|
|
|
  |
||
| ||||||
|
|
Copyright (C) 1989, 1995 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 Ghostscript General Public License for full details.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
This file, drivers.doc, describes the interface between Ghostscript and
device drivers.
For an overview of Ghostscript and a list of the documentation files, see
README.
********
******** Adding a driver ********
********
To add a driver to Ghostscript, all you need to do is edit devs.mak in
two places. The first is the list of devices, in the section headed
# -------------------------------- Catalog ------------------------------- #
Pick a name for your device, say smurf, and add smurf to the list.
(Device names must be 1 to 8 characters, consisting of only letters,
digits, and underscores, of which the first character must be a letter.
Case is significant: all current device names are lower case.)
The second is the section headed
# ---------------------------- Device drivers ---------------------------- #
Suppose the files containing the smurf driver are called joe and fred.
Then you should add the following lines:
# ------ The SMURF device ------ #
smurf_=joe.$(OBJ) fred.$(OBJ)
smurf.dev: $(smurf_)
$(SHP)gssetdev smurf $(smurf_)
joe.$(OBJ): joe.c ...and whatever it depends on
fred.$(OBJ): fred.c ...and whatever it depends on
If the smurf driver also needs special libraries, e.g., a library named
gorf, then the gssetdev line should look like
$(SHP)gssetdev smurf $(smurf_)
$(SHP)gsaddmod smurf -lib gorf
********
******** Keeping things simple
********
If you want to add a simple device (specifically, a black-and-white
printer), you probably don't need to read the rest of this document; just
use the code in an existing driver as a guide. The Epson and BubbleJet
drivers (gdevepsn.c and gdevbj10.c) are good models for dot-matrix printers,
which require presenting the data for many scan lines at once; the
DeskJet/LaserJet drivers (gdevdjet.c) are good models for laser printers,
which take a single scan line at a time but support data compression. For
color printers, the DeskJet 500C driver (gdevcdj.c) may be a good place to
start, although it is large and complex. gdevcdj.c is also a good example
of a device that has additional settable attributes (in this case, different
output quality modes that aren't just a matter of resolution).
On the other hand, if you're writing a driver for some more esoteric
device, you probably do need at least some of the information in the rest
of this document. It might be a good idea for you to read it in
conjunction with one of the existing drivers.
********
******** Driver structure ********
********
A device is represented by a structure divided into three parts:
- procedures that are shared by all instances of each device;
- parameters that are present in all devices but may be different
for each device or instance; and
- device-specific parameters that may be different for each instance.
Normally, the procedure structure is defined and initialized at compile
time. A prototype of the parameter structure (including both generic and
device-specific parameters) is defined and initialized at compile time,
but is copied and filled in when an instance of the device is created.
The gx_device_common macro defines the common structure elements, with the
intent that devices define and export a structure along the following
lines:
typedef struct smurf_device_s {
gx_device_common;
... device-specific parameters ...
} smurf_device;
smurf_device gs_smurf_device = {
sizeof(smurf_device), /* params_size */
0, /* static_procs, obsolete */
... generic parameter values ...
{ ... procedures ... }, /* std_procs */
... device-specific parameter values ...
};
The device structure instance *must* have the name gs_smurf_device, where
smurf is the device name used in devs.mak.
All the device procedures are called with the device as the first
argument. Since each device type is actually a different structure type,
the device procedures must be declared as taking a gx_device * as their
first argument, and must cast it to smurf_device * internally. For
example, in the code for the "memory" device, the first argument to all
routines is called dev, but the routines actually use md to reference
elements of the full structure, by virtue of the definition
#define md ((gx_device_memory *)dev)
(This is a cheap version of "object-oriented" programming: in C++, for
example, the cast would be unnecessary, and in fact the procedure table
would be constructed by the compiler.)
Structure definition
--------------------
This essentially duplicates the first part of the structure definition in
gxdevice.h.
typedef struct gx_device_s {
int params_size; /* size of this structure */
gx_device_procs *static_procs; /* (obsolete) */
const char *dname; /* the device name */
int width; /* width in pixels */
int height; /* height in pixels */
...
gx_device_color_info color_info; /* color information */
...
int is_open; /* true if device has been opened */
} gx_device;
The name in the structure should be the same as the name in devs.mak.
gx_device_common is a macro consisting of just the element definitions.
For sophisticated developers only
---------------------------------
If for any reason you need to change the definition of the basic device
structure, or add procedures, you must change the following places:
- This document and NEWS (if you want to keep the
documentation up to date).
- The definition of gx_device_common and/or the procedures
in gxdevice.h.
- Possibly, the default forwarding procedures in gxdevice.h
and gsdevice.c.
- The following devices that must have complete (non-defaulted)
procedure vectors:
- The null device in gsdevice.c.
- The command list "device" in gxclist.c. This is
not an actual device; it only defines procedures.
- The "memory" devices in gdevmem.h and gdevm*.c.
- The procedure record completion routine in gsdevice.c.
- The clip list accumulation "device" in gxacpath.c.
- The clipping "devices" in gxcpath.c and gxclip2.c.
- The hit detection "device" in zupath.c.
- The generic printer device macros in gdevprn.h.
- The generic printer device code in gdevprn.c.
- All the real devices in the standard Ghostscript distribution,
as listed in devs.mak. (All of them are supposed to use
the macros in gxdevice.h or gdevprn.h to initialize their
state, so you may not have to edit the source code for
them.)
- Any other drivers you have that aren't part of the standard
Ghostscript distribution.
You may also have to change the code for gx_default_get_params and/or
gx_default_put_params (in gsdparam.c).
Note that if all you are doing is adding optional procedures, you do NOT
have to modify any of the real device drivers listed in devs.mak;
Ghostscript will substitute the default procedures properly.
********
******** Types and coordinates ********
********
Coordinate system
-----------------
Since each driver specifies the initial transformation from user to device
coordinates, the driver can use any coordinate system it wants, as long as
a device coordinate will fit in an int. (This is only an issue on MS-DOS
systems, where ints are only 16 bits. User coordinates are represented as
floats.) Typically the coordinate system will have (0,0) in the upper
left corner, with X increasing to the right and Y increasing toward the
bottom. This happens to be the coordinate system that all the currently
supported devices use. However, there is supposed to be nothing in the
rest of Ghostscript that assumes this.
Drivers must check (and, if necessary, clip) the coordinate parameters
given to them: they should not assume the coordinates will be in bounds.
The fit_fill and fit_copy macros in gxdevice.h are very helpful in doing
this.
Color definition
----------------
Ghostscript represents colors internally as RGB or CMYK values. In
communicating with devices, however, it assumes that each device has a
palette of colors identified by integers (to be precise, elements of type
gx_color_index). Drivers may provide a uniformly spaced gray ramp or
color cube for halftoning, or they may do their own color approximation,
or both.
The color_info member of the device structure defines the color and
gray-scale capabilities of the device. Its type is defined as follows:
typedef struct gx_device_color_info_s {
int num_components; /* 1 = gray only, 3 = RGB, */
/* 4 = CMYK */
int depth; /* # of bits per pixel */
gx_color_value max_gray; /* # of distinct gray levels -1 */
gx_color_value max_rgb; /* # of distinct color levels -1 */
/* (only relevant if num_comp. > 1) */
gx_color_value dither_gray; /* size of gray ramp for halftoning */
gx_color_value dither_rgb; /* size of color cube ditto */
/* (only relevant if num_comp. > 1) */
} gx_device_color_info;
The following macros (in gxdevice.h) provide convenient shorthands for
initializing this structure for ordinary black-and-white or color devices:
#define dci_black_and_white ...
#define dci_color(depth,maxv,dither) ...
The idea is that a device has a certain number of gray levels (max_gray +1)
and a certain number of colors (max_rgb +1) that it can produce directly.
When Ghostscript wants to render a given RGB or CMYK color as a device
color, it first tests whether the color is a gray level. (If
num_components is 1, it converts all colors to gray levels.) If so:
- If max_gray is large (>= 31), Ghostscript asks the device to
approximate the gray level directly. If the device returns a valid
gx_color_index, Ghostscript uses it. Otherwise, Ghostscript assumes that
the device can represent dither_gray distinct gray levels, equally spaced
along the diagonal of the color cube, and uses the two nearest ones to the
desired color for halftoning.
If the color is not a gray level:
- If max_rgb is large (>= 31), Ghostscript asks the device to
approximate the color directly. If the device returns a valid
gx_color_index, Ghostscript uses it. Otherwise, Ghostscript assumes that
the device can represent dither_rgb * dither_rgb * dither_rgb distinct
colors, equally spaced throughout the color cube, and uses two of the
nearest ones to the desired color for halftoning.
Types
-----
Here is a brief explanation of the various types that appear as parameters
or results of the drivers.
gx_color_value (defined in gxdevice.h)
This is the type used to represent RGB or CMYK color values. It is
currently equivalent to unsigned short. However, Ghostscript may use less
than the full range of the type to represent color values:
gx_color_value_bits is the number of bits actually used, and
gx_max_color_value is the maximum value (equal to 2^gx_max_color_value_bits
- 1).
gx_device (defined in gxdevice.h)
This is the device structure, as explained above.
gs_matrix (defined in gsmatrix.h)
This is a 2-D homogenous coordinate transformation matrix, used by
many Ghostscript operators.
gx_color_index (defined in gxdevice.h)
This is meant to be whatever the driver uses to represent a device
color. For example, it might be an index in a color map, or it might be
R, G, and B values packed into a single integer. Ghostscript doesn't ever
do any computations with gx_color_index values: it gets them from
map_rgb_color or map_cmyk_color and hands them back as arguments to
several other procedures. The special value gx_no_color_index (defined as
(gx_color_index)(-1)) means "transparent" for some of the procedures. The
type definition is simply:
typedef unsigned long gx_color_index;
gs_param_list (defined in gsparam.h)
This is a parameter list, which is used to read and set attributes
in a device. See the comments in gsparam.h, and the description of the
get_params and put_params procedures below, for more detail.
gx_bitmap (defined in gxbitmap.h)
This structure type represents a bitmap to be used as a tile for
filling a region (rectangle). Here is a copy of the relevant part of the
file:
/*
* Structure for describing stored bitmaps.
* Bitmaps are stored bit-big-endian (i.e., the 2^7 bit of the first
* byte corresponds to x=0), as a sequence of bytes (i.e., you can't
* do word-oriented operations on them if you're on a little-endian
* platform like the Intel 80x86 or VAX). Each scan line must start on
* a (32-bit) word boundary, and hence is padded to a word boundary,
* although this should rarely be of concern, since the raster and width
* are specified individually. The first scan line corresponds to y=0
* in whatever coordinate system is relevant.
*
* For bitmaps used as halftone tiles, we may replicate the tile in
* X and/or Y, but it is still valuable to know the true tile dimensions.
*/
typedef struct gx_bitmap_s {
byte *data;
int raster; /* bytes per scan line */
gs_int_point size; /* width, height */
gx_bitmap_id id;
ushort rep_width, rep_height; /* true size of tile */
} gx_bitmap;
********
******** Coding conventions ********
********
While most drivers (especially printer drivers) follow a very similar
template, there is one important coding convention that is not obvious from
reading the code for existing drivers: Driver procedures must not use
malloc to allocate any storage that stays around after the procedure
returns. Instead, they must use gs_malloc and gs_free, which have slightly
different calling conventions. (The prototypes for these are in
gsmemory.h, which is included in gx.h, which is included in gdevprn.h.)
This is necessary so that Ghostscript can clean up all allocated memory
before exiting, which is essential in environments that provide only
single-address-space multi-tasking (specifically, Microsoft Windows).
char *gs_malloc(uint num_elements, uint element_size,
const char *client_name);
Like calloc, but unlike malloc, gs_malloc takes an element count
and an element size. For structures, num_elements is 1 and element_size is
sizeof the structure; for byte arrays, num_elements is the number of bytes
and element_size is 1. Unlike calloc, gs_malloc does NOT clear the block
of storage.
The client_name is used for tracing and debugging. It must be a
real string, not NULL. Normally it is the name of the procedure in which
the call occurs.
void gs_free(char *data, uint num_elements, uint element_size,
const char *client_name);
Unlike free, gs_free demands that num_elements and element_size be
supplied. It also requires a client name, like gs_malloc.
All the driver procedures defined below that return int results return 0 on
success, or an appropriate negative error code in the case of error
conditions. The error codes are defined in gserrors.h. The relevant ones
for drivers are as follows:
gs_error_invalidfileaccess
An attempt to open a file failed.
gs_error_limitcheck
An otherwise valid parameter value was too large for
the implementation.
gs_error_rangecheck
A parameter was outside the valid range.
gs_error_VMerror
An attempt to allocate memory failed. (If this
happens, the procedure should release all memory it
allocated before it returns.)
If a driver does return an error, it should use the return_error
macro rather than a simple return statement, e.g.,
return_error(gs_error_VMerror);
This macro is defined in gx.h, which is automatically included by
gdevprn.h but not by gserrors.h.
********
******** Printer drivers ********
********
Printer drivers (which include drivers that write some kind of raster
file) are especially simple to implement. Of the driver procedures
defined in the next section, they only need implement two: map_rgb_color
(or map_cmyk_color) and map_color_rgb. In addition, they must implement a
print_page or print_page_copies procedure. There are a set of macros in
gdevprn.h that generate the device structure for such devices, of which
the simplest is prn_device; for an example, see gdevbj10.c. If you are
writing a printer driver, we suggest you start by reading gdevprn.h and
the subsection on "Color mapping" below; you may be able to ignore all the
rest of the driver procedures.
The print_page procedures are defined as follows:
int (*print_page)(P2(gx_device_printer *, FILE *))
int (*print_page_copies)(P3(gx_device_printer *, FILE *, int))
This procedure must read out the rendered image from the device and
write whatever is appropriate to the file. To read back one or more scan
lines of the image, the print_page procedure must call one of the following
procedures:
int gdev_prn_copy_scan_lines(P4(gx_device_printer *pdev, int y, byte *str,
uint size)
For this procedure, str is where the data should be copied to, and size is
the size of the buffer starting at str. This procedure returns the number
of scan lines copied, or <0 for an error.
int gdev_prn_get_bits(gx_device_printer *pdev, int y, byte *str,
byte **actual_data)
This procedure reads out exactly one scan line. If the scan line is
available in the correct format already, *actual_data is set to point to
it; otherwise, the scan line is copied to the buffer starting at str, and
*actual_data is set to str. This saves a copying step most of the time.
In either case, each row of the image is stored in the form described in
the comment under gx_bitmap above; each pixel takes the number of bits
specified as color_info.depth in the device structure, and holds values
returned by the device's map_{rgb,cmyk}_color procedure.
The print_page procedure can determine the number of bytes required to hold
a scan line by calling:
uint gdev_prn_raster(P1(gx_device_printer *))
For a very simple concrete example, we suggest reading the code in
bit_print_page in gdevbit.c.
If the device provides print_page, Ghostscript will call print_page the
requisite number of times to print the desired number of copies; if the
device provides print_page_copies, Ghostscript will call print_page_copies
once per page, passing it the desired number of copies.
********
******** Driver procedures ********
********
Most of the procedures that a driver may implement are optional. If a
device doesn't supply an optional procedure
|
|
|
Email addresses listed on this site may NOT be used for unsolicited commercial email. Ready-to-Run Software, Inc Privacy Statement Portions (c)Copyright, 1996-2005 by
Ready-to-Run
Software, Inc |
