xref: /xsrc/external/mit/libXfont/dist/doc/fontlib.xml

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  <article id='fontlib'>
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<!-- .EF 'Font Library Interface'\- % \-'July 27, 1991' -->
<!-- .OF 'Font Library Interface'\- % \-'July 27, 1991' -->
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    <articleinfo>
      <title>
	The X Font Library
      </title>
      <authorgroup>
	<author>
	  <firstname>Keith</firstname>
	  <surname>Packard</surname>
	  <affiliation>
	    <orgname>MIT X Consortium</orgname>
	  </affiliation>
	</author>
	<author>
	  <firstname>David</firstname>
	  <surname>Lemke</surname>
	  <affiliation>
	    <orgname>Network Computing Devices</orgname>
	  </affiliation>
	</author>
      </authorgroup>
      <releaseinfo>X Version 11, Release &fullrelvers;</releaseinfo>
      <copyright><year>1993</year><holder>Network Computing Devices</holder></copyright>

      <legalnotice>
	<para>
	  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 Network Computing
	  Devices not be used in advertising or publicity pertaining
	  to distribution of the software without specific, written
	  prior permission.  Network Computing Devices makes no
	  representations about the suitability of this software for
	  any purpose.  It is provided &ldquo;as is&rdquo; without
	  express or implied warranty.
	</para>
	<para role="multiLicensing">
	  Copyright &copy; 1993, 1994 X Consortium
	</para>
	<para>
	  Permission is hereby granted, free of charge, to any person
	  obtaining a copy of this software and associated
	  documentation files (the &ldquo;Software&rdquo;), to deal in the
	  Software without restriction, including without limitation
	  the rights to use, copy, modify, merge, publish, distribute,
	  sublicense, and/or sell copies of the Software, and to
	  permit persons to whom the Software is furnished to do so,
	  subject to the following conditions:
	</para><para>
	  The above copyright notice and this permission notice shall be
	  included in all copies or substantial portions of the Software.
	</para><para>
	  THE SOFTWARE IS PROVIDED &ldquo;AS IS&rdquo;, WITHOUT WARRANTY
	  OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO
	  THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
	  PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE X
	  CONSORTIUM BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
	  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
	  OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
	  SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
	</para><para>
	  Except as contained in this notice, the name of the X
	  Consortium shall not be used in advertising or otherwise to
	  promote the sale, use or other dealings in this Software
	  without prior written authorization from the X Consortium.
	</para>
        <para>X Window System is a trademark of The Open Group.</para>
      </legalnotice>
      <pubdate>July 27, 1991</pubdate>
    </articleinfo>

    <warning>
      <para>
	This document has not been updated since X11R6, and is likely
	to be somewhat out of date for the current libXfont.
      </para>
    </warning>

    <para>
      This document describes the data structures and interfaces for
      using the X Font library.  It is intended as a reference for
      programmers building X and Font servers.  You may want to refer
      to the following documents:
    </para>

    <itemizedlist>
      <listitem>
	<para>
	  <citetitle pubwork="article">Definition of the Porting Layer for
	    the X v11 Sample Server</citetitle> for a discussion on how this
	  library interacts with the X server
	</para>
      </listitem>
      <listitem>
	<para>
	  <olink targetdoc='xfs-design' targetptr='xfs-design'>
	  <citetitle pubwork="article">Font Server Implementation
	    Overview</citetitle></olink> which discusses the design of the font
	  server.
	</para>
      </listitem>
      <listitem>
	<para>
	  <citetitle pubwork="article">Bitmap Distribution Format</citetitle>
	  which covers the contents of the bitmap font files which this
	  library reads; although the library is capable of reading other
	  formats as well, including non-bitmap fonts.
	</para>
      </listitem>
      <listitem>
	<para>
	  <olink targetdoc='fsproto' targetptr='fsproto'>
	  <citetitle pubwork="article">The X Font Service Protocol</citetitle>
	  </olink>
	  for a description of the constraints placed  on the design by
	  including support for this font service mechanism.
	</para>
      </listitem>
    </itemizedlist>

    <para>
      This document assumes the reader is familiar with the X server design,
      the X protocol as it relates to fonts and the C programming language.
      As with most MIT produced documentation, this relies heavily on the
      source code, so have a listing handy.
    </para>

    <sect1 id='Requirements_for_the_Font_library'>
<title>Requirements for the Font library</title>

      <para>
	To avoid miles of duplicate code in the X server, the font server
	and the various font manipulation tools, the font library should
	provide interfaces appropriate for all of these	tasks.  In
	particular, the X server and font server should be able to both
	use the library to access disk based fonts, and	to communicate
	with a font server.  By providing a general library, we hoped to
	avoid duplicating code between the X server and font server.
      </para>

      <para>
	Another requirement is that the X server (or even a font server)
	be able to continue servicing requests from other clients while
	awaiting a response from the font server on behalf of one client.
	This is the strongest requirement placed on the	font library, and
	has warped the design in curious ways.	Because both the X server
	and font server are single threaded, the font library must not
	suspend internally, rather it returns an indication of suspension
	to the application which continues processing other things, until
	the font data is ready, at which time it restarts the suspended
	request.
      </para>

      <para>
	Because the code for reading and manipulating bitmap font data is
	used by the font applications <command>mkfontdir</command> and
	<command>bdftopcf</command>, the font library includes
	bitmap-font specific interfaces which those applications use,
	instead of the more general interfaces used by the X and font
	servers, which are unaware of the source of the font data.
	These routines will be refered to as the bitmap font access
	methods.
      </para>

    </sect1>

    <sect1 id='General_Font_Library_Interface_details'>
<title>General Font Library Interface details.</title>

      <para>
	To avoid collision between the #define name space for errors, the Font
	library defines a new set of return values:
      </para>

      <programlisting remap='.nf'>
#define AllocError      80
#define StillWorking    81
#define FontNameAlias   82
#define BadFontName     83
#define Suspended       84
#define Successful      85
#define BadFontPath     86
#define BadCharRange    87
#define BadFontFormat   88
#define FPEResetFailed  89
      </programlisting> <!-- .fi -->

      <para>
	Whenever a routine returns <errorname>Suspended</errorname>,
	the font library will notify the caller (via the ClientSignal
	interface described below) who should then reinvoke the same routine
	again with the same arguments.
      </para>

    </sect1>

    <sect1 id='Font_Path_Elements'>
<title>Font Path Elements</title>

      <para>
	At the center of the general font access methods used by X and
	<command>xfs</command> is the Font Path Element data structure.
	Like most structures in the X server, this contains a collection
	of data and some function pointers for manipulating this data:
      </para>

      <programlisting remap='.nf'>
/* External view of font paths */
typedef struct _FontPathElement {
    int         name_length;
    char       *name;
    int         type;
    int         refcount;
    pointer     private;
} FontPathElementRec, *FontPathElementPtr;

typedef struct _FPEFunctions {
    int         (*name_check) ( /* name */ );
    int         (*init_fpe) ( /* fpe */ );
    int         (*reset_fpe) ( /* fpe */ );
    int         (*free_fpe) ( /* fpe */ );
    int         (*open_font) (  /* client, fpe, flags,
                        name, namelen, format,
                        fid,  ppfont, alias */ );
    int         (*close_font) ( /* pfont */ );
    int         (*list_fonts) ( /* client, fpe, pattern,
                        patlen, maxnames, paths */ );
    int         (*start_list_fonts_with_info) (
                        /* client, fpe, name, namelen,
                           maxnames, data */ );
    int         (*list_next_font_with_info) (
                        /* client, fpe, name, namelen,
                           info, num, data */ );
    int         (*wakeup_fpe) ( /* fpe, mask */ );
    int         (*client_died) ( /* client, fpe */ );
} FPEFunctionsRec, FPEFunctions;
      </programlisting> <!-- .fi -->

      <para>
	The function pointers are split out from the data structure to
	save memory; additionally, this avoids any complications when
	initializing the data structure as there would not be any way
	to discover the appropriate function to	call (a chicken and
	egg problem).
      </para>

      <para>
	When a font path type is initialized, it passes the function
	pointers to the server which are then stored in an
	<structname>FPEFunctionsRec</structname>. Each function is
	described below in turn.
      </para>

      <sect2 id='name_check'>
	<title>(*name_check)</title>

	<para>
	  Each new font path member is passed to this function; if
	  the return value is <errorname>Successful</errorname>, then
	  the FPE recognises the format of the string.  This does not
	  guarantee that the FPE will be able to successfully use this
	  member.  For example, the disk-based font directory file
	  <filename>fonts.dir</filename> may be corrupted, this will
	  not be detected until the font path is initialized.  This
	  routine never returns <errorname>Suspended</errorname>.
	</para>
      </sect2>

      <sect2 id='init_fpe'>
	<title>(*init_fpe)</title>

	<para>
	  Initialize a new font path element.  This function prepares
	  a new font path element for other requests: the disk font
	  routine reads the <filename>fonts.dir</filename> and
	  <filename>fonts.alias</filename> files into the internal
	  format, while the font server routine connects to the
	  requested font server and prepares for using it.  This
	  routine returns <errorname>Successful</errorname> if
	  everything went OK, otherwise the return value indicates the
	  source of the problem.  This routine never returns
	  <errorname>Suspended</errorname>.
	</para>
      </sect2>

      <sect2 id='reset_fpe'>
	<title>(*reset_fpe)</title>

	<para>
	  When the X font path is reset, and some of the new members
	  are also in the old font path, this function is called to
	  reinitialize those FPEs.  This routine returns
	  <errorname>Successful</errorname> if everything went OK.  It
	  returns <errorname>FPEResetFailed</errorname> if (for some
	  reason) the reset failed, and the caller should remove the
	  old FPE and simply create a new one in its place.  This is
	  used by the disk-based fonts routine as resetting the
	  internal directory structures would be more complicated than
	  simply having destroying the old and creating a new.
	</para>
      </sect2>

      <sect2 id='free_fpe'>
	<title>(*free_fpe)</title>

	<para>
	  When the server is finished with an FPE, this function is
	  called to dispose of any internal state.  It should return
	  <errorname>Successful</errorname>, unless something terrible
	  happens.
	</para>
      </sect2>

      <sect2 id='open_font'>
	<title>(*open_font)</title>

	<para>
	  This routine requests that a font be opened.  The <parameter
	  class='function'>client</parameter> argument is used by the
	  font library only in connection with suspending/restarting
	  the request.  The <parameter class='function'>flags</parameter>
	  argument specifies some behaviour for the library and can be
	  any of:
	</para>

	<programlisting remap='.nf'>
/* OpenFont flags */
#define FontLoadInfo    0x0001
#define FontLoadProps   0x0002
#define FontLoadMetrics 0x0004
#define FontLoadBitmaps 0x0008
#define FontLoadAll     0x000f
#define FontOpenSync    0x0010
	</programlisting> <!-- .fi -->

	<para>
	  The various fields specify which portions of the font should
	  be loaded at this time.  When <constant>FontOpenSync</constant>
	  is specified, this routine will not return until all of the
	  requested portions are loaded.  Otherwise, this routine may
	  return <errorname>Suspended</errorname>.  When the presented
	  font name is actually an alias for some other font name,
	  <errorname>FontNameAlias</errorname> is returned, and the
	  actual font name is stored in the location pointed to by the
	  <parameter class='function'>alias</parameter> argument as a
	  null-terminated string.
	</para>
      </sect2>

      <sect2 id='close_font'>
	<title>(*close_font)</title>

	<para>
	  When the server is finished with a font, this routine
	  disposes of any internal state and frees the font data
	  structure.
	</para>
      </sect2>

      <sect2 id='list_fonts'>
	<title>(*list_fonts)</title>

	<para>
	  The <parameter class='function'>paths</parameter> argument is
	  a data structure which will be filled with all of the font names
	  from this directory which match the specified
	  <parameter class='function'>pattern</parameter>.  At
	  most <parameter class='function'>maxnames</parameter> will be added.
	  This routine may return <errorname>Suspended</errorname>.
	</para>
      </sect2>

      <sect2 id='start_list_fonts_with_info'>
	<title>(*start_list_fonts_with_info)</title>

	<para>
	  This routine sets any internal state for a verbose listing of
	  all fonts matching the specified pattern.  This routine may
	  return <errorname>Suspended</errorname>.
	</para>
      </sect2>

      <sect2 id='list_next_font_with_info'>
	<title>(*list_next_font_with_info)</title>

	<para>
	  To avoid storing huge amounts of data, the interface for
	  ListFontsWithInfo allows the server to get one reply at a time
	  and forward that to the client.  When the font name returned
	  is actually an alias for some other font,
	  <errorname>FontNameAlias</errorname> will be returned.  The
	  actual font name is return instead, and the font alias which
	  matched the pattern is returned in the location pointed to by
	  data as a null-terminated string.  The caller can then get the
	  information by recursively listing that font name with a
	  maxnames of 1.  When <errorname>Successful</errorname> is
	  returned, the matching font name is returned, and a
	  FontInfoPtr is stored in the location pointed to by
	  <parameter class='function'>data</parameter>.
	  <parameter class='function'>Data</parameter> must be initialized
	  with a pointer to a FontInfoRec allocated by the caller.  When the
	  pointer pointed to by <parameter class='function'>data</parameter>
	  is not left pointing at that storage, the caller mustn't free the
	  associated property data. This routine may return
	  <errorname>Suspended</errorname>.
	</para>
      </sect2>

      <sect2 id='wakeup_fpe'>
	<title>(*wakeup_fpe)</title>

	<para>
	  Whenever an FPE function has returned
	  <errorname>Suspended</errorname>, this routine is called
	  whenever the application wakes up from waiting for input
	  (from <citerefentry><refentrytitle>select</refentrytitle>
	    <manvolnum>2</manvolnum></citerefentry>).  This
	    <parameter class='function'>mask</parameter> argument should be
	    the value returned from <function>select(2)</function>.
	</para>
      </sect2>

      <sect2 id='client_died'>
	<title>(*client_died)</title>

	<para>
	  When an FPE function has returned <errorname>Suspended</errorname>
	  and the associated client is being destroyed, this function
	  allows the font library to dispose of any state associated
	  with that client.
	</para>
      </sect2>
    </sect1>

    <sect1 id='Fonts'>
<title>Fonts</title>

      <para>
	The data structure which actually contains the font information has
	changed significantly since previous releases; it now attempts to
	hide the actual storage format for the data from the application,
	providing accessor functions to get at the data.  This allows a
	range of internal details for different font sources.  The structure
	is split into two pieces, so that ListFontsWithInfo can share
	information from the font when it has been loaded.  The
	<structname>FontInfo</structname> structure, then, contains only
	information germane to LFWI.
      </para>

      <programlisting remap='.nf'>
typedef struct _FontInfo {
    unsigned short firstCol;            /* range of glyphs for this font */
    unsigned short lastCol;
    unsigned short firstRow;
    unsigned short lastRow;
    unsigned short defaultCh;           /* default character index */
    unsigned int noOverlap:1;           /* no combination of glyphs overlap */
    unsigned int terminalFont:1;        /* Character cell font */
    unsigned int constantMetrics:1;     /* all metrics are the same */
    unsigned int constantWidth:1;       /* all character widths are the same*/
    unsigned int inkInside:1;           /* all ink inside character cell */
    unsigned int inkMetrics:1;          /* font has ink metrics */
    unsigned int allExist:1;            /* no missing chars in range */
    unsigned int drawDirection:2;       /* left-to-right/right-to-left*/
    unsigned int cachable:1;            /* font needn't be opened each time*/
    unsigned int anamorphic:1;          /* font is strangely scaled */
    short       maxOverlap;             /* maximum overlap amount */
    short       pad;                    /* unused */
    xCharInfo   maxbounds;              /* glyph metrics maximums */
    xCharInfo   minbounds;              /* glyph metrics minimums */
    xCharInfo   ink_maxbounds;          /* ink metrics maximums */
    xCharInfo   ink_minbounds;          /* ink metrics minimums */
    short       fontAscent;             /* font ascent amount */
    short       fontDescent;            /* font descent amount */
    int         nprops;                 /* number of font properties */
    FontPropPtr props;                  /* font properties */
    char       *isStringProp;           /* boolean array */
}           FontInfoRec, *FontInfoPtr;
      </programlisting> <!-- .fi -->

      <para>
	The font structure, then, contains a font info record, the format of
	the bits in each bitmap and the functions which access the font
	records (which are stored in an opaque format hung off of
	<structfield>fontPrivate</structfield>).
      </para>

      <programlisting remap='.nf'>
typedef struct _Font {
    int         refcnt;
    FontInfoRec info;
    char        bit;                    /* bit order: LSBFirst/MSBFirst */
    char        byte;                   /* byte order: LSBFirst/MSBFirst */
    char        glyph;                  /* glyph pad: 1, 2, 4 or 8 */
    char        scan;                   /* glyph scan unit: 1, 2 or 4 */
    fsBitmapFormat format;              /* FS-style format (packed) */
    int         (*get_glyphs)  ( /* font, count, chars, encoding, count, glyphs */ );
    int         (*get_metrics) ( /* font, count, chars, encoding, count, glyphs */ );
    int         (*get_bitmaps) ( /* client, font, flags, format,
                                    flags, nranges, ranges, data_sizep,
                                    num_glyphsp, offsetsp, glyph_datap,
                                    free_datap */ );
    int         (*get_extents) ( /* client, font, flags, nranges,
                                    ranges, nextentsp, extentsp */);
    void        (*unload_font) ( /* font */ );
    FontPathElementPtr fpe;             /* FPE associated with this font */
    pointer     svrPrivate;             /* X/FS private data */
    pointer     fontPrivate;            /* private to font */
    pointer     fpePrivate;             /* private to FPE */
    int         maxPrivate;             /* devPrivates (see below) */
    pointer     *devPrivates;           /*  ... */
}           FontRec, *FontPtr;
      </programlisting> <!-- .fi -->

      <para>
	Yes, there are several different private pointers in the
	<structfield>Font</structfield> structure; they were added
	haphazardly until the devPrivate pointers were added.  Future
	releases may remove some (or all) of the specific pointers,
	leaving only the <structfield>devPrivates</structfield>mechanism.
      </para>

      <para>
	There are two similar interfaces implemented -
	<structfield>get_glyphs</structfield>/<structfield>get_metrics</structfield>
	and
	<structfield>get_bitmaps</structfield>/<structfield>get_extents</structfield>.
	Too little time caused the font-server specific interfaces to
	be placed in the font library (and portions duplicated in each
	renderer) instead of having them integrated into the font server
	itself.  This may change.  The X server uses only
	<structfield>get_glyphs</structfield>/<structfield>get_metrics</structfield>,
	and those will not change dramatically.  Each of the routines
	is described below.
      </para>

      <sect2 id='get_glyphs'>
	<title>(*get_glyphs)</title>

	<para>
	  This routine returns <structname>CharInfoPtrs</structname>
	  for each of the requested characters in the font.  If the
	  character does not exist in the font, the default character
	  will be returned, unless no default character exists in
	  which case that character is skipped.  Thus, the number of
	  glyphs returned will not always be the same as the number of
	  characters passed in.
	</para>
      </sect2>

      <sect2 id='get_metrics'>
	<title>(*get_metrics)</title>

	<para>
	  This is similar to <structfield>(*get_glyphs)</structfield>
	  except that pointers to <structname>xCharInfo</structname>
	  structures are returned, and, if the font has ink metrics,
	  those are returned instead of the bitmap metrics.
	</para>
      </sect2>

      <sect2 id='get_bitmaps'>
<title>(*get_bitmaps)</title>

	<para>
	  This packs the glyph image data in the requested
	  <parameter class='function'>format</parameter> and returns it.  The
	  <parameter class='function'>ranges</parameter>/<parameter class='function'>nranges</parameter>
	  argument specify the set of glyphs from the font to pack together.
	</para>
      </sect2>

      <sect2 id='get_extents'>
	<title>(*get_extents)</title>

	<para>
	  This returns the metrics for the specified font from the
	  specified <parameter class='function'>ranges</parameter>.
	</para>

      </sect2>

      <sect2 id='unload_font'>
	<title>(*unload_font)</title>

	<para>
	  This is called from the FPE routine
	  <function>(*close_font)</function>, and so should not ever be
	  called from the application.
	</para>
      </sect2>

      <sect2 id='maxPrivate'>
<title>maxPrivate</title>

	<para>
	  When initializing a new font structure,
	  <structfield>maxPrivate</structfield> should be set to -1 so
	  that the <function>FontSetPrivate()</function> macro works
	  properly with an index of 0.  Initializing
	  <structfield>maxPrivate</structfield> to 0 can cause
	  problems if the server tries to set something at index 0.
	</para>
      </sect2>
    </sect1>
  </article>