doc/fontconfig-devel.sgml

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  13 <!ENTITY fcobjecttype SYSTEM "fcobjecttype.sgml">
  14 <!ENTITY fcpattern SYSTEM "fcpattern.sgml">
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  23     Copyright © 2003 Keith Packard
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  43 <article>
  44         <title>Fontconfig Developers Reference, Version &version; </title>
  45         <artheader>
  46                 <author>
  47                         <firstname>Keith</firstname>
  48                         <surname>Packard</surname>
  49                         <affiliation><orgname>
  50                                 HP Cambridge Research Lab
  51                         </orgname></affiliation>
  52                 </author>
  53                 <authorinitials>KRP</authorinitials>
  54                 <productname>Fontconfig</productname>
  55                 <productnumber>&version;</productnumber>
  56                 <LegalNotice>
  57                         <simpara>
  58 Copyright © 2002 Keith Packard
  59                         </simpara><simpara>
  60 Permission to use, copy, modify, distribute, and sell this software and its
  61 documentation for any purpose is hereby granted without fee, provided that
  62 the above copyright notice appear in all copies and that both that
  63 copyright notice and this permission notice appear in supporting
  64 documentation, and that the name of Keith Packard not be used in
  65 advertising or publicity pertaining to distribution of the software without
  66 specific, written prior permission.  Keith Packard makes no
  67 representations about the suitability of this software for any purpose.  It
  68 is provided "as is" without express or implied warranty.
  69                         </simpara><simpara>
  70 KEITH PACKARD DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
  71 INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO
  72 EVENT SHALL KEITH PACKARD BE LIABLE FOR ANY SPECIAL, INDIRECT OR
  73 CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE,
  74 DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
  75 TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
  76 PERFORMANCE OF THIS SOFTWARE.
  77                         </simpara>
  78                 </LegalNotice>
  79         </artheader>
  80 <sect1><title>DESCRIPTION</title>
  81   <para>
  82 Fontconfig is a library designed to provide system-wide font configuration,
  83 customization and application access.
  84   </para>
  85 </sect1>
  86 <sect1><title>FUNCTIONAL OVERVIEW</title>
  87   <para>
  88 Fontconfig contains two essential modules, the configuration module which
  89 builds an internal configuration from XML files and the matching module
  90 which accepts font patterns and returns the nearest matching font.
  91   </para>
  92   <sect2><title>FONT CONFIGURATION</title>
  93     <para>
  94 The configuration module consists of the FcConfig datatype, libexpat and
  95 FcConfigParse which walks over an XML tree and ammends a configuration with
  96 data found within.  From an external perspective, configuration of the
  97 library consists of generating a valid XML tree and feeding that to
  98 FcConfigParse.  The only other mechanism provided to applications for
  99 changing the running configuration is to add fonts and directories to the
 100 list of application-provided font files.
 101     </para><para>
 102 The intent is to make font configurations relatively static, and shared by
 103 as many applications as possible.  It is hoped that this will lead to more
 104 stable font selection when passing names from one application to another.
 105 XML was chosen as a configuration file format because it provides a format
 106 which is easy for external agents to edit while retaining the correct
 107 structure and syntax.
 108     </para><para>
 109 Font configuration is separate from font matching; applications needing to
 110 do their own matching can access the available fonts from the library and
 111 perform private matching.  The intent is to permit applications to pick and
 112 choose appropriate functionality from the library instead of forcing them to
 113 choose between this library and a private configuration mechanism.  The hope
 114 is that this will ensure that configuration of fonts for all applications
 115 can be centralized in one place.  Centralizing font configuration will
 116 simplify and regularize font installation and customization.
 117     </para>
 118   </sect2>
 119   <sect2>
 120     <title>FONT PROPERTIES</title>
 121     <para>
 122 While font patterns may contain essentially any properties, there are some
 123 well known properties with associated types.  Fontconfig uses some of these
 124 properties for font matching and font completion.  Others are provided as a
 125 convenience for the applications rendering mechanism.
 126     </para>
 127     <programlisting>
 128                  Property Definitions
 129
 130     Property       CPP Symbol           Type    Description
 131     ----------------------------------------------------
 132     family         FC_FAMILY            String  Font family names
 133     familylang     FC_FAMILYLANG        String  Language cooresponding to
 134                                                 each family name
 135     style          FC_STYLE             String  Font style. Overrides weight
 136                                                 and slant
 137     stylelang      FC_STYLELANG         String  Language cooresponding to
 138                                                 each style name
 139     fullname       FC_FULLNAME          String  Font face full name where
 140                                                 different from family and
 141                                                 family + style
 142     fullnamelang   FC_FULLNAMELANG      String  Language cooresponding to
 143                                                 each fullname
 144     slant          FC_SLANT             Int     Italic, oblique or roman
 145     weight         FC_WEIGHT            Int     Light, medium, demibold,
 146                                                 bold or black
 147     size           FC_SIZE              Double  Point size
 148     aspect         FC_ASPECT            Double  Stretches glyphs horizontally
 149                                                 before hinting
 150     pixelsize      FC_PIXEL_SIZE        Double  Pixel size
 151     spacing        FC_SPACING           Int     Proportional, dual-width,
 152                                                 monospace or charcell
 153     foundry        FC_FOUNDRY           String  Font foundry name
 154     antialias      FC_ANTIALIAS         Bool    Whether glyphs can be
 155                                                 antialiased
 156     hinting        FC_HINTING           Bool    Whether the rasterizer should
 157                                                 use hinting
 158     verticallayout FC_VERTICAL_LAYOUT   Bool    Use vertical layout
 159     autohint       FC_AUTOHINT          Bool    Use autohinter instead of
 160                                                 normal hinter
 161     globaladvance  FC_GLOBAL_ADVANCE    Bool    Use font global advance data
 162     file           FC_FILE              String  The filename holding the font
 163     index          FC_INDEX             Int     The index of the font within
 164                                                 the file
 165     ftface         FC_FT_FACE           FT_Face Use the specified FreeType
 166                                                 face object
 167     rasterizer     FC_RASTERIZER        String  Which rasterizer is in use
 168     outline        FC_OUTLINE           Bool    Whether the glyphs are outlines
 169     scalable       FC_SCALABLE          Bool    Whether glyphs can be scaled
 170     scale          FC_SCALE             Double  Scale factor for point->pixel
 171                                                 conversions
 172     dpi            FC_DPI               Double  Target dots per inch
 173     rgba           FC_RGBA              Int     unknown, rgb, bgr, vrgb,
 174                                                 vbgr, none - subpixel geometry
 175     minspace       FC_MINSPACE          Bool    Eliminate leading from line
 176                                                 spacing
 177     charset        FC_CHARSET           CharSet Unicode chars encoded by
 178                                                 the font
 179     lang           FC_LANG              String  List of RFC-3066-style
 180                                                 languages this font supports
 181     </programlisting>
 182   </sect2>
 183 </sect1>
 184 <sect1><title>Datatypes</title>
 185   <para>
 186 Fontconfig uses abstract datatypes to hide internal implementation details
 187 for most data structures.  A few structures are exposed where appropriate.
 188   </para>
 189   <sect2><title>FcChar8, FcChar16, FcChar32, FcBool</title>
 190     <para>
 191 These are primitive datatypes; the FcChar* types hold precisely the number
 192 of bits stated (if supported by the C implementation).  FcBool holds
 193 one of two CPP symbols: FcFalse or FcTrue.
 194     </para>
 195   </sect2>
 196   <sect2><title>FcMatrix</title>
 197     <para>
 198 An FcMatrix holds an affine transformation, usually used to reshape glyphs.
 199 A small set of matrix operations are provided to manipulate these.
 200     <programlisting>
 201         typedef struct _FcMatrix {
 202                 double xx, xy, yx, yy;
 203         } FcMatrix;
 204     </programlisting>
 205     </para>
 206   </sect2>
 207   <sect2><title>FcCharSet</title>
 208     <para>
 209 An FcCharSet is an abstract type that holds the set of encoded unicode chars
 210 in a font.  Operations to build and compare these sets are provided.
 211     </para>
 212   </sect2>
 213   <sect2><title>FcType</title>
 214     <para>
 215 Tags the kind of data stored in an FcValue.
 216     </para>
 217   </sect2>
 218   <sect2><title>FcValue</title>
 219     <para>
 220 An FcValue object holds a single value with one of a number of different
 221 types.  The 'type' tag indicates which member is valid.
 222     <programlisting>
 223         typedef struct _FcValue {
 224                 FcType type;
 225                 union {
 226                         const FcChar8 *s;
 227                         int i;
 228                         FcBool b;
 229                         double d;
 230                         const FcMatrix *m;
 231                         const FcCharSet *c;
 232                 } u;
 233         } FcValue;
 234     </programlisting>
 235     <programlisting>
 236                   FcValue Members
 237
 238         Type            Union member    Datatype
 239         --------------------------------
 240         FcTypeVoid      (none)          (none)
 241         FcTypeInteger   i               int
 242         FcTypeDouble    d               double
 243         FcTypeString    s               char *
 244         FcTypeBool      b               b
 245         FcTypeMatrix    m               FcMatrix *
 246         FcTypeCharSet   c               FcCharSet *
 247     </programlisting>
 248     </para>
 249   </sect2>
 250   <sect2><title>FcPattern</title>
 251     <para>
 252 holds a set of names with associated value lists; each name refers to a
 253 property of a font.  FcPatterns are used as inputs to the matching code as
 254 well as holding information about specific fonts.  Each property can hold
 255 one or more values; conventionally all of the same type, although the
 256 interface doesn't demand that.
 257     </para>
 258   </sect2>
 259   <sect2><title>FcFontSet</title>
 260     <para>
 261     <programlisting>
 262         typedef struct _FcFontSet {
 263                 int nfont;
 264                 int sfont;
 265                 FcPattern **fonts;
 266         } FcFontSet;
 267     </programlisting>
 268 An FcFontSet contains a list of FcPatterns.  Internally fontconfig uses this
 269 data structure to hold sets of fonts.  Externally, fontconfig returns the
 270 results of listing fonts in this format.  'nfont' holds the number of
 271 patterns in the 'fonts' array; 'sfont' is used to indicate the size of that
 272 array.
 273     </para>
 274   </sect2>
 275   <sect2><title>FcStrSet, FcStrList</title>
 276     <para>
 277 FcStrSet holds a list of strings that can be appended to and enumerated.
 278 Its unique characteristic is that the enumeration works even while strings
 279 are appended during enumeration.  FcStrList is used during enumeration to
 280 safely and correctly walk the list of strings even while that list is edited
 281 in the middle of enumeration.
 282     </para>
 283   </sect2>
 284   <sect2><title>FcObjectSet</title>
 285     <para>
 286       <programlisting>
 287         typedef struct _FcObjectSet {
 288                 int nobject;
 289                 int sobject;
 290                 const char **objects;
 291         } FcObjectSet;
 292       </programlisting>
 293 holds a set of names and is used to specify which fields from fonts are
 294 placed in the the list of returned patterns when listing fonts.
 295     </para>
 296   </sect2>
 297   <sect2><title>FcObjectType</title>
 298     <para>
 299       <programlisting>
 300         typedef struct _FcObjectType {
 301                 const char *object;
 302                 FcType type;
 303         } FcObjectType;
 304       </programlisting>
 305 marks the type of a pattern element generated when parsing font names.
 306 Applications can add new object types so that font names may contain the new
 307 elements.
 308     </para>
 309   </sect2>
 310   <sect2><title>FcConstant</title>
 311     <para>
 312       <programlisting>
 313         typedef struct _FcConstant {
 314             const FcChar8 *name;
 315             const char *object;
 316             int value;
 317         } FcConstant;
 318       </programlisting>
 319 Provides for symbolic constants for new pattern elements.  When 'name' is
 320 seen in a font name, an 'object' element is created with value 'value'.
 321     </para>
 322   </sect2>
 323   <sect2><title>FcBlanks</title>
 324     <para>
 325 holds a list of Unicode chars which are expected to be blank; unexpectedly
 326 blank chars are assumed to be invalid and are elided from the charset
 327 associated with the font.
 328     </para>
 329   </sect2>
 330   <sect2><title>FcFileCache</title>
 331     <para>
 332 holds the per-user cache information for use while loading the font
 333 database. This is built automatically for the current configuration when
 334 that is loaded.  Applications must always pass '0' when one is requested.
 335     </para>
 336   </sect2>
 337   <sect2><title>FcConfig</title>
 338     <para>
 339 holds a complete configuration of the library; there is one default
 340 configuration, other can be constructed from XML data structures.  All
 341 public entry points that need global data can take an optional FcConfig*
 342 argument; passing 0 uses the default configuration.  FcConfig objects hold two
 343 sets of fonts, the first contains those specified by the configuration, the
 344 second set holds those added by the application at run-time.  Interfaces
 345 that need to reference a particulat set use one of the FcSetName enumerated
 346 values.
 347     </para>
 348   </sect2>
 349   <sect2><title>FcSetName</title>
 350     <para>
 351 Specifies one of the two sets of fonts available in a configuration;
 352 FcSetSystem for those fonts specified in the configuration and
 353 FcSetApplication which holds fonts provided by the application.
 354     </para>
 355   </sect2>
 356   <sect2><title>FcResult</title>
 357     <para>
 358 Used as a return type for functions manipulating FcPattern objects.
 359     <programlisting>
 360       FcResult Values
 361         Result Code             Meaning
 362         -----------------------------------------------------------
 363         FcResultMatch           Object exists with the specified ID
 364         FcResultNoMatch         Object doesn't exist at all
 365         FcResultTypeMismatch    Object exists, but the type doesn't match
 366         FcResultNoId            Object exists, but has fewer values
 367                                 than specified
 368     </programlisting>
 369     </para>
 370   </sect2>
 371   <sect2><title>FcAtomic</title>
 372     <para>
 373 Used for locking access to config files.  Provides a safe way to update
 374 configuration files.
 375     </para>
 376   </sect2>
 377 </sect1>
 378 <sect1><title>FUNCTIONS</title>
 379   <para>
 380 These are grouped by functionality, often using the main datatype being
 381 manipulated.
 382   </para>
 383   <sect2><title>Initialization</title>
 384     <para>
 385 These functions provide some control over how the library is initialized.
 386     </para>
 387     &fcinit;
 388   </sect2>
 389   <sect2><title>FcPattern</title>
 390     <para>
 391 An FcPattern is an opaque type that holds both patterns to match against the
 392 available fonts, as well as the information about each font.
 393     </para>
 394     &fcpattern;
 395   </sect2>
 396   <sect2><title>FcFontSet</title>
 397     <para>
 398 An FcFontSet simply holds a list of patterns; these are used to return the
 399 results of listing available fonts.
 400     </para>
 401     &fcfontset;
 402   </sect2>
 403   <sect2><title>FcObjectSet</title>
 404     <para>
 405 An FcObjectSet holds a list of pattern property names; it is used to
 406 indiciate which properties are to be returned in the patterns from
 407 FcFontList.
 408     </para>
 409     &fcobjectset;
 410   </sect2>
 411   <sect2><title>FreeType specific functions</title>
 412     <para>
 413 While the fontconfig library doesn't insist that FreeType be used as the
 414 rasterization mechanism for fonts, it does provide some convenience
 415 functions.
 416     </para>
 417     &fcfreetype;
 418   </sect2>
 419   <sect2><title>FcValue</title>
 420     <para>
 421 FcValue is a structure containing a type tag and a union of all possible
 422 datatypes.  The tag is an enum of type
 423 <emphasis>FcType</emphasis>
 424 and is intended to provide a measure of run-time
 425 typechecking, although that depends on careful programming.
 426     </para>
 427     &fcvalue;
 428   </sect2>
 429   <sect2><title>FcCharSet</title>
 430     <para>
 431 An FcCharSet is a boolean array indicating a set of unicode chars.  Those
 432 associated with a font are marked constant and cannot be edited.
 433 FcCharSets may be reference counted internally to reduce memory consumption;
 434 this may be visible to applications as the result of FcCharSetCopy may
 435 return it's argument, and that CharSet may remain unmodifiable.
 436     </para>
 437     &fccharset;
 438   </sect2>
 439   <sect2><title>FcMatrix</title>
 440     <para>
 441 FcMatrix structures hold an affine transformation in matrix form.
 442     </para>
 443     &fcmatrix;
 444   </sect2>
 445   <sect2><title>FcConfig</title>
 446     <para>
 447 An FcConfig object holds the internal representation of a configuration.
 448 There is a default configuration which applications may use by passing 0 to
 449 any function using the data within an FcConfig.
 450     </para>
 451     &fcconfig;
 452   </sect2>
 453   <sect2><title>FcObjectType</title>
 454     <para>
 455 Provides for applcation-specified font name object types so that new
 456 pattern elements can be generated from font names.
 457     </para>
 458     &fcobjecttype;
 459   </sect2>
 460   <sect2><title>FcConstant</title>
 461     <para>
 462 Provides for application-specified symbolic constants for font names.
 463     </para>
 464     &fcconstant;
 465   </sect2>
 466   <sect2><title>FcBlanks</title>
 467     <para>
 468 An FcBlanks object holds a list of Unicode chars which are expected to
 469 be blank when drawn.  When scanning new fonts, any glyphs which are
 470 empty and not in this list will be assumed to be broken and not placed in
 471 the FcCharSet associated with the font.  This provides a significantly more
 472 accurate CharSet for applications.
 473     </para>
 474     &fcblanks;
 475   </sect2>
 476   <sect2><title>FcAtomic</title>
 477     <para>
 478 These functions provide a safe way to update config files, allowing ongoing
 479 reading of the old config file while locked for writing and ensuring that a
 480 consistent and complete version of the config file is always available.
 481     </para>
 482     &fcatomic;
 483   </sect2>
 484   <sect2><title>File and Directory routines</title>
 485     <para>
 486 These routines work with font files and directories, including font
 487 directory cache files.
 488     </para>
 489     &fcfile;
 490   </sect2>
 491   <sect2><title>FcStrSet and FcStrList</title>
 492     <para>
 493 A data structure for enumerating strings, used to list directories while
 494 scanning the configuration as directories are added while scanning.
 495     </para>
 496     &fcstrset;
 497   </sect2>
 498   <sect2><title>String utilities</title>
 499     <para>
 500 Fontconfig manipulates many UTF-8 strings represented with the FcChar8 type.
 501 These functions are exposed to help applications deal with these UTF-8
 502 strings in a locale-insensitive manner.
 503     </para>
 504     &fcstring;
 505   </sect2>
 506 </sect1>
 507 </article>