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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<head>
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<title>Ghostscript's external font and text interface</title>
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<!-- $Id: Xfonts.htm,v 1.39 2005/10/20 19:46:23 ray Exp $ -->
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<!-- Originally: xfonts.txt -->
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<link rel="stylesheet" type="text/css" href="gs.css" title="Ghostscript Style">
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</head>
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<body>
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<!-- [1.1 begin headline] ================================================== -->
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<h1>Ghostscript's external font and text interface</h1>
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<!-- [1.1 end headline] ==================================================== -->
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<!-- [1.2 begin table of contents] ========================================= -->
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<h2>Table of contents</h2>
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<blockquote><ul>
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<li><a href="#Introduction">Introduction</a>
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<li><a href="#Types">Types</a>
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<li><a href="#Implementation_procedures">Implementation procedures</a>
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<li><a href="#Font_level_procedures">Font-level procedures</a>
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<ul>
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<li><a href="#lookup_font"><b><tt>lookup_font</tt></b></a>
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<li><a href="#char_xglyph"><b><tt>char_xglyph</tt></b></a>
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<li><a href="#char_metrics"><b><tt>char_metrics</tt></b></a>
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<li><a href="#render_char"><b><tt>render_char</tt></b></a>
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<li><a href="#release"><b><tt>release</tt></b></a>
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</ul>
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</ul></blockquote>
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<!-- [1.3 begin hint] ====================================================== -->
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<p>For other information, see the <a href="Readme.htm">Ghostscript
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overview</a>.
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<hr>
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<!-- [1.0 end visible header] ============================================== -->
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<!-- [2.0 begin contents] ================================================== -->
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<h2><a name="Introduction"></a>Introduction</h2>
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<p>
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Ghostscript can use the character rasterizer provided by the underlying
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operating system and window system; specifically, Adobe Type Manager (ATM)
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or a TrueType rasterizer under MS Windows, or the facilities provided by X
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Windows. This ability augments, but does not replace, Ghostscript's own
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Type 1 rasterizer: Ghostscript may still use its own rasterizer for very
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large characters, characters that are clipped or transformed in unusual
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ways, and for output to devices other than the screen.
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<p>
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Ghostscript connects to these platform facilities through a driver-like
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interface called the xfont (external font) interface. Current xfont
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implementations are associated directly with device drivers; in a future
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release, Ghostscript may separate them, so that (for example) it will be
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possible to use the platform rasterizer when writing to a file.
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<p>
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Please note that from this point, this file is likely to be useful only
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to a small number of Ghostscript porters and implementors.
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<hr>
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<h2><a name="Types"></a>Types</h2>
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<table cellpadding=0 cellspacing=0>
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<tr> <th valign=bottom align=left>Type
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<td>
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<th valign=bottom align=left>Declared /<br>defined in
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<td>
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<th valign=bottom align=left>Represents
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<tr> <td colspan=5><hr>
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<tr> <td valign=top><b><tt>gs_char</tt></b>
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<td>
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<td valign=top><b><tt>gsccode.h</tt></b>
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<td>
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<td valign=top>A character code that appears in a string. Currently
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it is always a single byte, but composite fonts or Unicode may
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require it to be wider in the future.
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<tr> <td valign=top><b><tt>gs_glyph</tt></b>
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<td>
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<td valign=top><b><tt>gsccode.h</tt></b>
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<td>
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<td valign=top>A character name like "period" or "epsilon". From
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the xfont implementation's point of view, it is just a handle;
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when necessary, Ghostscript provides a
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<b><tt>gs_proc_glyph_name_t</tt></b> procedure to
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convert it to a string name.
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<tr> <td valign=top><b><tt>gs_proc_glyph_name_t</tt></b>
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<td>
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<td valign=top><b><tt>gsccode.h</tt></b>
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<td>
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<td valign=top>A procedure that maps a <b><tt>gs_glyph</tt></b> to
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its string name; see the <b><tt>char_xglyph</tt></b> procedure.
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<tr> <td valign=top><b><tt>gx_xglyph</tt></b>
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<td>
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<td valign=top><b><tt>gsxfont.h</tt></b>
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<td>
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<td valign=top>A character or glyph code that can be used with a
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specific platform font. Normally it will be a character code
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that the implementation of <b><tt>render_char</tt></b> will
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turn into a 1-character string and give to the platform's
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"display string" operation.
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<tr> <td valign=top><b><tt>gx_xfont_procs</tt></b>
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<td>
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<td valign=top><b><tt>gsxfont.h</tt></b>, <b><tt>gxxfont.h</tt></b>
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<td>
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<td valign=top>The xfont analogue of
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<b><tt>gx_device_procs</tt></b>, the type of the
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procedure record that defines an xfont implementation.
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<tr> <td valign=top><b><tt>gx_xfont</tt></b>
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<td>
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<td valign=top><b><tt>gsxfont.h</tt></b>, <b><tt>gxxfont.h</tt></b>
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<td>
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<td valign=top>The gxfont analogue of <b><tt>gx_device</tt></b>,
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the type of the basic structure for an xfont.
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<tr> <td valign=top>(<b><tt>encoding_index</tt></b>)
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<td>
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<td>
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<td>
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<td valign=top>Not really a type, although it probably should be:
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an <b><tt>int</tt></b> used to indicate the
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<b><tt>Encoding</tt></b> used by a font. Defined values are
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<dl compact>
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<dt>0<dd><b><tt>StandardEncoding</tt></b>
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<dt>1<dd><b><tt>ISOLatin1Encoding</tt></b>
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<dt>2<dd><b><tt>SymbolEncoding</tt></b>
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<dt>3<dd><b><tt>DingbatsEncoding</tt></b>
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<dt>-1<dd>Other encoding
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</dl>
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</table>
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<hr>
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<h2><a name="Implementation_procedures"></a>Implementation
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procedures</h2>
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<p>
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All the procedures that return <b><tt>int</tt></b> results return 0 on
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success, or an appropriate negative error code for error conditions; these
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error codes are defined in <b><tt>gserrors.h</tt></b>. The relevant ones
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are the same as for <a href="Drivers.htm">drivers</a>, and as with drivers,
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if an implementation procedure returns an error, it should use the
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<b><tt>return_error</tt></b> macro -- defined in <b><tt>gx.h</tt></b>,
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which is automatically included by <b><tt>gdevprn.h</tt></b> but not by
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<b><tt>gserrors.h</tt></b> -- rather than a simple <b><tt>return</tt></b>
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statement, for instance
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<blockquote>
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<b><tt>return_error(gs_error_VMerror);</tt></b>
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</blockquote>
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<h2><a name="Font_level_procedures"></a>Font-level procedures</h2>
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<h3><a name="lookup_font"></a><b><tt>lookup_font</tt></b></h3>
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<dl>
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<dt><b><tt>gx_xfont *(*lookup_font)(P7(gx_device *dev,
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const byte *fname, uint len, int encoding_index,
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const gs_uid *puid, const gs_matrix *pmat,
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const gs_memory_procs *mprocs))</tt></b>
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<dd>Look up a font name, <b><tt>UniqueID</tt></b>, and matrix, and return
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an xfont, or <b><tt>NULL</tt></b> if no suitable xfont exists. Use
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<b><tt>mprocs</tt></b> to allocate the xfont and any subsidiary data
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structures. The matrix is the <b><tt>FontMatrix</tt></b> concatenated with
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the CTM, so (roughly speaking) the font size in pixels is
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<b><tt>pmat -> yy</tt></b> × 1000 for a normal
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Type 1 font.
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<p>
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Note that this is the only implementation procedure that does not take an
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xfont * as its first argument. In fact, callers of
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<b><tt>lookup_font</tt></b> must use the <b><tt>get_xfont_device</tt></b>
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driver procedure to get the correct device to pass as the first argument to
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<b><tt>lookup_font</tt></b>.
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</dl>
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<h3><a name="char_xglyph"></a><b><tt>char_xglyph</tt></b></h3>
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<dl>
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<dt><b><tt>gx_xglyph (*char_xglyph)(P5(gx_xfont *xf, gs_char chr,
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int encoding_index, gs_glyph glyph,
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gs_proc_glyph_name_t glyph_name))</tt></b>
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<dd>Convert a character name to an xglyph code. In the case of
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<b><tt>glyphshow</tt></b>, <b><tt>chr</tt></b> may be
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<b><tt>gs_no_char</tt></b>; for an ordinary <b><tt>show</tt></b> operation,
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if the character code is invalid, <b><tt>glyph</tt></b> may be
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<b><tt>gs_no_glyph</tt></b>.
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</dl>
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<h3><a name="char_metrics"></a><b><tt>char_metrics</tt></b></h3>
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<dl>
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<dt><b><tt>int (*char_metrics)(P5(gx_xfont *xf, gx_xglyph xg,
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int wmode, gs_int_point *pwidth,
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gs_int_rect *pbbox))</tt></b>
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<dd>Get the metrics for a character. If the metrics are unavailable,
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return 1.
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</dl>
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<h3><a name="render_char"></a><b><tt>render_char</tt></b></h3>
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<dl>
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<dt><b><tt>int (*render_char)(P7(gx_xfont *xf, gx_xglyph xg,
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gx_device *target, int x, int y, gx_color_index color,
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int required))</tt></b>
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<dd>Render a character. <em>(x,y)</em> corresponds to the character
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origin. The target may be any Ghostscript device. A good implementation
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will check whether the target can handle this type of xfont directly (for
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instance by checking the target name), and if so, will render the character
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directly; otherwise, it will do what has to be done in the general case,
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namely, get a bitmap for the character and use the target's
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<b><tt>copy_mono</tt></b> operation. If <b><tt>required</tt></b> is false,
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the procedure should return an error if the rendering operation would be
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expensive, since in this case Ghostscript has already cached the bitmap and
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metrics from a previous call with <b><tt>required</tt></b>=true. If the
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operation cannot be done, return 1.
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</dl>
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<h3><a name="release"></a><b><tt>release</tt></b></h3>
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<dl>
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<dt><b><tt>int (*release)(P2(gx_xfont *xf,
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const gs_memory_procs *mprocs))</tt></b>
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<dd>Release any external resources associated with an xfont. If
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<b><tt>mprocs</tt></b> is not <b><tt>NULL</tt></b>, also free any storage
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allocated by <b><tt>lookup_font</tt></b> (including the xfont itself).
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</dl>
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<!-- [2.0 end contents] ==================================================== -->
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<!-- [3.0 begin visible trailer] =========================================== -->
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<hr>
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<p>
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<small>Copyright © 1996, 1997, 1998 Aladdin Enterprises. All
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rights reserved.</small>
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<p>
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This software is provided AS-IS with no warranty, either express or
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implied.
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This software is distributed under license and may not be copied,
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modified or distributed except as expressly authorized under the terms
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of the license contained in the file LICENSE in this distribution.
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For more information about licensing, please refer to
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http://www.ghostscript.com/licensing/. For information on
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commercial licensing, go to http://www.artifex.com/licensing/ or
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contact Artifex Software, Inc., 101 Lucas Valley Road #110,
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San Rafael, CA 94903, U.S.A., +1(415)492-9861.
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<p>
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<small>Ghostscript version 8.53, 20 October 2005
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<!-- [3.0 end visible trailer] ============================================= -->
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</body>
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</html>
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