Fontconfig Users Guide

3.1. FONT CONFIGURATION

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.

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.

3.2. 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.

3.3. 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.

3.4. 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:

5.1. <fontconfig>

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

5.2. <dir>

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

5.3. <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 1).

5.4. <include ignore_missing="no">

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.

5.5. <config>

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

5.6. <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.

5.7. <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.

5.8. <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.

5.9. <test qual="any" name="property" compare="eq">

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.

5.10. <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. 'mode' is one of:

5.11. <int>, <double>, <string>, <bool>

These elements hold a single value of the indicated type. <bool> elements hold either true or false.

5.12. <matrix>

This element holds the four <double> elements of an affine transformation.

5.13. <name>

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

5.14. <const>

Holds the name of a constant; these are always integers and serve as symbolic names for common font values:

5.15. <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.

5.16. <eq>, <not_eq>, <less>, <less_eq>, <more>, <more_eq>

These elements compare two values, producing a boolean result.

5.17. <not>

Inverts the boolean sense of its one expression element

5.18. <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.

5.19. <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 familys after the matching <family> and append the <default> families to the end of the family list.

5.20. <family>

Holds a single font family name

5.21. <prefer>, <accept>, <default>

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

6.1. 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/X11R6/lib/X11/fonts/truetype</dir>
<dir>/usr/X11R6/lib/X11/fonts/Type1</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>

<!--
	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>Verdana</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>Verdana</family></prefer>
</alias>
<alias>
	<family>monospace</family>
	<prefer><family>Andale Mono</family></prefer>
</alias>
</fontconfig>
    

6.2. 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>~/misc/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>