xref: /xsrc/external/mit/fontconfig/dist/doc/fontconfig-user.txt

                               fonts-conf

Name

   fonts.conf -- Font configuration files

Synopsis

   /etc/fonts/fonts.conf
   /etc/fonts/fonts.dtd
   /etc/fonts/conf.d
   $XDG_CONFIG_HOME/fontconfig/conf.d
   $XDG_CONFIG_HOME/fontconfig/fonts.conf
   ~/.fonts.conf.d
   ~/.fonts.conf

Description

   Fontconfig is a library designed to provide system-wide font
   configuration, customization and application access.

Functional Overview

   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.

Font Configuration

   The configuration module consists of the FcConfig datatype,
   libexpat and FcConfigParse which walks over an XML tree and
   amends 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.

   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.

   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.

Font Properties

   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.
  Property        Type    Description
  --------------------------------------------------------------
  family          String  Font family names
  familylang      String  Languages corresponding to each family
  style           String  Font style. Overrides weight and slant
  stylelang       String  Languages corresponding to each style
  fullname        String  Font full names (often includes style)
  fullnamelang    String  Languages corresponding to each fullname
  slant           Int     Italic, oblique or roman
  weight          Int     Light, medium, demibold, bold or black
  size            Double  Point size
  width           Int     Condensed, normal or expanded
  aspect          Double  Stretches glyphs horizontally before hinting
  pixelsize       Double  Pixel size
  spacing         Int     Proportional, dual-width, monospace or charcel
l
  foundry         String  Font foundry name
  antialias       Bool    Whether glyphs can be antialiased
  hinting         Bool    Whether the rasterizer should use hinting
  hintstyle       Int     Automatic hinting style
  verticallayout  Bool    Use vertical layout
  autohint        Bool    Use autohinter instead of normal hinter
  globaladvance   Bool    Use font global advance data (deprecated)
  file            String  The filename holding the font
  index           Int     The index of the font within the file
  ftface          FT_Face Use the specified FreeType face object
  rasterizer      String  Which rasterizer is in use (deprecated)
  outline         Bool    Whether the glyphs are outlines
  scalable        Bool    Whether glyphs can be scaled
  color           Bool    Whether any glyphs have color
  scale           Double  Scale factor for point->pixel conversions (dep
recated)
  dpi             Double  Target dots per inch
  rgba            Int     unknown, rgb, bgr, vrgb, vbgr,
                          none - subpixel geometry
  lcdfilter       Int     Type of LCD filter
  minspace        Bool    Eliminate leading from line spacing
  charset         CharSet Unicode chars encoded by the font
  lang            String  List of RFC-3066-style languages this
                          font supports
  fontversion     Int     Version number of the font
  capability      String  List of layout capabilities in the font
  fontformat      String  String name of the font format
  embolden        Bool    Rasterizer should synthetically embolden the f
ont
  embeddedbitmap  Bool    Use the embedded bitmap instead of the outline
  decorative      Bool    Whether the style is a decorative variant
  fontfeatures    String  List of the feature tags in OpenType to be ena
bled
  namelang        String  Language name to be used for the default value
 of
                          familylang, stylelang, and fullnamelang
  prgname         String  String  Name of the running program
  postscriptname  String  Font family name in PostScript
  fonthashint     Bool    Whether the font has hinting
  order           Int     Order number of the font

Font Matching

   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.

   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.

   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.

   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.

   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.

   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.

   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.

   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.

   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.

Font Names

   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:
        <families>-<point sizes>:<name1>=<values1>:<name2>=<values2>...

   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:
  Name                            Meaning
  ----------------------------------------------------------
  Times-12                        12 point Times Roman
  Times-12:bold                   12 point Times Bold
  Courier:italic                  Courier Italic in the default size
  Monospace:matrix=1 .1 0 1       The users preferred monospace font
                                  with artificial obliquing

   The '\', '-', ':' and ',' characters in family names must be
   preceded by a '\' character to avoid having them
   misinterpreted. Similarly, values containing '\', '=', '_', ':'
   and ',' must also have them preceded by a '\' character. The
   '\' characters are stripped out of the family name and values
   as the font name is read.

Debugging Applications

   To help diagnose font and applications problems, fontconfig is
   built with a large amount of internal debugging left enabled.
   It is controlled by means of the FC_DEBUG environment variable.
   The value of this variable is interpreted as a number, and each
   bit within that value controls different debugging messages.
  Name         Value    Meaning
  ---------------------------------------------------------
  MATCH            1    Brief information about font matching
  MATCHV           2    Extensive font matching information
  EDIT             4    Monitor match/test/edit execution
  FONTSET          8    Track loading of font information at startup
  CACHE           16    Watch cache files being written
  CACHEV          32    Extensive cache file writing information
  PARSE           64    (no longer in use)
  SCAN           128    Watch font files being scanned to build caches
  SCANV          256    Verbose font file scanning information
  MEMORY         512    Monitor fontconfig memory usage
  CONFIG        1024    Monitor which config files are loaded
  LANGSET       2048    Dump char sets used to construct lang values
  MATCH2        4096    Display font-matching transformation in patterns

   Add the value of the desired debug levels together and assign
   that (in base 10) to the FC_DEBUG environment variable before
   running the application. Output from these statements is sent
   to stdout.

Lang Tags

   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 ISO 639 language tag followed a hyphen and then by
   the ISO 3166 country code. The hyphen and country code may be
   elided.

   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. Languages with both two and three
   letter codes are provided with only the two letter code.

   For languages used in multiple territories with radically
   different character sets, fontconfig includes per-territory
   orthographies. This includes Azerbaijani, Kurdish, Pashto,
   Tigrinya and Chinese.

Configuration File Format

   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.

   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:
        <?xml version="1.0"?>
        <!DOCTYPE fontconfig SYSTEM "urn:fontconfig:fonts.dtd">
        <fontconfig>
        ...
        </fontconfig>

<fontconfig>

   This is the top level element for a font configuration and can
   contain <dir>, <cachedir>, <include>, <match> and <alias>
   elements in any order.

<dir prefix="default" salt="">

   This element contains a directory name which will be scanned
   for font files to include in the set of available fonts.

   If 'prefix' is set to "default" or "cwd", the current working
   directory will be added as the path prefix prior to the value.
   If 'prefix' is set to "xdg", the value in the XDG_DATA_HOME
   environment variable will be added as the path prefix. please
   see XDG Base Directory Specification for more details. If
   'prefix' is set to "relative", the path of current file will be
   added prior to the value.

   'salt' property affects to determine cache filename. this is
   useful for example when having different fonts sets on same
   path at container and share fonts from host on different font
   path.

<cachedir prefix="default">

   This element contains a directory name that is supposed to be
   stored or read the cache of font information. If multiple
   elements are specified in the configuration file, the directory
   that can be accessed first in the list will be used to store
   the cache files. If it starts with '~', it refers to a
   directory in the users home directory. If 'prefix' is set to
   "xdg", the value in the XDG_CACHE_HOME environment variable
   will be added as the path prefix. please see XDG Base Directory
   Specification for more details. The default directory is
   ``$XDG_CACHE_HOME/fontconfig'' and it contains the cache files
   named ``<hash value>-<architecture>.cache-<version>'', where
   <version> is the fontconfig cache file version number
   (currently 7).

<include ignore_missing="no" prefix="default">

   This element contains the name of an additional configuration
   file or directory. If a directory, every file within that
   directory starting with an ASCII digit (U+0030 - U+0039) and
   ending with the string ``.conf'' will be processed in sorted
   order. When the XML datatype is traversed by FcConfigParse, the
   contents of the file(s) will also be incorporated into the
   configuration by passing the filename(s) to
   FcConfigLoadAndParse. If 'ignore_missing' is set to "yes"
   instead of the default "no", a missing file or directory will
   elicit no warning message from the library. If 'prefix' is set
   to "xdg", the value in the XDG_CONFIG_HOME environment variable
   will be added as the path prefix. please see XDG Base Directory
   Specification for more details.

<config>

   This element provides a place to consolidate additional
   configuration information. <config> can contain <blank> and
   <rescan> elements in any order.

<description domain="fontconfig-conf">

   This element is supposed to hold strings which describe what a
   config is used for. This string can be translated through
   gettext. 'domain' needs to be set the proper name to apply
   then. fontconfig will tries to retrieve translations with
   'domain' from gettext.

<blank>

   Fonts often include "broken" glyphs which appear in the
   encoding but are drawn as blanks on the screen. Within the
   <blank> element, place each Unicode characters which is
   supposed to be blank in an <int> element. Characters outside of
   this set which are drawn as blank will be elided from the set
   of characters supported by the font.

<remap-dir prefix="default" as-path="" salt="">

   This element contains a directory name where will be mapped as
   the path 'as-path' in cached information. This is useful if the
   directory name is an alias (via a bind mount or symlink) to
   another directory in the system for which cached font
   information is likely to exist.

   'salt' property affects to determine cache filename as same as
   <dir> element.

<reset-dirs />

   This element removes all of fonts directories where added by
   <dir> elements. This is useful to override fonts directories
   from system to own fonts directories only.

<rescan>

   The <rescan> element holds an <int> 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.

<selectfont>

   This element is used to black/white list fonts from being
   listed or matched against. It holds acceptfont and rejectfont
   elements.

<acceptfont>

   Fonts matched by an acceptfont element are "whitelisted"; such
   fonts are explicitly included in the set of fonts used to
   resolve list and match requests; including them in this list
   protects them from being "blacklisted" by a rejectfont element.
   Acceptfont elements include glob and pattern elements which are
   used to match fonts.

<rejectfont>

   Fonts matched by an rejectfont element are "blacklisted"; such
   fonts are excluded from the set of fonts used to resolve list
   and match requests as if they didn't exist in the system.
   Rejectfont elements include glob and pattern elements which are
   used to match fonts.

<glob>

   Glob elements hold shell-style filename matching patterns
   (including ? and *) which match fonts based on their complete
   pathnames. This can be used to exclude a set of directories
   (/usr/share/fonts/uglyfont*), or particular font file types
   (*.pcf.gz), but the latter mechanism relies rather heavily on
   filenaming conventions which can't be relied upon. Note that
   globs only apply to directories, not to individual fonts.

<pattern>

   Pattern elements perform list-style matching on incoming fonts;
   that is, they hold a list of elements and associated values. If
   all of those elements have a matching value, then the pattern
   matches the font. This can be used to select fonts based on
   attributes of the font (scalable, bold, etc), which is a more
   reliable mechanism than using file extensions. Pattern elements
   include patelt elements.

<patelt name="property">

   Patelt elements hold a single pattern element and list of
   values. They must have a 'name' attribute which indicates the
   pattern element name. Patelt elements include int, double,
   string, matrix, bool, charset and const elements.

<match target="pattern">

   This element holds first a (possibly empty) list of <test>
   elements and then a (possibly empty) list of <edit> 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. If
   'target' is set to "scan", then this element applies when the
   font is scanned to build the fontconfig database.

<test qual="any" name="property" target="default" compare="eq">

   This element contains a single value which is compared with the
   target ('pattern', 'font', 'scan' or 'default') property
   "property" (substitute any of the property names seen above).
   'compare' can be one of "eq", "not_eq", "less", "less_eq",
   "more", "more_eq", "contains" or "not_contains". '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. 'ignore-blanks'
   takes a boolean value. if 'ignore-blanks' is set "true", any
   blanks in the string will be ignored on its comparison. this
   takes effects only when compare="eq" or compare="not_eq". When
   used in a <match target="font"> element, the target= attribute
   in the <test> element selects between matching the original
   pattern or the font. "default" selects whichever target the
   outer <match> element has selected.

<edit name="property" mode="assign" binding="weak">

   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 <test> elements, if so, the modification may
   affect the first matched value. Any values inserted into the
   property are given the indicated binding ("strong", "weak" or
   "same") with "same" binding using the value from the matched
   pattern element. 'mode' is one of:
  Mode                    With Match              Without Match
  ---------------------------------------------------------------------
  "assign"                Replace matching value  Replace all values
  "assign_replace"        Replace all values      Replace all values
  "prepend"               Insert before matching  Insert at head of list
  "prepend_first"         Insert at head of list  Insert at head of list
  "append"                Append after matching   Append at end of list
  "append_last"           Append at end of list   Append at end of list
  "delete"                Delete matching value   Delete all values
  "delete_all"            Delete all values       Delete all values

<int>, <double>, <string>, <bool>

   These elements hold a single value of the indicated type.
   <bool> elements hold either true or false. An important
   limitation exists in the parsing of floating point numbers --
   fontconfig requires that the mantissa start with a digit, not a
   decimal point, so insert a leading zero for purely fractional
   values (e.g. use 0.5 instead of .5 and -0.5 instead of -.5).

<matrix>

   This element holds four numerical expressions of an affine
   transformation. At their simplest these will be four <double>
   elements but they can also be more involved expressions.

<range>

   This element holds the two <int> elements of a range
   representation.

<charset>

   This element holds at least one <int> element of an Unicode
   code point or more.

<langset>

   This element holds at least one <string> element of a
   RFC-3066-style languages or more.

<name>

   Holds a property name. Evaluates to the first value from the
   property of the pattern. If the 'target' attribute is not
   present, it will default to 'default', in which case the
   property is returned from the font pattern during a
   target="font" match, and to the pattern during a
   target="pattern" match. The attribute can also take the values
   'font' or 'pattern' to explicitly choose which pattern to use.
   It is an error to use a target of 'font' in a match that has
   target="pattern".

<const>

   Holds the name of a constant; these are always integers and
   serve as symbolic names for common font values:
  Constant        Property        Value
  -------------------------------------
  thin            weight          0
  extralight      weight          40
  ultralight      weight          40
  light           weight          50
  demilight       weight          55
  semilight       weight          55
  book            weight          75
  regular         weight          80
  normal          weight          80
  medium          weight          100
  demibold        weight          180
  semibold        weight          180
  bold            weight          200
  extrabold       weight          205
  black           weight          210
  heavy           weight          210
  roman           slant           0
  italic          slant           100
  oblique         slant           110
  ultracondensed  width           50
  extracondensed  width           63
  condensed       width           75
  semicondensed   width           87
  normal          width           100
  semiexpanded    width           113
  expanded        width           125
  extraexpanded   width           150
  ultraexpanded   width           200
  proportional    spacing         0
  dual            spacing         90
  mono            spacing         100
  charcell        spacing         110
  unknown         rgba            0
  rgb             rgba            1
  bgr             rgba            2
  vrgb            rgba            3
  vbgr            rgba            4
  none            rgba            5
  lcdnone         lcdfilter       0
  lcddefault      lcdfilter       1
  lcdlight        lcdfilter       2
  lcdlegacy       lcdfilter       3
  hintnone        hintstyle       0
  hintslight      hintstyle       1
  hintmedium      hintstyle       2
  hintfull        hintstyle       3

<or>, <and>, <plus>, <minus>, <times>, <divide>

   These elements perform the specified operation on a list of
   expression elements. <or> and <and> are boolean, not bitwise.

<eq>, <not_eq>, <less>, <less_eq>, <more>, <more_eq>, <contains>,
<not_contains

   These elements compare two values, producing a boolean result.

<not>

   Inverts the boolean sense of its one expression element

<if>

   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.

<alias>

   Alias elements provide a shorthand notation for the set of
   common match operations needed to substitute one font family
   for another. They contain a <family> element followed by
   optional <prefer>, <accept> and <default> elements. Fonts
   matching the <family> element are edited to prepend the list of
   <prefer>ed families before the matching <family>, append the
   <accept>able families after the matching <family> and append
   the <default> families to the end of the family list.

<family>

   Holds a single font family name

<prefer>, <accept>, <default>

   These hold a list of <family> elements to be used by the
   <alias> element.

EXAMPLE CONFIGURATION FILE

System configuration file

   This is an example of a system-wide configuration file
<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "urn:fontconfig:fonts.dtd">
<!-- /etc/fonts/fonts.conf file to configure system font access -->
<fontconfig>
<!--
        Find fonts in these directories
-->
<dir>/usr/share/fonts</dir>
<dir>/usr/X11R6/lib/X11/fonts</dir>

<!--
        Accept deprecated 'mono' alias, replacing it with 'monospace'
-->
<match target="pattern">
        <test qual="any" name="family"><string>mono</string></test>
        <edit name="family" mode="assign"><string>monospace</string></ed
it>
</match>

<!--
        Names not including any well known alias are given 'sans-serif'
-->
<match target="pattern">
        <test qual="all" name="family" compare="not_eq"><string>sans-ser
if</string></test>
        <test qual="all" name="family" compare="not_eq"><string>serif</s
tring></test>
        <test qual="all" name="family" compare="not_eq"><string>monospac
e</string></test>
        <edit name="family" mode="append_last"><string>sans-serif</strin
g></edit>
</match>

<!--
        Load per-user customization file, but don't complain
        if it doesn't exist
-->
<include ignore_missing="yes" prefix="xdg">fontconfig/fonts.conf</includ
e>

<!--
        Load local customization files, but don't complain
        if there aren't any
-->
<include ignore_missing="yes">conf.d</include>
<include ignore_missing="yes">local.conf</include>

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

<!--
        Provide required aliases for standard names
        Do these after the users configuration file so that
        any aliases there are used preferentially
-->
<alias>
        <family>serif</family>
        <prefer><family>Times New Roman</family></prefer>
</alias>
<alias>
        <family>sans</family>
        <prefer><family>Arial</family></prefer>
</alias>
<alias>
        <family>monospace</family>
        <prefer><family>Andale Mono</family></prefer>
</alias>

<--
        The example of the requirements of OR operator;
        If the 'family' contains 'Courier New' OR 'Courier'
        add 'monospace' as the alternative
-->
<match target="pattern">
        <test name="family" compare="eq">
                <string>Courier New</string>
        </test>
        <edit name="family" mode="prepend">
                <string>monospace</string>
        </edit>
</match>
<match target="pattern">
        <test name="family" compare="eq">
                <string>Courier</string>
        </test>
        <edit name="family" mode="prepend">
                <string>monospace</string>
        </edit>
</match>

</fontconfig>

User configuration file

   This is an example of a per-user configuration file that lives
   in $XDG_CONFIG_HOME/fontconfig/fonts.conf
<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "urn:fontconfig:fonts.dtd">
<!-- $XDG_CONFIG_HOME/fontconfig/fonts.conf for per-user font configurat
ion -->
<fontconfig>

<!--
        Private font directory
-->
<dir prefix="xdg">fonts</dir>

<!--
        use rgb sub-pixel ordering to improve glyph appearance on
        LCD screens.  Changes affecting rendering, but not matching
        should always use target="font".
-->
<match target="font">
        <edit name="rgba" mode="assign"><const>rgb</const></edit>
</match>
<!--
        use WenQuanYi Zen Hei font when serif is requested for Chinese
-->
<match>
        <!--
                If you don't want to use WenQuanYi Zen Hei font for zh-t
w etc,
                you can use zh-cn instead of zh.
                Please note, even if you set zh-cn, it still matches zh.
                if you don't like it, you can use compare="eq"
                instead of compare="contains".
        -->
        <test name="lang" compare="contains">
                <string>zh</string>
        </test>
        <test name="family">
                <string>serif</string>
        </test>
        <edit name="family" mode="prepend">
                <string>WenQuanYi Zen Hei</string>
        </edit>
</match>
<!--
        use VL Gothic font when sans-serif is requested for Japanese
-->
<match>
        <test name="lang" compare="contains">
                <string>ja</string>
        </test>
        <test name="family">
                <string>sans-serif</string>
        </test>
        <edit name="family" mode="prepend">
                <string>VL Gothic</string>
        </edit>
</match>
</fontconfig>

Files

   fonts.conf 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.

   conf.d is the conventional name for a directory of additional
   configuration files managed by external applications or the
   local administrator. The filenames starting with decimal digits
   are sorted in lexicographic order and used as additional
   configuration files. All of these files are in XML format. The
   master fonts.conf file references this directory in an
   <include> directive.

   fonts.dtd is a DTD that describes the format of the
   configuration files.

   $XDG_CONFIG_HOME/fontconfig/conf.d and ~/.fonts.conf.d is the
   conventional name for a per-user directory of (typically
   auto-generated) configuration files, although the actual
   location is specified in the global fonts.conf file. please
   note that ~/.fonts.conf.d is deprecated now. it will not be
   read by default in the future version.

   $XDG_CONFIG_HOME/fontconfig/fonts.conf and ~/.fonts.conf is the
   conventional location for per-user font configuration, although
   the actual location is specified in the global fonts.conf file.
   please note that ~/.fonts.conf is deprecated now. it will not
   be read by default in the future version.

   $XDG_CACHE_HOME/fontconfig/*.cache-* and
   ~/.fontconfig/*.cache-* is the conventional repository of font
   information that isn't found in the per-directory caches. This
   file is automatically maintained by fontconfig. please note
   that ~/.fontconfig/*.cache-* is deprecated now. it will not be
   read by default in the future version.

Environment variables

   FONTCONFIG_FILE is used to override the default configuration
   file.

   FONTCONFIG_PATH is used to override the default configuration
   directory.

   FONTCONFIG_SYSROOT is used to set a default sysroot directory.

   FC_DEBUG is used to output the detailed debugging messages. see
   Debugging Applications section for more details.

   FC_DBG_MATCH_FILTER is used to filter out the patterns. this
   takes a comma-separated list of object names and effects only
   when FC_DEBUG has MATCH2. see Debugging Applications section
   for more details.

   FC_LANG is used to specify the default language as the weak
   binding in the query. if this isn't set, the default language
   will be determined from current locale.

   FONTCONFIG_USE_MMAP is used to control the use of mmap(2) for
   the cache files if available. this take a boolean value.
   fontconfig will checks if the cache files are stored on the
   filesystem that is safe to use mmap(2). explicitly setting this
   environment variable will causes skipping this check and
   enforce to use or not use mmap(2) anyway.

   SOURCE_DATE_EPOCH is used to ensure fc-cache(1) generates files
   in a deterministic manner in order to support reproducible
   builds. When set to a numeric representation of UNIX timestamp,
   fontconfig will prefer this value over using the modification
   timestamps of the input files in order to identify which cache
   files require regeneration. If SOURCE_DATE_EPOCH is not set (or
   is newer than the mtime of the directory), the existing
   behaviour is unchanged.

See Also

   fc-cat(1), fc-cache(1), fc-list(1), fc-match(1), fc-query(1),
   SOURCE_DATE_EPOCH.

Version

   Fontconfig version 2.14.1