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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="fonts-and-faces-custom-functions"></a>Customizing font functions</h2></div></div></div>
<p>
During shaping, HarfBuzz frequently needs to query font objects
to get at the contents and parameters of the glyphs in a font
file. It includes a built-in set of functions that is tailored
to working with OpenType fonts. However, as was the case with
Unicode functions in the buffers chapter, HarfBuzz also wants to
make it easy for you to assign a substitute set of font
functions if you are developing a program to work with a library
or platform that provides its own font functions.
</p>
<p>
Therefore, the HarfBuzz API defines a set of virtual
methods for accessing font-object properties, and you can
replace the defaults with your own selections without
interfering with the shaping process. Each font object in
HarfBuzz includes a structure called
<code class="literal">font_funcs</code> that serves as a vtable for the
font object. The virtual methods in
<code class="literal">font_funcs</code> are:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
<code class="function">hb_font_get_font_h_extents_func_t</code>: returns
the extents of the font for horizontal text.
</p></li>
<li class="listitem"><p>
<code class="function">hb_font_get_font_v_extents_func_t</code>: returns
the extents of the font for vertical text.
</p></li>
<li class="listitem"><p>
<code class="function">hb_font_get_nominal_glyph_func_t</code>: returns
the font's nominal glyph for a given code point.
</p></li>
<li class="listitem"><p>
<code class="function">hb_font_get_variation_glyph_func_t</code>: returns
the font's glyph for a given code point when it is followed by a
given Variation Selector.
</p></li>
<li class="listitem"><p>
<code class="function">hb_font_get_nominal_glyphs_func_t</code>: returns
the font's nominal glyphs for a series of code points.
</p></li>
<li class="listitem"><p>
<code class="function">hb_font_get_glyph_advance_func_t</code>: returns
the advance for a glyph.
</p></li>
<li class="listitem"><p>
<code class="function">hb_font_get_glyph_h_advance_func_t</code>: returns
the advance for a glyph for horizontal text.
</p></li>
<li class="listitem"><p>
<code class="function">hb_font_get_glyph_v_advance_func_t</code>:returns
the advance for a glyph for vertical text.
</p></li>
<li class="listitem"><p>
<code class="function">hb_font_get_glyph_advances_func_t</code>: returns
the advances for a series of glyphs.
</p></li>
<li class="listitem"><p>
<code class="function">hb_font_get_glyph_h_advances_func_t</code>: returns
the advances for a series of glyphs for horizontal text .
</p></li>
<li class="listitem"><p>
<code class="function">hb_font_get_glyph_v_advances_func_t</code>: returns
the advances for a series of glyphs for vertical text.
</p></li>
<li class="listitem"><p>
<code class="function">hb_font_get_glyph_origin_func_t</code>: returns
the origin coordinates of a glyph.
</p></li>
<li class="listitem"><p>
<code class="function">hb_font_get_glyph_h_origin_func_t</code>: returns
the origin coordinates of a glyph for horizontal text.
</p></li>
<li class="listitem"><p>
<code class="function">hb_font_get_glyph_v_origin_func_t</code>: returns
the origin coordinates of a glyph for vertical text.
</p></li>
<li class="listitem"><p>
<code class="function">hb_font_get_glyph_extents_func_t</code>: returns
the extents for a glyph.
</p></li>
<li class="listitem"><p>
<code class="function">hb_font_get_glyph_contour_point_func_t</code>:
returns the coordinates of a specific contour point from a glyph.
</p></li>
<li class="listitem"><p>
<code class="function">hb_font_get_glyph_name_func_t</code>: returns the
name of a glyph (from its glyph index).
</p></li>
<li class="listitem"><p>
<code class="function">hb_font_get_glyph_from_name_func_t</code>: returns
the glyph index that corresponds to a given glyph name.
</p></li>
</ul></div>
<p>
You can fetch the font-functions configuration for a font object
by calling <code class="function">hb_font_get_font_funcs()</code>:
</p>
<pre class="programlisting">
hb_font_funcs_t *ffunctions;
ffunctions = hb_font_get_font_funcs (font);
</pre>
<p>
The individual methods can each be replaced with their own setter
function, such as
<code class="function">hb_font_funcs_set_nominal_glyph_func(*ffunctions,
func, *user_data, destroy)</code>.
</p>
<p>
Font-functions structures can be reused for multiple font
objects, and can be reference counted with
<code class="function">hb_font_funcs_reference()</code> and
<code class="function">hb_font_funcs_destroy()</code>. Just like other
objects in HarfBuzz, you can set user-data for each
font-functions structure and assign a destroy callback for
it.
</p>
<p>
You can also mark a font-functions structure as immutable,
with <code class="function">hb_font_funcs_make_immutable()</code>. This
is especially useful if your code is a library or framework that
will have its own client programs. By marking your
font-functions structures as immutable, you prevent your client
programs from changing the configuration and introducing
inconsistencies and errors downstream.
</p>
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