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<chapter id="integration">
<title>Platform Integration Guide</title>
HarfBuzz was first developed for use with the GNOME and GTK
software stack commonly found in desktop Linux
distributions. Nevertheless, it can be used on other operating
systems and platforms, from iOS and macOS to Windows. It can also
be used with other application frameworks and components, such as
Android, Qt, or application-specific widget libraries.
This chapter will look at how HarfBuzz fits into a typical
text-rendering pipeline, and will discuss the APIs available to
integrate HarfBuzz with contemporary Linux, Mac, and Windows
software. It will also show how HarfBuzz integrates with popular
external libraries like FreeType and International Components for
Unicode (ICU) and describe the HarfBuzz language bindings for
On a GNOME system, HarfBuzz is designed to tie in with several
other common system libraries. The most common architecture uses
Pango at the layer directly "above" HarfBuzz; Pango is responsible
for text segmentation and for ensuring that each input
<type>hb_buffer_t</type> passed to HarfBuzz for shaping contains
Unicode code points that share the same segment properties
(namely, direction, language, and script, but also higher-level
properties like the active font, font style, and so on).
The layer directly "below" HarfBuzz is typically FreeType, which
is used to rasterize glyph outlines at the necessary optical size,
hinting settings, and pixel resolution. FreeType provides APIs for
accessing font and face information, so HarfBuzz includes
functions to create <type>hb_face_t</type> and
<type>hb_font_t</type> objects directly from FreeType
objects. HarfBuzz can use FreeType's built-in functions for
<structfield>font_funcs</structfield> vtable in an <type>hb_font_t</type>.
FreeType's output is bitmaps of the rasterized glyphs; on a
typical Linux system these will then be drawn by a graphics
library like Cairo, but those details are beyond HarfBuzz's
control. On the other hand, at the top end of the stack, Pango is
part of the larger GNOME framework, and HarfBuzz does include APIs
for working with key components of GNOME's higher-level libraries
&mdash; most notably GLib.
For other operating systems or application frameworks, the
critical integration points are where HarfBuzz gets font and face
information about the font used for shaping and where HarfBuzz
gets Unicode data about the input-buffer code points.
The font and face information is necessary for text shaping
because HarfBuzz needs to retrieve the glyph indices for
particular code points, and to know the extents and advances of
glyphs. Note that, in an OpenType variable font, both of those
types of information can change with different variation-axis
The Unicode information is necessary for shaping because the
properties of a code point (such as its General Category (gc),
Canonical Combining Class (ccc), and decomposition) can directly
impact the shaping moves that HarfBuzz performs.
<section id="integration-glib">
<title>GNOME integration, GLib, and GObject</title>
As mentioned in the preceding section, HarfBuzz offers
integration APIs to help client programs using the
GNOME and GTK framework commonly found in desktop Linux
GLib is the main utility library for GNOME applications. It
provides basic data types and conversions, file abstractions,
string manipulation, and macros, as well as facilities like
memory allocation and the main event loop.
Where text shaping is concerned, GLib provides several utilities
that HarfBuzz can take advantage of, including a set of
Unicode-data functions and a data type for script
information. Both are useful when working with HarfBuzz
buffers. To make use of them, you will need to include the
<filename>hb-glib.h</filename> header file.
GLib's <ulink
manipulation API</ulink> includes all the functionality
necessary to retrieve Unicode data for the
<structfield>unicode_funcs</structfield> structure of a HarfBuzz
The function <function>hb_glib_get_unicode_funcs()</function>
sets up a <type>hb_unicode_funcs_t</type> structure configured
with the GLib Unicode functions and returns a pointer to it.
You can attach this Unicode-functions structure to your buffer,
and it will be ready for use with GLib:
<programlisting language="C">
#include &lt;hb-glib.h&gt;
hb_unicode_funcs_t *glibufunctions;
glibufunctions = hb_glib_get_unicode_funcs();
hb_buffer_set_unicode_funcs(buf, glibufunctions);
For script information, GLib uses the
<type>GUnicodeScript</type> type. Like HarfBuzz's own
<type>hb_script_t</type>, this data type is an enumeration
of Unicode scripts, but text segments passed in from GLib code
will be tagged with a <type>GUnicodeScript</type>. Therefore,
when setting the script property on a <type>hb_buffer_t</type>,
you will need to convert between the <type>GUnicodeScript</type>
of the input provided by GLib and HarfBuzz's
<type>hb_script_t</type> type.
The <function>hb_glib_script_to_script()</function> function
takes an <type>GUnicodeScript</type> script identifier as its
sole argument and returns the corresponding <type>hb_script_t</type>.
The <function>hb_glib_script_from_script()</function> does the
reverse, taking an <type>hb_script_t</type> and returning the
<type>GUnicodeScript</type> identifier for GLib.
Finally, GLib also provides a reference-counted object type called <ulink
that is used for accessing raw memory segments with the benefits
of GLib's lifecycle management. HarfBuzz provides a
<function>hb_glib_blob_create()</function> function that lets
you create an <type>hb_blob_t</type> directly from a
<type>GBytes</type> object. This function takes only the
<type>GBytes</type> object as its input; HarfBuzz registers the
GLib <function>destroy</function> callback automatically.
The GNOME platform also features an object system called
GObject. For HarfBuzz, the main advantage of GObject is a
feature called <ulink
Introspection</ulink>. This is a middleware facility that can be
used to generate language bindings for C libraries. HarfBuzz uses it
to build its Python bindings, which we will look at in a separate section.
<section id="integration-freetype">
<title>FreeType integration</title>
FreeType is the free-software font-rendering engine included in
desktop Linux distributions, Android, ChromeOS, iOS, and multiple Unix
operating systems, and used by cross-platform programs like
Chrome, Java, and GhostScript. Used together, HarfBuzz can
perform shaping on Unicode text segments, outputting the glyph
IDs that FreeType should rasterize from the active font as well
as the positions at which those glyphs should be drawn.
HarfBuzz provides integration points with FreeType at the
face-object and font-object level and for the font-functions
virtual-method structure of a font object. To use the
FreeType-integration API, include the
<filename>hb-ft.h</filename> header.
In a typical client program, you will create your
<type>hb_face_t</type> face object and <type>hb_font_t</type>
font object from a FreeType <type>FT_Face</type>. HarfBuzz
provides a suite of functions for doing this.
In the most common case, you will want to use
<function>hb_ft_font_create_referenced()</function>, which
creates both an <type>hb_face_t</type> face object and
<type>hb_font_t</type> font object (linked to that face object),
and provides lifecycle management.
It is important to note,
though, that while HarfBuzz makes a distinction between its face and
font objects, FreeType's <type>FT_Face</type> does not. After
you create your <type>FT_Face</type>, you must set its size
parameter using <function>FT_Set_Char_Size()</function>, because
an <type>hb_font_t</type> is defined as an instance of an
<type>hb_face_t</type> with size specified.
<programlisting language="C">
#include &lt;hb-ft.h&gt;
FT_New_Face(ft_library, font_path, index, &amp;face);
FT_Set_Char_Size(face, 0, 1000, 0, 0);
hb_font_t *font = hb_ft_font_create(face);
<function>hb_ft_font_create_referenced()</function> is
the recommended function for creating an <type>hb_face_t</type> face
object. This function calls <function>FT_Reference_Face()</function>
before using the <type>FT_Face</type> and calls
<function>FT_Done_Face()</function> when it is finished using the
<type>FT_Face</type>. Consequently, your client program does not need
to worry about destroying the <type>FT_Face</type> while HarfBuzz
is still using it.
Although <function>hb_ft_font_create_referenced()</function> is
the recommended function, there is another variant for client code
where special circumstances make it necessary. The simpler
version of the function is <function>hb_ft_font_create()</function>,
which takes an <type>FT_Face</type> and an optional destroy callback
as its arguments. Because <function>hb_ft_font_create()</function>
does not offer lifecycle management, however, your client code will
be responsible for tracking references to the <type>FT_Face</type>
objects and destroying them when they are no longer needed. If you
do not have a valid reason for doing this, use
After you have created your font object from your
<type>FT_Face</type>, you can set or retrieve the
<structfield>load_flags</structfield> of the
<type>FT_Face</type> through the <type>hb_font_t</type>
object. HarfBuzz provides
<function>hb_ft_font_set_load_flags()</function> and
<function>hb_ft_font_get_load_flags()</function> for this
purpose. The ability to set the
<structfield>load_flags</structfield> through the font object
could be useful for enabling or disabling hinting, for example,
or to activate vertical layout.
HarfBuzz also provides a utility function called
<function>hb_ft_font_has_changed()</function> that you should
call whenever you have altered the properties of your underlying
<type>FT_Face</type>, as well as a
<function>hb_ft_get_face()</function> that you can call on an
<type>hb_font_t</type> font object to fetch its underlying <type>FT_Face</type>.
With an <type>hb_face_t</type> and <type>hb_font_t</type> both linked
to your <type>FT_Face</type>, you will typically also want to
use FreeType for the <structfield>font_funcs</structfield>
vtable of your <type>hb_font_t</type>. As a reminder, this
font-functions structure is the set of methods that HarfBuzz
will use to fetch important information from the font, such as
the advances and extents of individual glyphs.
All you need to do is call
<programlisting language="C">
and HarfBuzz will use FreeType for the font-functions in
As we noted above, an <type>hb_font_t</type> is derived from an
<type>hb_face_t</type> with size (and, perhaps, other
parameters, such as variation-axis coordinates)
specified. Consequently, you can reuse an <type>hb_face_t</type>
with several <type>hb_font_t</type> objects, and HarfBuzz
provides functions to simplify this.
The <function>hb_ft_face_create_referenced()</function>
function creates just an <type>hb_face_t</type> from a FreeType
<type>FT_Face</type> and, as with
<function>hb_ft_font_create_referenced()</function> above,
provides lifecycle management for the <type>FT_Face</type>.
Similarly, there is an <function>hb_ft_face_create()</function>
function variant that does not provide the lifecycle-management
feature. As with the font-object case, if you use this version
of the function, it will be your client code's respsonsibility
to track usage of the <type>FT_Face</type> objects.
A third variant of this function is
<function>hb_ft_face_create_cached()</function>, which is the
same as <function>hb_ft_face_create()</function> except that it
also uses the <structfield>generic</structfield> field of the
<type>FT_Face</type> structure to save a pointer to the newly
created <type>hb_face_t</type>. Subsequently, function calls
that pass the same <type>FT_Face</type> will get the same
<type>hb_face_t</type> returned &mdash; and the
<type>hb_face_t</type> will be correctly reference
counted. Still, as with
<function>hb_ft_face_create()</function>, your client code must
track references to the <type>FT_Face</type> itself, and destroy
it when it is unneeded.
<section id="integration-uniscribe">
<title>Uniscribe integration</title>
If your client program is running on Windows, HarfBuzz offers
an additional API that can help integrate with Microsoft's
Uniscribe engine and the Windows GDI.
Overall, the Uniscribe API covers a broader set of typographic
layout functions than HarfBuzz implements, but HarfBuzz's
shaping API can serve as a drop-in replacement for Uniscribe's shaping
functionality. In fact, one of HarfBuzz's design goals is to
accurately reproduce the same output for shaping a given text
segment that Uniscribe produces &mdash; even to the point of
duplicating known shaping bugs or deviations from the
specification &mdash; so you can be confident that your users'
documents with their existing fonts will not be affected adversely by
switching to HarfBuzz.
At a basic level, HarfBuzz's <function>hb_shape()</function>
function replaces both the <ulink url=""><function>ScriptShape()</function></ulink>
and <ulink
functions from Uniscribe.
However, whereas <function>ScriptShape()</function> returns the
glyphs and clusters for a shaped sequence and
<function>ScriptPlace()</function> returns the advances and
offsets for those glyphs, <function>hb_shape()</function>
handles both. After <function>hb_shape()</function> shapes a
buffer, the output glyph IDs and cluster IDs are returned as
an array of <structname>hb_glyph_info_t</structname> structures, and the
glyph advances and offsets are returned as an array of
<structname>hb_glyph_position_t</structname> structures.
Your client program only needs to ensure that it coverts
correctly between HarfBuzz's low-level data types (such as
<type>hb_position_t</type>) and Windows's corresponding types
(such as <type>GOFFSET</type> and <type>ABC</type>). Be sure you
read the <xref linkend="buffers-language-script-and-direction"
chapter for a full explanation of how HarfBuzz input buffers are
used, and see <xref linkend="shaping-buffer-output" /> for the
details of what <function>hb_shape()</function> returns in the
output buffer when shaping is complete.
Although <function>hb_shape()</function> itself is functionally
equivalent to Uniscribe's shaping routines, there are two
additional HarfBuzz functions you may want to use to integrate
the libraries in your code. Both are used to link HarfBuzz font
objects to the equivalent Windows structures.
The <function>hb_uniscribe_font_get_logfontw()</function>
function takes a <type>hb_font_t</type> font object and returns
a pointer to the <ulink
"logical font" that corresponds to it. A <type>LOGFONTW</type>
structure holds font-wide attributes, including metrics, size,
and style information.
In Uniscribe's model, the <type>SCRIPT_CACHE</type> holds the
device context, including the logical font that the shaping
functions apply.
The <function>hb_uniscribe_font_get_hfont()</function> function
also takes a <type>hb_font_t</type> font object, but it returns
an <type>HFONT</type> &mdash; a handle to the underlying logical
font &mdash; instead.
<type>LOGFONTW</type>s and <type>HFONT</type>s are both needed
by other Uniscribe functions.
As a final note, you may notice a reference to an optional
<literal>uniscribe</literal> shaper back-end in the <xref
linkend="configuration" /> section of the HarfBuzz manual. This
option is not a Uniscribe-integration facility.
Instead, it is a internal code path used in the
<command>hb-shape</command> command-line utility, which hands
shaping functionality over to Uniscribe entirely, when run on a
Windows system. That allows testing HarfBuzz's native output
against the Uniscribe engine, for tracking compatibility and
Because this back-end is only used when testing HarfBuzz
functionality, it is disabled by default when building the
HarfBuzz binaries.
<section id="integration-coretext">
<title>Core Text integration</title>
If your client program is running on macOS or iOS, HarfBuzz offers
an additional API that can help integrate with Apple's
Core Text engine and the underlying Core Graphics
framework. HarfBuzz does not attempt to offer the same
drop-in-replacement functionality for Core Text that it strives
for with Uniscribe on Windows, but you can still use HarfBuzz
to perform text shaping in native macOS and iOS applications.
Note, though, that if your interest is just in using fonts that
contain Apple Advanced Typography (AAT) features, then you do
not need to add Core Text integration. HarfBuzz natively
supports AAT features and will shape AAT fonts (on any platform)
automatically, without requiring additional work on your
part. This includes support for AAT-specific TrueType tables
such as <literal>mort</literal>, <literal>morx</literal>, and
<literal>kerx</literal>, which AAT fonts use instead of
<literal>GSUB</literal> and <literal>GPOS</literal>.
On a macOS or iOS system, the primary integration points offered
by HarfBuzz are for face objects and font objects.
The Apple APIs offer a pair of data structures that map well to
HarfBuzz's face and font objects. The Core Graphics API, which
is slightly lower-level than Core Text, provides
<ulink url=""><type>CGFontRef</type></ulink>, which enables access to typeface
properties, but does not include size information. Core Text's
<ulink url=""><type>CTFontRef</type></ulink> is analagous to a HarfBuzz font object,
with all of the properties required to render text at a specific
size and configuration.
Consequently, a HarfBuzz <type>hb_font_t</type> font object can
be hooked up to a Core Text <type>CTFontRef</type>, and a HarfBuzz
<type>hb_face_t</type> face object can be hooked up to a
You can create a <type>hb_face_t</type> from a
<type>CGFontRef</type> by using the
<function>hb_coretext_face_create()</function>. Subsequently,
you can retrieve the <type>CGFontRef</type> from a
<type>hb_face_t</type> with <function>hb_coretext_face_get_cg_font()</function>.
Likewise, you create a <type>hb_font_t</type> from a
<type>CTFontRef</type> by calling
<function>hb_coretext_font_create()</function>, and you can
fetch the associated <type>CTFontRef</type> from a
<type>hb_font_t</type> font object with
HarfBuzz also offers a <function>hb_font_set_ptem()</function>
that you an use to set the nominal point size on any
<type>hb_font_t</type> font object. Core Text uses this value to
implement optical scaling.
When integrating your client code with Core Text, it is
important to recognize that Core Text <literal>points</literal>
are not typographic points (standardized at 72 per inch) as the
term is used elsewhere in OpenType. Instead, Core Text points
are CSS points, which are standardized at 96 per inch.
HarfBuzz's font functions take this distinction into account,
but it can be an easy detail to miss in cross-platform
As a final note, you may notice a reference to an optional
<literal>coretext</literal> shaper back-end in the <xref
linkend="configuration" /> section of the HarfBuzz manual. This
option is not a Core Text-integration facility.
Instead, it is a internal code path used in the
<command>hb-shape</command> command-line utility, which hands
shaping functionality over to Core Text entirely, when run on a
macOS system. That allows testing HarfBuzz's native output
against the Core Text engine, for tracking compatibility and debugging.
Because this back-end is only used when testing HarfBuzz
functionality, it is disabled by default when building the
HarfBuzz binaries.
<section id="integration-icu">
<title>ICU integration</title>
Although HarfBuzz includes its own Unicode-data functions, it
also provides integration APIs for using the International
Components for Unicode (ICU) library as a source of Unicode data
on any supported platform.
The principal integration point with ICU is the
<type>hb_unicode_funcs_t</type> Unicode-functions structure
attached to a buffer. This structure holds the virtual methods
used for retrieving Unicode character properties, such as
General Category, Script, Combining Class, decomposition
mappings, and mirroring information.
To use ICU in your client program, you need to call
<function>hb_icu_get_unicode_funcs()</function>, which creates a
Unicode-functions structure populated with the ICU function for
each included method. Subsequently, you can attach the
Unicode-functions structure to your buffer:
<programlisting language="C">
hb_unicode_funcs_t *icufunctions;
icufunctions = hb_icu_get_unicode_funcs();
hb_buffer_set_unicode_funcs(buf, icufunctions);
and ICU will be used for Unicode-data access.
HarfBuzz also supplies a pair of functions
(<function>hb_icu_script_from_script()</function> and
<function>hb_icu_script_to_script()</function>) for converting
between ICU's and HarfBuzz's internal enumerations of Unicode
scripts. The <function>hb_icu_script_from_script()</function>
function converts from a HarfBuzz <type>hb_script_t</type> to an
ICU <type>UScriptCode</type>. The
<function>hb_icu_script_to_script()</function> function does the
reverse: converting from a <type>UScriptCode</type> identifier
to a <type>hb_script_t</type>.
By default, HarfBuzz's ICU support is built as a separate shared
library (<filename class="libraryfile"></filename>)
when compiling HarfBuzz from source. This allows client programs
that do not need ICU to link against HarfBuzz without unnecessarily
adding ICU as a dependency. You can also build HarfBuzz with ICU
support built directly into the main HarfBuzz shared library
(<filename class="libraryfile"></filename>),
by specifying the <literal>--with-icu=builtin</literal>
compile-time option.
<section id="integration-python">
<title>Python bindings</title>
As noted in the <xref linkend="integration-glib" /> section,
HarfBuzz uses a feature called <ulink
Introspection</ulink> (GI) to provide bindings for Python.
At compile time, the GI scanner analyzes the HarfBuzz C source
and builds metadata objects connecting the language bindings to
the C library. Your Python code can then use the HarfBuzz binary
through its Python interface.
HarfBuzz's Python bindings support Python 2 and Python 3. To use
them, you will need to have the <literal>pygobject</literal>
package installed. Then you should import
<literal>HarfBuzz</literal> from
<programlisting language="Python">
from gi.repository import HarfBuzz
and you can call HarfBuzz functions from Python. Sample code can
be found in the <filename></filename> script in the
HarfBuzz <filename>src</filename> directory.
Do note, however, that the Python API is subject to change
without advance notice. GI allows the bindings to be
automatically updated, which is one of its advantages, but you
may need to update your Python code.