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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="working-with-harfbuzz-clusters"></a>Working with HarfBuzz clusters</h2></div></div></div>

 When you add text to a HarfBuzz buffer, each code point must be
 assigned a cluster value.
 

 This cluster value is an arbitrary number; HarfBuzz uses it only
 to distinguish between clusters. Many client programs will use
 the index of each code point in the input text stream as the
 cluster value. This is for the sake of convenience; the actual
 value does not matter.
 

 Some of the shaping operations performed by HarfBuzz —
 such as reordering, composition, decomposition, and substitution
 — may alter the cluster values of some characters. The
 final cluster values in the buffer at the end of the shaping
 process will indicate to client programs which subsequences of
 glyphs represent a cluster and, therefore, must not be
 separated.
 

 In addition, client programs can query the final cluster values
 to discern other potentially important information about the
 glyphs in the output buffer (such as whether or not a ligature
 was formed).
 

 For example, if the initial sequence of cluster values was:
 
<pre class="programlisting">
 0,1,2,3,4
 </pre>

 and the final sequence of cluster values is:
 
<pre class="programlisting">
 0,0,3,3
 </pre>

 then there are two clusters in the output buffer: the first
 cluster includes the first two glyphs, and the second cluster
 includes the third and fourth glyphs. It is also evident that a
 ligature or conjunct has been formed, because there are fewer
 glyphs in the output buffer (four) than there were code points
 in the input buffer (five).
 

 Although client programs using HarfBuzz are free to assign
 initial cluster values in any manner they choose to, HarfBuzz
 does offer some useful guarantees if the cluster values are
 assigned in a monotonic (either non-decreasing or non-increasing)
 order.
 

 For buffers in the left-to-right (LTR)
 or top-to-bottom (TTB) text flow direction,
 HarfBuzz will preserve the monotonic property: client programs
 are guaranteed that monotonically increasing initial cluster
 values will be returned as monotonically increasing final
 cluster values.
 

 For buffers in the right-to-left (RTL)
 or bottom-to-top (BTT) text flow direction,
 the directionality of the buffer itself is reversed for final
 output as a matter of design. Therefore, HarfBuzz inverts the
 monotonic property: client programs are guaranteed that
 monotonically increasing initial cluster values will be
 returned as monotonically decreasing final
 cluster values.
 

 Client programs can adjust how HarfBuzz handles clusters during
 shaping by setting the
 <code class="literal">cluster_level</code> of the
 buffer. HarfBuzz offers three levels of
 clustering support for this property:
 
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
Level 0 is the default and
	reproduces the behavior of the old HarfBuzz library.
	

	 The distinguishing feature of level 0 behavior is that, at
	 the beginning of processing the buffer, all code points that
	 are categorized as marks,
	 modifier symbols, or
	 Emoji extended pictographic modifiers,
	 as well as the Zero Width Joiner and
	 Zero Width Non-Joiner code points, are
	 assigned the cluster value of the closest preceding code
	 point from different category. 
	

	 In essence, whenever a base character is followed by a mark
	 character or a sequence of mark characters, those marks are
	 reassigned to the same initial cluster value as the base
	 character. This reassignment is referred to as
	 "merging" the affected clusters. This behavior is based on
	 the Grapheme Cluster Boundary specification in <a class="ulink" href="https://www.unicode.org/reports/tr29/#Regex_Definitions" target="_top">Unicode
	 Technical Report 29</a>.
	

	 Client programs can specify level 0 behavior for a buffer by
	 setting its <code class="literal">cluster_level</code> to
	 <code class="literal">HB_BUFFER_CLUSTER_LEVEL_MONOTONE_GRAPHEMES</code>. 
	
</li>
<li class="listitem">

	 Level 1 tweaks the old behavior
	 slightly to produce better results. Therefore, level 1
	 clustering is recommended for code that is not required to
	 implement backward compatibility with the old HarfBuzz.
	

	 Level 1 differs from level 0 by not merging the 
	 clusters of marks and other modifier code points with the
	 preceding "base" code point's cluster. By preserving the
	 separate cluster values of these marks and modifier code
	 points, script shapers can perform additional operations
	 that might lead to improved results (for example, reordering
	 a sequence of marks).
	

	 Client programs can specify level 1 behavior for a buffer by
	 setting its <code class="literal">cluster_level</code> to
	 <code class="literal">HB_BUFFER_CLUSTER_LEVEL_MONOTONE_CHARACTERS</code>. 
	
</li>
<li class="listitem">

	 Level 2 differs significantly in how it
	 treats cluster values. In level 2, HarfBuzz never merges
	 clusters.
	

	 This difference can be seen most clearly when HarfBuzz processes
	 ligature substitutions and glyph decompositions. In level 0 
	 and level 1, ligatures and glyph decomposition both involve
	 merging clusters; in level 2, neither of these operations
	 triggers a merge.
	

	 Client programs can specify level 2 behavior for a buffer by
	 setting its <code class="literal">cluster_level</code> to
	 <code class="literal">HB_BUFFER_CLUSTER_LEVEL_CHARACTERS</code>. 
	
</li>
</ul></div>

 As mentioned earlier, client programs using HarfBuzz often
 assign initial cluster values in a buffer by reusing the indices
 of the code points in the input text. This gives a sequence of
 cluster values that is monotonically increasing (for example,
 0,1,2,3,4). 
 

 It is not required that the cluster values
 in a buffer be monotonically increasing. However, if the initial
 cluster values in a buffer are monotonic and the buffer is
 configured to use cluster level 0 or 1, then HarfBuzz
 guarantees that the final cluster values in the shaped buffer
 will also be monotonic. No such guarantee is made for cluster
 level 2.
 

 In levels 0 and 1, HarfBuzz implements the following conceptual
 model for cluster values:
 
<div class="itemizedlist"><ul class="itemizedlist compact" style="list-style-type: disc; ">
<li class="listitem">
 If the sequence of input cluster values is monotonic, the
	 sequence of cluster values will remain monotonic.
	</li>
<li class="listitem">
 Each cluster value represents a single cluster.
	</li>
<li class="listitem">
 Each cluster contains one or more glyphs and one or more
 characters.
	</li>
</ul></div>

 In practice, this model offers several benefits. Assuming that
 the initial cluster values were monotonically increasing
 and distinct before shaping began, then, in the final output:
 
<div class="itemizedlist"><ul class="itemizedlist compact" style="list-style-type: disc; ">
<li class="listitem">
	 All adjacent glyphs having the same final cluster
	 value belong to the same cluster.
	</li>
<li class="listitem">
 Each character belongs to the cluster that has the highest
	 cluster value not larger than its
	 initial cluster value.
	</li>
</ul></div>
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