<para>
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 "morx" table,
+ 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
- "kerx" table, then it is used for positioning; if not, but
+ <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 "kern" table, then HarfBuzz will
- use the "kern" table. If there is no "kerx", no GPOS, and no
- "kern", HarfBuzz will fall back to positioning marks itself.
+ 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.
</para>
<para>
With a well-behaved OpenType font, you expect GDEF, GSUB, and
also applies the <literal>calt</literal>,
<literal>clig</literal>, <literal>curs</literal>,
<literal>dist</literal>, <literal>kern</literal>,
- <literal>liga</literal>, <literal>rclt</literal>,
- and <literal>frac</literal> features.
+ <literal>liga</literal> and <literal>rclt</literal>, features.
</para>
<para>
+ 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
+ slash.
+ </para>
+ <para>
+ Some script-specific shaping models
+ (see <xref linkend="opentype-shaping-models" />) disable some of the
+ features listed above:
+ </para>
+ <itemizedlist>
+ <listitem>
+ <para>
+ Hangul: <literal>calt</literal>
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Indic: <literal>liga</literal>
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Khmer: <literal>liga</literal>
+ </para>
+ </listitem>
+ </itemizedlist>
+ <para>
If the text direction is vertical, HarfBuzz applies
the <literal>vert</literal> feature by default.
</para>