   Copyright (C) 1989, 1996 Aladdin Enterprises.  All rights reserved.
  
  This file is part of Aladdin Ghostscript.
  
  Aladdin Ghostscript is distributed with NO WARRANTY OF ANY KIND.  No author
  or distributor accepts any responsibility for the consequences of using it,
  or for whether it serves any particular purpose or works at all, unless he
  or she says so in writing.  Refer to the Aladdin Ghostscript Free Public
  License (the "License") for full details.
  
  Every copy of Aladdin Ghostscript must include a copy of the License,
  normally in a plain ASCII text file named PUBLIC.  The License grants you
  the right to copy, modify and redistribute Aladdin Ghostscript, but only
  under certain conditions described in the License.  Among other things, the
  License requires that the copyright notice and this notice be preserved on
  all copies.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

This file, drivers.txt, 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_)
	$(SETDEV) 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 entry should look like this:

smurf.dev: $(smurf_)
	$(SETDEV) smurf $(smurf_)
	$(ADDMOD) smurf -lib gorf

If, as will usually be the case, your driver is a printer driver (as
discussed below), the device entry should look like this:

smurf.dev: $(smurf_) page.dev
	$(SETPDEV) smurf $(smurf_)

or

smurf.dev: $(smurf_) page.dev
	$(SETPDEV) smurf $(smurf_)
	$(ADDMOD) 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, there are unfortunately no good models:
the two major color inkjet printer drivers, gdevcdj.c and gdevstc.c, are
far too complex to read.)

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.

Duplication of code, and sheer code volume, is a serious maintenance and
distribution problem for Ghostscript.  If your device is similar to an
existing one, try to implement your driver by adding some parameterization
to an existing driver rather than copying code.  gdevepsn.c and gdevdjet.c
are good examples of this approach.

********
******** 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.  Both of
these structures should be declared as const, but for backward
compatibility reasons are not.

The gx_device_common macro defines the common structure elements, with the
intent that devices define and export a structure along the following
lines.  Do not fill in the individual generic parameter values in the usual
way for C structures: use the macros defined for this purpose in gxdevice.h
or, if applicable, gdevprn.h.

	typedef struct smurf_device_s {
		gx_device_common;
		... device-specific parameters ...
	} smurf_device;
	smurf_device far_data gs_smurf_device = {
		... macro for generic parameter values ...,
		{ ... procedures ... },		/* std_procs */
		... device-specific parameter values if any ...
	};

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 mdev to reference elements of the
full structure, by virtue of the definition

	#define mdev ((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
--------------------

You should consult the definition of struct gx_device_s in gxdevice.h for
the complete details of the generic device structure.  Some of the most
important members of this structure for ordinary drivers are:

	const char *dname;		/* the device name */
	bool is_open;			/* true if device has been opened */
	gx_device_color_info color_info;	/* color information */
	int width;			/* width in pixels */
	int height;			/* height in pixels */

The name in the structure (dname) 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 declared in gxdevice.h
	    and implemented in gdevnfwd.c.
	- The device procedure record completion routines in gdevdflt.c.
	- Possibly, the default device implementation in gdevdflt.c and
	    gdevddrw.c.
	- If you are adding procedures that produce output, the bounding box
	    device in gdevbbox.c.
	- The following devices that must have complete (non-defaulted)
	    procedure vectors:
		- The null device in gdevnfwd.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 halftoning device in gdevht.c.
	- The clip list accumulation "device" in gxacpath.c.
	- The clipping "devices" in gxcpath.c and gxclip2.c.
	- The Pattern accumulation "device" in gxpcmap.c.
	- The generic printer device macros in gdevprn.h.
	- The generic printer device code in gdevprn.c.

You may also have to change the code for gx_default_get_params and/or
gx_default_put_params (in gsdparam.c).

You should not have to change any of the real devices in the standard
Ghostscript distribution (listed in devs.mak) or any of your own devices,
because all of them are supposed to use the macros in gxdevice.h or
gdevprn.h to define and initialize their state.

********
******** 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.)  Most current drivers use a coordinate system with (0,0) in the
upper left corner, with X increasing to the right and Y increasing toward
the bottom.  However, there is supposed to be nothing in the rest of
Ghostscript that assumes this, and indeed some drivers use a coordinate
system with (0,0) in the lower left corner.

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_tile_bitmap (defined in gxbitmap.h)
gx_strip_bitmap (defined in gxbitmap.h)

	These structure types represent bitmaps to be used as a tile for
filling a region (rectangle).  gx_tile_bitmap is an older type lacking shift
and rep_shift; gx_strip_bitmap has superseded it, and it should not be used
in new code.  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
 * (i.e., the dimensions prior to replication).  Requirements:
 *	width % rep_width = 0
 *	height % rep_height = 0
 *
 * For halftones at arbitrary angles, we provide for storing the halftone
 * data as a strip that must be shifted in X for different values of Y.
 * For an ordinary (non-shifted) halftone that has a repetition width of
 * W and a repetition height of H, the pixel at coordinate (X,Y)
 * corresponds to halftone pixel (X mod W, Y mod H), ignoring phase;
 * for a shifted halftone with shift S, the pixel at (X,Y) corresponds
 * to halftone pixel ((X + S * floor(Y/H)) mod W, Y mod H).  Requirements:
 *	strip_shift < rep_width
 *	strip_height % rep_height = 0
 *	shift = (strip_shift * (size.y / strip_height)) % rep_width
 */
typedef struct gx_strip_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 */
	ushort strip_height;
	ushort strip_shift;
	ushort shift;
} gx_strip_bitmap;

********
******** Coding conventions ********
********

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; they correspond
directly to the errors defined in the PostScript language reference
manuals.  The most common ones for drivers are:

	gs_error_invalidfileaccess
		An attempt to open a file failed.

	gs_error_ioerror
		An error occurred in reading or writing a file.

	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.

Allocating storage
------------------

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 (some versions of 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.

Driver instance allocation
--------------------------

All driver instances allocated by Ghostscript's standard allocator must
point to a "structure descriptor" that tells the garbage collector how to
trace pointers in the structure.  For drivers that are registered in the
normal way (using the makefile approach described above), no special care
is needed as long as instances are only created by calling the
gs_copydevice procedure defined in gsdevice.h.  If you have a need to
define devices that are not registered in this way, you must fill in the
stype member in any dynamically allocated instances with a pointer to the
same structure descriptor used to allocate the instance.  For more
information about structure descriptors, see gsmemory.h and gsstruct.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.  str need not be aligned.

    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.
str need not be aligned; however, if *actual_data is set to point to an
existing scan line, it will be aligned.  (See the description of the
get_bits procedure below for more details.)

In either case, each row of the image is stored in the form described
in the comment under gx_tile_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 <proc>, the entry in the
procedure structure may be either gx_default_<proc>, e.g.
gx_default_tile_rectangle, or NULL or 0.  (The device procedure must also
call the gx_default_ procedure if it doesn't implement the function for
particular values of the arguments.)  Since C compilers supply 0 as the
value for omitted structure elements, this convention means that
statically initialized procedure structures will continue to work even if
new (optional) members are added.

Life cycle
----------

A device instance start out life in a closed state.  In this state, no
output operations will occur.  Only the following procedures may be called:
	open_device
	get_initial_matrix
	get_params
	put_params

When setdevice installs a device instance in the graphics state, it checks
whether the instance is closed or open.  If the instance is closed,
setdevice calls the open routine, and then sets the state to open.

There is currently no user-accessible operation to close a device instance.
Device instances are only closed when they are about to be freed, which
occurs in three situations:

	- When a 'restore' occurs, if the instance was created since the
corresponding 'save';

	- By the garbage collector, if the instance is no longer accessible;

	- When Ghostscript exits (terminates).

Open/close/sync
---------------

int (*open_device)(P1(gx_device *)) [OPTIONAL]

	Open the device: do any initialization associated with making the
device instance valid.  This must be done before any output to the device.
The default implementation does nothing.

void (*get_initial_matrix)(P2(gx_device *, gs_matrix *)) [OPTIONAL]

	Construct the initial transformation matrix mapping user
coordinates (nominally 1/72" per unit) to device coordinates.  The default
procedure computes this from width, height, and x/y_pixels_per_inch on the
assumption that the origin is in the upper left corner, i.e.
		xx = x_pixels_per_inch/72, xy = 0,
		yx = 0, yy = -y_pixels_per_inch/72,
		tx = 0, ty = height.

int (*sync_output)(P1(gx_device *)) [OPTIONAL]

	Synchronize the device.  If any output to the device has been
buffered, send / write it now.  Note that this may be called several times
in the process of constructing a page, so printer drivers should NOT
implement this by printing the page.  The default implementation does
nothing.

int (*output_page)(P3(gx_device *, int num_copies, int flush)) [OPTIONAL]

	Output a fully composed page to the device.  The num_copies
argument is the number of copies that should be produced for a hardcopy
device.  (This may be ignored if the driver has some other way to specify
the number of copies.)  The flush argument is true for showpage, false for
copypage.  The default definition just calls sync_output.  Printer drivers
should implement this by printing and ejecting the page.

int (*close_device)(P1(gx_device *)) [OPTIONAL]

	Close the device: release any associated resources.  After this,
output to the device is no longer allowed.  The default implementation
does nothing.

Color/alpha mapping
-------------------

A given driver normally will implement either map_rgb_color or
map_cmyk_color, but not both.  Black-and-white drivers do not need to
implement either one.  Note that the map_xxx_color procedures must not
return gx_no_color_index (all 1's).

gx_color_index (*map_rgb_color)(P4(gx_device *, gx_color_value red,
  gx_color_value green, gx_color_value blue)) [OPTIONAL]

	Map a RGB color to a device color.  The range of legal values of
the RGB arguments is 0 to gx_max_color_value.  The default algorithm uses
the map_cmyk_color procedure if the driver supplies one, otherwise returns
1 if any of the values exceeds gx_max_color_value/2, 0 otherwise.

	Ghostscript assumes that for devices that have color capability
(i.e., color_info.num_components > 1), map_rgb_color returns a color index
for a gray level (as opposed to a non-gray color) iff red = green = blue.

gx_color_index (*map_cmyk_color)(P5(gx_device *, gx_color_value cyan,
  gx_color_value magenta, gx_color_value yellow, gx_color_value black))
  [OPTIONAL]

	Map a CMYK color to a device color.  The range of legal values of
the CMYK arguments is 0 to gx_max_color_value.  The default algorithm
calls the map_rgb_color procedure, with suitably transformed arguments.

	Ghostscript assumes that for devices that have color capability
(i.e., color_info.num_components > 1), map_cmyk_color returns a color
index for a gray level (as opposed to a non-gray color) iff cyan = magenta
= yellow.

int (*map_color_rgb)(P3(gx_device *, gx_color_index color,
  gx_color_value rgb[3])) [OPTIONAL]

	Map a device color code to RGB values.  The default algorithm
returns (0 if color==0 else gx_max_color_value) for all three components.

gx_color_index (*map_rgb_alpha_color)(P5(gx_device *, gx_color_value red,
  gx_color_value green, gx_color_value blue, gx_color_value alpha)) [OPTIONAL]

	Map a RGB color and an opacity value to a device color.  The range
of legal values of the RGB and alpha arguments is 0 to gx_max_color_value;
alpha = 0 means transparent, alpha = gx_max_color_value means fully
opaque.  The default is to use the map_rgb_color procedure and ignore
alpha.

	Note that if a driver implements map_rgb_alpha_color, it must also
implement map_rgb_color, and must implement them in such a way that
map_rgb_alpha_color(dev, r, g, b, gx_max_color_value) returns the same
value as map_rgb_color(dev, r, g, b).

	Note that there is no map_cmyk_alpha_color procedure.  CMYK
devices currently do not support variable opacity; alpha is ignored on
such devices.

typedef enum { go_text, go_graphics } graphic_object_type;
int (*get_alpha_bits)(P4(gx_device *dev, graphic_object_type type)) [OPTIONAL]

	Return the number of alpha (opacity) bits that should be used in
rendering an object of the given type.  The default value is 1; the only
values allowed are 1, 2, and 4.

Drawing
-------

All drawing operations use device coordinates and device color values.

int (*fill_rectangle)(P6(gx_device *, int x, int y,
  int width, int height, gx_color_index color))

	Fill a rectangle with a color.  The set of pixels filled is
{(px,py) | x <= px < x + width and y <= py < y + height}.  In other words,
the point (x,y) is included in the rectangle, as are (x+w-1,y), (x,y+h-1),
and (x+w-1,y+h-1), but *not* (x+w,y), (x,y+h), or (x+w,y+h).  If width <=
0 or height <= 0, fill_rectangle should return 0 without drawing anything.

	Note that fill_rectangle is the only non-optional procedure in the
driver interface.

int (*draw_line)(P6(gx_device *, int x0, int y0, int x1, int y1,
  gx_color_index color)) [OPTIONAL]

	Draw a minimum-thickness line from (x0,y0) to (x1,y1).  The
precise set of points to be filled is defined as follows.  First, if y1 <
y0, swap (x0,y0) and (x1,y1).  Then the line includes the point (x0,y0)
but not the point (x1,y1).  If x0=x1 and y0=y1, draw_line should return 0
without drawing anything.

Bitmap imaging
--------------

Bitmap (or pixmap) images are stored in memory in a nearly standard way.
The first byte corresponds to (0,0) in the image coordinate system: bits
(or polybit color values) are packed into it left-to-right.  There may be
padding at the end of each scan line: the distance from one scan line to
the next is always passed as an explicit argument.

int (*copy_mono)(P11(gx_device *, const unsigned char *data, int data_x,
  int raster, gx_bitmap_id id, int x, int y, int width, int height,
  gx_color_index color0, gx_color_index color1)) [OPTIONAL]

	Copy a monochrome image (similar to the PostScript image
operator).  Each scan line is raster bytes wide.  Copying begins at
(data_x,0) and transfers a rectangle of the given width at height to the
device at device coordinate (x,y).  (If the transfer should start at some
non-zero y value in the data, the caller can adjust the data address by
the appropriate multiple of the raster.)  The copying operation writes
device color color0 at each 0-bit, and color1 at each 1-bit: if color0 or
color1 is gx_no_color_index, the device pixel is unaffected if the image
bit is 0 or 1 respectively.  If id is different from gx_no_bitmap_id, it
identifies the bitmap contents unambiguously; a call with the same id will
always have the same data, raster, and data contents.

	This operation, with color0 = gx_no_color_index, is the workhorse
for text display in Ghostscript, so implementing it efficiently is very
important.

int (*tile_rectangle)(P10(gx_device *, const gx_tile_bitmap *tile,
  int x, int y, int width, int height,
  gx_color_index color0, gx_color_index color1,
  int phase_x, int phase_y)) [OPTIONAL] [OBSOLETE]

	This procedure is still supported, but has been superseded by
strip_tile_rectangle.  New drivers should implement strip_tile_rectangle; if
they cannot cope with non-zero shift values, they should test for this
explicitly and call the default implementation
(gx_default_strip_tile_rectangle) if shift != 0.  Clients should call
strip_tile_rectangle, not tile_rectangle.

int (*strip_tile_rectangle)(P10(gx_device *, const gx_strip_bitmap *tile,
  int x, int y, int width, int height,
  gx_color_index color0, gx_color_index color1,
  int phase_x, int phase_y)) [OPTIONAL]

	Tile a rectangle.  Tiling consists of doing multiple copy_mono
operations to fill the rectangle with copies of the tile.  The tiles are
aligned with the device coordinate system, to avoid "seams".
Specifically, the (phase_x, phase_y) point of the tile is aligned with the
origin of the device coordinate system.  (Note that this is backwards from
the PostScript definition of halftone phase.)  phase_x and phase_y are
guaranteed to be in the range [0..tile->width) and [0..tile->height)
respectively.

	If color0 and color1 are both gx_no_color_index, then the tile is
a color pixmap, not a bitmap: see the next section.

	This operation is the workhorse for halftone filling in Ghostscript,
so implementing it efficiently for solid tiles (i.e., where either color0
and color1 are both gx_no_color_index, for colored halftones, or neither one
is gx_no_color_index, for monochrome halftones) is very important.

Pixmap imaging
--------------

Pixmaps are just like bitmaps, except that each pixel occupies more than
one bit.  All the bits for each pixel are grouped together (this is
sometimes called "chunky" or "Z" format).  For copy_color, the number of
bits per pixel is given by the color_info.depth parameter in the device
structure: the legal values are 1, 2, 4, 8, 16, 24, or 32.  The pixel
values are device color codes (i.e., whatever it is that map_rgb_color
returns).

int (*copy_color)(P9(gx_device *, const unsigned char *data, int data_x,
  int raster, gx_bitmap_id id, int x, int y, int width, int height))
  [OPTIONAL]

	Copy a color image with multiple bits per pixel.  The raster is in
bytes, but x and width are in pixels, not bits.  If id is different from
gx_no_bitmap_id, it identifies the bitmap contents unambiguously; a call
with the same id will always have the same data, raster, and data contents.

We do not provide a separate procedure for tiling with a pixmap; instead,
tile_rectangle can also take colored tiles.  This is indicated by the
color0 and color1 arguments both being gx_no_color_index.  In this case,
as for copy_color, the raster and height in the "bitmap" are interpreted
as for real bitmaps, but the x and width are in pixels, not bits.

int (*copy_alpha)(P11(gx_device *dev, const unsigned char *data, int data_x,
  int raster, gx_bitmap_id id, int x, int y, int width, int height,
  gx_color_index color, int depth)) [OPTIONAL]

	Fill a given region with a given color modified by an individual
alpha value for each pixel.  depth, the number of bits per pixel, is
either 2 or 4, and in any case is always a value returned by a previous
call on the get_alpha_bits procedure.  Note that if get_alpha_bits always
returns 1, this procedure will never be called.

Bitmap/pixmap imaging
---------------------

int (*copy_rop)(P15(gx_device *dev,
  const byte *sdata, int sourcex, uint sraster, gx_bitmap_id id,
  const gx_color_index *scolors,
  const gx_tile_bitmap *texture, const gx_color_index *tcolors,
  int x, int y, int width, int height,
  int phase_x, int phase_y, int command)) [OPTIONAL]

	This procedure is still supported, but has been superseded by
strip_copy_rop.  New drivers should implement strip_copy_rop; if they cannot
cope with non-zero shift values in the texture, they should test for this
explicitly and call the default implementation (gx_default_strip_copy_rop)
if shift != 0.  Clients should call strip_copy_rop, not copy_rop.

int (*strip_copy_rop)(P15(gx_device *dev,
  const byte *sdata, int sourcex, uint sraster, gx_bitmap_id id,
  const gx_color_index *scolors,
  const gx_strip_bitmap *texture, const gx_color_index *tcolors,
  int x, int y, int width, int height,
  int phase_x, int phase_y, int command)) [OPTIONAL]

	Combine an optional source image S (as for copy_mono or copy_color)
and an optional texture T (a tile, as for tile_rectangle) with the existing
bitmap or pixmap D held by the driver, pixel by pixel, using any 3-input
Boolean operation as modified by "transparency" flags: schematically, set D
= f(D,S,T), computing f in RGB space rather than using actual device pixel
values.  S and T may each (independently) be a solid color, a bitmap with
"foreground" and "background" colors, or a pixmap.  This is a complex (and
currently rather slow) operation.  The arguments are as follows:

	dev - the device, as for all driver procedures

	sdata, sourcex, sraster, id, scolors - specify S, see below

	texture, tcolors - specify T, see below

	x, y, width, height - as for the other copy/fill procedures

	phase_x, phase_y - part of T specification, see below

	command - see below

S specification
...............

As noted above, the source (S) may be a solid color, a bitmap, or a pixmap.

If S is a solid color:

	- sdata, sourcex, sraster, and id are irrelevant.

	- scolors points to two gx_color_index values; scolors[0] =
	scolors[1] = the color.

If S is a bitmap:

	- sdata, sourcex, sraster, and id arguments are as for copy_mono or
	copy_color (data, data_x, raster, id), and specify a source bitmap.

	- scolors points to two gx_color_index values; scolors[0] is the
	background color (the color corresponding to 0-bits in the bitmap),
	scolors[1] is the foreground color (the color corresponding to
	1-bits in the bitmap).

If S is a pixmap:

	- sdata, sourcex, sraster, and id arguments are as for copy_mono or
	copy_color (data, data_x, raster, id), and specify a source pixmap
	whose depth is the same as the depth of the destination.

	- scolors is NULL.

Note that if the source is a bitmap with background=0 and foreground=1, and
the destination is 1 bit deep, then the source can be treated as a pixmap
(scolors=NULL).

T specification
...............

Similar to the source, the texture (T) may be a solid color, a bitmap, or a
pixmap.

If T is a solid color:

	- The texture pointer is irrelevant.

	- tcolors points to two gx_color_index values; tcolors[0] =
	tcolors[1] = the color.

If T is a bitmap:

	- The texture argument points to a gx_tile_bitmap, as for the
	tile_rectangle procedure.  Similarly, phase_x and phase_y specify
	the offset of the texture relative to the device coordinate system
	origin, again as for tile_rectangle.  The tile is a bitmap (1 bit
	per pixel).

	- tcolors points to two gx_color_index values; tcolors[0] is the
	background color (the color corresponding to 0-bits in the bitmap),
	tcolors[1] is the foreground color (the color corresponding to
	1-bits in the bitmap).

If T is a pixmap:

	- The texture argument pointer to a gx_tile_bitmap whose depth is
	the same as the depth of the destination.

	- tcolors is NULL.

Again, if the texture is a bitmap with background=0 and foreground=1, and
the destination depth is 1, the texture bitmap can be treated as a pixmap
(tcolors=NULL).

Note that while a source bitmap or pixmap has the same width and height as
the destination, a texture bitmap or pixmap has its own width and height
specified in the gx_tile_bitmap structure, and is replicated and/or clipped
as needed.

f specification
...............

Command indicates the raster operation and transparency as follows:

	bits 7-0: raster op
	bit 8: 0 if source opaque, 1 if source transparent
	bit 9: 0 if texture opaque, 1 if texture transparent
	bits ?-10: unused, must be 0

The raster operation follows the Microsoft and H-P specification.  It is an
8-element truth table that specifies the output value for each of the
possible 2x2x2 input values as follows:

	Bit	Texture	Source	Destination
	7	1	1	1
	6	1	1	0
	5	1	0	1
	4	1	0	0
	3	0	1	1
	2	0	1	0
	1	0	0	1
	0	0	0	0

Transparency affects the output in the following way.  A source or texture
pixel is considered transparent if its value is all 1's (e.g., 1 for
bitmaps, 0xffffff for 24-bit RGB pixmaps) *and* the corresponding
transparency bit is set in the command.  For each pixel, the result of the
Boolean operation is written into the destination iff neither the source nor
the texture pixel is transparent.  (Note that the H-P RasterOp
specification, on which this is based, specifies that if the source and
texture are both all 1's and the command specifies transparent source and
opaque texture, the result *should* be written in the output.  We think this
is an error in the documentation.)

Notes
.....

Note that copy_rop is defined to operate on pixels in RGB space, again
following the H-P and Microsoft specification.  For devices that don't use
RGB (or gray-scale with black=0, white=all 1's) as their native color
representation, the implementation of copy_rop must convert to RGB or gray
space, do the operation, and convert back (or do the equivalent of this).

Here are the copy_rop equivalents of the most important previous imaging
calls.  Note that rop3_S may be replaced by any other Boolean operation.
For monobit devices, we assume that black = 1.  We assume the following
declaration:
	static const gx_color_index white2[2] = { 1, 1 };

/* For all devices: */
(*fill_rectangle)(dev, x, y, w, h, color) ==>

	{ gx_color_index colors[2];
	  colors[0] = colors[1] = color;
	  (*dev_proc(dev, copy_rop))(dev, NULL, 0, 0, gx_no_bitmap_id, colors,
				     NULL, colors /*irrelevant*/,
				     x, y, w, h, 0, 0, rop3_S);
	}

/* For black-and-white devices only: */
(*copy_mono)(dev, base, sourcex, sraster, id,
	     x, y, w, h, (gx_color_index)0, (gx_color_index)1) ==>

	(*dev_proc(dev, copy_rop))(dev, base, sourcex, sraster, id, NULL,
				   NULL, white2 /*irrelevant*/,
				   x, y, w, h, 0, 0, rop3_S);

/* For color devices, where neither color0 nor color1 is gx_no_color_index: */
(*copy_mono)(dev, base, sourcex, sraster, id,
	     x, y, w, h, color0, color1) ==>

	{ gx_color_index colors[2];
	  colors[0] = color0, colors[1] = color1;
	  (*dev_proc(dev, copy_rop))(dev, base, sourcex, sraster, id, colors,
				     NULL, white2 /*irrelevant*/,
				     x, y, w, h, 0, 0, rop3_S);
	}

/* For black-and-white devices only: */
(*copy_mono)(dev, base, sourcex, sraster, id,
	     x, y, w, h, gx_no_color_index, (gx_color_index)1) ==>

	(*dev_proc(dev, copy_rop))(dev, base, sourcex, sraster, id, NULL,
				   NULL, white2 /*irrelevant*/,
				   x, y, w, h, 0, 0,
				   rop3_S | lop_S_transparent);

/* For all devices: */
(*copy_color)(dev, base, sourcex, sraster, id,
	      x, y, w, h) ==> [same as first copy_mono above]

/* For black-and-white devices only: */
(*tile_rectangle)(dev, tile, x, y, w, h,
		  (gx_color_index)0, (gx_color_index)1, px, py) ==>

	(*dev_proc(dev, copy_rop))(dev, NULL, 0, 0, gx_no_bitmap_id,
				   white2 /*irrelevant*/,
				   tile, NULL,
				   x, y, w, h, px, py, rop3_T)

Polygons
--------

In addition to the lowest-level drawing operations that take integer device
coordinates and pure device colors, the driver interface includes
higher-level operations that draw polygons using fixed-point coordinates,
possibly halftoned colors, and possibly a non-default logical operation.

The fill_* drawing operations all use the center-of-pixel rule: a pixel is
colored iff its center falls within the polygonal region being filled.  If a
pixel center (X+0.5,Y+0.5) falls exactly on the boundary, the pixel is
filled iff the boundary is horizontal and the filled region is above it, or
the boundary is not horizontal and the filled region is to the right of it.

int (*fill_trapezoid)(P10(gx_device *dev,
  fixed fx0, fixed fw0, fixed fy0,
  fixed fx1, fixed fw1, fixed fh, bool swap_axes,
  const gx_drawing_color *pdcolor, gs_logical_operation_t lop)) [OPTIONAL]

	Fill a trapezoid whose parallel sides are parallel to a coordinate
axis.  The corners are (fx0,fy0), (fx0+fw0,fy0), (fx1,fy0+fh), and
(fx1+fw1,fy0+fy).  We require fw0 >= 0, fw1 >= 0, and fh >= 0; if fw0 = 0 or
fw1 = 0, the trapezoid is actually a triangle.  If swap_axes is set, the
meanings of X and Y are interchanged.

int (*fill_parallelogram)(P9(gx_device *dev,
  fixed px, fixed py, fixed ax, fixed ay, fixed bx, fixed by,
  const gx_drawing_color *pdcolor, gs_logical_operation_t lop)) [OPTIONAL]

	Fill a parallelogram whose corners are (px,py), (px+ax,py+ay),
(px+bx,py+by), and (px+ax+bx,py+ay+by).  There are no constraints on the
values of any of the parameters, so the parallelogram may have any
orientation relative to the coordinate axes.

int (*fill_triangle)(P9(gx_device *dev,
  fixed px, fixed py, fixed ax, fixed ay, fixed bx, fixed by,
  const gx_drawing_color *pdcolor, gs_logical_operation_t lop)) [OPTIONAL]

	Fill a triangle whose corners are (px,py), (px+ax,py+ay), and
(px+bx,py+by).

int (*draw_thin_line)(P7(gx_device *dev,
  fixed fx0, fixed fy0, fixed fx1, fixed fy1,
  const gx_drawing_color *pdcolor, gs_logical_operation_t lop)) [OPTIONAL]

	Draw a one-pixel-wide line from (fx0,fy0) to (fx1,fy1).  This
replaces the older draw_line procedure, which required integer coordinates,
a pure color, and no logical operation.

High-level drawing
------------------

In addition to the low-level drawing operations described above, the driver
interface provides a set of high-level operations.  Normally these will have
their default implementation, which converts the high-level operation to the
low-level ones just described; however, drivers that generate high-level
output formats such as CGM, or communicate with devices that have firmware
for higher-level operations such as polygon fills, may implement these
high-level operations directly.

For more details, please consult the source code, specifically:
	gxpaint.h - defines gx_fill_params, gx_stroke_params
	gxfixed.h - defines fixed, gs_fixed_point (used by gx_*_params)
	gxistate.h - defines gs_imager_state (used by gx_*_params)
	gxline.h - defines gx_line_params (used by gs_imager_state)
	gslparam.h - defines line cap/join values (used by gx_line_params)
	gxmatrix.h - defines gs_matrix_fixed (used by gs_imager_state)
	g[s,x,z]path.h - defines gx_path
	g[x,z]cpath.h - defines gx_clip_path

int (*fill_path)(P6(gx_device *dev,
  const gx_imager_state *pis, gx_path *ppath, const gx_fill_params *params,
  const gx_drawing_color *pdcolor, const gx_clip_path *pcpath)) [OPTIONAL]

	Fill the given path, clipped by the given clip path, according to
the given parameters, with the given color.  The clip path pointer may be
NULL, meaning do not clip.

int (*stroke_path)(P6(gx_device *dev,
  const gx_imager_state *pis, gx_path *ppath, const gx_stroke_params *params,
  const gx_drawing_color *pdcolor, const gx_clip_path *pcpath)) [OPTIONAL]

	Stroke the given path, clipped by the given clip path, according to
the given parameters, with the given color.  The clip path pointer may be
NULL, meaning do not clip.

int (*fill_mask)(P13(gx_device *dev,
  const byte *data, int data_x, int raster, gx_bitmap_id id,
  int x, int y, int width, int height,
  const gx_drawing_color *pdcolor, int depth,
  int command, const gx_clip_path *pcpath)) [OPTIONAL]

	Color the 1-bits in the given mask (or according to the alpha
values, if depth > 1), clipped by the given clip path, with the given color
and logical operation.  The clip path pointer may be NULL, meaning do not
clip.  The parameters data, ..., height are as for copy_mono; depth is as
for copy_alpha; command is as for copy_rop.

High-level bitmap imaging
-------------------------

Similar to the high-level interface for fill/stroke graphics, a high-level
interface exists for bitmap images.  All of the procedures in this part of
the interface are optional; however, if any one of them is defined, they all
must be.  THIS INTERFACE IS NOT FULLY DEFINED YET, AND THE INFORMATION
PRESENTED HERE IS SUBJECT TO CHANGE.

A bitmap image is defined by a gs_image_t structure.  We only summarize this
structure here.

typedef struct gs_image_s {
	int Width;		/* Width of source image in pixels */
	int Height;		/* Height ditto */
	gs_matrix ImageMatrix;	/* transformation from 1x1 image space to */
				/* user space defined by imager CTM */
	int BitsPerComponent;
	const gs_color_space *ColorSpace;
	float Decode[8];	/* linear remapping of input values */
	bool Interpolate;	/* smooth image if true */
	bool ImageMask;		/* true = mask, false = solid image */
	bool adjust;		/* expand bits if mask */
	bool CombineWithColor;	/* use drawing color as RasterOp texture */
} gs_image_t;
typedef enum {
	gs_image_format_chunky = 0,
	gs_image_format_component_planar = 1
} gs_image_format_t;

For more details, consult the source code in
	gsiparam.h - defines parameters for an image

int (*begin_image)(P9(gx_device *dev,
  const gs_imager_state *pis, const gs_image_t *pim, gs_image_format_t format,
  gs_image_shape_t shape, const gx_drawing_color *pdcolor,
  const gx_clip_path *pcpath, gs_memory_t *memory, void **pinfo)) [OPTIONAL]

	Begin the transmission of an image.  Zero or more calls of
image_data will follow, and then a call of end_image.  The parameters of
begin_image are as follows:

	pis - pointer to an imager state.  The only relevant elements of the
imager state are the CTM (coordinate transformation matrix), the logical
operation (RasterOp/transparency), and the color rendering information.

	pim - pointer to the gs_image_t structure that defines the image
parameters.

	format - defines how pixels are represented for image_data.  See the
description of image_data below.

	shape - a bit mask that defines whether the client promises to
supply the full Width x Height array of pixels, or whether only a
subrectangle or irregular subset may be supplied; see the source code for
details.

	pdcolor - defines a drawing color, only needed for masks or if
CombineWithColor is true.

	pcpath - defines an optional clipping path, may be NULL.

	memory - defines the allocator to be used for allocating bookkeeping
information.

	pinfo - the implementation should return a pointer to its
bookkeeping structure here.

	begin_image is expected to allocate a structure for its bookkeeping
needs, using the allocator defined by the memory parameter, and return it in
*pinfo.  begin_image should not assume that the structures in *pim or
*pdcolor will survive the call on begin_image (except for the color space in
*pim->ColorSpace): it should copy any necessary parts of them into its own
bookkeeping structure.  It may, however, assume that *pis, *pcpath, and of
course *memory will live at least until end_image is called.

int (*image_data)(P8(gx_device *dev,
  void *info, const byte **planes, uint raster,
  int x, int y, int width, int height)) [OPTIONAL]

	This call provides more of the image source data.  Because of
banding, image_data will not necessarily receive complete rows, or all the
rows of an image; that is why the call supplies the source x, y, width, and
height parameters.  However, if x1/y1/w1/h1 and x2/y2/w2/h2 are the values
of these parameters for two successive calls on image_data, the following
properties are guaranteed:

	- 0 <= xi <= xi + wi <= params->Width.
	- 0 <= yi <= yi + hi <= params->Height.

	- If h1 > 1, then y2 = y1 + h1.

	- If h1 = 1, then either y2 = y1 + h1, or y2 = y1 and
	x2 = x1 + w1 and h2 = 1.

In other words, either a call passes 1 or more consecutive rows, or several
calls pass adjacent parts of a single row.  Note that the x/y/width/height
values refer to the source image data, not to the destination as they do for
copy_mono, copy_color, etc.

	The data for each row are packed big-endian within each byte.  The
raster (number of bytes per row) may include some padding at the end of each
row.  Note that for non-mask images, the input data may be in any color
space and may have any number of bits per component (1, 2, 4, 8, 12).
(Currently mask images always have 1 bit per component, but in the future,
they might allow multiple bits of alpha.)  Note also that each call of
image_data passes complete pixels: e.g., for a chunky image with 24 bits per
pixel, each call of image_data passes 3N bytes of data (specifically, 3 *
width * height).

	The interpretation of planes depends on the format argument of
begin_image.  If format is gs_image_format_chunky, planes[0] points to data
in "chunky" format, in which the components follow each other (e.g.,
RGBRGBRGB....)  If format is gs_image_format_component_planar, planes[0
.. N-1] point to data for the N components (e.g., N=3 for RGB data); each
plane contains samples for a single component, e.g., RR..., GG..., BB....
Note that the planes are divided by component, not by bit: for example, for
24-bit RGB data, N=3, with 8-bit values in each plane of data.  Someday we
may add gs_image_format_bit_planar, specifying that each plane would contain
only a single bit of the pixel value, but this is not currently implemented.

	image_data, unlike most other driver procedures that take bitmaps as
arguments, does not require the data to be aligned in any way.

int end_image(P3(gx_device *dev,
  void *info, bool draw_last)) [OPTIONAL]

	Finish processing an image, either because all data have been
supplied or because the caller has decided to abandon this image.  end_image
may be called at any time after begin_image.  It should free the info
structure and any subsidiary structures.  If draw_last is true, it should
finish drawing any buffered lines of the image.

	While there will almost never be more than one image enumeration in
progress -- i.e., after a begin_image, end_image will almost always be
called before the next begin_image -- driver code should not rely on this
property; in particular, it should store all information regarding the image
in the info structure, not in the driver structure.

	Note that if begin_image saves its parameters in the info structure,
it can decide on each call whether to use its own algorithms or to use the
default implementation.  (It may need to call gx_default_begin/end_image
partway through.)  [A later revision of this document may include an example
here.]

Reading bits back
-----------------

int (*get_bits)(P4(gx_device *, int y, byte *str, byte **actual_data))
  [OPTIONAL]

	Read one scan line of bits back from the device into the area
starting at str, namely, scan line y.  If the bits cannot be read back
(e.g., from a printer), return -1; otherwise return 0.  The contents of the
bits beyond the last valid bit in the scan line (as defined by the device
width) are unpredictable.  str need not be aligned in any particular way.

	If actual_data is NULL, the bits are always returned at str.  If
actual_data is not NULL, get_bits may either copy the bits to str and set
*actual_data = str, or it may leave the bits where they are and return a
pointer to them in *actual_data.  In the latter case, the bits are
guaranteed to start on a 32-bit boundary and to be padded to a multiple of
32 bits; also in this case, the bits are not guaranteed to still be there
after the next call on get_bits.

Parameters
----------

Devices may have an open-ended set of parameters, which are simply pairs
consisting of a name and a value.  The value may be of various types:
integer (int or long), boolean, float, string, name, null, array of integer,
or array of float.  For example, the Name of a device is a string; the
Margins of a device is an array of 2 floats.  See gsparam.h for more
details.

If a device has parameters other than the ones applicable to all devices
(or, in the case of printer devices, all printer devices), it must provide
get_params and put_params procedures.  If your device has parameters beyond
those of a straightforward display or printer, we strongly advise using the
_get_params and _put_params procedures in an existing device (for example,
gdevcdj.c or gdevbit.c) as a model for your own code.

int (*get_params)(P2(gx_device *dev, gs_param_list *plist)) [OPTIONAL]

	Read the parameters of the device into the parameter list at plist,
using the param_write_* macros/procedures defined in gsparam.h.
    
int (*put_params)(P2(gx_device *dev, gs_param_list *plist)) [OPTIONAL]

	Set the parameters of the device from the parameter list at plist,
using the param_read_* macros/procedures defined in gsparam.h.  All
put_params procedures must use a "two-phase commit" algorithm; see gsparam.h
for details.

External fonts
--------------

Drivers may include the ability to display text.  More precisely, they may
supply a set of procedures that in turn implement some font and text
handling capabilities.  These procedures are documented in another file,
xfonts.txt.  The link between the two is the driver procedure that
supplies the font/text procedures:

xfont_procs *(*get_xfont_procs)(P1(gx_device *dev)) [OPTIONAL]

	Return a structure of procedures for handling external fonts and
text display.  A NULL value means that this driver doesn't provide this
capability.

For technical reasons, a second procedure is also needed:

gx_device *(*get_xfont_device)(P1(gx_device *dev)) [OPTIONAL]

	Return the device that implements get_xfont_procs in a non-default
way for this device, if any.  Except for certain special internal devices,
this is always the device argument.

Page devices
------------

gx_device *(*get_page_device)(P1(gx_device *dev)) [OPTIONAL]

	According to the Adobe specifications, some devices are "page
devices" and some are not.  This procedure returns NULL if the device is
not a page device, or the device itself if it is a page device.  In the
case of forwarding devices, get_page_device returns the underlying page
device (or NULL if the underlying device is not a page device).

Miscellaneous
-------------

int (*get_band)(P3(gx_device *dev, int y, int *band_start)) [OPTIONAL]

	If the device is a band device, this procedure stores in *band_start
the scan line (device Y coordinate) of the band that includes the given Y
coordinate, and returns the number of scan lines in the band.  If the device
is not a band device, this procedure returns 0.  The latter is the default
implementation.
