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<chapter id="shaping-and-shape-plans">
<title>Shaping and shape plans</title>
Once you have your face and font objects configured as desired and
your input buffer is filled with the characters you need to shape,
all you need to do is call <function>hb_shape()</function>.
HarfBuzz will return the shaped version of the text in the same
buffer that you provided, but it will be in output mode. At that
point, you can iterate through the glyphs in the buffer, drawing
each one at the specified position or handing them off to the
appropriate graphics library.
For the most part, HarfBuzz's shaping step is straightforward from
the outside. But that doesn't mean there will never be cases where
you want to look under the hood and see what is happening on the
inside. HarfBuzz provides facilities for doing that, too.
<section id="shaping-buffer-output">
<title>Shaping and buffer output</title>
The <function>hb_shape()</function> function call takes four arguments: the font
object to use, the buffer of characters to shape, an array of
user-specified features to apply, and the length of that feature
array. The feature array can be NULL, so for the sake of
simplicity we will start with that case.
Internally, HarfBuzz looks at the tables of the font file to
determine where glyph classes, substitutions, and positioning
are defined, using that information to decide which
<emphasis>shaper</emphasis> to use (<literal>ot</literal> for
OpenType fonts, <literal>aat</literal> for Apple Advanced
Typography fonts, and so on). It also looks at the direction,
script, and language properties of the segment to figure out
which script-specific shaping model is needed (at least, in
shapers that support multiple options).
If a font has a GDEF table, then that is used for
glyph classes; if not, HarfBuzz will fall back to Unicode
categorization by code point. If a font has an AAT <literal>morx</literal> table,
then it is used for substitutions; if not, but there is a GSUB
table, then the GSUB table is used. If the font has an AAT
<literal>kerx</literal> table, then it is used for positioning; if not, but
there is a GPOS table, then the GPOS table is used. If neither
table is found, but there is a <literal>kern</literal> table, then HarfBuzz will
use the <literal>kern</literal> table. If there is no <literal>kerx</literal>, no GPOS, and no
<literal>kern</literal>, HarfBuzz will fall back to positioning marks itself.
With a well-behaved OpenType font, you expect GDEF, GSUB, and
GPOS tables to all be applied. HarfBuzz implements the
script-specific shaping models in internal functions, rather
than in the public API.
The algorithms
used for complex scripts can be quite involved; HarfBuzz tries
to be compatible with the OpenType Layout specification
and, wherever there is any ambiguity, HarfBuzz attempts to replicate the
output of Microsoft's Uniscribe engine. See the <ulink
Typography pages</ulink> for more detail.
In general, though, all that you need to know is that
<function>hb_shape()</function> returns the results of shaping
in the same buffer that you provided. The buffer's content type
will now be set to
<literal>HB_BUFFER_CONTENT_TYPE_GLYPHS</literal>, indicating
that it contains shaped output, rather than input text. You can
now extract the glyph information and positioning arrays:
<programlisting language="C">
hb_glyph_info_t *glyph_info = hb_buffer_get_glyph_infos(buf, &amp;glyph_count);
hb_glyph_position_t *glyph_pos = hb_buffer_get_glyph_positions(buf, &amp;glyph_count);
The glyph information array holds a <type>hb_glyph_info_t</type>
for each output glyph, which has two fields:
<parameter>codepoint</parameter> and
<parameter>cluster</parameter>. Whereas, in the input buffer,
the <parameter>codepoint</parameter> field contained the Unicode
code point, it now contains the glyph ID of the corresponding
glyph in the font. The <parameter>cluster</parameter> field is
an integer that you can use to help identify when shaping has
reordered, split, or combined code points; we will say more
about that in the next chapter.
The glyph positions array holds a corresponding
<type>hb_glyph_position_t</type> for each output glyph,
containing four fields: <parameter>x_advance</parameter>,
<parameter>x_offset</parameter>, and
<parameter>y_offset</parameter>. The advances tell you how far
you need to move the drawing point after drawing this glyph,
depending on whether you are setting horizontal text (in which
case you will have x advances) or vertical text (for which you
will have y advances). The x and y offsets tell you where to
move to start drawing the glyph; usually you will have both and
x and a y offset, regardless of the text direction.
Most of the time, you will rely on a font-rendering library or
other graphics library to do the actual drawing of glyphs, so
you will need to iterate through the glyphs in the buffer and
pass the corresponding values off.
<section id="shaping-opentype-features">
<title>OpenType features</title>
OpenType features enable fonts to include smart behavior,
implemented as "lookup" rules stored in the GSUB and GPOS
tables. The OpenType specification defines a long list of
standard features that fonts can use for these behaviors; each
feature has a four-character reserved name and a well-defined
semantic meaning.
Some OpenType features are defined for the purpose of supporting
complex-script shaping, and are automatically activated, but
only when a buffer's script property is set to a script that the
feature supports.
Other features are more generic and can apply to several (or
any) script, and shaping engines are expected to implement
them. By default, HarfBuzz activates several of these features
on every text run. They include <literal>abvm</literal>,
<literal>blwm</literal>, <literal>ccmp</literal>,
<literal>locl</literal>, <literal>mark</literal>,
<literal>mkmk</literal>, and <literal>rlig</literal>.
In addition, if the text direction is horizontal, HarfBuzz
also applies the <literal>calt</literal>,
<literal>clig</literal>, <literal>curs</literal>,
<literal>dist</literal>, <literal>kern</literal>,
<literal>liga</literal> and <literal>rclt</literal>, features.
Additionally, when HarfBuzz encounters a fraction slash
(<literal>U+2044</literal>), it looks backward and forward for decimal
digits (Unicode General Category = Nd), and enables features
<literal>numr</literal> on the sequence before the fraction slash,
<literal>dnom</literal> on the sequence after the fraction slash,
and <literal>frac</literal> on the whole sequence including the fraction
Some script-specific shaping models
(see <xref linkend="opentype-shaping-models" />) disable some of the
features listed above:
Hangul: <literal>calt</literal>
Indic: <literal>liga</literal>
Khmer: <literal>liga</literal>
If the text direction is vertical, HarfBuzz applies
the <literal>vert</literal> feature by default.
Still other features are designed to be purely optional and left
up to the application or the end user to enable or disable as desired.
You can adjust the set of features that HarfBuzz applies to a
buffer by supplying an array of <type>hb_feature_t</type>
features as the third argument to
<function>hb_shape()</function>. For a simple case, let's just
enable the <literal>dlig</literal> feature, which turns on any
"discretionary" ligatures in the font:
<programlisting language="C">
hb_feature_t userfeatures[1];
userfeatures[0].tag = HB_TAG('d','l','i','g');
userfeatures[0].value = 1;
userfeatures[0].start = HB_FEATURE_GLOBAL_START;
userfeatures[0].end = HB_FEATURE_GLOBAL_END;
<literal>HB_FEATURE_GLOBAL_END</literal> and
<literal>HB_FEATURE_GLOBAL_END</literal> are macros we can use
to indicate that the features will be applied to the entire
buffer. We could also have used a literal <literal>0</literal>
for the start and a <literal>-1</literal> to indicate the end of
the buffer (or have selected other start and end positions, if needed).
When we pass the <varname>userfeatures</varname> array to
<function>hb_shape()</function>, any discretionary ligature
substitutions from our font that match the text in our buffer
will get performed:
<programlisting language="C">
hb_shape(font, buf, userfeatures, num_features);
Just like we enabled the <literal>dlig</literal> feature by
setting its <parameter>value</parameter> to
<literal>1</literal>, you would disable a feature by setting its
<parameter>value</parameter> to <literal>0</literal>. Some
features can take other <parameter>value</parameter> settings;
be sure you read the full specification of each feature tag to
understand what it does and how to control it.
<section id="shaping-shaper-selection">
<title>Shaper selection</title>
The basic version of <function>hb_shape()</function> determines
its shaping strategy based on examining the capabilities of the
font file. OpenType font tables cause HarfBuzz to try the
<literal>ot</literal> shaper, while AAT font tables cause HarfBuzz to try the
<literal>aat</literal> shaper.
In the real world, however, a font might include some unusual
mix of tables, or one of the tables might simply be broken for
the script you need to shape. So, sometimes, you might not
want to rely on HarfBuzz's process for deciding what to do, and
just tell <function>hb_shape()</function> what you want it to try.
<function>hb_shape_full()</function> is an alternate shaping
function that lets you supply a list of shapers for HarfBuzz to
try, in order, when shaping your buffer. For example, if you
have determined that HarfBuzz's attempts to work around broken
tables gives you better results than the AAT shaper itself does,
you might move the AAT shaper to the end of your list of
preferences and call <function>hb_shape_full()</function>
<programlisting language="C">
char *shaperprefs[3] = {"ot", "default", "aat"};
hb_shape_full(font, buf, userfeatures, num_features, shaperprefs);
to get results you are happier with.
You may also want to call
<function>hb_shape_list_shapers()</function> to get a list of
the shapers that were built at compile time in your copy of HarfBuzz.
<section id="shaping-plans-and-caching">
<title>Plans and caching</title>
Internally, HarfBuzz uses a structure called a shape plan to
track its decisions about how to shape the contents of a
buffer. The <function>hb_shape()</function> function builds up the shape plan by
examining segment properties and by inspecting the contents of
the font.
This process can involve some decision-making and
trade-offs — for example, HarfBuzz inspects the GSUB and GPOS
lookups for the script and language tags set on the segment
properties, but it falls back on the lookups under the
<literal>DFLT</literal> tag (and sometimes other common tags)
if there are actually no lookups for the tag requested.
HarfBuzz also includes some work-arounds for
handling well-known older font conventions that do not follow
OpenType or Unicode specifications, for buggy system fonts, and for
peculiarities of Microsoft Uniscribe. All of that means that a
shape plan, while not something that you should edit directly in
client code, still might be an object that you want to
inspect. Furthermore, if resources are tight, you might want to
cache the shape plan that HarfBuzz builds for your buffer and
font, so that you do not have to rebuild it for every shaping call.
You can create a cacheable shape plan with
<function>hb_shape_plan_create_cached(face, props,
user_features, num_user_features, shaper_list)</function>, where
<parameter>face</parameter> is a face object (not a font object,
notably), <parameter>props</parameter> is an
<parameter>user_features</parameter> is an array of
<type>hb_feature_t</type>s (with length
<parameter>num_user_features</parameter>), and
<parameter>shaper_list</parameter> is a list of shapers to try.
Shape plans are objects in HarfBuzz, so there are
reference-counting functions and user-data attachment functions
you can
use. <function>hb_shape_plan_reference(shape_plan)</function>
increases the reference count on a shape plan, while
<function>hb_shape_plan_destroy(shape_plan)</function> decreases
the reference count, destroying the shape plan when the last
reference is dropped.
You can attach user data to a shaper (with a key) using the
function, optionally supplying a <function>destroy</function>
callback to use. You can then fetch the user data attached to a
shape plan with
<function>hb_shape_plan_get_user_data(shape_plan, key)</function>.