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<title>The Ghostscript library</title>
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The Ghostscript library
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<h2>Table of contents</h2>
<blockquote><ul>
<li><a href="#GS_library">The Ghostscript library</a>
<li><a href="#PS_operator_API">PostScript operator API</a>
<ul>
<li><a href="#Patterns">Patterns</a>
<li><a href="#Lower_level_API">Lower-level API</a>
</ul>
<li><a href="#Full_example">A full example</a>
</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="GS_library"></a>The Ghostscript library</h1>
<p>
This document describes the Ghostscript library, a set of procedures
to implement the graphics and filtering capabilities that are primitive
operations in the PostScript language and in Adobe Portable Document Format
(PDF).
<p>
Ghostscript is actually two programs: a language interpreter, and a
graphics library. The library provides, in the form of C procedures, all
the graphics functions of the language, that is, approximately those
facilities listed in section 8.1 of the
<em><a href="http://cseng.awl.com/bookdetail.qry?ISBN=0-201-18127-4&ptype=0">PostScript
Language Reference Manual</a></em>, Second Edition, starting with the
graphics state operators. In addition, the library provides some
lower-level graphics facilities that offer higher performance in exchange
for less generality.
<hr>
<h1><a name="PS_operator_API"></a>PostScript operator API</h1>
<p>
The highest level of the library, which is the one that most clients will
use, directly implements the PostScript graphics operators with procedures
named <b><tt>gs_XXX</tt></b>, for instance <b><tt>gs_moveto</tt></b> and
<b><tt>gs_fill</tt></b>. Nearly all of these procedures take graphics
state objects as their first arguments, such as
<blockquote><b><tt>
int gs_moveto(gs_state *, double, double);
</tt></b></blockquote>
<p>
Nearly every procedure returns an integer code which is >= 0 for a
successful return or <0 for a failure. The failure codes correspond
directly to PostScript errors, and are defined in
<b><tt>gserrors.h</tt></b>.
<p>
The library implements all the operators in the following sections of the
<em><a href="http://cseng.awl.com/bookdetail.qry?ISBN=0-201-18127-4&ptype=0">PostScript
Language Reference Manual</a></em>, Second Edition, with the indicated
omissions and with the APIs defined in the indicated <b><tt>.h</tt></b>
files. A header of the form <b><em>A.h(B.h)</em></b> indicates that
<b><em>A.h</em></b> is included in <b><em>B.h</em></b>, so
<b><em>A.h</em></b> need not be included explicitly if <b><em>B.h</em></b>
is included. Operators marked with * in the "omissions" column are not
implemented directly; the library provides lower-level procedures that can
be used to implement the operator.
<p>
There are slight differences in the operators that return multiple values,
since C's provisions for this are awkward. Also, the control structure for
the operators involving callback procedures (<b><tt>pathforall</tt></b>,
<b><tt>image</tt></b>, <b><tt>colorimage</tt></b>,
<b><tt>imagemask</tt></b>) is partly inverted: the client calls a procedure
to set up an enumerator object, and then calls another procedure for each
iteration. The <b><tt>...show</tt></b> operators,
<b><tt>charpath</tt></b>, and <b><tt>stringwidth</tt></b> also use an
inverted control structure.
<blockquote><table cellpadding=0 cellspacing=0>
<tr valign=bottom>
<th align=left>Section<br>(operators)
<td>
<th align=left>Headers
<td>
<th align=left>Omissions
<tr> <td colspan=5><hr>
<tr valign=top> <td>Graphics state -- device-independent
<td>
<td><b><tt>gscolor.h</tt></b>(<b><tt>gsstate.h</tt></b>)<br><b><tt>gscolor1.h</tt></b><br><b><tt>gscolor2.h</tt></b><br><b><tt>gscspace.h</tt></b><br><b><tt>gshsb.h</tt></b><br><b><tt>gsline.h</tt></b>(<b><tt>gsstate.h</tt></b>)<br><b><tt>gsstate.h</tt></b>
<td>
<td>
<tr> <td>
<tr valign=top> <td>Graphics state -- device-dependent
<td>
<td><b><tt>gscolor.h</tt></b>(<b><tt>gsstate.h</tt></b>)<br><b><tt>gscolor1.h</tt></b><br><b><tt>gscolor2.h</tt></b><br><b><tt>gsht.h</tt></b>(<b><tt>gsht1.h</tt></b>,<b><tt>gsstate.h</tt></b>)<br><b><tt>gsht1.h</tt></b><br><b><tt>gsline.h</tt></b>(<b><tt>gsstate.h</tt></b>)
<td>
<td>
<tr> <td>
<tr valign=top> <td>Coordinate system and matrix
<td>
<td><b><tt>gscoord.h</tt></b><br><b><tt>gsmatrix.h</tt></b>
<td>
<td><b><tt>*matrix</tt></b>, <b><tt>*identmatrix</tt></b>, <b><tt>*concatmatrix</tt></b>, <b><tt>*invertmatrix</tt></b>
<tr> <td>
<tr valign=top> <td>Path construction
<td>
<td><b><tt>gspath.h</tt></b><br><b><tt>gspath2.h</tt></b>
<td>
<td><b><tt>*arct</tt></b>, <b><tt>*pathforall</tt></b>, <b><tt>ustrokepath</tt></b>, <b><tt>uappend</tt></b>, <b><tt>upath</tt></b>, <b><tt>ucache</tt></b>
<tr> <td>
<tr valign=top> <td>Painting
<td>
<td><b><tt>gsimage.h</tt></b><br><b><tt>gspaint.h</tt></b><br><b><tt>gspath2.h</tt></b>
<td>
<td><b><tt>*image</tt></b>, <b><tt>*colorimage</tt></b>, <b><tt>*imagemask</tt></b>, <b><tt>ufill</tt></b>, <b><tt>ueofill</tt></b>, <b><tt>ustroke</tt></b>
<tr> <td>
<tr valign=top> <td>Form and pattern
<td>
<td><b><tt>gscolor2.h</tt></b>
<td>
<td><b><tt>execform</tt></b>
<tr> <td>
<tr valign=top> <td>Device setup and output
<td>
<td><b><tt>gsdevice.h</tt></b>
<td>
<td><b><tt>*showpage</tt></b>, <b><tt>*set</tt></b>/<b><tt>currentpagedevice</tt></b>
<tr> <td>
<tr valign=top> <td>Character and font
<td>
<td><b><tt>gschar.h</tt></b><br><b><tt>gsfont.h</tt></b>
<td>
<td>*(all the <b><tt>show</tt></b> operators), <b><tt>definefont</tt></b>, <b><tt>undefinefont</tt></b>, <b><tt>findfont</tt></b>, <b><tt>*scalefont</tt></b>, <b><tt>*makefont</tt></b>, <b><tt>selectfont</tt></b>, <b><tt>[Global]FontDirectory</tt></b>, <b><tt>Standard</tt></b>/<b><tt>ISOLatin1Encoding</tt></b>, <b><tt>findencoding</tt></b>
</table></blockquote>
<p>
The following procedures from the list above operate differently from their
PostScript operator counterparts, as explained here:
<dl>
<dt><b><tt>gs_makepattern(gscolor2.h)</tt></b>
<dd>Takes an explicit current color, rather than using the current color in
the graphics state. Takes an explicit allocator for allocating the pattern
implementation. See below for more details on
<b><tt>gs_makepattern</tt></b>.
</dl>
<dl>
<dt><b><tt>gs_setpattern(gscolor2.h)</tt></b>
<dt><b><tt>gs_setcolor(gscolor2.h)</tt></b>
<dt><b><tt>gs_currentcolor(gscolor2.h)</tt></b>
<dd>Use <b><tt>gs_client_color</tt></b> rather than a set of color
parameter values. See below for more details on
<b><tt>gs_setpattern</tt></b>.
</dl>
<dl>
<dt><b><tt>gs_currentdash_length/pattern/offset(gsline.h)</tt></b>
<dd>Splits up <b><tt>currentdash</tt></b> into three separate procedures.
</dl>
<dl>
<dt><b><tt>gs_screen_init/currentpoint/next/install(gsht.h)</tt></b>
<dd>Provide an "enumeration style" interface to <b><tt>setscreen</tt></b>.
(<b><tt>gs_setscreen</tt></b> is also implemented.)
</dl>
<dl>
<dt><b><tt>gs_rotate/scale/translate(gscoord.h)</tt></b>
<dt><b><tt>gs_[i][d]transform(gscoord.h)</tt></b>
<dd>These always operate on the graphics state CTM. The corresponding
operations on free-standing matrices are in <b><tt>gsmatrix.h</tt></b> and
have different names.
</dl>
<dl>
<dt><b><tt>gs_path_enum_alloc/init/next/cleanup(gspath.h)</tt></b>
<dd>Provide an "enumeration style" implementation of
<b><tt>pathforall</tt></b>.
</dl>
<dl>
<dt><b><tt>gs_image_enum_alloc(gsimage.h)</tt></b>
<dt><b><tt>gs_image_init/next/cleanup(gsimage.h)</tt></b>
<dd>Provide an "enumeration style" interface to the equivalent of
<b><tt>image</tt></b>, <b><tt>imagemask</tt></b>, and
<b><tt>colorimage</tt></b>. In the <b><tt>gs_image_t</tt></b>,
<b><tt>ColorSpace</tt></b> provides an explicit color space, rather than
using the current color space in the graphics state;
<b><tt>ImageMask</tt></b> distinguishes <b><tt>imagemask</tt></b> from
<b><tt>[color]image</tt></b>.
</dl>
<dl>
<dt><b><tt>gs_get/putdeviceparams(gsdevice.h)</tt></b>
<dd>Take a <b><tt>gs_param_list</tt></b> for specifying or receiving the
parameter values. See <b><tt>gsparam.h</tt></b> for more details.
</dl>
<dl>
<dt><b><tt>gs_show_enum_alloc/release(gschar.h)</tt></b>
<dt><b><tt>gs_xxxshow_[n_]init(gschar.h)</tt></b>
<dt><b><tt>gs_show_next(gschar.h)</tt></b>
<dd>Provide an "enumeration style" interface to writing text. Note that
control returns to the caller if the character must be rasterized.
</dl>
<p>
This level of the library also implements the following operators from other
sections of the Manual:
<blockquote><table cellpadding=0 cellspacing=0>
<tr valign=bottom>
<th align=left>Section<br>(operators)
<td>
<th align=left>Headers
<td>
<th align=left>Operators
<tr> <td colspan=5><hr>
<tr valign=top> <td>Interpreter parameter
<td>
<td><b><tt>gsfont.h</tt></b>
<td>
<td><b><tt>cachestatus</tt></b>, <b><tt>setcachelimit</tt></b>, <b><tt>*set/currentcacheparams</tt></b>
<tr valign=top> <td>Display PostScript
<td>
<td><b><tt>gsstate.h</tt></b>
<td>
<td><b><tt>set/currenthalftonephase</tt></b>
</table></blockquote>
<p>
In order to obtain the full PostScript Level 2 functionality listed above,
<b><tt>FEATURE_DEVS</tt></b> must be set in the makefile to include at least the following:
<blockquote><b><tt>
FEATURE_DEVS=patcore.dev cmykcore.dev psl2core.dev dps2core.dev ciecore.dev path1core.dev hsbcore.dev
</tt></b></blockquote>
<p>
The <b><tt>*lib.mak</tt></b> makefiles mentioned below do not always
include all of these features.
<p>
Files named <b><tt>gs*.c</tt></b> implement the higher level of the
graphics library. As might be expected, all procedures, variables, and
structures available at this level begin with "<b><tt>gs_</tt></b>".
Structures that appear in these interfaces, but whose definitions may be
hidden from clients, also have names beginning with "<b><tt>gs_</tt></b>",
that is, the prefix, not the implementation, reflects at what level the
abstraction is made available.
<h2><a name="Patterns"></a>Patterns</h2>
<p>
Patterns are the most complicated PostScript language objects that the
library API deals with. As in PostScript, defining a pattern color and
using the color are two separate operations.
<p>
<b><tt>gs_makepattern</tt></b> defines a pattern color. Its arguments are as follows:
<blockquote><table cellpadding=0 cellspacing=0>
<tr valign=top> <td><b><tt>gs_client_color *</tt></b>
<td>
<td>The resulting <b><tt>Pattern</tt></b> color is stored here. This is different from PostScript, which has no color objects <em>per se</em>, and hence returns a modified copy of the dictionary.
<tr valign=top> <td><b><tt>const gs_client_pattern *</tt></b>
<td>
<td>The analogue of the original <b><tt>Pattern</tt></b> dictionary, described in detail just below.
<tr valign=top> <td><b><tt>const gs_matrix *</tt></b>
<td>
<td>Corresponds to the matrix argument of the <b><tt>makepattern</tt></b> operator.
<tr valign=top> <td><b><tt>gs_state *</tt></b>
<td>
<td>The current graphics state.
<tr valign=top> <td><b><tt>gs_memory_t *</tt></b>
<td>
<td>The allocator to use for allocating the saved data for the
<b><tt>Pattern</tt></b> color. If this is
<b><tt>NULL</tt></b>, <b><tt>gs_makepattern</tt></b> uses the
same allocator that allocated the graphics state. Library
clients should probably always use <b><tt>NULL</tt></b>.
</table></blockquote>
<p>
The <b><tt>gs_client_pattern</tt></b> structure defined in
<b><tt>gscolor2.h</tt></b> corresponds to the <b><tt>Pattern</tt></b>
dictionary that is the argument to the PostScript language
<b><tt>makepattern</tt></b> operator. This structure has one extra member,
<b><tt>void *client_data</tt></b>, which is a place for clients to
store a pointer to additional data for the <b><tt>PaintProc</tt></b>; this
provides the same functionality as putting additional keys in the
<b><tt>Pattern</tt></b> dictionary at the PostScript language level. The
<b><tt>PaintProc</tt></b> is an ordinary C procedure that takes as
parameters a <b><tt>gs_client_color *</tt></b>, which is the
<b><tt>Pattern</tt></b> color that is being used for painting, and a
<b><tt>gs_state *</tt></b>, which is the same graphics state that
would be presented to the <b><tt>PaintProc</tt></b> in PostScript.
Currently the <b><tt>gs_client_color *</tt></b> is always the current
color in the graphics state, but the <b><tt>PaintProc</tt></b> should not
rely on this. The <b><tt>PaintProc</tt></b> can retrieve the
<b><tt>gs_client_pattern *</tt></b> from the
<b><tt>gs_client_color *</tt></b> with the
<b><tt>gs_getpattern</tt></b> procedure, also defined in
<b><tt>gscolor2.h</tt></b>, and from there, it can retrieve the
<b><tt>client_data</tt></b> pointer.
<p>
The normal way to set a <b><tt>Pattern</tt></b> color is to call
<b><tt>gs_setpattern</tt></b> with the graphics state and with the
<b><tt>gs_client_color</tt></b> returned by <b><tt>gs_makepattern</tt></b>.
After that, one can use <b><tt>gs_setcolor</tt></b> to set further
<b><tt>Pattern</tt></b> colors (colored, or uncolored with the same
underlying color space); the rules are the same as those in PostScript.
Note that for <b><tt>gs_setpattern</tt></b>, the
<b><tt>paint.values</tt></b> in the <b><tt>gs_client_color</tt></b> must be
filled in for uncolored patterns; this corresponds to the additional
arguments for the PostScript <b><tt>setpattern</tt></b> operator in the
uncolored case.
<p>
There is a special procedure <b><tt>gs_makebitmappattern</tt></b> for creating bitmap-based
patterns. Its API is documented in <b><tt>gscolor2.h</tt></b>; its implementation, in
<b><tt>gspcolor.c</tt></b>, may be useful as an example of a pattern using a particularly
simple <b><tt>PaintProc.</tt></b>
<h2><a name="Lower_level_API"></a>Lower-level API</h2>
<p>
Files named <b><tt>gx*.c</tt></b> implement the lower level of the graphics
library. The interfaces at the <b><tt>gx</tt></b> level are less stable,
and expose more of the implementation detail, than those at the
<b><tt>gs</tt></b> level: in particular, the <b><tt>gx</tt></b> interfaces
generally use device coordinates in an internal fixed-point representation,
as opposed to the <b><tt>gs</tt></b> interfaces that use floating point
user coordinates. Named entities at this level begin with
<b><tt>gx_</tt></b>.
<p>
Files named <b><tt>gz*.c</tt></b> and <b><tt>gz*.h</tt></b> are internal to
the Ghostscript implementation, and are not designed to be called by
clients.
<hr>
<h1><a name="Full_example"></a>A full example</h1>
<p>
The file <b><tt>gslib.c</tt></b> in the Ghostscript fileset is a complete
example program that initializes the library and produces output using it;
files named <b><tt>*lib.mak</tt></b> (such as <b><tt>ugcclib.mak</tt></b>
and <b><tt>bclib.mak</tt></b>) are makefiles using <b><tt>gslib.c</tt></b>
as the main program. The following annotated excerpts from this file are
intended to provide a roadmap for applications that call the library.
<blockquote><pre>/* Capture stdin/out/err before gs.h redefines them. */
#include <stdio.h>
static FILE *real_stdin, *real_stdout, *real_stderr;
static void
get_real(void)
{ real_stdin = stdin, real_stdout = stdout, real_stderr = stderr;
}</pre></blockquote>
<p>
Any application using Ghostscript should include the fragment above at the
very beginning of the main program.
<blockquote><pre>#include "gx.h"</pre></blockquote>
<p>
The <b><tt>gx.h</tt></b> header includes a wealth of declarations related
to the Ghostscript memory manager, portability machinery, debugging
framework, and other substrate facilities. Any application file that calls
any Ghostscript API functions should probably include <b><tt>gx.h</tt></b>.
<blockquote><pre>/* Configuration information imported from gconfig.c. */
extern gx_device *gx_device_list[];
/* Other imported procedures */
/* from gsinit.c */
extern void gs_lib_init(P1(FILE *));
extern void gs_lib_finit(P2(int, int));
/* from gsalloc.c */
extern gs_ref_memory_t *ialloc_alloc_state(P2(gs_memory_t *, uint));</pre></blockquote>
<p>
The externs above are needed for initializing the library.
<blockquote><pre> gs_ref_memory_t *imem;
#define mem ((gs_memory_t *)imem)
gs_state *pgs;
gx_device *dev = gx_device_list[0];
gp_init();
get_real();
gs_stdin = real_stdin;
gs_stdout = real_stdout;
gs_stderr = real_stderr;
gs_lib_init(stdout);
....
imem = ialloc_alloc_state(&gs_memory_default, 20000);
imem->space = 0;
....
pgs = gs_state_alloc(mem);</pre></blockquote>
<p>
The code above initializes the library and its memory manager. <b><tt>pgs</tt></b> now
points to the graphics state that will be passed to the drawing routines in
the library.
<blockquote><pre> gs_setdevice_no_erase(pgs, dev); /* can't erase yet */
{ gs_point dpi;
gs_screen_halftone ht;
gs_dtransform(pgs, 72.0, 72.0, &dpi);
ht.frequency = min(fabs(dpi.x), fabs(dpi.y)) / 16.001;
ht.angle = 0;
ht.spot_function = odsf;
gs_setscreen(pgs, &ht);
}</pre></blockquote>
<p>
The code above initializes the default device and sets a default halftone
screen. (For brevity, we have omitted the definition of odsf, the spot
function.)
<blockquote><pre> /* gsave and grestore (among other places) assume that */
/* there are at least 2 gstates on the graphics stack. */
/* Ensure that now. */
gs_gsave(pgs);</pre></blockquote>
<p>
The call above completes initializing the graphics state. When the program
is finished, it should execute:
<blockquote><pre> gs_lib_finit(0, 0);</pre></blockquote>
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<font size=2>
<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
</font>
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