Sun Mar 2 14:16:17 2003 Owen Taylor <otaylor@redhat.com>

[fontconfig.git] / doc / fontconfig-user.sgml

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<article>
<artheader>
  <title>Fontconfig Users Guide</title>
  <titleabbrev>Fontconfig</titleabbrev>
  <author><firstname>Keith</><surname>Packard</></author>
  <authorinitials>krp</authorinitials>
</artheader>
<sect1><title>NAME</title>
  <para>
  fontconfig - Font configuration and customization library
  </para>
</sect1>
<sect1><title>DESCRIPTION</title>
  <para>
Fontconfig is a library designed to provide system-wide font configuration,
customization and application access.
  </para>
</sect1>
<sect1><title>FUNCTIONAL OVERVIEW</title>
  <para>
Fontconfig contains two essential modules, the configuration module which
builds an internal configuration from XML files and the matching module
which accepts font patterns and returns the nearest matching font.
  </para>
  <sect2><title>FONT CONFIGURATION</title>
    <para>
The configuration module consists of the FcConfig datatype, libexpat and
FcConfigParse which walks over an XML tree and ammends a configuration with
data found within.  From an external perspective, configuration of the
library consists of generating a valid XML tree and feeding that to
FcConfigParse.  The only other mechanism provided to applications for
changing the running configuration is to add fonts and directories to the
list of application-provided font files.  
    </para><para>
The intent is to make font configurations relatively static, and shared by
as many applications as possible.  It is hoped that this will lead to more
stable font selection when passing names from one application to another.
XML was chosen as a configuration file format because it provides a format
which is easy for external agents to edit while retaining the correct
structure and syntax.
    </para><para>
Font configuration is separate from font matching; applications needing to
do their own matching can access the available fonts from the library and
perform private matching.  The intent is to permit applications to pick and
choose appropriate functionality from the library instead of forcing them to
choose between this library and a private configuration mechanism.  The hope
is that this will ensure that configuration of fonts for all applications
can be centralized in one place.  Centralizing font configuration will
simplify and regularize font installation and customization.
    </para>
  </sect2>
  <sect2>
    <title>FONT PROPERTIES</title>
    <para>
While font patterns may contain essentially any properties, there are some
well known properties with associated types.  Fontconfig uses some of these
properties for font matching and font completion.  Others are provided as a
convenience for the applications rendering mechanism.
    </para>
    <table>
      <title>Property Definitions</title>
        <tgroup cols=3 align=left colsep=1 rowsep=1>
        <colspec colname=Property>
        <colspec colname=Type>
        <colspec colname=Description>
        <thead>
          <row>
            <entry>Property</entry>
            <entry>Type</entry>
            <entry>Description</entry>
          </row>
        </thead>
        <tbody>
<row><entry>family</entry><entry>String</entry><entry>Font family name</entry></row>
<row><entry>style</entry><entry>String</entry><entry>Font style. Overrides weight and slant</entry></row>
<row><entry>slant</entry><entry>Int</entry><entry>Italic, oblique or roman</entry></row>
<row><entry>weight</entry><entry>Int</entry><entry>Light, medium, demibold, bold or black</entry></row>
<row><entry>size</entry><entry>Double</entry><entry>Point size</entry></row>
<row><entry>aspect</entry><entry>Double</entry><entry>Stretches glyphs horizontally before hinting</entry></row>
<row><entry>pixelsize</entry><entry>Double</entry><entry>Pixel size</entry></row>
<row><entry>spacing</entry><entry>Int</entry><entry>Proportional, monospace or charcell</entry></row>
<row><entry>foundry</entry><entry>String</entry><entry>Font foundry name</entry></row>
<row><entry>antialias</entry><entry>Bool</entry><entry>Whether glyphs can be antialiased</entry></row>
<row><entry>hinting</entry><entry>Bool</entry><entry>Whether the rasterizer should use hinting</entry></row>
<row><entry>verticallayout</entry><entry>Bool</entry><entry>Use vertical layout</entry></row>
<row><entry>autohint</entry><entry>Bool</entry><entry>Use autohinter instead of normal hinter</entry></row>
<row><entry>globaladvance</entry><entry>Bool</entry><entry>Use font global advance data</entry></row>
<row><entry>file</entry><entry>String</entry><entry>The filename holding the font</entry></row>
<row><entry>index</entry><entry>Int</entry><entry>The index of the font within the file</entry></row>
<row><entry>ftface</entry><entry>FT_Face</entry><entry>Use the specified FreeType face object</entry></row>
<row><entry>rasterizer</entry><entry>String</entry><entry>Which rasterizer is in use</entry></row>
<row><entry>outline</entry><entry>Bool</entry><entry>Whether the glyphs are outlines</entry></row>
<row><entry>scalable</entry><entry>Bool</entry><entry>Whether glyphs can be scaled</entry></row>
<row><entry>scale</entry><entry>Double</entry><entry>Scale factor for point->pixel conversions</entry></row>
<row><entry>dpi</entry><entry>Double</entry><entry>Target dots per inch</entry></row>
<row><entry>rgba</entry><entry>Int</entry><entry>unknown, rgb, bgr, vrgb, vbgr, none - subpixel geometry</entry></row>
<row><entry>minspace</entry><entry>Bool</entry><entry>Eliminate leading from line spacing</entry></row>
<row><entry>charset</entry><entry>CharSet</entry><entry>Unicode chars encoded by the font</entry></row>
<row><entry>lang</entry><entry>String</entry><entry>List of RFC-3066-style languages this font supports</entry></row>
      </tbody>
      </tgroup>
    </table>
  </sect2>
  <sect2>
  <title>FONT MATCHING</title>
    <para>
Fontconfig performs matching by measuring the distance from a provided
pattern to all of the available fonts in the system.  The closest matching
font is selected.  This ensures that a font will always be returned, but
doesn't ensure that it is anything like the requested pattern.
    </para><para> 
Font matching starts with an application constructed pattern.  The desired
attributes of the resulting font are collected together in a pattern.  Each
property of the pattern can contain one or more values; these are listed in
priority order; matches earlier in the list are considered "closer" than
matches later in the list.
    </para><para>
The initial pattern is modified by applying the list of editing instructions
specific to patterns found in the configuration; each consists of a match
predicate and a set of editing operations.  They are executed in the order
they appeared in the configuration.  Each match causes the associated
sequence of editing operations to be applied.
    </para><para>
After the pattern has been edited, a sequence of default substitutions are
performed to canonicalize the set of available properties; this avoids the
need for the lower layers to constantly provide default values for various
font properties during rendering.
    </para><para>
The canonical font pattern is finally matched against all available fonts.
The distance from the pattern to the font is measured for each of several
properties: foundry, charset, family, lang, spacing, pixelsize, style,
slant, weight, antialias, rasterizer and outline.  This list is in priority
order -- results of comparing earlier elements of this list weigh more
heavily than later elements.
    </para><para>
There is one special case to this rule; family names are split into two
bindings; strong and weak.  Strong family names are given greater precedence
in the match than lang elements while weak family names are given lower
precedence than lang elements.  This permits the document language to drive
font selection when any document specified font is unavailable.
    </para><para>
The pattern representing that font is augmented to include any properties
found in the pattern but not found in the font itself; this permits the
application to pass rendering instructions or any other data through the
matching system.  Finally, the list of editing instructions specific to
fonts found in the configuration are applied to the pattern.  This modified
pattern is returned to the application.
    </para><para>
The return value contains sufficient information to locate and rasterize the
font, including the file name, pixel size and other rendering data.  As
none of the information involved pertains to the FreeType library,
applications are free to use any rasterization engine or even to take
the identified font file and access it directly.
    </para><para>
The match/edit sequences in the configuration are performed in two passes
because there are essentially two different operations necessary -- the
first is to modify how fonts are selected; aliasing families and adding
suitable defaults.  The second is to modify how the selected fonts are
rasterized.  Those must apply to the selected font, not the original pattern
as false matches will often occur.
    </para>
  </sect2>
  <sect2><title>FONT NAMES</title>
    <para>
Fontconfig provides a textual representation for patterns that the library
can both accept and generate.  The representation is in three parts, first a
list of family names, second a list of point sizes and finally a list of
additional properties:
    </para>
    <programlisting>
        &lt;families&gt;-&lt;point sizes&gt;:&lt;name1&gt;=&lt;values1&gt;:&lt;name2&gt;=&lt;values2&gt;...
    </programlisting>
    <para>
Values in a list are separated with commas.  The name needn't include either
families or point sizes; they can be elided.  In addition, there are
symbolic constants that simultaneously indicate both a name and a value.
Here are some examples:
    </para>
    <table colsep=0 rowsep=0>
      <title>Sample Font Names</title>
      <tgroup cols=2 align=left colsep=0 rowsep=0>
        <thead><row>
          <entry>Name</entry>
          <entry>Meaning</entry>
        </row></thead>
        <tbody>
<row><entry>Times-12</entry><entry>12 point Times Roman</entry></row>
<row><entry>Times-12:bold</entry><entry>12 point Times Bold</entry></row>
<row><entry>Courier:italic</entry><entry>Courier Italic in the default size</entry></row>
<row><entry>Monospace:matrix=1 .1 0 1</entry><entry>The users preferred monospace font
with artificial obliquing</entry></row>
        </tbody>
      </tgroup>
    </table>
  </sect2>
</sect1>
<sect1><title>LANG TAGS</title>
  <para>
Each font in the database contains a list of languages it supports.  This is
computed by comparing the Unicode coverage of the font with the orthography
of each language.  Languages are tagged using an RFC-3066 compatible naming
and occur in two parts -- the ISO639 language tag followed a hyphen and then
by the ISO 3166 country code.  The hyphen and country code may be elided.
  </para><para>
Fontconfig has orthographies for several languages built into the library.
No provision has been made for adding new ones aside from rebuilding the
library.  It currently supports 122 of the 139 languages named in ISO 639-1,
141 of the languages with two-letter codes from ISO 639-2 and another 30
languages with only three-letter codes.
  </para>
</sect1>
<sect1><title>CONFIGURATION FILE FORMAT</title>
  <para>
Configuration files for fontconfig are stored in XML format; this
format makes external configuration tools easier to write and ensures that
they will generate syntactically correct configuration files.  As XML
files are plain text, they can also be manipulated by the expert user using
a text editor.
  </para><para>
The fontconfig document type definition resides in the external entity
"fonts.dtd"; this is normally stored in the default font configuration
directory (/etc/fonts).  Each configuration file should contain the
following structure:
    <programlisting>
        &lt;?xml version="1.0"?&gt;
        &lt;!DOCTYPE fontconfig SYSTEM "fonts.dtd"&gt;
        &lt;fontconfig&gt;
        ...
        &lt;/fontconfig&gt;
    </programlisting>
  </para>
  <sect2><title>&lt;fontconfig&gt;</title><para>
This is the top level element for a font configuration and can contain
&lt;dir&gt;, &lt;cache&gt;, &lt;include&gt;, &lt;match&gt; and &lt;alias&gt; elements in any order.
  </para></sect2>
  <sect2><title>&lt;dir&gt;</title><para>
This element contains a directory name which will be scanned for font files
to include in the set of available fonts.
  </para></sect2>
  <sect2><title>&lt;cache&gt;</title><para>
This element contains a file name for the per-user cache of font
information.  If it starts with '~', it refers to a file in the users
home directory.  This file is used to hold information about fonts that
isn't present in the per-directory cache files.  It is automatically
maintained by the fontconfig library.  The default for this file 
is ``~/.fonts.cache-&lt;version&gt;'', where &lt;version&gt; is the font configuration
file version number (currently 1).
  </para></sect2>
  <sect2><title>&lt;include ignore_missing="no"&gt;</title><para>
This element contains the name of an additional configuration file.  When
the XML datatype is traversed by FcConfigParse, the contents of the file
will also be incorporated into the configuration by passing the filename to
FcConfigLoadAndParse.  If 'ignore_missing' is set to "yes" instead of the
default "no", a missing file will elicit no warning message from the library.
  </para></sect2>
  <sect2><title>&lt;config&gt;</title><para>
This element provides a place to consolodate additional configuration
information.  &lt;config&gt; can contain &lt;blank&gt; and &lt;rescan&gt; elements in any
order.
  </para></sect2>
  <sect2><title>&lt;blank&gt;</title><para>
Fonts often include "broken" glyphs which appear in the encoding but are
drawn as blanks on the screen.  Within the &lt;blank&gt; element, place each
Unicode characters which is supposed to be blank in an &lt;int&gt; element.
Characters outside of this set which are drawn as blank will be elided from
the set of characters supported by the font.
  </para></sect2>
  <sect2><title>&lt;rescan&gt;</title><para>
The &lt;rescan&gt; element holds an &lt;int&gt; element which indicates the default
interval between automatic checks for font configuration changes.
Fontconfig will validate all of the configuration files and directories and
automatically rebuild the internal datastructures when this interval passes.
  </para></sect2>
  <sect2><title>&lt;match target="pattern"&gt;</title><para>
This element holds first a (possibly empty) list of &lt;test&gt; elements and then
a (possibly empty) list of &lt;edit&gt; elements.  Patterns which match all of the
tests are subjected to all the edits.  If 'target' is set to "font" instead
of the default "pattern", then this element applies to the font name
resulting from a match rather than a font pattern to be matched.
  </para></sect2>
  <sect2><title>&lt;test qual="any" name="property" compare="eq"&gt;</title><para>
This element contains a single value which is compared with the pattern
property "property" (substitute any of the property names seen 
above). 'compare' can be one of "eq", "not_eq", "less", "less_eq", "more", or
"more_eq".  'qual' may either be the default, "any", in which case the match
succeeds if any value associated with the property matches the test value, or
"all", in which case all of the values associated with the property must
match the test value.
  </para></sect2>
  <sect2><title>&lt;edit name="property" mode="assign" binding="weak"&gt;</title><para>
This element contains a list of expression elements (any of the value or
operator elements).  The expression elements are evaluated at run-time and
modify the property "property".  The modification depends on whether
"property" was matched by one of the associated &lt;test&gt; elements, if so, the
modification may affect the first matched value.  Any values inserted into
the property are given the indicated binding. 'mode' is one of:
    <table>
      <title>Edit Element Modes</title>
        <tgroup cols=3 align=left colsep=0 rowsep=0>
        <thead>
          <row>
            <entry>Mode</entry>
            <entry>Operation With Match</entry>
            <entry>Operation Without Match</entry>
          </row>
        </thead>
        <tbody>
<row><entry>"assign"</entry><entry>Replace matching value</entry><entry>Replace all values</entry></row>
<row><entry>"assign_replace"</entry><entry>Replace all values</entry><entry>Replace all values</entry></row>
<row><entry>"prepend"</entry><entry>Insert before matching value</entry><entry>Insert at head of list</entry></row>
<row><entry>"prepend_first"</entry><entry>Insert at head of list</entry><entry>Insert at head of list</entry></row>
<row><entry>"append"</entry><entry>Append after matching value</entry><entry>Append at end of list</entry></row>
<row><entry>"append_last"</entry><entry>Append at end of list</entry><entry>Append at end of list</entry></row>
        </tbody>
      </tgroup>
    </table>
  </para></sect2>
  <sect2><title>&lt;int&gt;, &lt;double&gt;, &lt;string&gt;, &lt;bool&gt;</title><para>
These elements hold a single value of the indicated type.  &lt;bool&gt; elements
hold either true or false.
  </para></sect2>
  <sect2><title>&lt;matrix&gt;</title><para>
This element holds the four &lt;double&gt; elements of an affine
transformation.
  </para></sect2>
  <sect2><title>&lt;name&gt;</title><para>
Holds a property name.  Evaluates to the first value from the property of
the font, not the pattern.
  </para></sect2>
  <sect2><title>&lt;const&gt;</title><para>
Holds the name of a constant; these are always integers and serve as
symbolic names for common font values:
    <table>
      <title>Symbolic Constants</title>
        <tgroup cols=3 align=left colsep=0 rowsep=0>
        <thead>
          <row>
            <entry>Constant</entry>
            <entry>Property</entry>
            <entry>CPP Symbol</entry>
          </row>
        </thead>
        <tbody>
<row><entry>light</entry><entry>weight</entry></row>
<row><entry>medium</entry><entry>weight</entry></row>
<row><entry>demibold</entry><entry>weight</entry></row>
<row><entry>bold</entry><entry>weight</entry></row>
<row><entry>black</entry><entry>weight</entry></row>
<row><entry>roman</entry><entry>slant</entry></row>
<row><entry>italic</entry><entry>slant</entry></row>
<row><entry>oblique</entry><entry>slant</entry></row>
<row><entry>proportional</entry><entry>spacing</entry></row>
<row><entry>mono</entry><entry>spacing</entry></row>
<row><entry>charcell</entry><entry>spacing</entry></row>
<row><entry>unknown</entry><entry>rgba</entry></row>
<row><entry>rgb</entry><entry>rgba</entry></row>
<row><entry>bgr</entry><entry>rgba</entry></row>
<row><entry>vrgb</entry><entry>rgba</entry></row>
<row><entry>vbgr</entry><entry>rgba</entry></row>
<row><entry>none</entry><entry>rgba</entry></row>
        </tbody>
      </tgroup>
    </table>
      </para>
    </sect2>
  <sect2><title>&lt;or&gt;,
                &lt;and&gt;,
                &lt;plus&gt;,
                &lt;minus&gt;,
                &lt;times&gt;,
                &lt;divide&gt;</title><para>
These elements perform the specified operation on a list of expression
elements.  &lt;or&gt; and &lt;and&gt; are boolean, not bitwise.
      </para>
    </sect2>
  <sect2><title>&lt;eq&gt;,
                &lt;not_eq&gt;,
                &lt;less&gt;,
                &lt;less_eq&gt;,
                &lt;more&gt;,
                &lt;more_eq&gt;</title><para>
These elements compare two values, producing a boolean result.
  </para></sect2>
  <sect2><title>&lt;not&gt;</title><para>
Inverts the boolean sense of its one expression element
  </para></sect2>
  <sect2><title>&lt;if&gt;</title><para>
This element takes three expression elements; if the value of the first is
true, it produces the value of the second, otherwise it produces the value
of the third.
  </para></sect2>
  <sect2><title>&lt;alias&gt;</title><para>
Alias elements provide a shorthand notation for the set of common match
operations needed to substitute one font family for another.  They contain a
&lt;family&gt; element followed by optional &lt;prefer&gt;, &lt;accept&gt; and &lt;default&gt;
elements.  Fonts matching the &lt;family&gt; element are edited to prepend the
list of &lt;prefer&gt;ed families before the matching &lt;family&gt;, append the
&lt;accept&gt;able familys after the matching &lt;family&gt; and append the &lt;default&gt;
families to the end of the family list.
  </para></sect2>
  <sect2><title>&lt;family&gt;</title><para>
Holds a single font family name
  </para></sect2>
  <sect2><title>&lt;prefer&gt;, &lt;accept&gt;, &lt;default&gt;</title><para>
These hold a list of &lt;family&gt; elements to be used by the &lt;alias&gt; element.
&lt;/article&gt;
  </para></sect2>
</sect1>
<sect1><title>EXAMPLE CONFIGURATION FILE</title>
  <sect2><title>System configuration file</title>
    <para>
This is an example of a system-wide configuration file
    </para>
    <programlisting>
&lt;?xml version="1.0"?&gt;
&lt;!DOCTYPE fontconfig SYSTEM "fonts.dtd"&gt;
&lt;!-- /etc/fonts/fonts.conf file to configure system font access --&gt;
&lt;fontconfig&gt;
&lt;!-- 
        Find fonts in these directories
--&gt;
&lt;dir&gt;/usr/X11R6/lib/X11/fonts/truetype&lt;/dir&gt;
&lt;dir&gt;/usr/X11R6/lib/X11/fonts/Type1&lt;/dir&gt;

&lt;!--
        Accept deprecated 'mono' alias, replacing it with 'monospace'
--&gt;
&lt;match target="pattern"&gt;
        &lt;test qual="any" name="family"&gt;&lt;string&gt;mono&lt;/string&gt;&lt;/test&gt;
        &lt;edit name="family" mode="assign"&gt;&lt;string&gt;monospace&lt;/string&gt;&lt;/edit&gt;
&lt;/match&gt;

&lt;!--
        Names not including any well known alias are given 'sans'
--&gt;
&lt;match target="pattern"&gt;
        &lt;test qual="all" name="family" mode="not_eq"&gt;sans&lt;/test&gt;
        &lt;test qual="all" name="family" mode="not_eq"&gt;serif&lt;/test&gt;
        &lt;test qual="all" name="family" mode="not_eq"&gt;monospace&lt;/test&gt;
        &lt;edit name="family" mode="append_last"&gt;&lt;string&gt;sans&lt;/string&gt;&lt;/edit&gt;
&lt;/match&gt;

&lt;!--
        Load per-user customization file, but don't complain
        if it doesn't exist
--&gt;
&lt;include ignore_missing="yes"&gt;~/.fonts.conf&lt;/include&gt;

&lt;!--
        Alias well known font names to available TrueType fonts.
        These substitute TrueType faces for similar Type1
        faces to improve screen appearance.
--&gt;
&lt;alias&gt;
        &lt;family&gt;Times&lt;/family&gt;
        &lt;prefer&gt;&lt;family&gt;Times New Roman&lt;/family&gt;&lt;/prefer&gt;
        &lt;default&gt;&lt;family&gt;serif&lt;/family&gt;&lt;/default&gt;
&lt;/alias&gt;
&lt;alias&gt;
        &lt;family&gt;Helvetica&lt;/family&gt;
        &lt;prefer&gt;&lt;family&gt;Verdana&lt;/family&gt;&lt;/prefer&gt;
        &lt;default&gt;&lt;family&gt;sans&lt;/family&gt;&lt;/default&gt;
&lt;/alias&gt;
&lt;alias&gt;
        &lt;family&gt;Courier&lt;/family&gt;
        &lt;prefer&gt;&lt;family&gt;Courier New&lt;/family&gt;&lt;/prefer&gt;
        &lt;default&gt;&lt;family&gt;monospace&lt;/family&gt;&lt;/default&gt;
&lt;/alias&gt;

&lt;!--
        Provide required aliases for standard names
        Do these after the users configuration file so that
        any aliases there are used preferentially
--&gt;
&lt;alias&gt;
        &lt;family&gt;serif&lt;/family&gt;
        &lt;prefer&gt;&lt;family&gt;Times New Roman&lt;/family&gt;&lt;/prefer&gt;
&lt;/alias&gt;
&lt;alias&gt;
        &lt;family&gt;sans&lt;/family&gt;
        &lt;prefer&gt;&lt;family&gt;Verdana&lt;/family&gt;&lt;/prefer&gt;
&lt;/alias&gt;
&lt;alias&gt;
        &lt;family&gt;monospace&lt;/family&gt;
        &lt;prefer&gt;&lt;family&gt;Andale Mono&lt;/family&gt;&lt;/prefer&gt;
&lt;/alias&gt;
&lt;/fontconfig&gt;
    </programlisting>
  </sect2>
  <sect2><title>User configuration file</title>
    <para>
This is an example of a per-user configuration file that lives in
~/.fonts.conf
    </para>
    <programlisting>
&lt;?xml version="1.0"?&gt;
&lt;!DOCTYPE fontconfig SYSTEM "fonts.dtd"&gt;
&lt;!-- ~/.fonts.conf for per-user font configuration --&gt;
&lt;fontconfig&gt;

&lt;!--
        Private font directory
--&gt;
&lt;dir&gt;~/misc/fonts&lt;/dir&gt;

&lt;!--
        use rgb sub-pixel ordering to improve glyph appearance on
        LCD screens.  Changes affecting rendering, but not matching
        should always use target="font".
--&gt;
&lt;match target="font"&gt;
        &lt;edit name="rgba" mode="assign"&gt;&lt;const&gt;rgb&lt;/const&gt;&lt;/edit&gt;
&lt;/match&gt;
&lt;/fontconfig&gt;
    </programlisting>
  </sect2>
</sect1>
<sect1><title>FILES</title>
  <para>
<emphasis>fonts.conf</emphasis>
contains configuration information for the fontconfig library
consisting of directories to look at for font information as well as
instructions on editing program specified font patterns before attempting to
match the available fonts.  It is in xml format.
  </para>
  <para>
<emphasis>fonts.dtd</emphasis>
is a DTD that describes the format of the configuration files.
  </para>
  <para>
<emphasis>~/.fonts.conf</emphasis>
is the conventional location for per-user font configuration, although the
actual location is specified in the global fonts.conf file.
  </para>
  <para>
<emphasis> ~/.fonts.cache-*</emphasis>
is the conventional repository of font information that isn't found in the
per-directory caches.  This file is automatically maintained by fontconfig.
  </para>
</sect1>
</article>