Aminet 2/Aminet AMIGA CDROM (1994)(Walnut Creek)[Feb 1994][W.O. 44790-1].iso/Aminet/text/print/ghostscript2_6_1.lha/drivers.doc

Text File | 1993-05-26 | 28KB | 679 lines

   Copyright (C) 1989, 1990, 1991, 1992, 1993 Aladdin Enterprises.
     All rights reserved.

This file is part of Ghostscript.

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 Ghostscript General Public License for full details.

Everyone is granted permission to copy, modify and redistribute
Ghostscript, but only under the conditions described in the Ghostscript
General Public License.  A copy of this license is supposed to have been
given to you along with 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.

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

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 500 C driver (gdevcdj.c) is
a good place to start.

On the other hand, if you're writing a driver for some more esoteric
device, or want to do something like add new settable attributes (besides
page size and resolution), 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
        { ... procedures ... },        * procs
        ... generic parameter values ...
        ... 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 structure definition in gxdevice.h.

typedef struct gx_device_s {
    int params_size;        /* size of this structure */
    gx_device_procs *procs;        /* pointer to procedure structure */
    char *name;            /* the device name */
    int width;            /* width in pixels */
    int height;            /* height in pixels */
    float x_pixels_per_inch;    /* x density */
    float y_pixels_per_inch;    /* y density */
    gs_rect margin_inches;        /* margins around imageable area, */
                    /* in inches */
    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.
    - The null device in gsdevice.c.  (Note that this device does
        not allow procedure defaulting.)
    - The tracing "device" in gstdev.c.  (Ditto.)
    - The command list "device" in gxclist.c.  (Ditto.)
    - The clip list accumulation and clipping "devices" in gxcpath.c.
        (Ditto.)
    - The "memory" devices in gdevmem.h and gdevmem*.c.  (Ditto.)
    - 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.  (Most of the printer devices are
        created with the macros in gdevprn.h, 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_props and/or
gx_default_put_props (in gsdevice.c).  Note that if all you are doing
is adding optional procedures, you do NOT have to modify any device
drivers other than the ones specifically listed above; Ghostscript
will substitute the default procedures properly.

********
******** 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
gs.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.

    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.

********
******** 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 { 1, 1, 1, 0, 2, 0 }
#define dci_color(depth,maxv,dither) { 3, depth, maxv, maxv, dither, 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 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
gx_color_value, 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
gx_color_value, 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 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.  Ghostscript
doesn't ever do any computations with these 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_prop_item (defined in gsprops.h)

    This is an element of a property list, which is used to read and
set attributes in a device.  See the comments in gsprops.h, and the
description of the get_props and put_props 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;

********
******** Driver procedures ********
********

All the procedures 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.

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
----------

Ghostscript "opens" and "closes" drivers explicitly; a driver can assume
that no output operations will be done through it while it is closed.
Ghostscript keeps track of whether a given driver is open, so a driver
will never be opened when it is already open, or closed when it is already
closed.

The following are the only driver procedures that may be called when the
driver is closed:
    open_device
    get_initial_matrix
    get_props
    put_props

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 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.

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.

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.

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))

    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 is the workhorse for text display in Ghostscript,
so implementing it efficiently is very important.

int (*tile_rectangle)(P10(gx_device *, const gx_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.

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).  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))

    Copy a color image with multiple bits per pixel.  The raster is in
bytes, but x and width are in pixels, not bits.  If the device doesn't
actually support color, this is OPTIONAL; the default is equivalent to
copy_mono with color0 = 0 and color1 = 1.  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.

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.

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, starting with scan line y.  If the bits cannot be
read back (e.g., from a printer), return -1; otherwise return a value
as described below.  The contents of the bits beyond the last valid
bit in the scan line (as defined by the device width) are
unpredictable.

    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 point 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.

Properties
----------

Devices may have an open-ended set of properties, which are simply pairs
consisting of a name and a value.  The value may be of various types:
integer, boolean, float, string, array of integer, or array of float.

Property lists are somewhat complex.  If your device has properties beyond
those of a straightforward display or printer, we strongly advise using
the code for the default implementation of get_props and put_props in
gsdevice.c as a model for your own code.

int (*get_props)(P2(gx_device *dev, gs_prop_item *plist)) [OPTIONAL]

    Read all the properties of the device into the property list at
plist.  Return the number of properties.  See gsprops.h for more details,
gx_default_get_props in gsdevice.c for an example.

    If plist is NULL, just return the number of properties plus the
total number of elements in all array-valued properties.  This is how the
getdeviceprops operator finds out how much storage to allocate for the
property list.

int (*put_props)(P3(gx_device *dev, gs_prop_item *plist,
  int count)) [OPTIONAL]

    Set the properties of the device from the property list at plist.
Return 0 if everything was OK, an error code
(gs_error_undefined/typecheck/rangecheck/limitcheck) if some property had
an invalid type or out-of-range value.  See gsprops.h for more details,
gx_default_put_props in gsdevice.c for an example.

    Changing device properties may require closing the device and
reopening it.  If this is the case, the put_props procedure should just
close the device; a higher-level routine (gs_putdeviceprops) will reopen
it.

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.doc.  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.