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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>Clusters: HarfBuzz Manual</title> <meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> <link rel="home" href="index.html" title="HarfBuzz Manual"> <link rel="up" href="pt01.html" title="Part I. User's manual"> <link rel="prev" href="shaping-plans-and-caching.html" title="Plans and caching"> <link rel="next" href="working-with-harfbuzz-clusters.html" title="Working with HarfBuzz clusters"> <meta name="generator" content="GTK-Doc V1.32 (XML mode)"> <link rel="stylesheet" href="style.css" type="text/css"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table class="navigation" id="top" width="100%" summary="Navigation header" cellpadding="2" cellspacing="5"><tr valign="middle"> <td width="100%" align="left" class="shortcuts"></td> <td><a accesskey="h" href="index.html"><img src="home.png" width="16" height="16" border="0" alt="Home"></a></td> <td><a accesskey="u" href="pt01.html"><img src="up.png" width="16" height="16" border="0" alt="Up"></a></td> <td><a accesskey="p" href="shaping-plans-and-caching.html"><img src="left.png" width="16" height="16" border="0" alt="Prev"></a></td> <td><a accesskey="n" href="working-with-harfbuzz-clusters.html"><img src="right.png" width="16" height="16" border="0" alt="Next"></a></td> </tr></table> <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> <div class="footer"> <hr>Generated by GTK-Doc V1.32</div> </body> </html>