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
   ~/.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 charcell
  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
  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
  outline         Bool    Whether the glyphs are outlines
  scalable        Bool    Whether glyphs can be scaled
  scale           Double  Scale factor for point->pixel conversions
  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
  embolden        Bool    Rasterizer should synthetically embolden 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 preceeded by a '\'
character to avoid having them misinterpreted. Similarly, values containing '\
', '=', '_', ':' and ',' must also have them preceeded 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
  OBJTYPES      4096    Display message when value typechecks fail


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 "fonts.dtd">
        <fontconfig>
        ...
        </fontconfig>


<fontconfig>

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

<dir>

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

<cache>

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-<version>'', where
<version> is the font configuration file version number (currently 2).

<include ignore_missing="no">

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.

<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", 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. 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


<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 the four <double> elements of an affine transformation.

<name>

Holds a property name. Evaluates to the first value from the property of the
font, not the 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
  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>

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 "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'
-->
<match target="pattern">
        <test qual="all" name="family" mode="not_eq">sans</test>
        <test qual="all" name="family" mode="not_eq">serif</test>
        <test qual="all" name="family" mode="not_eq">monospace</test>
        <edit name="family" mode="append_last"><string>sans</string></edit>
</match>

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

<!--
        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>
</fontconfig>


User configuration file

This is an example of a per-user configuration file that lives in ~/.fonts.conf

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<!-- ~/.fonts.conf for per-user font configuration -->
<fontconfig>

<!--
        Private font directory
-->
<dir>~/.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>
</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.

~/.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.

~/.fonts.conf is the conventional location for per-user font configuration,
although the actual location is specified in the global fonts.conf file.

~/.fonts.cache-* is the conventional repository of font information that isn't
found in the per-directory caches. This file is automatically maintained by
fontconfig.

See Also

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

Version

Fontconfig version 2.8.0