Font Configuration and Customization for Open Source Systems - 白皮书翻译和深入

font config 作为一个linux系统的font的配置的基本要素，如何使用的一份手册翻译：

本文尝试从font config手册出发，介绍font config的基本概念和基本元素，以期待在了解font config的使用的同时，了解font config的概念和原始。

1 写在之前，必须弄清楚什么是fontconfig

1.1 Fontconfig是字體相關的计算机程序库，用于配置、定制全系统的字体，或將字體提供給应用程序使用。

https://zh.wikipedia.org/wiki/Fontconfig

1.2 freedesktop 是如何定义fontconfig的功能的

About Fontconfig

Fontconfig can:

discover new fonts when installed automatically, removing a common source of configuration problems.
perform font name substitution, so that appropriate alternative fonts can be selected if fonts are missing.
identify the set of fonts required to completely cover a set of languages.
have GUI configuration tools built as it uses an XML-based configuration file (though with autodiscovery, we believe this need is minimized).
efficiently and quickly find the fonts you need among the set of fonts you have installed, even if you have installed thousands of fonts, while minimzing memory usage.
be used in concert with the X Render Extension and FreeType to implement high quality, anti-aliased and subpixel rendered text on a display.
Fontconfig does not:


render the fonts themselves (this is left to FreeType or other rendering mechanisms)
depend on the X Window System in any fashion, so that printer only applications do not have such dependencies.

【franlin】上述解释比较细节到运用了，有说明如果font缺少的情况下如何匹配，已经fontconfig的XML的配置属性。

3 更详细fontconfig的介绍还是要看文档：

Font configuration and customization has traditionally been left to each application.
Fontconfig is a library designed to provide a common system that can serve to ease
application development and provide users with the ability to confidently install new
fonts with the expectation that they will be used by most applications. Fontconfig
provides the ability for multiple configuration interfaces to affect a wide range of
systems without requiring custom code for each new system. Fontconfig provides a
range of services to allow applications to pick those appropriate without being forced
to use the entire interface.Wide acceptance of the Fontconfig mechanisms will improve
system consistency without requiring a radical redesign of existing applications.

上述这段是fontconfig开发者，Keith Packard自己写的，如果你还有问题可以email 给他：[email protected]
【Franklin 案】fontconfig是一个通用库，这个库是用来给应用最大的方便安装新的字体，配置和定制字体，查找字体而不需要重新设计接口和改变应用本身。

https://keithp.com/~keithp/talks/guadec2002/fontconfig.pdf

2 Design 基本设计思路

Fontconfig is designed to configure and customize the use of fonts stored within the local filesystem. As
the requirements and expectations of applications using the font configuration mechanism differ,
Fontconfig is designed to be useful at several levels. Applications can get just the list of font directories
or can use the complete font naming and matching system. Porting an existing system with a custom font
configuration mechanism can start with the lowest level and migrate in the future to using more of the
capabilities of this system, supplanting existing mechanisms within the application

fontconfig 的使用情景比较弹性，根据应用的需要可以做各个层次的工作。因为他主要是配置的工作。

2 Configuration Files 配置文件：

To ensure that every system using Fontconfig shares a common configuration, the Fontconfig library
always uses the same configuration files by default. Font config searches for the fonts.conf
configuration file along the directories specified in the FONTCONFIG_PATH environment variable, and
also in /etc/fonts.

Fontconfig 库总是会使用默认的文件配置来适应不同的系统。主要是通过fonts.conf这个文件来做的，而这个文件的搜寻路径通过两个方式

1 如果是没有设定，默认的话请放在

/etc/fonts 

文件夹下就好

2 或者通过环境变量FONTCONFIG_PATH 来设定

由於 /etc/fonts/fonts.conf 是 X window 的預設設定，系統更新時，可能會將您所設定過的檔案覆蓋掉，強烈建議不直接更改。系統全域設定建議寫於 /etc/fonts/conf.d 資料夾內，或是 local.conf；個人設定則寫在 ~/.fonts 資料夾內，並自訂名稱，規則如下。

fonts-conf

Name

fonts.conf -- Font configuration files

Synopsis

/etc/fonts/fonts.conf /etc/fonts/fonts.dtd /etc/fonts/conf.d $XDG_CONFIG_HOME/fontconfig/conf.d $XDG_CONFIG_HOME/fontconfig/fonts.conf ~/.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 (deprecated) 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 (deprecated) outline Bool Whether the glyphs are outlines scalable Bool Whether glyphs can be scaled color Bool Whether any glyphs have color scale Double Scale factor for point->pixel conversions (deprecated) 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 fontformat String String name of the font format embolden Bool Rasterizer should synthetically embolden the font embeddedbitmap Bool Use the embedded bitmap instead of the outline decorative Bool Whether the style is a decorative variant fontfeatures String List of the feature tags in OpenType to be enabled namelang String Language name to be used for the default value of familylang, stylelang, and fullnamelang prgname String String Name of the running program postscriptname 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:

-:=:=...

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 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.

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 MATCH2 4096 Display font-matching transformation in patterns

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:

...

This is the top level element for a font configuration and can contain 

, , ,  and  elements in any order.

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.

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 ``-.cache-'', where  is the font configuration file version number (currently 5).

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.

This element provides a place to consolidate additional configuration information.  can contain  and  elements in any order.

Fonts often include "broken" glyphs which appear in the encoding but are drawn as blanks on the screen. Within the  element, place each Unicode characters which is supposed to be blank in an element. Characters outside of this set which are drawn as blank will be elided from the set of characters supported by the font.

The  element holds an  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.

This element is used to black/white list fonts from being listed or matched against. It holds acceptfont and rejectfont elements.

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.

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 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 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 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.

This element holds first a (possibly empty) list of  elements and then a (possibly empty) list of  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.

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 element, the target= attribute in the element selects between matching the original pattern or the font. "default" selects whichever target the outer element has selected.

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  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

, , , 

These elements hold a single value of the indicated type.  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).

This element holds four numerical expressions of an affine transformation. At their simplest these will be four  elements but they can also be more involved expressions.

This element holds the two  elements of a range representation.

This element holds at least one  element of an Unicode code point or more.

This element holds at least one  element of a RFC-3066-style languages or more.

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".

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 demilight weight 55 semilight weight 55 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

, , , , , 

These elements perform the specified operation on a list of expression elements.  and  are boolean, not bitwise.

, , , , , , , 

These elements compare two values, producing a boolean result.

Inverts the boolean sense of its one expression element

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 elements provide a shorthand notation for the set of common match operations needed to substitute one font family for another. They contain a  element followed by optional , and  elements. Fonts matching the  element are edited to prepend the list of ed families before the matching , append the able families after the matching and append the  families to the end of the family list.

Holds a single font family name

, , 

These hold a list of  elements to be used by the  element.

EXAMPLE CONFIGURATION FILE

System configuration file

This is an example of a system-wide configuration file

/usr/share/fonts /usr/X11R6/lib/X11/fonts mono monospace sans-serif serif monospace sans-serif fontconfig/fonts.conf conf.d local.conf Times Times New Roman serif Helvetica Arial sans Courier Courier New monospace serif Times New Roman sans Arial monospace Andale Mono <-- The example of the requirements of OR operator; If the 'family' contains 'Courier New' OR 'Courier' add 'monospace' as the alternative --> Courier New monospace Courier monospace

User configuration file

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

fonts rgb zh serif WenQuanYi Zen Hei ja sans-serif VL Gothic

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 directive.

fonts.dtd is a DTD that describes the format of the configuration files.

$XDG_CONFIG_HOME/fontconfig/conf.d and ~/.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. please note that ~/.fonts.conf.d is deprecated now. it will not be read by default in the future version.

$XDG_CONFIG_HOME/fontconfig/fonts.conf and ~/.fonts.conf is the conventional location for per-user font configuration, although the actual location is specified in the global fonts.conf file. please note that ~/.fonts.conf is deprecated now. it will not be read by default in the future version.

$XDG_CACHE_HOME/fontconfig/*.cache-* and ~/.fontconfig/*.cache-* is the conventional repository of font information that isn't found in the per-directory caches. This file is automatically maintained by fontconfig. please note that ~/.fontconfig/*.cache-* is deprecated now. it will not be read by default in the future version.

Environment variables

FONTCONFIG_FILE is used to override the default configuration file.

FONTCONFIG_PATH is used to override the default configuration directory.

FC_DEBUG is used to output the detailed debugging messages. see Debugging Applications section for more details.

FC_DBG_MATCH_FILTER is used to filter out the patterns. this takes a comma-separated list of object names and effects only when FC_DEBUG has MATCH2. see Debugging Applications section for more details.

FONTCONFIG_USE_MMAP is used to control the use of mmap(2) for the cache files if available. this take a boolean value. fontconfig will checks if the cache files are stored on the filesystem that is safe to use mmap(2). explicitly setting this environment variable will causes skipping this check and enforce to use or not use mmap(2) anyway.

See Also

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

Version

Fontconfig version 2.12.1

===============================================================================================================

REF：

1 Font configuration (简体中文)

https://wiki.archlinux.org/index.php/Font_configuration_(%E7%AE%80%E4%BD%93%E4%B8%AD%E6%96%87)

2 Font configuration

https://wiki.archlinux.org/index.php/Font_configuration

3 字体渲览

http://wiki.ubuntu.org.cn/%E5%AD%97%E4%BD%93%E6%B8%B2%E6%9F%93

4 维基百科

https://zh.wikipedia.org/wiki/Fontconfig

5 HowtoCustomFontswithFontconfig

 http://wiki.ubuntu-tw.org/index.php?title=HowtoCustomFontswithFontconfig

6 user manul

https://www.freedesktop.org/software/fontconfig/fontconfig-user.html

7 about 

https://www.freedesktop.org/wiki/Software/fontconfig/About/

8 function review and  源码概略

https://www.freedesktop.org/software/fontconfig/fontconfig-devel/x19.html

9 

https://www.freedesktop.org/wiki/Software/fontconfig/

10 font

https://www.freedesktop.org/software/fontconfig/front.html