>Clusters</h2></div></div></div>
 <div class="toc"><dl class="toc">
 <dt><span class="section"><a href="clusters.html#clusters-and-shaping">Clusters and shaping</a></span></dt>
 <dt><span class="section"><a href="working-with-harfbuzz-clusters.html">Working with HarfBuzz clusters</a></span></dt>
 <dt><span class="section"><a href="a-clustering-example-for-levels-0-and-1.html">A clustering example for levels 0 and 1</a></span></dt>
 <dt><span class="section"><a href="reordering-in-levels-0-and-1.html">Reordering in levels 0 and 1</a></span></dt>
 <dt><span class="section"><a href="the-distinction-between-levels-0-and-1.html">The distinction between levels 0 and 1</a></span></dt>
 <dt><span class="section"><a href="level-2.html">Level 2</a></span></dt>
 <dd><dl>
 <dt><span class="section"><a href="level-2.html#ligatures-with-combining-marks-in-level-2">Ligatures with combining marks in level 2</a></span></dt>
 <dt><span class="section"><a href="level-2.html#reordering-in-level-2">Reordering in level 2</a></span></dt>
 <dt><span class="section"><a href="level-2.html#other-considerations-in-level-2">Other considerations in level 2</a></span></dt>
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 <div class="section">
 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
 <a name="clusters-and-shaping"></a>Clusters and shaping</h2></div></div></div>
 <p>
       In text shaping, a <span class="emphasis"><em>cluster</em></span> is a sequence of
       characters that needs to be treated as a single, indivisible
       unit. A single letter or symbol can be a cluster of its
       own. Other clusters correspond to longer subsequences of the
       input code points — such as a ligature or conjunct form
       — and require the shaper to ensure that the cluster is not
       broken during the shaping process.
     </p>
 <p>
       A cluster is distinct from a <span class="emphasis"><em>grapheme</em></span>,
       which is the smallest unit of meaning in a writing system or
       script.
     </p>
 <p>
       The definitions of the two terms are similar. However, clusters
       are only relevant for script shaping and glyph layout. In
       contrast, graphemes are a property of the underlying script, and
       are of interest when client programs implement orthographic
       or linguistic functionality.
     </p>
 <p>
       For example, two individual letters are often two separate
       graphemes. When two letters form a ligature, however, they
       combine into a single glyph. They are then part of the same
       cluster and are treated as a unit by the shaping engine —
       even though the two original, underlying letters remain separate
       graphemes.
     </p>
 <p>
       HarfBuzz is concerned with clusters, <span class="emphasis"><em>not</em></span>
       with graphemes — although client programs using HarfBuzz
       may still care about graphemes for other reasons from time to time.
     </p>
 <p>
       During the shaping process, there are several shaping operations
       that may merge adjacent characters (for example, when two code
       points form a ligature or a conjunct form and are replaced by a
       single glyph) or split one character into several (for example,
       when decomposing a code point through the
       <code class="literal">ccmp</code> feature). Operations like these alter
       clusters; HarfBuzz tracks the changes to ensure that no clusters
       get lost or broken during shaping.
     </p>
 <p>
       HarfBuzz records cluster information independently from how
       shaping operations affect the individual glyphs returned in an
       output buffer. Consequently, a client program using HarfBuzz can
       utilize the cluster information to implement features such as:
     </p>
 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 <li class="listitem"><p>
 	  Correctly positioning the cursor within a shaped text run,
 	  even when characters have formed ligatures, composed or
 	  decomposed, reordered, or undergone other shaping operations.
 	</p></li>
 <li class="listitem"><p>
 	  Correctly highlighting a text selection that includes some,
 	  but not all, of the characters in a word.
 	</p></li>
 <li class="listitem"><p>
 	  Applying text attributes (such as color or underlining) to
 	  part, but not all, of a word.
 	</p></li>
 <li class="listitem"><p>
 	  Generating output document formats (such as PDF) with
 	  embedded text that can be fully extracted.
 	</p></li>
 <li class="listitem"><p>
 	  Determining the mapping between input characters and output
 	  glyphs, such as which glyphs are ligatures.
 	</p></li>
 <li class="listitem"><p>
 	  Performing line-breaking, justification, and other
 	  line-level or paragraph-level operations that must be done
 	  after shaping is complete, but which require examining
 	  character-level properties.
 	</p></li>
 </ul></div>
 </div>
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	<div class="chapter">
	<div class="titlepage"><div><div><h2 class="title">
	<a name="clusters"></a>Clusters</h2></div></div></div>
	<div class="toc"><dl class="toc">
	<dt><span class="section"><a href="clusters.html#clusters-and-shaping">Clusters and shaping</a></span></dt>
	<dt><span class="section"><a href="working-with-harfbuzz-clusters.html">Working with HarfBuzz clusters</a></span></dt>
	<dt><span class="section"><a href="a-clustering-example-for-levels-0-and-1.html">A clustering example for levels 0 and 1</a></span></dt>
	<dt><span class="section"><a href="reordering-in-levels-0-and-1.html">Reordering in levels 0 and 1</a></span></dt>
	<dt><span class="section"><a href="the-distinction-between-levels-0-and-1.html">The distinction between levels 0 and 1</a></span></dt>
	<dt><span class="section"><a href="level-2.html">Level 2</a></span></dt>
	<dd><dl>
	<dt><span class="section"><a href="level-2.html#ligatures-with-combining-marks-in-level-2">Ligatures with combining marks in level 2</a></span></dt>
	<dt><span class="section"><a href="level-2.html#reordering-in-level-2">Reordering in level 2</a></span></dt>
	<dt><span class="section"><a href="level-2.html#other-considerations-in-level-2">Other considerations in level 2</a></span></dt>
	</dl></dd>
	</dl></div>
	<div class="section">
	<div class="titlepage"><div><div><h2 class="title" style="clear: both">
	<a name="clusters-and-shaping"></a>Clusters and shaping</h2></div></div></div>
	<p>
	In text shaping, a <span class="emphasis"><em>cluster</em></span> is a sequence of
	characters that needs to be treated as a single, indivisible
	unit. A single letter or symbol can be a cluster of its
	own. Other clusters correspond to longer subsequences of the
	input code points — such as a ligature or conjunct form
	— and require the shaper to ensure that the cluster is not
	broken during the shaping process.
	</p>
	<p>
	A cluster is distinct from a <span class="emphasis"><em>grapheme</em></span>,
	which is the smallest unit of meaning in a writing system or
	script.
	</p>
	<p>
	The definitions of the two terms are similar. However, clusters
	are only relevant for script shaping and glyph layout. In
	contrast, graphemes are a property of the underlying script, and
	are of interest when client programs implement orthographic
	or linguistic functionality.
	</p>
	<p>
	For example, two individual letters are often two separate
	graphemes. When two letters form a ligature, however, they
	combine into a single glyph. They are then part of the same
	cluster and are treated as a unit by the shaping engine —
	even though the two original, underlying letters remain separate
	graphemes.
	</p>
	<p>
	HarfBuzz is concerned with clusters, <span class="emphasis"><em>not</em></span>
	with graphemes — although client programs using HarfBuzz
	may still care about graphemes for other reasons from time to time.
	</p>
	<p>
	During the shaping process, there are several shaping operations
	that may merge adjacent characters (for example, when two code
	points form a ligature or a conjunct form and are replaced by a
	single glyph) or split one character into several (for example,
	when decomposing a code point through the
	<code class="literal">ccmp</code> feature). Operations like these alter
	clusters; HarfBuzz tracks the changes to ensure that no clusters
	get lost or broken during shaping.
	</p>
	<p>
	HarfBuzz records cluster information independently from how
	shaping operations affect the individual glyphs returned in an
	output buffer. Consequently, a client program using HarfBuzz can
	utilize the cluster information to implement features such as:
	</p>
	<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
	<li class="listitem"><p>
	Correctly positioning the cursor within a shaped text run,
	even when characters have formed ligatures, composed or
	decomposed, reordered, or undergone other shaping operations.
	</p></li>
	<li class="listitem"><p>
	Correctly highlighting a text selection that includes some,
	but not all, of the characters in a word.
	</p></li>
	<li class="listitem"><p>
	Applying text attributes (such as color or underlining) to
	part, but not all, of a word.
	</p></li>
	<li class="listitem"><p>
	Generating output document formats (such as PDF) with
	embedded text that can be fully extracted.
	</p></li>
	<li class="listitem"><p>
	Determining the mapping between input characters and output
	glyphs, such as which glyphs are ligatures.
	</p></li>
	<li class="listitem"><p>
	Performing line-breaking, justification, and other
	line-level or paragraph-level operations that must be done
	after shaping is complete, but which require examining
	character-level properties.
	</p></li>
	</ul></div>
	</div>
	</div>
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