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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="setting-buffer-properties"></a>Setting buffer properties</h2></div></div></div>
<p>
Buffers containing input characters still need several
properties set before HarfBuzz can shape their text correctly.
</p>
<p>
Initially, all buffers are set to the
<code class="literal">HB_BUFFER_CONTENT_TYPE_INVALID</code> content
type. After adding text, the buffer should be set to
<code class="literal">HB_BUFFER_CONTENT_TYPE_UNICODE</code> instead, which
indicates that it contains un-shaped input
characters. After shaping, the buffer will have the
<code class="literal">HB_BUFFER_CONTENT_TYPE_GLYPHS</code> content type.
</p>
<p>
<code class="function">hb_buffer_add_utf8()</code> and the
other UTF functions set the content type of their buffer
automatically. But if you are reusing a buffer you may want to
check its state with
<code class="function">hb_buffer_get_content_type(buffer)</code>. If
necessary you can set the content type with
</p>
<pre class="programlisting">
hb_buffer_set_content_type(buf, HB_BUFFER_CONTENT_TYPE_UNICODE);
</pre>
<p>
to prepare for shaping.
</p>
<p>
Buffers also need to carry information about the script,
language, and text direction of their contents. You can set
these properties individually:
</p>
<pre class="programlisting">
hb_buffer_set_direction(buf, HB_DIRECTION_LTR);
hb_buffer_set_script(buf, HB_SCRIPT_LATIN);
hb_buffer_set_language(buf, hb_language_from_string("en", -1));
</pre>
<p>
However, since these properties are often repeated for
multiple text runs, you can also save them in a
<code class="literal">hb_segment_properties_t</code> for reuse:
</p>
<pre class="programlisting">
hb_segment_properties_t *savedprops;
hb_buffer_get_segment_properties (buf, savedprops);
...
hb_buffer_set_segment_properties (buf2, savedprops);
</pre>
<p>
HarfBuzz also provides getter functions to retrieve a buffer's
direction, script, and language properties individually.
</p>
<p>
HarfBuzz recognizes four text directions in
<span class="type">hb_direction_t</span>: left-to-right
(<code class="literal">HB_DIRECTION_LTR</code>), right-to-left (<code class="literal">HB_DIRECTION_RTL</code>),
top-to-bottom (<code class="literal">HB_DIRECTION_TTB</code>), and
bottom-to-top (<code class="literal">HB_DIRECTION_BTT</code>). For the
script property, HarfBuzz uses identifiers based on the
<a class="ulink" href="https://unicode.org/iso15924/" target="_top">ISO 15924
standard</a>. For languages, HarfBuzz uses tags based on the
<a class="ulink" href="https://tools.ietf.org/html/bcp47" target="_top">IETF BCP 47</a> standard.
</p>
<p>
Helper functions are provided to convert character strings into
the necessary script and language tag types.
</p>
<p>
Two additional buffer properties to be aware of are the
"invisible glyph" and the replacement code point. The
replacement code point is inserted into buffer output in place of
any invalid code points encountered in the input. By default, it
is the Unicode <code class="literal">REPLACEMENT CHARACTER</code> code
point, <code class="literal">U+FFFD</code> "�". You can change this with
</p>
<pre class="programlisting">
hb_buffer_set_replacement_codepoint(buf, replacement);
</pre>
<p>
passing in the replacement Unicode code point as the
<em class="parameter"><code>replacement</code></em> parameter.
</p>
<p>
The invisible glyph is used to replace all output glyphs that
are invisible. By default, the standard space character
<code class="literal">U+0020</code> is used; you can replace this (for
example, when using a font that provides script-specific
spaces) with
</p>
<pre class="programlisting">
hb_buffer_set_invisible_glyph(buf, replacement_glyph);
</pre>
<p>
Do note that in the <em class="parameter"><code>replacement_glyph</code></em>
parameter, you must provide the glyph ID of the replacement you
wish to use, not the Unicode code point.
</p>
<p>
HarfBuzz supports a few additional flags you might want to set
on your buffer under certain circumstances. The
<code class="literal">HB_BUFFER_FLAG_BOT</code> and
<code class="literal">HB_BUFFER_FLAG_EOT</code> flags tell HarfBuzz
that the buffer represents the beginning or end (respectively)
of a text element (such as a paragraph or other block). Knowing
this allows HarfBuzz to apply certain contextual font features
when shaping, such as initial or final variants in connected
scripts.
</p>
<p>
<code class="literal">HB_BUFFER_FLAG_PRESERVE_DEFAULT_IGNORABLES</code>
tells HarfBuzz not to hide glyphs with the
<code class="literal">Default_Ignorable</code> property in Unicode. This
property designates control characters and other non-printing
code points, such as joiners and variation selectors. Normally
HarfBuzz replaces them in the output buffer with zero-width
space glyphs (using the "invisible glyph" property discussed
above); setting this flag causes them to be printed, which can
be helpful for troubleshooting.
</p>
<p>
Conversely, setting the
<code class="literal">HB_BUFFER_FLAG_REMOVE_DEFAULT_IGNORABLES</code> flag
tells HarfBuzz to remove <code class="literal">Default_Ignorable</code>
glyphs from the output buffer entirely. Finally, setting the
<code class="literal">HB_BUFFER_FLAG_DO_NOT_INSERT_DOTTED_CIRCLE</code>
flag tells HarfBuzz not to insert the dotted-circle glyph
(<code class="literal">U+25CC</code>, "◌"), which is normally
inserted into buffer output when broken character sequences are
encountered (such as combining marks that are not attached to a
base character).
</p>
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