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<title>Ghostscript's external font and text interface</title>
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Ghostscript's external font and text interface
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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>
<ul>
<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>
</ul></blockquote>
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<p>For other information, see the <a href="Readme.htm">Ghostscript
overview</a>.
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<h1><a name="Introduction"></a>Introduction</h1>
<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>
<h1><a name="Types"></a>Types</h1>
<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>
<h1><a name="Implementation_procedures"></a>Implementation procedures</h1>
<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>Copyright © 1996, 1997, 1998 Aladdin Enterprises. All rights reserved.
<p>This file is part of AFPL Ghostscript. See the
<a href="Public.htm">Aladdin Free Public License</a> (the "License") for
full details of the terms of using, copying, modifying, and redistributing
AFPL Ghostscript.
<p>
Ghostscript version 6.50, 2 December 2000
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