The Ghostscript library

## Table of contents

> - The Ghostscript library
> - PostScript operator API
>   - Patterns
>   - Lower-level API
> - A full example

For other information, see the Ghostscript overview.

# The Ghostscript library

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).

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 PostScript Language Reference Manual, 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.

# 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`, for instance `gs_moveto` and `gs_fill`. Nearly all of these procedures take graphics state objects as their first arguments, such as

> `int gs_moveto(gs_state *, double, double);`

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 `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 callback procedures (`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 (operators) |  | Headers |  | Omissions
> Graphics state -- device-independent |  | `gscolor.h`(`gsstate.h`) `gscolor1.h` `gscolor2.h` `gscspace.h` `gshsb.h` `gsline.h`(`gsstate.h`) `gsstate.h` |  |
> Graphics state -- 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 |  | `gscoord.h` `gsmatrix.h` |  | `*matrix`, `*identmatrix`, `*concatmatrix`, `*invertmatrix`
> Path construction |  | `gspath.h` `gspath2.h` |  | `*arct`, `*pathforall`, `ustrokepath`, `uappend`, `upath`, `ucache`
> Painting |  | `gsimage.h` `gspaint.h` `gspath2.h` |  | `*image`, `*colorimage`, `*imagemask`, `ufill`, `ueofill`, `ustroke`
> Form and pattern |  | `gscolor2.h` |  | `execform`
> Device setup and output |  | `gsdevice.h` |  | `*showpage`, `*set`/`currentpagedevice`
> Character and font |  | `gschar.h` `gsfont.h` |  | *(all the `show` operators), `definefont`, `undefinefont`, `findfont`, `*scalefont`, `*makefont`, `selectfont`, `[Global]FontDirectory`, `Standard`/`ISOLatin1Encoding`, `findencoding`

The following procedures from the list above operate differently from their PostScript operator counterparts, as explained here:

`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 three 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.

This level of the library also implements the following operators from other sections of the Manual:

> Section (operators) |  | Headers |  | Operators
> Interpreter parameter |  | `gsfont.h` |  | `cachestatus`, `setcachelimit`, `*set/currentcacheparams`
> Display PostScript |  | `gsstate.h` |  | `set/currenthalftonephase`

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_`", that is, the prefix, not the implementation, reflects at what level the abstraction is made available.

## 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` (such as `ugcclib.mak` and `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 <stdio.h>
>     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 fragment above 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 externs above 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 code above 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 code 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 call above completes initializing the graphics state. When the program is finished, it should execute:

>             gs_lib_finit(0, 0);

Copyright © 1996, 1997, 1998 Aladdin Enterprises. All rights reserved.

This file is part of GNU Ghostscript. See the GNU General Public License (the "License") for full details of the terms of using, copying, modifying, and redistributing GNU Ghostscript.

GNU Ghostscript version 6.53, 13 February 2002