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<h1>Ghostscript's external font and text interface</h1>
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<h2>Table of contents</h2>
<blockquote><ul>
<li><a href="#Introduction">Introduction</a>
<li><a href="#Types">Types</a>
<li><a href="#Implementation_procedures">Implementation procedures</a>
<li><a href="#Font_level_procedures">Font-level procedures</a>
<ul>
<li><a href="#lookup_font"><b><tt>lookup_font</tt></b></a>
<li><a href="#char_xglyph"><b><tt>char_xglyph</tt></b></a>
<li><a href="#char_metrics"><b><tt>char_metrics</tt></b></a>
<li><a href="#render_char"><b><tt>render_char</tt></b></a>
<li><a href="#release"><b><tt>release</tt></b></a>
</ul>
</ul></blockquote>
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<p>For other information, see the <a href="Readme.htm">Ghostscript
overview</a>.
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<h2><a name="Introduction"></a>Introduction</h2>
<p>
Ghostscript can use the character rasterizer provided by the underlying
operating system and window system; specifically, Adobe Type Manager (ATM)
or a TrueType rasterizer under MS Windows, or the facilities provided by X
Windows. This ability augments, but does not replace, Ghostscript's own
Type 1 rasterizer: Ghostscript may still use its own rasterizer for very
large characters, characters that are clipped or transformed in unusual
ways, and for output to devices other than the screen.
<p>
Ghostscript connects to these platform facilities through a driver-like
interface called the xfont (external font) interface. Current xfont
implementations are associated directly with device drivers; in a future
release, Ghostscript may separate them, so that (for example) it will be
possible to use the platform rasterizer when writing to a file.
<p>
Please note that from this point, this file is likely to be useful only
to a small number of Ghostscript porters and implementors.
<hr>
<h2><a name="Types"></a>Types</h2>
<table cellpadding=0 cellspacing=0>
<tr> <th valign=bottom align=left>Type
<td>
<th valign=bottom align=left>Declared /<br>defined in
<td>
<th valign=bottom align=left>Represents
<tr> <td colspan=5><hr>
<tr> <td valign=top><b><tt>gs_char</tt></b>
<td>
<td valign=top><b><tt>gsccode.h</tt></b>
<td>
<td valign=top>A character code that appears in a string. Currently
it is always a single byte, but composite fonts or Unicode may
require it to be wider in the future.
<tr> <td valign=top><b><tt>gs_glyph</tt></b>
<td>
<td valign=top><b><tt>gsccode.h</tt></b>
<td>
<td valign=top>A character name like "period" or "epsilon". From
the xfont implementation's point of view, it is just a handle;
when necessary, Ghostscript provides a
<b><tt>gs_proc_glyph_name_t</tt></b> procedure to
convert it to a string name.
<tr> <td valign=top><b><tt>gs_proc_glyph_name_t</tt></b>
<td>
<td valign=top><b><tt>gsccode.h</tt></b>
<td>
<td valign=top>A procedure that maps a <b><tt>gs_glyph</tt></b> to
its string name; see the <b><tt>char_xglyph</tt></b> procedure.
<tr> <td valign=top><b><tt>gx_xglyph</tt></b>
<td>
<td valign=top><b><tt>gsxfont.h</tt></b>
<td>
<td valign=top>A character or glyph code that can be used with a
specific platform font. Normally it will be a character code
that the implementation of <b><tt>render_char</tt></b> will
turn into a 1-character string and give to the platform's
"display string" operation.
<tr> <td valign=top><b><tt>gx_xfont_procs</tt></b>
<td>
<td valign=top><b><tt>gsxfont.h</tt></b>, <b><tt>gxxfont.h</tt></b>
<td>
<td valign=top>The xfont analogue of
<b><tt>gx_device_procs</tt></b>, the type of the
procedure record that defines an xfont implementation.
<tr> <td valign=top><b><tt>gx_xfont</tt></b>
<td>
<td valign=top><b><tt>gsxfont.h</tt></b>, <b><tt>gxxfont.h</tt></b>
<td>
<td valign=top>The gxfont analogue of <b><tt>gx_device</tt></b>,
the type of the basic structure for an xfont.
<tr> <td valign=top>(<b><tt>encoding_index</tt></b>)
<td>
<td>
<td>
<td valign=top>Not really a type, although it probably should be:
an <b><tt>int</tt></b> used to indicate the
<b><tt>Encoding</tt></b> used by a font. Defined values are
<dl compact>
<dt>0<dd><b><tt>StandardEncoding</tt></b>
<dt>1<dd><b><tt>ISOLatin1Encoding</tt></b>
<dt>2<dd><b><tt>SymbolEncoding</tt></b>
<dt>3<dd><b><tt>DingbatsEncoding</tt></b>
<dt>-1<dd>Other encoding
</dl>
</table>
<hr>
<h2><a name="Implementation_procedures"></a>Implementation
procedures</h2>
<p>
All the procedures that return <b><tt>int</tt></b> results return 0 on
success, or an appropriate negative error code for error conditions; these
error codes are defined in <b><tt>gserrors.h</tt></b>. The relevant ones
are the same as for <a href="Drivers.htm">drivers</a>, and as with drivers,
if an implementation procedure returns an error, it should use the
<b><tt>return_error</tt></b> macro -- defined in <b><tt>gx.h</tt></b>,
which is automatically included by <b><tt>gdevprn.h</tt></b> but not by
<b><tt>gserrors.h</tt></b> -- rather than a simple <b><tt>return</tt></b>
statement, for instance
<blockquote>
<b><tt>return_error(gs_error_VMerror);</tt></b>
</blockquote>
<h2><a name="Font_level_procedures"></a>Font-level procedures</h2>
<h3><a name="lookup_font"></a><b><tt>lookup_font</tt></b></h3>
<dl>
<dt><b><tt>gx_xfont *(*lookup_font)(P7(gx_device *dev,
const byte *fname, uint len, int encoding_index,
const gs_uid *puid, const gs_matrix *pmat,
const gs_memory_procs *mprocs))</tt></b>
<dd>Look up a font name, <b><tt>UniqueID</tt></b>, and matrix, and return
an xfont, or <b><tt>NULL</tt></b> if no suitable xfont exists. Use
<b><tt>mprocs</tt></b> to allocate the xfont and any subsidiary data
structures. The matrix is the <b><tt>FontMatrix</tt></b> concatenated with
the CTM, so (roughly speaking) the font size in pixels is
<b><tt>pmat -> yy</tt></b> × 1000 for a normal
Type 1 font.
<p>
Note that this is the only implementation procedure that does not take an
xfont * as its first argument. In fact, callers of
<b><tt>lookup_font</tt></b> must use the <b><tt>get_xfont_device</tt></b>
driver procedure to get the correct device to pass as the first argument to
<b><tt>lookup_font</tt></b>.
</dl>
<h3><a name="char_xglyph"></a><b><tt>char_xglyph</tt></b></h3>
<dl>
<dt><b><tt>gx_xglyph (*char_xglyph)(P5(gx_xfont *xf, gs_char chr,
int encoding_index, gs_glyph glyph,
gs_proc_glyph_name_t glyph_name))</tt></b>
<dd>Convert a character name to an xglyph code. In the case of
<b><tt>glyphshow</tt></b>, <b><tt>chr</tt></b> may be
<b><tt>gs_no_char</tt></b>; for an ordinary <b><tt>show</tt></b> operation,
if the character code is invalid, <b><tt>glyph</tt></b> may be
<b><tt>gs_no_glyph</tt></b>.
</dl>
<h3><a name="char_metrics"></a><b><tt>char_metrics</tt></b></h3>
<dl>
<dt><b><tt>int (*char_metrics)(P5(gx_xfont *xf, gx_xglyph xg,
int wmode, gs_int_point *pwidth,
gs_int_rect *pbbox))</tt></b>
<dd>Get the metrics for a character. If the metrics are unavailable,
return 1.
</dl>
<h3><a name="render_char"></a><b><tt>render_char</tt></b></h3>
<dl>
<dt><b><tt>int (*render_char)(P7(gx_xfont *xf, gx_xglyph xg,
gx_device *target, int x, int y, gx_color_index color,
int required))</tt></b>
<dd>Render a character. <em>(x,y)</em> corresponds to the character
origin. The target may be any Ghostscript device. A good implementation
will check whether the target can handle this type of xfont directly (for
instance by checking the target name), and if so, will render the character
directly; otherwise, it will do what has to be done in the general case,
namely, get a bitmap for the character and use the target's
<b><tt>copy_mono</tt></b> operation. If <b><tt>required</tt></b> is false,
the procedure should return an error if the rendering operation would be
expensive, since in this case Ghostscript has already cached the bitmap and
metrics from a previous call with <b><tt>required</tt></b>=true. If the
operation cannot be done, return 1.
</dl>
<h3><a name="release"></a><b><tt>release</tt></b></h3>
<dl>
<dt><b><tt>int (*release)(P2(gx_xfont *xf,
const gs_memory_procs *mprocs))</tt></b>
<dd>Release any external resources associated with an xfont. If
<b><tt>mprocs</tt></b> is not <b><tt>NULL</tt></b>, also free any storage
allocated by <b><tt>lookup_font</tt></b> (including the xfont itself).
</dl>
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<p>
<small>Copyright © 1996, 1997, 1998 Aladdin Enterprises. All
rights reserved.</small>
<p>
This software is provided AS-IS with no warranty, either express or
implied.
This software is distributed under license and may not be copied,
modified or distributed except as expressly authorized under the terms
of the license contained in the file LICENSE in this distribution.
For more information about licensing, please refer to
http://www.ghostscript.com/licensing/. For information on
commercial licensing, go to http://www.artifex.com/licensing/ or
contact Artifex Software, Inc., 101 Lucas Valley Road #110,
San Rafael, CA 94903, U.S.A., +1(415)492-9861.
<p>
<small>Ghostscript version 8.53, 20 October 2005
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