doc/fontconfig-devel.sgml

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  25     fontconfig/doc/local-fontconfig-devel.sgml
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  27     Copyright © 2003 Keith Packard
  28
  29     Permission to use, copy, modify, distribute, and sell this software and its
  30     documentation for any purpose is hereby granted without fee, provided that
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  47 <article>
  48         <title>Fontconfig Developers Reference, Version &version; </title>
  49         <artheader>
  50                 <author>
  51                         <firstname>Keith</firstname>
  52                         <surname>Packard</surname>
  53                         <affiliation><orgname>
  54                                 HP Cambridge Research Lab
  55                         </orgname></affiliation>
  56                 </author>
  57                 <authorinitials>KRP</authorinitials>
  58                 <productname>Fontconfig</productname>
  59                 <productnumber>&version;</productnumber>
  60                 <LegalNotice>
  61                         <simpara>
  62 Copyright © 2002 Keith Packard
  63                         </simpara><simpara>
  64 Permission to use, copy, modify, distribute, and sell this software and its
  65 documentation for any purpose is hereby granted without fee, provided that
  66 the above copyright notice appear in all copies and that both that
  67 copyright notice and this permission notice appear in supporting
  68 documentation, and that the name of the author(s) not be used in
  69 advertising or publicity pertaining to distribution of the software without
  70 specific, written prior permission.  The authors make no
  71 representations about the suitability of this software for any purpose.  It
  72 is provided "as is" without express or implied warranty.
  73                         </simpara><simpara>
  74 THE AUTHOR(S) DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
  75 INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO
  76 EVENT SHALL THE AUTHOR(S) BE LIABLE FOR ANY SPECIAL, INDIRECT OR
  77 CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE,
  78 DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
  79 TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
  80 PERFORMANCE OF THIS SOFTWARE.
  81                         </simpara>
  82                 </LegalNotice>
  83         </artheader>
  84 <sect1><title>DESCRIPTION</title>
  85   <para>
  86 Fontconfig is a library designed to provide system-wide font configuration,
  87 customization and application access.
  88   </para>
  89 </sect1>
  90 <sect1><title>FUNCTIONAL OVERVIEW</title>
  91   <para>
  92 Fontconfig contains two essential modules, the configuration module which
  93 builds an internal configuration from XML files and the matching module
  94 which accepts font patterns and returns the nearest matching font.
  95   </para>
  96   <sect2><title>FONT CONFIGURATION</title>
  97     <para>
  98 The configuration module consists of the FcConfig datatype, libexpat and
  99 FcConfigParse which walks over an XML tree and ammends a configuration with
 100 data found within.  From an external perspective, configuration of the
 101 library consists of generating a valid XML tree and feeding that to
 102 FcConfigParse.  The only other mechanism provided to applications for
 103 changing the running configuration is to add fonts and directories to the
 104 list of application-provided font files.
 105     </para><para>
 106 The intent is to make font configurations relatively static, and shared by
 107 as many applications as possible.  It is hoped that this will lead to more
 108 stable font selection when passing names from one application to another.
 109 XML was chosen as a configuration file format because it provides a format
 110 which is easy for external agents to edit while retaining the correct
 111 structure and syntax.
 112     </para><para>
 113 Font configuration is separate from font matching; applications needing to
 114 do their own matching can access the available fonts from the library and
 115 perform private matching.  The intent is to permit applications to pick and
 116 choose appropriate functionality from the library instead of forcing them to
 117 choose between this library and a private configuration mechanism.  The hope
 118 is that this will ensure that configuration of fonts for all applications
 119 can be centralized in one place.  Centralizing font configuration will
 120 simplify and regularize font installation and customization.
 121     </para>
 122   </sect2>
 123   <sect2>
 124     <title>FONT PROPERTIES</title>
 125     <para>
 126 While font patterns may contain essentially any properties, there are some
 127 well known properties with associated types.  Fontconfig uses some of these
 128 properties for font matching and font completion.  Others are provided as a
 129 convenience for the applications rendering mechanism.
 130     </para>
 131     <programlisting>
 132                  Property Definitions
 133
 134     Property       CPP Symbol           Type    Description
 135     ----------------------------------------------------
 136     family         FC_FAMILY            String  Font family names
 137     familylang     FC_FAMILYLANG        String  Language cooresponding to
 138                                                 each family name
 139     style          FC_STYLE             String  Font style. Overrides weight
 140                                                 and slant
 141     stylelang      FC_STYLELANG         String  Language cooresponding to
 142                                                 each style name
 143     fullname       FC_FULLNAME          String  Font face full name where
 144                                                 different from family and
 145                                                 family + style
 146     fullnamelang   FC_FULLNAMELANG      String  Language cooresponding to
 147                                                 each fullname
 148     slant          FC_SLANT             Int     Italic, oblique or roman
 149     weight         FC_WEIGHT            Int     Light, medium, demibold,
 150                                                 bold or black
 151     size           FC_SIZE              Double  Point size
 152     width          FC_WIDTH             Int     Condensed, normal or expanded
 153     aspect         FC_ASPECT            Double  Stretches glyphs horizontally
 154                                                 before hinting
 155     pixelsize      FC_PIXEL_SIZE        Double  Pixel size
 156     spacing        FC_SPACING           Int     Proportional, dual-width,
 157                                                 monospace or charcell
 158     foundry        FC_FOUNDRY           String  Font foundry name
 159     antialias      FC_ANTIALIAS         Bool    Whether glyphs can be
 160                                                 antialiased
 161     hinting        FC_HINTING           Bool    Whether the rasterizer should
 162                                                 use hinting
 163     hintstyle      FC_HINT_STYLE        Int     Automatic hinting style
 164     verticallayout FC_VERTICAL_LAYOUT   Bool    Use vertical layout
 165     autohint       FC_AUTOHINT          Bool    Use autohinter instead of
 166                                                 normal hinter
 167     globaladvance  FC_GLOBAL_ADVANCE    Bool    Use font global advance data
 168     file           FC_FILE              String  The filename holding the font
 169     index          FC_INDEX             Int     The index of the font within
 170                                                 the file
 171     ftface         FC_FT_FACE           FT_Face Use the specified FreeType
 172                                                 face object
 173     rasterizer     FC_RASTERIZER        String  Which rasterizer is in use
 174     outline        FC_OUTLINE           Bool    Whether the glyphs are outlines
 175     scalable       FC_SCALABLE          Bool    Whether glyphs can be scaled
 176     scale          FC_SCALE             Double  Scale factor for point->pixel
 177                                                 conversions
 178     dpi            FC_DPI               Double  Target dots per inch
 179     rgba           FC_RGBA              Int     unknown, rgb, bgr, vrgb,
 180                                                 vbgr, none - subpixel geometry
 181     lcdfilter      FC_LCD_FILTER        Int     Type of LCD filter
 182     minspace       FC_MINSPACE          Bool    Eliminate leading from line
 183                                                 spacing
 184     charset        FC_CHARSET           CharSet Unicode chars encoded by
 185                                                 the font
 186     lang           FC_LANG              LangSet Set of RFC-3066-style
 187                                                 languages this font supports
 188     fontversion    FC_FONTVERSION       Int     Version number of the font
 189     capability     FC_CAPABILITY        String  List of layout capabilities in
 190                                                 the font
 191     embolden       FC_EMBOLDEN          Bool    Rasterizer should
 192                                                 synthetically embolden the font
 193     </programlisting>
 194   </sect2>
 195 </sect1>
 196 <sect1><title>Datatypes</title>
 197   <para>
 198 Fontconfig uses abstract datatypes to hide internal implementation details
 199 for most data structures.  A few structures are exposed where appropriate.
 200   </para>
 201   <sect2><title>FcChar8, FcChar16, FcChar32, FcBool</title>
 202     <para>
 203 These are primitive datatypes; the FcChar* types hold precisely the number
 204 of bits stated (if supported by the C implementation).  FcBool holds
 205 one of two CPP symbols: FcFalse or FcTrue.
 206     </para>
 207   </sect2>
 208   <sect2><title>FcMatrix</title>
 209     <para>
 210 An FcMatrix holds an affine transformation, usually used to reshape glyphs.
 211 A small set of matrix operations are provided to manipulate these.
 212     <programlisting>
 213         typedef struct _FcMatrix {
 214                 double xx, xy, yx, yy;
 215         } FcMatrix;
 216     </programlisting>
 217     </para>
 218   </sect2>
 219   <sect2><title>FcCharSet</title>
 220     <para>
 221 An FcCharSet is an abstract type that holds the set of encoded unicode chars
 222 in a font.  Operations to build and compare these sets are provided.
 223     </para>
 224   </sect2>
 225   <sect2><title>FcLangSet</title>
 226     <para>
 227 An FcLangSet is an abstract type that holds the set of languages supported
 228 by a font.  Operations to build and compare these sets are provided. These
 229 are computed for a font based on orthographic information built into the
 230 fontconfig library. Fontconfig has orthographies for all of the ISO 639-1
 231 languages except for MS, NA, PA, PS, QU, RN, RW, SD, SG, SN, SU and ZA. If
 232 you have orthographic information for any of these languages, please submit
 233 them.
 234     </para>
 235   </sect2>
 236   <sect2><title>FcLangResult</title>
 237     <para>
 238 An FcLangResult is an enumeration used to return the results of comparing
 239 two language strings or FcLangSet objects. FcLangEqual means the
 240 objects match language and territory. FcLangDifferentTerritory means
 241 the objects match in language but differ in territory.
 242 FcLangDifferentLang means the objects differ in language.
 243     </para>
 244   </sect2>
 245   <sect2><title>FcType</title>
 246     <para>
 247 Tags the kind of data stored in an FcValue.
 248     </para>
 249   </sect2>
 250   <sect2><title>FcValue</title>
 251     <para>
 252 An FcValue object holds a single value with one of a number of different
 253 types.  The 'type' tag indicates which member is valid.
 254     <programlisting>
 255         typedef struct _FcValue {
 256                 FcType type;
 257                 union {
 258                         const FcChar8 *s;
 259                         int i;
 260                         FcBool b;
 261                         double d;
 262                         const FcMatrix *m;
 263                         const FcCharSet *c;
 264                         void *f;
 265                         const FcLangSet *l;
 266                 } u;
 267         } FcValue;
 268     </programlisting>
 269     <programlisting>
 270                   FcValue Members
 271
 272         Type            Union member    Datatype
 273         --------------------------------
 274         FcTypeVoid      (none)          (none)
 275         FcTypeInteger   i               int
 276         FcTypeDouble    d               double
 277         FcTypeString    s               FcChar8 *
 278         FcTypeBool      b               b
 279         FcTypeMatrix    m               FcMatrix *
 280         FcTypeCharSet   c               FcCharSet *
 281         FcTypeFTFace    f               void * (FT_Face)
 282         FcTypeLangSet   l               FcLangSet *
 283     </programlisting>
 284     </para>
 285   </sect2>
 286   <sect2><title>FcPattern</title>
 287     <para>
 288 holds a set of names with associated value lists; each name refers to a
 289 property of a font.  FcPatterns are used as inputs to the matching code as
 290 well as holding information about specific fonts.  Each property can hold
 291 one or more values; conventionally all of the same type, although the
 292 interface doesn't demand that.
 293     </para>
 294   </sect2>
 295   <sect2><title>FcFontSet</title>
 296     <para>
 297     <programlisting>
 298         typedef struct _FcFontSet {
 299                 int nfont;
 300                 int sfont;
 301                 FcPattern **fonts;
 302         } FcFontSet;
 303     </programlisting>
 304 An FcFontSet contains a list of FcPatterns.  Internally fontconfig uses this
 305 data structure to hold sets of fonts.  Externally, fontconfig returns the
 306 results of listing fonts in this format.  'nfont' holds the number of
 307 patterns in the 'fonts' array; 'sfont' is used to indicate the size of that
 308 array.
 309     </para>
 310   </sect2>
 311   <sect2><title>FcStrSet, FcStrList</title>
 312     <para>
 313 FcStrSet holds a list of strings that can be appended to and enumerated.
 314 Its unique characteristic is that the enumeration works even while strings
 315 are appended during enumeration.  FcStrList is used during enumeration to
 316 safely and correctly walk the list of strings even while that list is edited
 317 in the middle of enumeration.
 318     </para>
 319   </sect2>
 320   <sect2><title>FcObjectSet</title>
 321     <para>
 322       <programlisting>
 323         typedef struct _FcObjectSet {
 324                 int nobject;
 325                 int sobject;
 326                 const char **objects;
 327         } FcObjectSet;
 328       </programlisting>
 329 holds a set of names and is used to specify which fields from fonts are
 330 placed in the the list of returned patterns when listing fonts.
 331     </para>
 332   </sect2>
 333   <sect2><title>FcObjectType</title>
 334     <para>
 335       <programlisting>
 336         typedef struct _FcObjectType {
 337                 const char *object;
 338                 FcType type;
 339         } FcObjectType;
 340       </programlisting>
 341 marks the type of a pattern element generated when parsing font names.
 342 Applications can add new object types so that font names may contain the new
 343 elements.
 344     </para>
 345   </sect2>
 346   <sect2><title>FcConstant</title>
 347     <para>
 348       <programlisting>
 349         typedef struct _FcConstant {
 350             const FcChar8 *name;
 351             const char *object;
 352             int value;
 353         } FcConstant;
 354       </programlisting>
 355 Provides for symbolic constants for new pattern elements.  When 'name' is
 356 seen in a font name, an 'object' element is created with value 'value'.
 357     </para>
 358   </sect2>
 359   <sect2><title>FcBlanks</title>
 360     <para>
 361 holds a list of Unicode chars which are expected to be blank; unexpectedly
 362 blank chars are assumed to be invalid and are elided from the charset
 363 associated with the font.
 364     </para>
 365   </sect2>
 366   <sect2><title>FcFileCache</title>
 367     <para>
 368 holds the per-user cache information for use while loading the font
 369 database. This is built automatically for the current configuration when
 370 that is loaded.  Applications must always pass '0' when one is requested.
 371     </para>
 372   </sect2>
 373   <sect2><title>FcConfig</title>
 374     <para>
 375 holds a complete configuration of the library; there is one default
 376 configuration, other can be constructed from XML data structures.  All
 377 public entry points that need global data can take an optional FcConfig*
 378 argument; passing 0 uses the default configuration.  FcConfig objects hold two
 379 sets of fonts, the first contains those specified by the configuration, the
 380 second set holds those added by the application at run-time.  Interfaces
 381 that need to reference a particulat set use one of the FcSetName enumerated
 382 values.
 383     </para>
 384   </sect2>
 385   <sect2><title>FcSetName</title>
 386     <para>
 387 Specifies one of the two sets of fonts available in a configuration;
 388 FcSetSystem for those fonts specified in the configuration and
 389 FcSetApplication which holds fonts provided by the application.
 390     </para>
 391   </sect2>
 392   <sect2><title>FcResult</title>
 393     <para>
 394 Used as a return type for functions manipulating FcPattern objects.
 395     <programlisting>
 396       FcResult Values
 397         Result Code             Meaning
 398         -----------------------------------------------------------
 399         FcResultMatch           Object exists with the specified ID
 400         FcResultNoMatch         Object doesn't exist at all
 401         FcResultTypeMismatch    Object exists, but the type doesn't match
 402         FcResultNoId            Object exists, but has fewer values
 403                                 than specified
 404         FcResultOutOfMemory     Malloc failed
 405     </programlisting>
 406     </para>
 407   </sect2>
 408   <sect2><title>FcAtomic</title>
 409     <para>
 410 Used for locking access to config files.  Provides a safe way to update
 411 configuration files.
 412     </para>
 413   </sect2>
 414   <sect2><title>FcCache</title>
 415     <para>
 416 Holds information about the fonts contained in a single directory. Normal
 417 applications need not worry about this as caches for font access are
 418 automatically managed by the library. Applications dealing with cache
 419 management may want to use some of these objects in their work, however the
 420 included 'fc-cache' program generally suffices for all of that.
 421     </para>
 422   </sect2>
 423 </sect1>
 424 <sect1><title>FUNCTIONS</title>
 425   <para>
 426 These are grouped by functionality, often using the main datatype being
 427 manipulated.
 428   </para>
 429   <sect2><title>Initialization</title>
 430     <para>
 431 These functions provide some control over how the library is initialized.
 432     </para>
 433     &fcinit;
 434   </sect2>
 435   <sect2><title>FcPattern</title>
 436     <para>
 437 An FcPattern is an opaque type that holds both patterns to match against the
 438 available fonts, as well as the information about each font.
 439     </para>
 440     &fcpattern;
 441     &fcformat;
 442   </sect2>
 443   <sect2><title>FcFontSet</title>
 444     <para>
 445 An FcFontSet simply holds a list of patterns; these are used to return the
 446 results of listing available fonts.
 447     </para>
 448     &fcfontset;
 449   </sect2>
 450   <sect2><title>FcObjectSet</title>
 451     <para>
 452 An FcObjectSet holds a list of pattern property names; it is used to
 453 indiciate which properties are to be returned in the patterns from
 454 FcFontList.
 455     </para>
 456     &fcobjectset;
 457   </sect2>
 458   <sect2><title>FreeType specific functions</title>
 459     <para>
 460 While the fontconfig library doesn't insist that FreeType be used as the
 461 rasterization mechanism for fonts, it does provide some convenience
 462 functions.
 463     </para>
 464     &fcfreetype;
 465   </sect2>
 466   <sect2><title>FcValue</title>
 467     <para>
 468 FcValue is a structure containing a type tag and a union of all possible
 469 datatypes.  The tag is an enum of type
 470 <emphasis>FcType</emphasis>
 471 and is intended to provide a measure of run-time
 472 typechecking, although that depends on careful programming.
 473     </para>
 474     &fcvalue;
 475   </sect2>
 476   <sect2><title>FcCharSet</title>
 477     <para>
 478 An FcCharSet is a boolean array indicating a set of unicode chars.  Those
 479 associated with a font are marked constant and cannot be edited.
 480 FcCharSets may be reference counted internally to reduce memory consumption;
 481 this may be visible to applications as the result of FcCharSetCopy may
 482 return it's argument, and that CharSet may remain unmodifiable.
 483     </para>
 484     &fccharset;
 485   </sect2>
 486   <sect2><title>FcLangSet</title>
 487     <para>
 488 An FcLangSet is a set of language names (each of which include language and
 489 an optional territory). They are used when selecting fonts to indicate which
 490 languages the fonts need to support. Each font is marked, using language
 491 orthography information built into fontconfig, with the set of supported
 492 languages.
 493     </para>
 494     &fclangset;
 495   </sect2>
 496   <sect2><title>FcMatrix</title>
 497     <para>
 498 FcMatrix structures hold an affine transformation in matrix form.
 499     </para>
 500     &fcmatrix;
 501   </sect2>
 502   <sect2><title>FcConfig</title>
 503     <para>
 504 An FcConfig object holds the internal representation of a configuration.
 505 There is a default configuration which applications may use by passing 0 to
 506 any function using the data within an FcConfig.
 507     </para>
 508     &fcconfig;
 509   </sect2>
 510   <sect2><title>FcObjectType</title>
 511     <para>
 512 Provides for applcation-specified font name object types so that new
 513 pattern elements can be generated from font names.
 514     </para>
 515     &fcobjecttype;
 516   </sect2>
 517   <sect2><title>FcConstant</title>
 518     <para>
 519 Provides for application-specified symbolic constants for font names.
 520     </para>
 521     &fcconstant;
 522   </sect2>
 523   <sect2><title>FcBlanks</title>
 524     <para>
 525 An FcBlanks object holds a list of Unicode chars which are expected to
 526 be blank when drawn.  When scanning new fonts, any glyphs which are
 527 empty and not in this list will be assumed to be broken and not placed in
 528 the FcCharSet associated with the font.  This provides a significantly more
 529 accurate CharSet for applications.
 530     </para>
 531     &fcblanks;
 532   </sect2>
 533   <sect2><title>FcAtomic</title>
 534     <para>
 535 These functions provide a safe way to update config files, allowing ongoing
 536 reading of the old config file while locked for writing and ensuring that a
 537 consistent and complete version of the config file is always available.
 538     </para>
 539     &fcatomic;
 540   </sect2>
 541   <sect2><title>File and Directory routines</title>
 542     <para>
 543 These routines work with font files and directories, including font
 544 directory cache files.
 545     </para>
 546     &fcfile;
 547     &fcdircache;
 548   </sect2>
 549   <sect2><title>FcCache routines</title>
 550     <para>
 551 These routines work with font directory caches, accessing their contents in
 552 limited ways. It is not expected that normal applications will need to use
 553 these functions.
 554     </para>
 555     &fccache;
 556   </sect2>
 557   <sect2><title>FcStrSet and FcStrList</title>
 558     <para>
 559 A data structure for enumerating strings, used to list directories while
 560 scanning the configuration as directories are added while scanning.
 561     </para>
 562     &fcstrset;
 563   </sect2>
 564   <sect2><title>String utilities</title>
 565     <para>
 566 Fontconfig manipulates many UTF-8 strings represented with the FcChar8 type.
 567 These functions are exposed to help applications deal with these UTF-8
 568 strings in a locale-insensitive manner.
 569     </para>
 570     &fcstring;
 571   </sect2>
 572 </sect1>
 573 </article>