Copyright (C) 1989, 1996 Aladdin Enterprises. All rights reserved. This file is part of GNU Ghostscript. GNU Ghostscript is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY. No author or distributor accepts responsibility to anyone for the consequences of using it or for whether it serves any particular purpose or works at all, unless he says so in writing. Refer to the GNU General Public License for full details. Everyone is granted permission to copy, modify and redistribute GNU Ghostscript, but only under the conditions described in the GNU General Public License. A copy of this license is supposed to have been given to you along with GNU Ghostscript so you can know your rights and responsibilities. It should be in a file named COPYING. Among other things, the copyright notice and this notice must be preserved on all copies. Aladdin Enterprises is not affiliated with the Free Software Foundation or the GNU Project. GNU Ghostscript, as distributed by Aladdin Enterprises, does not depend on any other GNU software. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - This file, lib.txt, describes the Ghostscript library, a collection of C procedures that implement the primitive graphic operations of the Ghostscript language. For an overview of Ghostscript and a list of the documentation files, see README. ******** ******** The Ghostscript library ******** ******** 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, i.e., approximately those facilities listed in section 6.2 of the PostScript manual 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. PostScript operator API ----------------------- The highest level of the library, which is the one that most clients will use, directly implements the PostScript graphics operators with procedures named gs_XXX, e.g., gs_moveto, gs_fill. Nearly all of these procedures take a graphics state object as their first argument, e.g., int gs_moveto(gs_state *, double, double); Nearly all procedures return 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 gserrors.h. The library implements all the operators in the following sections of the PostScript Language Reference Manual, Second Edition, with the indicated omissions and with the APIs defined in the indicated .h files. A header of the form A.h(B.h) indicates that A.h is included in B.h, so A.h need not be included explicitly if B.h 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. 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 (a) callback procedure(s) (pathforall, image, colorimage, imagemask) is partly inverted: the client calls a procedure to set up an enumerator object, and then calls another procedure for each iteration. The ...show operators, charpath, and stringwidth also use an inverted control structure. Section Headers Omissions ------- ------- --------- Graphics State Operators -- Device Independent gscolor.h(gsstate.h) gscolor1.h gscolor2.h gscspace.h gshsb.h gsline.h(gsstate.h) gsstate.h Graphics State Operators -- Device Dependent gscolor.h(gsstate.h) gscolor1.h gscolor2.h gsht.h(gsht1.h,gsstate.h) gsht1.h gsline.h(gsstate.h) Coordinate System and Matrix Operators gscoord.h *matrix, *identmatrix, gsmatrix.h *concatmatrix, *invertmatrix Path Construction Operators gspath.h *arct, *pathforall, gspath2.h ustrokepath, uappend, upath, ucache Painting Operators gsimage.h *image, *colorimage, *imagemask, gspaint.h ufill, ueofill, ustroke gspath2.h Form and Pattern Operators gscolor2.h execform Device Setup and Output Operators gsdevice.h *showpage, *set/currentpagedevice Character and Font Operators gschar.h *(all the show operators), gsfont.h definefont, undefinefont, findfont, *scalefont, *makefont, selectfont, [Global]FontDirectory, Standard/ISOLatin1Encoding, findencoding << -------------------------------- end -------------------------------- >> The following procedures in the above list operate differently from their PostScript operator counterparts: Procedure(header) Difference(s) ----------------- ------------- gs_makepattern(gscolor2.h) 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 gs_makepattern. gs_setpattern(gscolor2.h) gs_setcolor(gscolor2.h) gs_currentcolor(gscolor2.h) Use gs_client_color rather than a set of color parameter values. See below for more details on gs_setpattern. gs_currentdash_length/pattern/offset(gsline.h) Splits up currentdash into 3 separate procedures. gs_screen_init/currentpoint/next/install(gsht.h) Provide an "enumeration style" interface to setscreen. (gs_setscreen is also implemented.) gs_rotate/scale/translate(gscoord.h) gs_[i][d]transform(gscoord.h) These always operate on the graphics state CTM. The corresponding operations on free-standing matrices are in gsmatrix.h and have different names. gs_path_enum_alloc/init/next/cleanup(gspath.h) Provide an "enumeration style" implementation of pathforall. gs_image_enum_alloc(gsimage.h) gs_image_init/next/cleanup(gsimage.h) Provide an "enumeration style" interface to the equivalent of image, imagemask, and colorimage. In the gs_image_t, ColorSpace provides an explicit color space, rather than using the current color space in the graphics state; ImageMask distinguishes imagemask from [color]image. gs_get/putdeviceparams(gsdevice.h) Take a gs_param_list for specifying or receiving the parameter values. See gsparam.h for more details. gs_show_enum_alloc/release(gschar.h) gs_xxxshow_[n_]init(gschar.h) gs_show_next(gschar.h) Provide an "enumeration style" interface to writing text. Note that control returns to the caller if the character must be rasterized. << -------------------------------- end -------------------------------- >> This level of the library also implements the following operators from other sections of the Manual: Section Headers Operators ------- ------- --------- Interpreter Parameter Operators gsfont.h cachestatus, setcachelimit, *set/currentcacheparams Display PostScript Operators gsstate.h set/currenthalftonephase << -------------------------------- end -------------------------------- >> In order to obtain the full PostScript Level 2 functionality listed above, FEATURE_DEVS must be set in the makefile to include at least the following: FEATURE_DEVS=patcore.dev cmykcore.dev psl2core.dev dps2core.dev ciecore.dev\ path1core.dev hsbcore.dev The *lib.mak makefiles mentioned below do not always include all of these features. Files named gs*.c implement the higher level of the graphics library. As might be expected, all procedures, variables, and structures available at this level begin with gs_. Structures that appear in these interfaces, but whose definitions may be hidden from clients, also have names beginning with gs_, i.e., the prefix reflects at what level the abstraction is made available, not the implementation. Patterns -------- 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. gs_makepattern defines a pattern color. Its arguments are as follows: gs_client_color * -- the resulting Pattern color is stored here. This is different from PostScript, which has no color objects per se and hence returns a modified copy of the dictionary. const gs_client_pattern * -- the analogue of the original Pattern dictionary, described in detail just below. const gs_matrix * -- corresponds to the matrix argument of the makepattern operator. gs_state * -- the current graphics state. gs_memory_t * -- the allocator to use for allocating the saved data for the Pattern color. If this is NULL, gs_makepattern uses the same allocator that allocated the graphics state. Library clients should probably always use NULL. The gs_client_pattern structure defined in gscolor2.h corresponds to the Pattern dictionary that is the argument to the PostScript language makepattern operator. This structure has one extra member, void *client_data, which is a place for clients to store a pointer to additional data for the PaintProc; this provides the same functionality as putting additional keys in the Pattern dictionary at the PostScript language level. The PaintProc is an ordinary C procedure that takes as parameters a gs_client_color *, which is the Pattern color that is being used for painting, and a gs_state *, which is the same graphics state that would be presented to the PaintProc in PostScript. Currently the gs_client_color * is always the current color in the graphics state, but the PaintProc should not rely on this. The PaintProc can retrieve the gs_client_pattern * from the gs_client_color * with the gs_getpattern procedure, also defined in gscolor2.h, and from there, it can retrieve the client_data pointer. The normal way to set a Pattern color is to call gs_setpattern with the graphics state and with the gs_client_color returned by gs_makepattern. After that, one can use gs_setcolor to set further Pattern colors (colored, or uncolored with the same underlying color space); the rules are the same as those in PostScript. Note that for gs_setpattern, the paint.values in the gs_client_color must be filled in for uncolored patterns; this corresponds to the additional arguments for the PostScript setpattern operator in the uncolored case. There is a special procedure gs_makebitmappattern for creating bitmap-based patterns. Its API is documented in gscolor2.h; its implementation, in gspcolor.c, may be useful as an example of a pattern using a particularly simple PaintProc. Lower-level API --------------- Files named gx*.c implement the lower level of the graphics library. The interfaces at the gx level are less stable, and expose more of the implementation detail, than those at the gs level: in particular, the gx interfaces generally use device coordinates in an internal fixed-point representation, as opposed to the gs interfaces that use floating point user coordinates. Named entities at this level begin with gx_. Files named gz*.c and gz*.h are internal to the Ghostscript implementation, and are not designed to be called by clients. A full example -------------- The file gslib.c in the Ghostscript fileset is a complete example program that initializes the library and produces output using it; files named *lib.mak (e.g., ugcclib.mak, bclib.mak) are makefiles using gslib.c as the main program. The following annotated excerpts from this file are intended to provide a roadmap for applications that call the library. /* Capture stdin/out/err before gs.h redefines them. */ #include static FILE *real_stdin, *real_stdout, *real_stderr; static void get_real(void) { real_stdin = stdin, real_stdout = stdout, real_stderr = stderr; } Any application using Ghostscript should include the above fragment at the very beginning of the main program. #include "gx.h" The gx.h 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 gx.h. /* 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)); The above externs are needed for initializing the library. 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); The above code initializes the library and its memory manager. pgs now points to the graphics state that will be passed to the drawing routines in the library. 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); } The above initializes the default device and sets a default halftone screen. (For brevity, we have omitted the definition of odsf, the spot function.) /* gsave and grestore (among other places) assume that */ /* there are at least 2 gstates on the graphics stack. */ /* Ensure that now. */ gs_gsave(pgs); The above call completes initializing the graphics state. When the program is finished, it should execute: gs_lib_finit(0, 0);