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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="shaping-opentype-features"></a>OpenType features</h2></div></div></div>
<p>
OpenType features enable fonts to include smart behavior,
implemented as "lookup" rules stored in the GSUB and GPOS
tables. The OpenType specification defines a long list of
standard features that fonts can use for these behaviors; each
feature has a four-character reserved name and a well-defined
semantic meaning.
</p>
<p>
Some OpenType features are defined for the purpose of supporting
script-specific shaping, and are automatically activated, but
only when a buffer's script property is set to a script that the
feature supports.
</p>
<p>
Other features are more generic and can apply to several (or
any) script, and shaping engines are expected to implement
them. By default, HarfBuzz activates several of these features
on every text run. They include <code class="literal">abvm</code>,
<code class="literal">blwm</code>, <code class="literal">ccmp</code>,
<code class="literal">locl</code>, <code class="literal">mark</code>,
<code class="literal">mkmk</code>, and <code class="literal">rlig</code>.
</p>
<p>
In addition, if the text direction is horizontal, HarfBuzz
also applies the <code class="literal">calt</code>,
<code class="literal">clig</code>, <code class="literal">curs</code>,
<code class="literal">dist</code>, <code class="literal">kern</code>,
<code class="literal">liga</code> and <code class="literal">rclt</code>, features.
</p>
<p>
Additionally, when HarfBuzz encounters a fraction slash
(<code class="literal">U+2044</code>), it looks backward and forward for decimal
digits (Unicode General Category = Nd), and enables features
<code class="literal">numr</code> on the sequence before the fraction slash,
<code class="literal">dnom</code> on the sequence after the fraction slash,
and <code class="literal">frac</code> on the whole sequence including the fraction
slash.
</p>
<p>
Some script-specific shaping models
(see <a class="xref" href="opentype-shaping-models.html" title="OpenType shaping models">the section called “OpenType shaping models”</a>) disable some of the
features listed above:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
Hangul: <code class="literal">calt</code>
</p></li>
<li class="listitem"><p>
Indic: <code class="literal">liga</code>
</p></li>
<li class="listitem"><p>
Khmer: <code class="literal">liga</code>
</p></li>
</ul></div>
<p>
If the text direction is vertical, HarfBuzz applies
the <code class="literal">vert</code> feature by default.
</p>
<p>
Still other features are designed to be purely optional and left
up to the application or the end user to enable or disable as desired.
</p>
<p>
You can adjust the set of features that HarfBuzz applies to a
buffer by supplying an array of <span class="type">hb_feature_t</span>
features as the third argument to
<code class="function">hb_shape()</code>. For a simple case, let's just
enable the <code class="literal">dlig</code> feature, which turns on any
"discretionary" ligatures in the font:
</p>
<pre class="programlisting">
hb_feature_t userfeatures[1];
userfeatures[0].tag = HB_TAG('d','l','i','g');
userfeatures[0].value = 1;
userfeatures[0].start = HB_FEATURE_GLOBAL_START;
userfeatures[0].end = HB_FEATURE_GLOBAL_END;
</pre>
<p>
<code class="literal">HB_FEATURE_GLOBAL_END</code> and
<code class="literal">HB_FEATURE_GLOBAL_END</code> are macros we can use
to indicate that the features will be applied to the entire
buffer. We could also have used a literal <code class="literal">0</code>
for the start and a <code class="literal">-1</code> to indicate the end of
the buffer (or have selected other start and end positions, if needed).
</p>
<p>
When we pass the <code class="varname">userfeatures</code> array to
<code class="function">hb_shape()</code>, any discretionary ligature
substitutions from our font that match the text in our buffer
will get performed:
</p>
<pre class="programlisting">
hb_shape(font, buf, userfeatures, num_features);
</pre>
<p>
Just like we enabled the <code class="literal">dlig</code> feature by
setting its <em class="parameter"><code>value</code></em> to
<code class="literal">1</code>, you would disable a feature by setting its
<em class="parameter"><code>value</code></em> to <code class="literal">0</code>. Some
features can take other <em class="parameter"><code>value</code></em> settings;
be sure you read the full specification of each feature tag to
understand what it does and how to control it.
</p>
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