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24 <a name="working-with-harfbuzz-clusters"></a>Working with HarfBuzz clusters</h2></div></div></div>
26 When you add text to a HarfBuzz buffer, each code point must be
27 assigned a <span class="emphasis"><em>cluster value</em></span>.
30 This cluster value is an arbitrary number; HarfBuzz uses it only
31 to distinguish between clusters. Many client programs will use
32 the index of each code point in the input text stream as the
33 cluster value. This is for the sake of convenience; the actual
34 value does not matter.
37 Some of the shaping operations performed by HarfBuzz —
38 such as reordering, composition, decomposition, and substitution
39 — may alter the cluster values of some characters. The
40 final cluster values in the buffer at the end of the shaping
41 process will indicate to client programs which subsequences of
42 glyphs represent a cluster and, therefore, must not be
46 In addition, client programs can query the final cluster values
47 to discern other potentially important information about the
48 glyphs in the output buffer (such as whether or not a ligature
52 For example, if the initial sequence of cluster values was:
54 <pre class="programlisting">
58 and the final sequence of cluster values is:
60 <pre class="programlisting">
64 then there are two clusters in the output buffer: the first
65 cluster includes the first two glyphs, and the second cluster
66 includes the third and fourth glyphs. It is also evident that a
67 ligature or conjunct has been formed, because there are fewer
68 glyphs in the output buffer (four) than there were code points
69 in the input buffer (five).
72 Although client programs using HarfBuzz are free to assign
73 initial cluster values in any manner they choose to, HarfBuzz
74 does offer some useful guarantees if the cluster values are
75 assigned in a monotonic (either non-decreasing or non-increasing)
79 For buffers in the left-to-right (LTR)
80 or top-to-bottom (TTB) text flow direction,
81 HarfBuzz will preserve the monotonic property: client programs
82 are guaranteed that monotonically increasing initial cluster
83 values will be returned as monotonically increasing final
87 For buffers in the right-to-left (RTL)
88 or bottom-to-top (BTT) text flow direction,
89 the directionality of the buffer itself is reversed for final
90 output as a matter of design. Therefore, HarfBuzz inverts the
91 monotonic property: client programs are guaranteed that
92 monotonically increasing initial cluster values will be
93 returned as monotonically <span class="emphasis"><em>decreasing</em></span> final
97 Client programs can adjust how HarfBuzz handles clusters during
98 shaping by setting the
99 <code class="literal">cluster_level</code> of the
100 buffer. HarfBuzz offers three <span class="emphasis"><em>levels</em></span> of
101 clustering support for this property:
103 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
104 <li class="listitem">
105 <p><span class="emphasis"><em>Level 0</em></span> is the default and
106 reproduces the behavior of the old HarfBuzz library.
109 The distinguishing feature of level 0 behavior is that, at
110 the beginning of processing the buffer, all code points that
111 are categorized as <span class="emphasis"><em>marks</em></span>,
112 <span class="emphasis"><em>modifier symbols</em></span>, or
113 <span class="emphasis"><em>Emoji extended pictographic</em></span> modifiers,
114 as well as the <span class="emphasis"><em>Zero Width Joiner</em></span> and
115 <span class="emphasis"><em>Zero Width Non-Joiner</em></span> code points, are
116 assigned the cluster value of the closest preceding code
117 point from <span class="emphasis"><em>different</em></span> category.
120 In essence, whenever a base character is followed by a mark
121 character or a sequence of mark characters, those marks are
122 reassigned to the same initial cluster value as the base
123 character. This reassignment is referred to as
124 "merging" the affected clusters. This behavior is based on
125 the Grapheme Cluster Boundary specification in <a class="ulink" href="https://www.unicode.org/reports/tr29/#Regex_Definitions" target="_top">Unicode
126 Technical Report 29</a>.
129 Client programs can specify level 0 behavior for a buffer by
130 setting its <code class="literal">cluster_level</code> to
131 <code class="literal">HB_BUFFER_CLUSTER_LEVEL_MONOTONE_GRAPHEMES</code>.
134 <li class="listitem">
136 <span class="emphasis"><em>Level 1</em></span> tweaks the old behavior
137 slightly to produce better results. Therefore, level 1
138 clustering is recommended for code that is not required to
139 implement backward compatibility with the old HarfBuzz.
142 Level 1 differs from level 0 by not merging the
143 clusters of marks and other modifier code points with the
144 preceding "base" code point's cluster. By preserving the
145 separate cluster values of these marks and modifier code
146 points, script shapers can perform additional operations
147 that might lead to improved results (for example, reordering
148 a sequence of marks).
151 Client programs can specify level 1 behavior for a buffer by
152 setting its <code class="literal">cluster_level</code> to
153 <code class="literal">HB_BUFFER_CLUSTER_LEVEL_MONOTONE_CHARACTERS</code>.
156 <li class="listitem">
158 <span class="emphasis"><em>Level 2</em></span> differs significantly in how it
159 treats cluster values. In level 2, HarfBuzz never merges
163 This difference can be seen most clearly when HarfBuzz processes
164 ligature substitutions and glyph decompositions. In level 0
165 and level 1, ligatures and glyph decomposition both involve
166 merging clusters; in level 2, neither of these operations
170 Client programs can specify level 2 behavior for a buffer by
171 setting its <code class="literal">cluster_level</code> to
172 <code class="literal">HB_BUFFER_CLUSTER_LEVEL_CHARACTERS</code>.
177 As mentioned earlier, client programs using HarfBuzz often
178 assign initial cluster values in a buffer by reusing the indices
179 of the code points in the input text. This gives a sequence of
180 cluster values that is monotonically increasing (for example,
184 It is not <span class="emphasis"><em>required</em></span> that the cluster values
185 in a buffer be monotonically increasing. However, if the initial
186 cluster values in a buffer are monotonic and the buffer is
187 configured to use cluster level 0 or 1, then HarfBuzz
188 guarantees that the final cluster values in the shaped buffer
189 will also be monotonic. No such guarantee is made for cluster
193 In levels 0 and 1, HarfBuzz implements the following conceptual
194 model for cluster values:
196 <div class="itemizedlist"><ul class="itemizedlist compact" style="list-style-type: disc; ">
197 <li class="listitem"><p>
198 If the sequence of input cluster values is monotonic, the
199 sequence of cluster values will remain monotonic.
201 <li class="listitem"><p>
202 Each cluster value represents a single cluster.
204 <li class="listitem"><p>
205 Each cluster contains one or more glyphs and one or more
210 In practice, this model offers several benefits. Assuming that
211 the initial cluster values were monotonically increasing
212 and distinct before shaping began, then, in the final output:
214 <div class="itemizedlist"><ul class="itemizedlist compact" style="list-style-type: disc; ">
215 <li class="listitem"><p>
216 All adjacent glyphs having the same final cluster
217 value belong to the same cluster.
219 <li class="listitem"><p>
220 Each character belongs to the cluster that has the highest
221 cluster value <span class="emphasis"><em>not larger than</em></span> its
222 initial cluster value.
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