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 namesfamilylang      String  Languages corresponding to each familystyle           String  Font style. Overrides weight and slantstylelang       String  Languages corresponding to each stylefullname        String  Font full names (often includes style)fullnamelang    String  Languages corresponding to each fullnameslant           Int     Italic, oblique or romanweight          Int     Light, medium, demibold, bold or blacksize            Double  Point sizewidth           Int     Condensed, normal or expandedaspect          Double  Stretches glyphs horizontally before hintingpixelsize       Double  Pixel sizespacing         Int     Proportional, dual-width, monospace or charcellfoundry         String  Font foundry nameantialias       Bool    Whether glyphs can be antialiasedhinting         Bool    Whether the rasterizer should use hintinghintstyle       Int     Automatic hinting styleverticallayout  Bool    Use vertical layoutautohint        Bool    Use autohinter instead of normal hinterglobaladvance   Bool    Use font global advance data (deprecated)file            String  The filename holding the fontindex           Int     The index of the font within the fileftface          FT_Face Use the specified FreeType face objectrasterizer      String  Which rasterizer is in use (deprecated)outline         Bool    Whether the glyphs are outlinesscalable        Bool    Whether glyphs can be scaledcolor           Bool    Whether any glyphs have colorscale           Double  Scale factor for point->pixel conversions (deprecated)dpi             Double  Target dots per inchrgba            Int     unknown, rgb, bgr, vrgb, vbgr,none - subpixel geometrylcdfilter       Int     Type of LCD filterminspace        Bool    Eliminate leading from line spacingcharset         CharSet Unicode chars encoded by the fontlang            String  List of RFC-3066-style languages thisfont supportsfontversion     Int     Version number of the fontcapability      String  List of layout capabilities in the fontfontformat      String  String name of the font formatembolden        Bool    Rasterizer should synthetically embolden the fontembeddedbitmap  Bool    Use the embedded bitmap instead of the outlinedecorative      Bool    Whether the style is a decorative variantfontfeatures    String  List of the feature tags in OpenType to be enablednamelang        String  Language name to be used for the default value offamilylang, stylelang, and fullnamelangprgname         String  String  Name of the running programpostscriptname  String  Font family name in PostScript

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 RomanTimes-12:bold                   12 point Times BoldCourier:italic                  Courier Italic in the default sizeMonospace:matrix=1 .1 0 1       The users preferred monospace fontwith 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.

<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">

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 "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.

<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 font configuration file version number (currently 5).

<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.

<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.

<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          0extralight      weight          40ultralight      weight          40light           weight          50demilight       weight          55semilight       weight          55book            weight          75regular         weight          80normal          weight          80medium          weight          100demibold        weight          180semibold        weight          180bold            weight          200extrabold       weight          205black           weight          210heavy           weight          210roman           slant           0italic          slant           100oblique         slant           110ultracondensed  width           50extracondensed  width           63condensed       width           75semicondensed   width           87normal          width           100semiexpanded    width           113expanded        width           125extraexpanded   width           150ultraexpanded   width           200proportional    spacing         0dual            spacing         90mono            spacing         100charcell        spacing         110unknown         rgba            0rgb             rgba            1bgr             rgba            2vrgb            rgba            3vbgr            rgba            4none            rgba            5lcdnone         lcdfilter       0lcddefault      lcdfilter       1lcdlight        lcdfilter       2lcdlegacy       lcdfilter       3hintnone        hintstyle       0hintslight      hintstyle       1hintmedium      hintstyle       2hintfull        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.

System configuration file

This is an example of a system-wide configuration file

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "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></edit>
</match><!--Names not including any well known alias are given 'sans-serif'
-->
<match target="pattern"><test qual="all" name="family" mode="not_eq"><string>sans-serif</string></test><test qual="all" name="family" mode="not_eq"><string>serif</string></test><test qual="all" name="family" mode="not_eq"><string>monospace</string></test><edit name="family" mode="append_last"><string>sans-serif</string></edit>
</match><!--Load per-user customization file, but don't complainif it doesn't exist
-->
<include ignore_missing="yes" prefix="xdg">fontconfig/fonts.conf</include><!--Load local customization files, but don't complainif 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 Type1faces 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 namesDo these after the users configuration file so thatany 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" mode="eq"><string>Courier New</string></test><edit name="family" mode="prepend"><string>monospace</string></edit>
</match>
<match target="pattern"><test name="family" mode="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 "fonts.dtd">
<!-- $XDG_CONFIG_HOME/fontconfig/fonts.conf for per-user font configuration -->
<fontconfig><!--Private font directory
-->
<dir prefix="xdg">fonts</dir><!--use rgb sub-pixel ordering to improve glyph appearance onLCD screens.  Changes affecting rendering, but not matchingshould 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-tw 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>