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 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.
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 Definitions
Property CPP Symbol Type Description
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family FC_FAMILY String Font family name
style FC_STYLE String Font style. Overrides weight
and slant
slant FC_SLANT Int Italic, oblique or roman
weight FC_WEIGHT Int Light, medium, demibold,
bold or black
size FC_SIZE Double Point size
aspect FC_ASPECT Double Stretches glyphs horizontally
before hinting
pixelsize FC_PIXEL_SIZE Double Pixel size
spacing FC_SPACING Int Proportional, 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
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
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
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
dpi FC_DPI Double Target dots per inch
rgba FC_RGBA Int unknown, rgb, bgr, vrgb,
vbgr, none - subpixel geometry
minspace FC_MINSPACE Bool Eliminate leading from line
spacing
charset FC_CHARSET CharSet Unicode chars encoded by
the font
lang FC_LANG String List of RFC-3066-style
languages this font supports