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.
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.
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 application's rendering mechanism.
Property Definitions
Property C Preprocessor Symbol Type Description
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family FC_FAMILY String Font family names
familylang FC_FAMILYLANG String Language corresponding to
each family name
style FC_STYLE String Font style. Overrides weight
and slant
stylelang FC_STYLELANG String Language corresponding to
each style name
fullname FC_FULLNAME String Font face full name where
different from family and
family + style
fullnamelang FC_FULLNAMELANG String Language corresponding to
each fullname
slant FC_SLANT Int Italic, oblique or roman
weight FC_WEIGHT Int Light, medium, demibold,
bold or black
size FC_SIZE Double Point size
width FC_WIDTH Int Condensed, normal or expanded
aspect FC_ASPECT Double Stretches glyphs horizontally
before hinting
pixelsize FC_PIXEL_SIZE Double Pixel size
spacing FC_SPACING Int Proportional, dual-width,
monospace or charcell
foundry FC_FOUNDRY String Font foundry name
antialias FC_ANTIALIAS Bool Whether glyphs can be
antialiased
hinting FC_HINTING Bool Whether the rasterizer should
use hinting
hintstyle FC_HINT_STYLE Int Automatic hinting style
verticallayout FC_VERTICAL_LAYOUT Bool Use vertical layout
autohint FC_AUTOHINT Bool Use autohinter instead of
normal hinter
globaladvance FC_GLOBAL_ADVANCE Bool Use font global advance data (deprecated)
file FC_FILE String The filename holding the font
index FC_INDEX Int The index of the font within
the file
ftface FC_FT_FACE FT_Face Use the specified FreeType
face object
rasterizer FC_RASTERIZER String Which rasterizer is in use (deprecated)
outline FC_OUTLINE Bool Whether the glyphs are outlines
scalable FC_SCALABLE Bool Whether glyphs can be scaled
scale FC_SCALE Double Scale factor for point->pixel
conversions (deprecated)
symbol FC_SYMBOL Bool Whether font uses MS symbol-font encoding
color FC_COLOR Bool Whether any glyphs have color
dpi FC_DPI Double Target dots per inch
rgba FC_RGBA Int unknown, rgb, bgr, vrgb,
vbgr, none - subpixel geometry
lcdfilter FC_LCD_FILTER Int Type of LCD filter
minspace FC_MINSPACE Bool Eliminate leading from line
spacing
charset FC_CHARSET CharSet Unicode chars encoded by
the font
lang FC_LANG LangSet Set of RFC-3066-style
languages this font supports
fontversion FC_FONTVERSION Int Version number of the font
capability FC_CAPABILITY String List of layout capabilities in
the font
fontformat FC_FONTFORMAT String String name of the font format
embolden FC_EMBOLDEN Bool Rasterizer should
synthetically embolden the font
embeddedbitmap FC_EMBEDDED_BITMAP Bool Use the embedded bitmap instead
of the outline
decorative FC_DECORATIVE Bool Whether the style is a decorative
variant
fontfeatures FC_FONT_FEATURES String List of extra feature tags in
OpenType to be enabled
namelang FC_NAMELANG String Language name to be used for the
default value of familylang,
stylelang and fullnamelang
prgname FC_PRGNAME String Name of the running program
hash FC_HASH String SHA256 hash value of the font data
with "sha256:" prefix (deprecated)
postscriptname FC_POSTSCRIPT_NAME String Font name in PostScript