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<chapter id="getting-started">
<title>Getting started with HarfBuzz</title>
<section>
<title>An overview of the HarfBuzz shaping API</title>
<para>
The core of the HarfBuzz shaping API is the function
<function>hb_shape()</function>. This function takes a font, a
buffer containing a string of Unicode codepoints and
(optionally) a list of font features as its input. It replaces
the codepoints in the buffer with the corresponding glyphs from
the font, correctly ordered and positioned, and with any of the
optional font features applied.
</para>
<para>
In addition to holding the pre-shaping input (the Unicode
codepoints that comprise the input string) and the post-shaping
output (the glyphs and positions), a HarfBuzz buffer has several
properties that affect shaping. The most important are the
text-flow direction (e.g., left-to-right, right-to-left,
top-to-bottom, or bottom-to-top), the script tag, and the
language tag.
</para>
<para>
For input string buffers, flags are available to denote when the
buffer represents the beginning or end of a paragraph, to
indicate whether or not to visibly render Unicode <literal>Default
Ignorable</literal> codepoints, and to modify the cluster-merging
behavior for the buffer. For shaped output buffers, the
individual X and Y offsets and <literal>advances</literal>
(the logical dimensions) of each glyph are
accessible. HarfBuzz also flags glyphs as
<literal>UNSAFE_TO_BREAK</literal> if breaking the string at
that glyph (e.g., in a line-breaking or hyphenation process)
would require re-shaping the text.
</para>
<para>
HarfBuzz also provides methods to compare the contents of
buffers, join buffers, normalize buffer contents, and handle
invalid codepoints, as well as to determine the state of a
buffer (e.g., input codepoints or output glyphs). Buffer
lifecycles are managed and all buffers are reference-counted.
</para>
<para>
Although the default <function>hb_shape()</function> function is
sufficient for most use cases, a variant is also provide that
lets you specify which of HarfBuzz's shapers to use on a buffer.
</para>
<para>
HarfBuzz can read TrueType fonts, TrueType collections, OpenType
fonts, and OpenType collections. Functions are provided to query
font objects about metrics, Unicode coverage, available tables and
features, and variation selectors. Individual glyphs can also be
queried for metrics, variations, and glyph names. OpenType
variable fonts are supported, and HarfBuzz allows you to set
variation-axis coordinates on font objects.
</para>
<para>
HarfBuzz provides glue code to integrate with various other
libraries, including FreeType, GObject, and CoreText. Support
for integrating with Uniscribe and DirectWrite is experimental
at present.
</para>
</section>
<section>
<title>Terminology</title>
<para>
</para>
<variablelist>
<?dbfo list-presentation="blocks"?>
<varlistentry>
<term>script</term>
<listitem>
<para>
In text shaping, a <emphasis>script</emphasis> is a
writing system: a set of symbols, rules, and conventions
that is used to represent a language or multiple
languages.
</para>
<para>
In general computing lingo, the word "script" can also
be used to mean an executable program (usually one
written in a human-readable programming language). For
the sake of clarity, HarfBuzz documents will always use
more specific terminology when referring to this
meaning, such as "Python script" or "shell script." In
all other instances, "script" refers to a writing system.
</para>
<para>
For developers using HarfBuzz, it is important to note
the distinction between a script and a language. Most
scripts are used to write a variety of different
languages, and many languages may be written in more
than one script.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>shaper</term>
<listitem>
<para>
In HarfBuzz, a <emphasis>shaper</emphasis> is a
handler for a specific script-shaping model. HarfBuzz
implements separate shapers for Indic, Arabic, Thai and
Lao, Khmer, Myanmar, Tibetan, Hangul, Hebrew, the
Universal Shaping Engine (USE), and a default shaper for
non-complex scripts.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>cluster</term>
<listitem>
<para>
In text shaping, a <emphasis>cluster</emphasis> is a
sequence of codepoints that must be treated as an
indivisible unit. Clusters can include code-point
sequences that form a ligature or base-and-mark
sequences. Tracking and preserving clusters is important
when shaping operations might separate or reorder
code points.
</para>
<para>
HarfBuzz provides three cluster
<emphasis>levels</emphasis> that implement different
approaches to the problem of preserving clusters during
shaping operations.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>grapheme</term>
<listitem>
<para>
In linguistics, a <emphasis>grapheme</emphasis> is one
of the indivisible units that make up a writing system or
script. Often, graphemes are individual symbols (letters,
numbers, punctuation marks, logograms, etc.) but,
depending on the writing system, a particular grapheme
might correspond to a sequence of several Unicode code
points.
</para>
<para>
In practice, HarfBuzz and other text-shaping engines
are not generally concerned with graphemes. However, it
is important for developers using HarfBuzz to recognize
that there is a difference between graphemes and shaping
clusters (see above). The two concepts may overlap
frequently, but there is no guarantee that they will be
identical.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>syllable</term>
<listitem>
<para>
In linguistics, a <emphasis>syllable</emphasis> is an
a sequence of sounds that makes up a building block of a
particular language. Every language has its own set of
rules describing what constitutes a valid syllable.
</para>
<para>
For text-shaping purposes, the various definitions of
"syllable" are important because script-specific shaping
operations may be applied at the syllable level. For
example, a reordering rule might specify that a vowel
mark be reordered to the beginning of the syllable.
</para>
<para>
Syllables will consist of one or more Unicode code
points. The definition of a syllable for a particular
writing system might correspond to how HarfBuzz
identifies clusters (see above) for the same writing
system. However, it is important for developers using
HarfBuzz to recognize that there is a difference between
syllables and shaping clusters. The two concepts may
overlap frequently, but there is no guarantee that they
will be identical.
</para>
</listitem>
</varlistentry>
</variablelist>
</section>
<section>
<title>A simple shaping example</title>
<para>
Below is the simplest HarfBuzz shaping example possible.
</para>
<orderedlist numeration="arabic">
<listitem>
<para>
Create a buffer and put your text in it.
</para>
</listitem>
</orderedlist>
<programlisting language="C">
#include <hb.h>
hb_buffer_t *buf;
buf = hb_buffer_create();
hb_buffer_add_utf8(buf, text, -1, 0, -1);
</programlisting>
<orderedlist numeration="arabic">
<listitem override="2">
<para>
Set the script, language and direction of the buffer.
</para>
</listitem>
</orderedlist>
<programlisting language="C">
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));
</programlisting>
<orderedlist numeration="arabic">
<listitem override="3">
<para>
Create a face and a font, using FreeType for now.
</para>
</listitem>
</orderedlist>
<programlisting language="C">
#include <hb-ft.h>
FT_New_Face(ft_library, font_path, index, &face);
FT_Set_Char_Size(face, 0, 1000, 0, 0);
hb_font_t *font = hb_ft_font_create(face);
</programlisting>
<orderedlist numeration="arabic">
<listitem override="4">
<para>
Shape!
</para>
</listitem>
</orderedlist>
<programlisting>
hb_shape(font, buf, NULL, 0);
</programlisting>
<orderedlist numeration="arabic">
<listitem override="5">
<para>
Get the glyph and position information.
</para>
</listitem>
</orderedlist>
<programlisting language="C">
hb_glyph_info_t *glyph_info = hb_buffer_get_glyph_infos(buf, &glyph_count);
hb_glyph_position_t *glyph_pos = hb_buffer_get_glyph_positions(buf, &glyph_count);
</programlisting>
<orderedlist numeration="arabic">
<listitem override="6">
<para>
Iterate over each glyph.
</para>
</listitem>
</orderedlist>
<programlisting language="C">
for (i = 0; i < glyph_count; ++i) {
glyphid = glyph_info[i].codepoint;
x_offset = glyph_pos[i].x_offset / 64.0;
y_offset = glyph_pos[i].y_offset / 64.0;
x_advance = glyph_pos[i].x_advance / 64.0;
y_advance = glyph_pos[i].y_advance / 64.0;
draw_glyph(glyphid, cursor_x + x_offset, cursor_y + y_offset);
cursor_x += x_advance;
cursor_y += y_advance;
}
</programlisting>
<orderedlist numeration="arabic">
<listitem override="7">
<para>
Tidy up.
</para>
</listitem>
</orderedlist>
<programlisting language="C">
hb_buffer_destroy(buf);
hb_font_destroy(hb_ft_font);
</programlisting>
<para>
This example shows enough to get us started using HarfBuzz. In
the sections that follow, we will use the remainder of
HarfBuzz's API to refine and extend the example and improve its
text-shaping capabilities.
</para>
</section>
</chapter>
