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[Docs] Usermanual; add Fonts And Faces chapter.

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<!ENTITY version SYSTEM "version.xml">
]>
<chapter id="fonts-and-faces">
<title>Fonts and faces</title>
<section id="using-freetype">
<title>Fonts, faces, and output</title>
<para>
In the previous chapter, we saw how to set up a buffer and fill
it with text as Unicode code points. In order to shape this
buffer text with HarfBuzz, you will need also need a font
object.
</para>
<para>
HarfBuzz provides abstractions to help you cache and reuse the
heavier parts of working with binary fonts, so we will look at
how to do that. We will also look at how to work with the
FreeType font-rendering library and at how you can customize
HarfBuzz to work with other libraries.
</para>
<para>
Finally, we will look at how to work with OpenType variable
fonts, the latest update to the OpenType font format, and at
some other recent additions to OpenType.
</para>
<section id="fonts-and-faces-objects">
<title>Font and face objects</title>
<para>
The outcome of shaping a run of text depends on the contents of
a specific font file (such as the substitutions and positioning
moves in the 'GSUB' and 'GPOS' tables), so HarfBuzz makes
accessing those internals fast.
</para>
<para>
An <type>hb_face_t</type> represents a <emphasis>face</emphasis>
in HarfBuzz. This data type is a wrapper around an
<type>hb_blob_t</type> blob that holds the contents of a binary
fotn file. Since HarfBuzz supports TrueType Collections and
OpenType Collections (each of which can include multiple
typefaces), a HarfBuzz face also requires an index number
specifying which typeface in the file you want to use. Most of
the font files you will encounter in the wild include just a
single face, however, so most of the time you would pass in
<literal>0</literal> as the index when you create a face:
</para>
<programlisting language="C">
hb_blob_t* blob = hb_blob_create_from_file(file);
...
hb_face_t* face = hb_face_create(blob, 0);
</programlisting>
<para>
On its own, a face object is not quite ready to use for
shaping. The typeface must be set to a specific point size in
order for some details (such as hinting) to work. In addition,
if the font file in question is an OpenType Variable Font, then
you may need to specify one or variation-axis settings (or a
named instance) in order to get the output you need.
</para>
<para>
In HarfBuzz, you do this by creating a <emphasis>font</emphasis>
object from your face.
</para>
<para>
Font objects also have the advantage of being considerably
lighter-weight than face objects (remember that a face contains
the contents of a binary font file mapped into memory). As a
result, you can cache and reuse a font object, but you could
also create a new one for each additional size you needed.
Creating new fonts incurs some additional overhead, of course,
but whether or not it is excessive is your call in the end. In
contrast, face objects are substantially larger, and you really
should cache them and reuse them whenever possible.
</para>
<para>
You can create a font object from a face object:
</para>
<programlisting language="C">
hb_font_t* hb_font = hb_font_create(hb_face);
</programlisting>
<para>
After creating a font, there are a few properties you should
set. Many fonts enable and disable hints based on the size it
is used at, so setting this is important for font
objects. <function>hb_font_set_ppem(font, x_ppem,
y_ppem)</function> sets the pixels-per-EM value of the font. You
can also set the point size of the font with
<function>hb_font_set_ppem(font, ptem)</function>. HarfBuzz uses the
industry standard 72 points per inch.
</para>
<para>
HarfBuzz lets you specify the degree subpixel precision you want
through a scaling factor. You can set horizontal and
vertical scaling factors on the
font by calling <function>hb_font_set_scale(font, x_scale,
y_scale)</function>.
</para>
<para>
There may be times when you are handed a font object and need to
access the face object that it comes from. For that, you can call
</para>
<programlisting language="C">
hb_face = hb_font_get_face(hb_font);
</programlisting>
<para>
You can also create a font object from an existing font object
using the <function>hb_font_create_sub_font()</function>
function. This creates a child font object that is initiated
with the same attributes as its parent; it can be used to
quickly set up a new font for the purpose of overriding a specific
font-functions method.
</para>
<para>
All face objects and font objects are lifecycle-managed by
HarfBuzz. After creating a face, you increase its reference
count with <function>hb_face_reference(face)</function> and
decrease it with
<function>hb_face_destroy(face)</function>. Likewise, you
increase the reference count on a font with
<function>hb_font_reference(font)</function> and decrease it
with <function>hb_font_destroy(font)</function>.
</para>
<para>
You can also attach user data to face objects and font objects.
</para>
</section>
<section id="fonts-and-faces-custom-functions">
<title>Customizing font functions</title>
<para>
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.
</para>
<para>
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
<literal>font_funcs</literal> that serves as a vtable for the
font object. The virtual methods in
<literal>font_funcs</literal> are:
</para>
<itemizedlist>
<listitem>
<para>
<function>hb_font_get_font_h_extents_func_t</function>: returns
the extents of the font for horizontal text.
</para>
</listitem>
<listitem>
<para>
<function>hb_font_get_font_v_extents_func_t</function>: returns
the extents of the font for vertical text.
</para>
</listitem>
<listitem>
<para>
<function>hb_font_get_nominal_glyph_func_t</function>: returns
the font's nominal glyph for a given code point.
</para>
</listitem>
<listitem>
<para>
<function>hb_font_get_variation_glyph_func_t</function>: returns
the font's glyph for a given code point when it is followed by a
given Variation Selector.
</para>
</listitem>
<listitem>
<para>
<function>hb_font_get_nominal_glyphs_func_t</function>: returns
the font's nominal glyphs for a series of code points.
</para>
</listitem>
<listitem>
<para>
<function>hb_font_get_glyph_advance_func_t</function>: returns
the advance for a glyph.
</para>
</listitem>
<listitem>
<para>
<function>hb_font_get_glyph_h_advance_func_t</function>: returns
the advance for a glyph for horizontal text.
</para>
</listitem>
<listitem>
<para>
<function>hb_font_get_glyph_v_advance_func_t</function>:returns
the advance for a glyph for vertical text.
</para>
</listitem>
<listitem>
<para>
<function>hb_font_get_glyph_advances_func_t</function>: returns
the advances for a series of glyphs.
</para>
</listitem>
<listitem>
<para>
<function>hb_font_get_glyph_h_advances_func_t</function>: returns
the advances for a series of glyphs for horizontal text .
</para>
</listitem>
<listitem>
<para>
<function>hb_font_get_glyph_v_advances_func_t</function>: returns
the advances for a series of glyphs for vertical text.
</para>
</listitem>
<listitem>
<para>
<function>hb_font_get_glyph_origin_func_t</function>: returns
the origin coordinates of a glyph.
</para>
</listitem>
<listitem>
<para>
<function>hb_font_get_glyph_h_origin_func_t</function>: returns
the origin coordinates of a glyph for horizontal text.
</para>
</listitem>
<listitem>
<para>
<function>hb_font_get_glyph_v_origin_func_t</function>: returns
the origin coordinates of a glyph for vertical text.
</para>
</listitem>
<listitem>
<para>
<function>hb_font_get_glyph_extents_func_t</function>: returns
the extents for a glyph.
</para>
</listitem>
<listitem>
<para>
<function>hb_font_get_glyph_contour_point_func_t</function>:
returns the coordinates of a specific contour point from a glyph.
</para>
</listitem>
<listitem>
<para>
<function>hb_font_get_glyph_name_func_t</function>: returns the
name of a glyph (from its glyph index).
</para>
</listitem>
<listitem>
<para>
<function>hb_font_get_glyph_from_name_func_t</function>: returns
the glyph index that corresponds to a given glyph name.
</para>
</listitem>
</itemizedlist>
<para>
You can fetch the font-functions configuration for a font object
by calling <function>hb_font_get_font_funcs()</function>:
</para>
<programlisting language="C">
hb_font_funcs_t *ffunctions;
ffunctions = hb_font_get_font_funcs (font);
</programlisting>
<para>
The individual methods can each be replaced with their own setter
function, such as
<function>hb_font_funcs_set_nominal_glyph_func(*ffunctions,
func, *user_data, destroy)</function>.
</para>
<para>
Font-functions structures can be reused for multiple font
objects, and can be reference counted with
<function>hb_font_funcs_reference()</function> and
<function>hb_font_funcs_destroy()</function>. Just like other
objects in HarfBuzz, you can set user-data for each
font-functions structure and assign a destroy callback for
it.
</para>
<para>
You can also mark a font-functions structure as immutable,
with <function>hb_font_funcs_make_immutable()</function>. 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.
</para>
</section>
<section id="fonts-and-faces-native-opentype">
<title>Font objects and HarfBuzz's native OpenType implementation</title>
<para>
By default, whenever HarfBuzz creates a font object, it will
configure the font to use a built-in set of font functions that
supports contemporary OpenType font internals. If you want to
work with OpenType or TrueType fonts, you should be able to use
these functions without difficulty.
</para>
<para>
Many of the methods in the font-functions structure deal with
the fundamental properties of glyphs that are required for
shaping text: extents (the maximums and minimums on each axis),
origins (the <literal>(0,0)</literal> coordinate point which
glyphs are drawn in reference to), and advances (the amount that
the cursor needs to be moved after drawing each glyph, including
any empty space for the glyph's side bearings).
</para>
<para>
As you can see in the list of functions, there are separate "horizontal"
and "vertical" variants depending on whether the text is set in
the horizontal or vertical direction. For some scripts, fonts
that are designed to support text set horizontally or vertically (for
example, in Japanese) may include metrics for both text
directions. When fonts don't include this information, HarfBuzz
does its best to transform what the font provides.
</para>
<para>
In addition to the direction-specific functions, HarfBuzz
provides some higher-level functions for fetching information
like extents and advances for a glyph. If you call
</para>
<programlisting language="C">
hb_font_get_glyph_advance_for_direction(font, direction, extents);
</programlisting>
<para>
then you can provide any <type>hb_direction_t</type> as the
<parameter>direction</parameter> parameter, and HarfBuzz will
use the correct function variant for the text direction. There
are similar higher-level versions of the functions for fetching
extents, origin coordinates, and contour-point
coordinates. There are also addition and subtraction functions
for moving points with respect to the origin.
</para>
<para>
There are also methods for fetching the glyph ID that
corresponds to a Unicode code point (possibly when followed by a
variation-selector code point), fetching the glyph name from the
font, and fetching the glyph ID that corresponds to a glyph name
you already have.
</para>
<para>
HarfBuzz also provides functions for converting between glyph
names and string
variables. <function>hb_font_glyph_to_string(font, glyph, s,
size)</function> retrieves the name for the glyph ID
<parameter>glyph</parameter> from the font object. It generates a
generic name of the form <literal>gidDDD</literal> (where DDD is
the glyph index) if there is no name for the glyph in the
font. The <function>hb_font_glyph_from_string(font, s, len,
glyph)</function> takes an input string <parameter>s</parameter>
and looks for a glyph with that name in the font, returning its
glyph ID in the <parameter>glyph</parameter>
output parameter. It automatically parses
<literal>gidDDD</literal> and <literal>uniUUUU</literal> strings.
</para>
</section>
<!-- Commenting out FreeType integration section-holder for now. May move
to the full-blown Integration Chapter. -->
<!-- <section id="fonts-and-faces-freetype">
<title>Using FreeType</title>
<para>
</para>
</section>
<section id="using-harfbuzzs-native-opentype-implementation">
<title>Using HarfBuzz's native OpenType implementation</title>
<para>
</para>
</section>
<section id="using-your-own-font-functions">
<title>Using your own font functions</title>
</section> -->
<section id="fonts-and-faces-variable">
<title>Working with OpenType Variable Fonts</title>
<para>
If you are working with OpenType Variable Fonts, there are a few
additional functions you should use to specify the
variation-axis settings of your font object. Without doing so,
your variable font's font object can still be used, but only at
the default setting for every axis (which, of course, is
sometimes what you want, but does not cover general usage).
</para>
<para>
HarfBuzz manages variation settings in the
<type>hb_variation_t</type> data type, which holds a <property>tag</property> for the
variation-axis identifier tag and a <property>value</property> for its
setting. You can retrieve the list of variation axes in a font
binary from the face object (not from a font object, notably) by
calling <function>hb_ot_var_get_axis_count(face)</function> to
find the number of axes, then using
<function>hb_ot_var_get_axis_infos()</function> to collect the
axis structures:
</para>
<programlisting language="C">
axes = hb_ot_var_get_axis_count(face);
...
hb_ot_var_get_axis_infos(face, 0, axes, axes_array);
</programlisting>
<para>
For each axis returned in the array, you can can access the
identifier in its <property>tag</property>. HarfBuzz also has
tag definitions predefined for the five standard axes specified
in OpenType (<literal>ital</literal> for italic,
<literal>opsz</literal> for optical size,
<literal>slnt</literal> for slant, <literal>wdth</literal> for
width, and <literal>wght</literal> for weight). Each axis also
has a <property>min_value</property>, a
<property>default_value</property>, and a <property>max_value</property>.
</para>
<para>
To set your font object's variation settings, you call the
<function>hb_font_set_variations()</function> function with an
array of <type>hb_variation_t</type> variation settings. Let's
say our font has weight and width axes. We need to specify each
of the axes by tag and assign a value on the axis:
</para>
<programlisting language="C">
unsigned int variation_count = 2;
hb_variation_t variation_data[variation_count];
variation_data[0].tag = HB_OT_TAG_VAR_AXIS_WIDTH;
variation_data[1].tag = HB_OT_TAG_VAR_AXIS_WEIGHT;
variation_data[0].value = 80;
variation_data[1].value = 750;
...
hb_font_set_variations(font, variation_data, variation_count);
</programlisting>
<para>
That should give us a slightly condensed font ("normal" on the
<literal>wdth</literal> axis is 100) at a noticeably bolder
weight ("regular" is 400 on the <literal>wght</literal> axis).
</para>
<para>
In practice, though, you should always check that the value you
want to set on the axis is within the
[<property>min_value</property>,<property>max_value</property>]
range actually implemented in the font's variation axis. After
all, a font might only provide lighter-than-regular weights, and
setting a heavier value on the <literal>wght</literal> axis will
not change that.
</para>
<para>
Once your variation settings are specified on your font object,
however, shaping with a variable font is just like shaping a
static font.
</para>
</section>
</chapter>
</chapter>