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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>A clustering example for levels 0 and 1: 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="clusters.html" title="Clusters"> <link rel="prev" href="working-with-harfbuzz-clusters.html" title="Working with HarfBuzz clusters"> <link rel="next" href="reordering-in-levels-0-and-1.html" title="Reordering in levels 0 and 1"> <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="clusters.html"><img src="up.png" width="16" height="16" border="0" alt="Up"></a></td> <td><a accesskey="p" href="working-with-harfbuzz-clusters.html"><img src="left.png" width="16" height="16" border="0" alt="Prev"></a></td> <td><a accesskey="n" href="reordering-in-levels-0-and-1.html"><img src="right.png" width="16" height="16" border="0" alt="Next"></a></td> </tr></table> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="a-clustering-example-for-levels-0-and-1"></a>A clustering example for levels 0 and 1</h2></div></div></div> <p> The basic shaping operations affect clusters in a predictable manner when using level 0 or level 1: </p> <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> <li class="listitem"><p> When two or more clusters <span class="emphasis"><em>merge</em></span>, the resulting merged cluster takes as its cluster value the <span class="emphasis"><em>minimum</em></span> of the incoming cluster values. </p></li> <li class="listitem"><p> When a cluster <span class="emphasis"><em>decomposes</em></span>, all of the resulting child clusters inherit as their cluster value the cluster value of the parent cluster. </p></li> <li class="listitem"><p> When a character is <span class="emphasis"><em>reordered</em></span>, the reordered character and all clusters that the character moves past as part of the reordering are merged into one cluster. </p></li> </ul></div> <p> The functionality, guarantees, and benefits of level 0 and level 1 behavior can be seen with some examples. First, let us examine what happens with cluster values when shaping involves cluster merging with ligatures and decomposition. </p> <p> Let's say we start with the following character sequence (top row) and initial cluster values (bottom row): </p> <pre class="programlisting"> A,B,C,D,E 0,1,2,3,4 </pre> <p> During shaping, HarfBuzz maps these characters to glyphs from the font. For simplicity, let us assume that each character maps to the corresponding, identical-looking glyph: </p> <pre class="programlisting"> A,B,C,D,E 0,1,2,3,4 </pre> <p> Now if, for example, <code class="literal">B</code> and <code class="literal">C</code> form a ligature, then the clusters to which they belong "merge". This merged cluster takes for its cluster value the minimum of all the cluster values of the clusters that went in to the ligature. In this case, we get: </p> <pre class="programlisting"> A,BC,D,E 0,1 ,3,4 </pre> <p> because 1 is the minimum of the set {1,2}, which were the cluster values of <code class="literal">B</code> and <code class="literal">C</code>. </p> <p> Next, let us say that the <code class="literal">BC</code> ligature glyph decomposes into three components, and <code class="literal">D</code> also decomposes into two components. Whenever a cluster decomposes, its components each inherit the cluster value of their parent: </p> <pre class="programlisting"> A,BC0,BC1,BC2,D0,D1,E 0,1 ,1 ,1 ,3 ,3 ,4 </pre> <p> Next, if <code class="literal">BC2</code> and <code class="literal">D0</code> form a ligature, then their clusters (cluster values 1 and 3) merge into <code class="literal">min(1,3) = 1</code>: </p> <pre class="programlisting"> A,BC0,BC1,BC2D0,D1,E 0,1 ,1 ,1 ,1 ,4 </pre> <p> Note that the entirety of cluster 3 merges into cluster 1, not just the <code class="literal">D0</code> glyph. This reflects the fact that the cluster <span class="emphasis"><em>must</em></span> be treated as an indivisible unit. </p> <p> At this point, cluster 1 means: the character sequence <code class="literal">BCD</code> is represented by glyphs <code class="literal">BC0,BC1,BC2D0,D1</code> and cannot be broken down any further. </p> </div> <div class="footer"> <hr>Generated by GTK-Doc V1.32</div> </body> </html>