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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>
<p>
When you add text to a HarfBuzz buffer, each code point must be
assigned a <span class="emphasis"><em>cluster value</em></span>.
</p>
<p>
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.
</p>
<p>
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.
</p>
<p>
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).
</p>
<p>
For example, if the initial sequence of cluster values was:
</p>
<pre class="programlisting">
0,1,2,3,4
</pre>
<p>
and the final sequence of cluster values is:
</p>
<pre class="programlisting">
0,0,3,3
</pre>
<p>
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).
</p>
<p>
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.
</p>
<p>
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.
</p>
<p>
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 <span class="emphasis"><em>decreasing</em></span> final
cluster values.
</p>
<p>
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 <span class="emphasis"><em>levels</em></span> of
clustering support for this property:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<p><span class="emphasis"><em>Level 0</em></span> is the default and
reproduces the behavior of the old HarfBuzz library.
</p>
<p>
The distinguishing feature of level 0 behavior is that, at
the beginning of processing the buffer, all code points that
are categorized as <span class="emphasis"><em>marks</em></span>,
<span class="emphasis"><em>modifier symbols</em></span>, or
<span class="emphasis"><em>Emoji extended pictographic</em></span> modifiers,
as well as the <span class="emphasis"><em>Zero Width Joiner</em></span> and
<span class="emphasis"><em>Zero Width Non-Joiner</em></span> code points, are
assigned the cluster value of the closest preceding code
point from <span class="emphasis"><em>different</em></span> category.
</p>
<p>
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>.
</p>
<p>
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>.
</p>
</li>
<li class="listitem">
<p>
<span class="emphasis"><em>Level 1</em></span> 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.
</p>
<p>
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).
</p>
<p>
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>.
</p>
</li>
<li class="listitem">
<p>
<span class="emphasis"><em>Level 2</em></span> differs significantly in how it
treats cluster values. In level 2, HarfBuzz never merges
clusters.
</p>
<p>
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.
</p>
<p>
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>.
</p>
</li>
</ul></div>
<p>
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).
</p>
<p>
It is not <span class="emphasis"><em>required</em></span> 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.
</p>
<p>
In levels 0 and 1, HarfBuzz implements the following conceptual
model for cluster values:
</p>
<div class="itemizedlist"><ul class="itemizedlist compact" style="list-style-type: disc; ">
<li class="listitem"><p>
If the sequence of input cluster values is monotonic, the
sequence of cluster values will remain monotonic.
</p></li>
<li class="listitem"><p>
Each cluster value represents a single cluster.
</p></li>
<li class="listitem"><p>
Each cluster contains one or more glyphs and one or more
characters.
</p></li>
</ul></div>
<p>
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:
</p>
<div class="itemizedlist"><ul class="itemizedlist compact" style="list-style-type: disc; ">
<li class="listitem"><p>
All adjacent glyphs having the same final cluster
value belong to the same cluster.
</p></li>
<li class="listitem"><p>
Each character belongs to the cluster that has the highest
cluster value <span class="emphasis"><em>not larger than</em></span> its
initial cluster value.
</p></li>
</ul></div>
</div>
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