fontconfig/doc/fontconfig-devel.sgml

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    Copyright <20> 2003 Keith Packard
   
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<article>
	<title>Fontconfig Developers Reference, Version &version; </title>
	<artheader>
		<author>
			<firstname>Keith</firstname>
			<surname>Packard</surname>
			<affiliation><orgname>
				HP Cambridge Research Lab
			</orgname></affiliation>
		</author>
		<authorinitials>KRP</authorinitials>
		<productname>Fontconfig</productname>
		<productnumber>&version;</productnumber>
		<LegalNotice>
			<simpara>		
Copyright <20> 2002 Keith Packard
			</simpara><simpara>
Permission to use, copy, modify, distribute, and sell this software and its
documentation for any purpose is hereby granted without fee, provided that
the above copyright notice appear in all copies and that both that
copyright notice and this permission notice appear in supporting
documentation, and that the name of Keith Packard not be used in
advertising or publicity pertaining to distribution of the software without
specific, written prior permission.  Keith Packard makes no
representations about the suitability of this software for any purpose.  It
is provided "as is" without express or implied warranty.
			</simpara><simpara>
KEITH PACKARD DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO
EVENT SHALL KEITH PACKARD BE LIABLE FOR ANY SPECIAL, INDIRECT OR
CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE,
DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
PERFORMANCE OF THIS SOFTWARE.
			</simpara>
		</LegalNotice>
	</artheader>
<sect1><title>DESCRIPTION</title>
  <para>
Fontconfig is a library designed to provide system-wide font configuration,
customization and application access.
  </para>
</sect1>
<sect1><title>FUNCTIONAL OVERVIEW</title>
  <para>
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.
  </para>
  <sect2><title>FONT CONFIGURATION</title>
    <para>
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.  
    </para><para>
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.
    </para><para>
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.
    </para>
  </sect2>
  <sect2>
    <title>FONT PROPERTIES</title>
    <para>
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.
    </para>
    <programlisting>
                 Property Definitions

    Property    CPP Symbol      Type    Description
    ----------------------------------------------------
    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
    </programlisting>
  </sect2>
</sect1>
<sect1><title>Datatypes</title>
  <para>
Fontconfig uses abstract datatypes to hide internal implementation details
for most data structures.  A few structures are exposed where appropriate.
  </para>
  <sect2><title>FcChar8, FcChar16, FcChar32, FcBool</title>
    <para>
These are primitive datatypes; the FcChar* types hold precisely the number
of bits stated (if supported by the C implementation).  FcBool holds
one of two CPP symbols: FcFalse or FcTrue.
    </para>
  </sect2>
  <sect2><title>FcMatrix</title>
    <para>
An FcMatrix holds an affine transformation, usually used to reshape glyphs.
A small set of matrix operations are provided to manipulate these.
    <programlisting>
        typedef struct _FcMatrix {
                double xx, xy, yx, yy;
        } FcMatrix;
    </programlisting>
    </para>
  </sect2>
  <sect2><title>FcCharSet</title>
    <para>
An FcCharSet is an abstract type that holds the set of encoded unicode chars
in a font.  Operations to build and compare these sets are provided.
    </para>
  </sect2>
  <sect2><title>FcType</title>
    <para>
Tags the kind of data stored in an FcValue.
    </para>
  </sect2>
  <sect2><title>FcValue</title>
    <para>
An FcValue object holds a single value with one of a number of different
types.  The 'type' tag indicates which member is valid.
    <programlisting>
        typedef struct _FcValue {
                FcType type;
                union {
                        const FcChar8 *s;
                        int i;
                        FcBool b;
                        double d;
                        const FcMatrix *m;
                        const FcCharSet *c;
                } u;
        } FcValue;
    </programlisting>
    <programlisting>
                  FcValue Members

        Type            Union member    Datatype
        --------------------------------
        FcTypeVoid      (none)          (none)
        FcTypeInteger   i               int
        FcTypeDouble    d               double
        FcTypeString    s               char *
        FcTypeBool      b               b
        FcTypeMatrix    m               FcMatrix *
        FcTypeCharSet   c               FcCharSet *
    </programlisting>
    </para>
  </sect2>
  <sect2><title>FcPattern</title>
    <para>
holds a set of names with associated value lists; each name refers to a
property of a font.  FcPatterns are used as inputs to the matching code as
well as holding information about specific fonts.  Each property can hold
one or more values; conventionally all of the same type, although the
interface doesn't demand that.
    </para>
  </sect2>
  <sect2><title>FcFontSet</title>
    <para>
    <programlisting>
        typedef struct _FcFontSet {
                int nfont;
                int sfont;
                FcPattern **fonts;
        } FcFontSet;
    </programlisting>
An FcFontSet contains a list of FcPatterns.  Internally fontconfig uses this
data structure to hold sets of fonts.  Externally, fontconfig returns the
results of listing fonts in this format.  'nfont' holds the number of
patterns in the 'fonts' array; 'sfont' is used to indicate the size of that
array.
    </para>
  </sect2>
  <sect2><title>FcStrSet, FcStrList</title>
    <para>
FcStrSet holds a list of strings that can be appended to and enumerated.
Its unique characteristic is that the enumeration works even while strings
are appended during enumeration.  FcStrList is used during enumeration to
safely and correctly walk the list of strings even while that list is edited
in the middle of enumeration.
    </para>
  </sect2>
  <sect2><title>FcObjectSet</title>
    <para>
      <programlisting>
        typedef struct _FcObjectSet {
                int nobject;
                int sobject;
                const char **objects;
        } FcObjectSet;
      </programlisting>
holds a set of names and is used to specify which fields from fonts are
placed in the the list of returned patterns when listing fonts.
    </para>
  </sect2>
  <sect2><title>FcObjectType</title>
    <para>
      <programlisting>
        typedef struct _FcObjectType {
                const char *object;
                FcType type;
        } FcObjectType;
      </programlisting>
marks the type of a pattern element generated when parsing font names.
Applications can add new object types so that font names may contain the new
elements.
    </para>
  </sect2>
  <sect2><title>FcConstant</title>
    <para>
      <programlisting>
        typedef struct _FcConstant {
            const FcChar8 *name;
            const char *object;
            int value;
        } FcConstant;
      </programlisting>
Provides for symbolic constants for new pattern elements.  When 'name' is
seen in a font name, an 'object' element is created with value 'value'.
    </para>
  </sect2>
  <sect2><title>FcBlanks</title>
    <para>
holds a list of Unicode chars which are expected to be blank; unexpectedly
blank chars are assumed to be invalid and are elided from the charset
associated with the font.
    </para>
  </sect2>
  <sect2><title>FcFileCache</title>
    <para>
holds the per-user cache information for use while loading the font
database. This is built automatically for the current configuration when
that is loaded.  Applications must always pass '0' when one is requested.
    </para>
  </sect2>
  <sect2><title>FcConfig</title>
    <para>
holds a complete configuration of the library; there is one default
configuration, other can be constructed from XML data structures.  All
public entry points that need global data can take an optional FcConfig*
argument; passing 0 uses the default configuration.  FcConfig objects hold two
sets of fonts, the first contains those specified by the configuration, the
second set holds those added by the application at run-time.  Interfaces
that need to reference a particulat set use one of the FcSetName enumerated
values.
    </para>
  </sect2>
  <sect2><title>FcSetName</title>
    <para>
Specifies one of the two sets of fonts available in a configuration;
FcSetSystem for those fonts specified in the configuration and
FcSetApplication which holds fonts provided by the application.
    </para>
  </sect2>
  <sect2><title>FcResult</title>
    <para>
Used as a return type for functions manipulating FcPattern objects.
    <programlisting>
      FcResult Values
        Result Code             Meaning
        -----------------------------------------------------------
        FcResultMatch           Object exists with the specified ID
        FcResultNoMatch         Object doesn't exist at all
        FcResultTypeMismatch    Object exists, but the type doesn't match
        FcResultNoId            Object exists, but has fewer values
                                than specified
    </programlisting>
    </para>
  </sect2>
  <sect2><title>FcAtomic</title>
    <para>
Used for locking access to config files.  Provides a safe way to update
configuration files.
    </para>
  </sect2>
</sect1>
<sect1><title>FUNCTIONS</title>
  <para>
These are grouped by functionality, often using the main datatype being
manipulated.
  </para>
  <sect2><title>Initialization</title>
    <para>
These functions provide some control over how the library is initialized.
    </para>
    &fcinit;
  </sect2>
  <sect2><title>FcPattern</title>
    <para>
An FcPattern is an opaque type that holds both patterns to match against the
available fonts, as well as the information about each font.
    </para>
    &fcpattern;
  </sect2>
  <sect2><title>FcFontSet</title>
    <para>
An FcFontSet simply holds a list of patterns; these are used to return the
results of listing available fonts.
    </para>
    &fcfontset;
  </sect2>
  <sect2><title>FcObjectSet</title>
    <para>
An FcObjectSet holds a list of pattern property names; it is used to
indiciate which properties are to be returned in the patterns from
FcFontList.
    </para>
    &fcobjectset;
  </sect2>
  <sect2><title>FreeType specific functions</title>
    <para>
While the fontconfig library doesn't insist that FreeType be used as the
rasterization mechanism for fonts, it does provide some convenience
functions.
    </para>
    &fcfreetype;
  </sect2>
  <sect2><title>FcValue</title>
    <para>
FcValue is a structure containing a type tag and a union of all possible
datatypes.  The tag is an enum of type 
<emphasis>FcType</emphasis>
and is intended to provide a measure of run-time
typechecking, although that depends on careful programming.
    </para>
    &fcvalue;
  </sect2>
  <sect2><title>FcCharSet</title>
    <para>
An FcCharSet is a boolean array indicating a set of unicode chars.  Those
associated with a font are marked constant and cannot be edited.
FcCharSets may be reference counted internally to reduce memory consumption;
this may be visible to applications as the result of FcCharSetCopy may
return it's argument, and that CharSet may remain unmodifiable.
    </para>
    &fccharset;
  </sect2>
  <sect2><title>FcMatrix</title>
    <para>
FcMatrix structures hold an affine transformation in matrix form.
    </para>
    &fcmatrix;
  </sect2>
  <sect2><title>FcConfig</title>
    <para>
An FcConfig object holds the internal representation of a configuration.
There is a default configuration which applications may use by passing 0 to
any function using the data within an FcConfig.
    </para>
    &fcconfig;
  </sect2>
  <sect2><title>FcObjectType</title>
    <para>
Provides for applcation-specified font name object types so that new
pattern elements can be generated from font names.
    </para>
    &fcobjecttype;
  </sect2>
  <sect2><title>FcConstant</title>
    <para>
Provides for application-specified symbolic constants for font names.
    </para>
    &fcconstant;
  </sect2>
  <sect2><title>FcBlanks</title>
    <para>
An FcBlanks object holds a list of Unicode chars which are expected to
be blank when drawn.  When scanning new fonts, any glyphs which are
empty and not in this list will be assumed to be broken and not placed in
the FcCharSet associated with the font.  This provides a significantly more
accurate CharSet for applications.
    </para>
    &fcblanks;
  </sect2>
  <sect2><title>FcAtomic</title>
    <para>
These functions provide a safe way to update config files, allowing ongoing
reading of the old config file while locked for writing and ensuring that a
consistent and complete version of the config file is always available.
    </para>
    &fcatomic;
  </sect2>
  <sect2><title>File and Directory routines</title>
    <para>
These routines work with font files and directories, including font
directory cache files.
    </para>
    &fcfile;
  </sect2>
  <sect2><title>FcStrSet and FcStrList</title>
    <para>
A data structure for enumerating strings, used to list directories while
scanning the configuration as directories are added while scanning.
    </para>
    &fcstrset;
  </sect2>
  <sect2><title>String utilities</title>
    <para>
Fontconfig manipulates many UTF-8 strings represented with the FcChar8 type.
These functions are exposed to help applications deal with these UTF-8
strings in a locale-insensitive manner.
    </para>
    &fcstring;
  </sect2>
</sect1>
</article>