To add a driver to Ghostscript, first pick a name for your device, say
"smurf". (Device names must be 1 to 8 characters, begin
with a letter, and consist only of letters, digits, and underscores. Case
is significant: all current device names are lower case.) Then all you
need do is edit contrib.mak in two places.
The list of devices, in the section headed "Catalog". Add
smurf to the list.
The section headed "Device drivers".
Suppose the files containing the smurf driver are called
"joe" and "fred". Then you should add the
following lines:
and whatever joe.c and fred.c depend on.
If the smurf driver also needs special libraries, for instance a library
named "gorf", then the entry should look like this:
Note that the space before the :, and the explicit compilation rules for the
.c files, are required for portability,
Keeping things simple
If you want to add a simple device (specifically, a monochrome 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 Canon 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 in 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 volume of code, 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 by copying code to create an entirely new
source module. 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 (normally) 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 the latter is 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.
The device structure instance must have the name
gs_smurf_device, where smurf is the device
name used in contrib.mak. gx_device_common
is a macro consisting only of the element definitions.
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
refer to elements of the full structure, using the following standard
initialization statement at the beginning of each procedure:
(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 contrib.mak.
For sophisticated developers only
If for any reason you need to change the definition of the basic device
structure, or to add procedures, you must change the following places:
This document and the news document (if you want
to keep the documentation up to date).
The definition of gx_device_common and the procedures
in gxdevcli.h.
Possibly, the default forwarding procedures declared in
gxdevice.h and implemented in
gdevnfwd.c.
The device procedure record completion routines in
gdevdflt.c.
You may also have to change the code for
gx_default_get_params 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
and contrib.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.
Coordinates and types
Coordinate system
Since each driver specifies the initial transformation from user
coordinates 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 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
Between the Ghostscript graphics library and the device, colors are
represented in three forms. Color components in a color space (Gray, RGB,
DeviceN, etc.) represented as frac values. Device colorants
are represented as gx_color_value values. For many
procedures, colors are represented in a type called
gx_color_index.
All three types are described in more detail in Types
The color_info member of the device structure defines the
color and gray-scale capabilities of the device. Its type is defined as
follows:
/*
* The enlarged color model information structure: Some of the
* information that was implicit in the component number in
* the earlier conventions (component names, polarity, mapping
* functions) are now explicitly provided.
*
* Also included is some information regarding the encoding of
* color information into gx_color_index. Some of this information
* was previously gathered indirectly from the mapping
* functions in the existing code, specifically to speed up the
* halftoned color rendering operator (see
* gx_dc_ht_colored_fill_rectangle in gxcht.c). The information
* is now provided explicitly because such optimizations are
* more critical when the number of color components is large.
*
* Note: no pointers have been added to this structure, so there
* is no requirement for a structure descriptor.
*/
typedef struct gx_device_color_info_s {
/*
* max_components is the maximum number of components for all
* color models supported by this device. This does not include
* any alpha components.
*/
int max_components;
/*
* The number of color components. This does not include any
* alpha-channel information, which may be integrated into
* the gx_color_index but is otherwise passed as a separate
* component.
*/
int num_components;
/*
* Polarity of the components of the color space, either
* additive or subtractive. This is used to interpret transfer
* functions and halftone threshold arrays. Possible values
* are GX_CM_POLARITY_ADDITIVE or GX_CM_POLARITY_SUBTRACTIVE
*/
gx_color_polarity_t polarity;
/*
* The number of bits of gx_color_index actually used.
* This must be <= sizeof(gx_color_index), which is usually 64.
*/
byte depth;
/*
* Index of the gray color component, if any. The max_gray and
* dither_gray values apply to this component only; all other
* components use the max_color and dither_color values.
*
* This will be GX_CINFO_COMP_NO_INDEX if there is no gray
* component.
*/
byte gray_index;
/*
* max_gray and max_color are the number of distinct native
* intensity levels, less 1, for the gray and all other color
* components, respectively. For nearly all current devices
* that support both gray and non-gray components, the two
* parameters have the same value.
*
* dither_grays and dither_colors are the number of intensity
* levels between which halftoning can occur, for the gray and
* all other color components, respectively. This is
* essentially redundant information: in all reasonable cases,
* dither_grays = max_gray + 1 and dither_colors = max_color + 1.
* These parameters are, however, extensively used in the
* current code, and thus have been retained.
*
* Note that the non-gray values may now be relevant even if
* num_components == 1. This simplifies the handling of devices
* with configurable color models which may be set for a single
* non-gray color model.
*/
gx_color_value max_gray; /* # of distinct color levels -1 */
gx_color_value max_color;
gx_color_value dither_grays;
gx_color_value dither_colors;
/*
* Information to control super-sampling of objects to support
* anti-aliasing.
*/
gx_device_anti_alias_info anti_alias;
/*
* Flag to indicate if gx_color_index for this device may be divided
* into individual fields for each component. This is almost always
* the case for printers, and is the case for most modern displays
* as well. When this is the case, halftoning may be performed
* separately for each component, which greatly simplifies processing
* when the number of color components is large.
*
* If the gx_color_index is separable in this manner, the comp_shift
* array provides the location of the low-order bit for each
* component. This may be filled in by the client, but need not be.
* If it is not provided, it will be calculated based on the values
* in the max_gray and max_color fields as follows:
*
* comp_shift[num_components - 1] = 0,
* comp_shift[i] = comp_shift[i + 1]
* + ( i == gray_index ? ceil(log2(max_gray + 1))
* : ceil(log2(max_color + 1)) )
*
* The comp_mask and comp_bits fields should be left empty by the client.
* They will be filled in during initialization using the following
* mechanism:
*
* comp_bits[i] = ( i == gray_index ? ceil(log2(max_gray + 1))
* : ceil(log2(max_color + 1)) )
*
* comp_mask[i] = (((gx_color_index)1 << comp_bits[i]) - 1)
* << comp_shift[i]
*
* (For current devices, it is almost always the case that
* max_gray == max_color, if the color model contains both gray and
* non-gray components.)
*
* If separable_and_linear is not set, the data in the other fields
* is unpredictable and should be ignored.
*/
gx_color_enc_sep_lin_t separable_and_linear;
byte comp_shift[GX_DEVICE_COLOR_MAX_COMPONENTS];
byte comp_bits[GX_DEVICE_COLOR_MAX_COMPONENTS];
gx_color_index comp_mask[GX_DEVICE_COLOR_MAX_COMPONENTS];
/*
* Pointer to name for the process color model.
*/
const char * cm_name;
} gx_device_color_info;
Note: See Changing color_info data before changing
any information in the color_info structure for a device.
It is recommended that the values for this structure be defined using one
of the standard macros provided for this purpose. This allows for future
changes to be made to the structure without changes being required in the
actual device code.
The following macros (in gxdevcli.h) provide
convenient shorthands for initializing this structure for ordinary
black-and-white or color devices:
The #define dci_black_and_white macro defines a
single bit monochrome device (For example: a typical monochrome printer device.)
The #define dci_color(depth,maxv,dither) macro can be used
to define a 24 bit RGB device or a 4 or 32 bit CMYK device.
The #define dci_extended_alpha_values macro (in
gxdevcli.h)
specifies most of the current fields in the structure. However this macro allows
only the default setting for the comp_shift, comp_bits, and comp_mask fields
to be set. Any device which requires a non-default setting for these fields
has to correctly these fields during the device open procedure.
See
Separable and linear fields> and
Changing color_info data.
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 color space color value as a device color, it first tests
whether the color is a gray level and 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.
Separable and linear fields
The three fields comp_shift, comp_bits, and
comp_mask are only used if the separable_and_linear
field is set to GX_CINFO_SEP_LIN. In this situation a gx_color_index
value must represent a combination created by or'ing bits for each of the devices's
output colorants. The comp_shift array defines the location
(shift count) of each colorants bits in the output gx_color_index value. The
comp_bits array defines the number of bits for each colorant.
The comp_mask array contains a mask which can be used to isolate
the bits for each colorant. These fields must be set if the device supports
more than four colorants.
Changing color_info data
For most devices, the information in the device's color_info
structure is defined by the various device definition macros and the data remains
constant during the entire existence of the device. In general the Ghostscript
graphics assumes that the information is constant. However some devices want
to modify the data in this structure.
The device's put_params procedure may change
color_info field values.
After the data has been modified then the
device should be closed (via a call to gs_closedevice). Closing
the device will erase the current page so these changes should only be made
before anything has been drawn on a page.
The device's open_device procedure may change
color_info field values. These changes should be done before
calling any other procedures are called.
The Ghostscript graphics library
uses some of the data in color_info to set the default
procedures for the
get_color_mapping_procs,
get_color_comp_index,
encode_color, and
decode_color procedures.
These default procedures are set when the
device is originally created. If any changes are made to the
color_info fields then the device's open_device
procedure
has responsibility for insuring that the correct procedures are contained
in the device structure. (For an example, see the display device open procedure
display_open and its subroutine display_set_color_format
(in gdevdisp).
Types
Here is a brief explanation of the various types that appear as parameters
or results of the drivers.
This is the type used to represent color values for the input to the
color model mapping procedures. It is currently defined as a short. It has a
range of frac_0 to frac_1.
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.
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 encode_color and
hands them back as arguments to several other procedures.
The special
value gx_no_color_index (defined as
(~(gx_color_index)(0)) ) means "transparent" for some of
the procedures.
The size of gx_color_index can be either 32 or 64 bits. The
choice depends upon the architecture of the CPU and the compiler. The default
type definition is simply:
typedef unsigned long gx_color_index;
However if GX_COLOR_INDEX_TYPE is defined, then it is used
as the type for gx_color_index.
typedef GX_COLOR_INDEX_TYPE gx_color_index;
The smaller size (32 bits) may produce more efficient or faster executing
code. The larger size (64 bits) is needed for representing either more
bits per component or more components. An example of the later case is
a device that supports 8 bit contone colorants using a DeviceCMYK process
color model with its four colorants and also supports additional spot
colorants.
Currently autoconf attempts to find a 64 bit type definition for the
compiler being used, and if a 64 bit type is found then
GX_COLOR_INDEX_TYPE is set to the type.
For Microsoft and the MSVC compiler, GX_COLOR_INDEX_TYPE will
be set to unsigned _int64 if USE_LARGE_COLOR_INDEX
is set to 1 either on the make command line or by editing the definition
in msvc32.mak
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, rather than a simple return statement it
should use the return_error macro defined in gx.h, which is automatically included by gdevprn.h but not by gserrors.h. For example
return_error(gs_error_VMerror);
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).
Like calloc, but unlike malloc,
gs_malloc takes an element count and an element size. For
structures, num_elements is 1 andi
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.
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
i
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 registered in the normal way
(using the makefile approach described above), no special care is needed as
long as instances are created only 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.
The printer driver must implement a print_page or
print_page_copies procedure. There are 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)(gx_device_printer *, FILE *)
int (*print_page_copies)(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(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
encode_color procedure.
The print_page procedure can determine the number of bytes
required to hold a scan line by calling:
uint gdev_prn_raster(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 WXYZ, the entry
in the procedure structure may be either gx_default_WXYZ,
for instance 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 continue to work even if new (optional)
members are added.
Life cycle
A device instance begins life in a closed state. In this state, no output
operations will occur. Only the following procedures may be called:
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 no user-accessible operation to close a device instance. This is
not an oversight -- it is required in order to enforce the following
invariant:
If a device instance is the current device in any graphics state,
it must be open (have is_open set to true).
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 and is in a VM being restored. I.e.,
if the instance was created in local VM since a save, it
will always be closed and freed by the corresponding
restore; if it was created in global VM, it will only be
closed by the outermost restore, regardless of the save
level at the time the instance was created.
By the garbage collector, if the instance is no longer accessible.
When Ghostscript exits (terminates).
Open, close, sync, copy
int (*open_device)(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. NOTE: Clients should never call
a device's open_device procedure directly: they should
always call gs_opendevice instead.
int (*finish_copydevice)(gx_device *dev, const gx_device
*from_dev)[OPTIONAL]
Perform any cleanup
required after copydevice has created a new device instance
by copying from_dev. If the copy operation should not be
allowed, this procedure should return an error; the copy will be freed. The
default implementation allows copying the device prototype, but does not
allow copying device instances, because instances may contain internal
pointers that should not be shared between copies, and there is no way to
determine this from outside the device. NOTE: Clients should never
call a device's finish_copydevice procedure: this procedure
is only intended for use by gs_copydevice[2].
Construct the initial transformation matrix mapping user coordinates
(nominally 1/72 inch per unit) to device coordinates. The default
procedure computes this from width, height, and
[xy]_pixels_per_inch on the assumption that
the origin is in the upper left corner, that is
xx = x_pixels_per_inch/72, xy = 0, yx = 0, yy = -y_pixels_per_inch/72, tx = 0, ty = height.
int (*sync_output)(gx_device *)[OPTIONAL]
Synchronize the device. If any output to the device has been
buffered, send or 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)(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)(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. NOTE: Clients should never call a device's
close_device procedure directly: they should always call
gs_closedevice instead.
Color and alpha mapping
Note that code in the Ghostscript library may cache the results of calling
one or more of the color mapping procedures. If the result returned by any
of these procedures would change (other than as a result of a change made by
the driver's put_params procedure), the driver must call
gx_device_decache_colors(dev).
The map_rgb_color, map_color_rgb, and
map_cmyk_color are obsolete. They have been left
in the device procedure list for backward compatibility. See the
encode_color and decode_color procedures
below. To insure that older device drivers are changed to use the new
encode_color and decode_color
procedures,
the parameters for the older procedures have been changed to
match the new procedures. To minimize changes in devices that have
already been written, the map_rgb_color and map_cmyk_color routines
are used as the default value for the encode_color routine. The
map_cmyk_color routine is used if the number of components is four.
The map_rgb_color routine is used if the number of components is one
or three. This works okay for RGB and CMYK process color model devices.
However this does not work properly for gray devices. The encode_color
routine for a gray device is only passed one component. Thus the
map_rgb_color routine must be modified to only use a single input (instead
of three). (See the encode_color and decode_color routines below.)
Colors can be specified to the Ghostscript graphics library in a variety
of forms. For example, there are a wide variety of color spaces that can
be used such as Gray, RGB, CMYK, DeviceN, Separation, Indexed, CIEbasedABC,
etc. The graphics library converts the various input color space
values into four base color spaces: Gray, RGB, CMYK, and DeviceN. The
DeviceN color space allows for specifying values for individual device
colorants or spot colors.
Colors are converted by the device in a two step process. The first step
is to convert a color in one of the base color spaces (Gray, RGB, CMYK,
or DeviceN) into values for each device colorant. This transformation is
done via a set of procedures provided by the device. These procedures are
provided by the get_color_mapping_procs device procedure.
Between the first and second steps, the graphics library applies transfer
functions to the device colorants. Where needed, the output of the results
after the transfer functions is used by the graphics library for halftoning.
In the second step, the device procedure encode_color is
used to convert the transfer function results into a
gx_color_index value.
The gx_color_index values are passed to specify colors
to various routines.
The choice of the encoding for a gx_color_index is
up to the device. Common choices are indexes into a color palette or
several integers packed together into a single value. The manner of this
encoding is usually opaque to the graphics library. The only exception to this
statement occurs when halftoning 5 or more colorants. In this case the
graphics library assumes that if a colorant values is zero then the
bits associated with the colorant in the gx_color_index
value are zero.
int get_color_comp_index(const gx_device * dev, const char * pname,
int name_size, int src_index)[OPTIONAL]
This procedure returns the device colorant number of the given name.
The possible return values are -1, 0 to
GX_DEVICE_COLOR_MAX_COMPONENTS - 1, or
GX_DEVICE_COLOR_MAX_COMPONENTS. A value of -1 indicates that
the specified name is not a colorant for the device. A value of 0 to
GX_DEVICE_COLOR_MAX_COMPONENTS - 1 indicates the colorant number
of the given name. A value of GX_DEVICE_COLOR_MAX_COMPONENTS
indicates that the given name is a valid colorant name for the device but the
colorant is not currently being used. This is used for implementing names
which are in SeparationColorNames but not in SeparationOrder.
The default procedure returns results based upon process color model
of DeviceGray, DeviceRGB, or DeviceCMYK selected by
color_info.num_components. This procedure must be
defined if another process color model is used by the device or spot colors are
supported by the device.
This procedure returns a list of three procedures. These procedures
are used to translate values in either Gray, RGB, or CMYK color spaces
into device colorant values. A separate procedure is not required for the
DeviceN and Separation color spaces since these already represent
device colorants.
The default procedure returns a list of procedures based upon
color_info.num_components. These procedures are appropriate
for DeviceGray, DeviceRGB, or DeviceCMYK process color model devices. A
procedure must be defined if another process color model is used by the
device or spot colors are to be supported.
Map a set of device color values into a gx_color_index
value. The range of legal values of the
arguments is 0 to gx_max_color_value. The default procedure
packs bits into a gx_color_index value based upon the
values in color_info.depth and
color_info.num_components.
Note that the encode_color procedure
must not return gx_no_color_index (all 1s).
int (*decode_color)(gx_device *, gx_color_index color,
gx_color_value * CV)[OPTIONAL]
This is the inverse of the encode_color procedure.
Map a gx_color_index value to color values. The default
procedure unpacks bits from the gx_color_index value based upon
the values in color_info.depth and
color_info.num_components.
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
encode_color procedure and ignore alpha.
Note that if a driver implements map_rgb_alpha_color, it
must also implement encode_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
encode_color(dev, CV).
int (*map_color_rgb_alpha)(gx_device *,
gx_color_index color, gx_color_value rgba[4])[OPTIONAL]
Map a device color code to RGB and alpha values. The default
implementation calls map_color_rgb and fills in
gx_max_color_value for alpha.
Note that if a driver implements map_color_rgb_alpha, it
must also implement decode_color, and must implement them
in such a way that the first 3 values returned by
map_color_rgb_alpha are the same as the values returned by
decode_color.
Note that only RGB devices currently support variable opacity; alpha is ignored
on other devices. The PDF 1.4 transparency features are supported on all devices.
This procedure is no longer used: it is replaced by the
color_info.anti_alias member of the driver structure. However, it still
appears in the driver procedure vector for backward compatibility. It
should never be called, and drivers should not implement it.
Pixel-level drawing
This group of drawing operations specifies data at the pixel level. All
drawing operations use device coordinates and device color values.
int (*fill_rectangle)(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.
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)(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 and
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)(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)(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 (that is, 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, 32, 40, 48, 56, or 64. The pixel
values are device color codes (that is, whatever it is that
encode_color returns).
int (*copy_color)(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.
Compositing
In addition to direct writing of opaque pixels, devices must also support
compositing. Currently two kinds of compositing are defined
(RasterOp and alpha-based), but more may be added in the
future.
THIS AREA OF THE INTERFACE IS SOMEWHAT UNSTABLE: USE AT YOUR OWN
RISK.
int (*copy_alpha)(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]
This procedure is somewhat misnamed: it was added to the interface
before we really understood alpha channel and compositing.
Fill a given region with a given color modified by an individual alpha
value for each pixel. For each pixel, this is equivalent to
alpha-compositing with a source pixel whose alpha value is obtained from
the pixmap (data, data_x, and
raster) and whose color is the given color (which has
not been premultiplied by the alpha value), using the Sover
rule. depth, the number of bits per alpha value, 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.
Create a new device (called a "compositing device" or "compositor")
that will composite data written to it with the device's existing data,
according to the compositing function defined by *pcte.
Devices will normally implement this in one of the following standard ways:
Devices that don't do any imaging and don't forward any imaging
operations (for example, the null device, the hit detection device, and the
clipping list accumulation device) simply return themselves, which
effectively ignores the compositing function.
"Leaf" devices that do imaging and have no special optimizations for
compositing (for example, some memory devices) ask the
gs_composite_t to create a default compositor.
Leaf devices that can implement some kinds of compositing operation
efficiently (for example, monobit memory devices and RasterOp) inspect the
type and values of *pcte to determine whether it specifies
such an operation: if so, they create a specialized compositor, and if not,
they ask the gs_composite_t to create a default compositor.
Other kinds of forwarding devices, which don't fall into any of these
categories, require special treatment. In principle, what they do is ask
their target to create a compositor, and then create and return a copy of
themselves with the target's new compositor as the target of the copy.
There is a possible default implementation of this approach: if the
original device was D with target T, and T creates a
compositor C, then the default implementation creates a device
F that for each operation temporarily changes D's target to
C, forwards the operation to D, and then changes D's
target back to T. However, the Ghostscript library currently only
creates a compositor with an imaging forwarding device as target in a few
specialized situations (banding, and bounding box computation), and these
are handled as special cases.
Note that the compositor may have a different color space, color
representation, or bit depth from the device to which it is compositing.
For example, alpha-compositing devices use standard-format chunky color
even if the underlying device doesn't.
Closing a compositor frees all of its storage, including the compositor
itself. However, since the create_compositor call may
return the same device, clients must check for this case, and only call the
close procedure if a separate device was created.
[strip_]copy_rop WILL BE SUPERSEDED BY COMPOSITORS
int (*copy_rop)(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)(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:
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).
The texture specification T
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 points 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 or clipped as needed.
The function specification f
"Command" indicates the raster operation and transparency as follows:
Bits
7-0
raster op
8
0 if source opaque, 1 if source transparent
9
0 if texture opaque, 1 if texture transparent
?-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 2×2×2 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 1s (for instance, 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 HP
RasterOp specification, on which this is based, specifies that if the
source and texture are both all 1s 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
copy_rop is defined to operate on pixels in RGB space,
again following the HP and Microsoft specification. For devices that
don't use RGB (or gray-scale with black = 0, white = all 1s) 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. We assume the
declaration:
static const gx_color_index white2[2] = { 1, 1 };
Note that rop3_S may be replaced by any other Boolean operation.
For monobit devices, we assume that black = 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)
Polygon-level drawing
In addition to the pixel-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.
Fill a trapezoid. The bottom and top edges are parallel to the x
axis, and are defined by ybot and ytop,
respectively. The left and right edges are defined by left
and right. Both of these represent lines (gs_fixed_edge
is defined in gxdevcli.h and consists
of gs_fixed_pointstart and end points).
The y coordinates of these lines need not have any specific relation to
ybot and ytop. The routine is defined this way so
that the filling algorithm can subdivide edges and still guarantee
that the exact same pixels will be filled. If
swap_axes is set, the meanings of X and Y are
interchanged.
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.
Draw a one-pixel-wide line from (fx0,fy0) to
(fx1,fy1).
int (*draw_line)(gx_device *dev, int x0, int y0,
int x1, int y1, gx_color_index color)[OPTIONAL] [OBSOLETE]
This procedure is no longer used: it is replaced by the draw_thin_line
procedure. However, still appears in the driver procedure vector for
backward compatibility. It should never be called, and drivers should not
implement it.
High-level drawing
In addition to the lower-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:
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.
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 not to clip.
int (*fill_mask)(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.
Images
Similar to the high-level interface for fill and stroke graphics, a high-level
interface exists for bitmap images. The procedures in this part of the
interface are optional.
Bitmap images come in a variety of types, corresponding closely (but not
precisely) to the PostScript ImageTypes. The generic or common part of all
bitmap images is defined by:
Of course, standard PostScript images don't have an alpha component. For
more details, consult the source code in gsiparam.h and gsiparm*.h,
which define parameters for an image.
The begin[_typed_]image driver procedures create image
enumeration structures. The common part of these structures consists of:
typedef struct gx_image_enum_common_s {
const gx_image_type_t *image_type;
const gx_image_enum_procs_t *procs;
gx_device *dev;
gs_id id;
int num_planes;
int plane_depths[gs_image_max_planes]; /* [num_planes] */
int plane_widths[gs_image_max_planes] /* [num_planes] */
} gx_image_enum_common_t;
where procs consists of:
typedef struct gx_image_enum_procs_s {
/*
* Pass the next batch of data for processing.
*/
#define image_enum_proc_plane_data(proc)\
int proc(gx_device *dev,\
gx_image_enum_common_t *info, const gx_image_plane_t *planes,\
int height)
image_enum_proc_plane_data((*plane_data));
/*
* End processing an image, freeing the enumerator.
*/
#define image_enum_proc_end_image(proc)\
int proc(gx_device *dev,\
gx_image_enum_common_t *info, bool draw_last)
image_enum_proc_end_image((*end_image));
/*
* Flush any intermediate buffers to the target device.
* We need this for situations where two images interact
* (currently, only the mask and the data of ImageType 3).
* This procedure is optional (may be 0).
*/
#define image_enum_proc_flush(proc)\
int proc(gx_image_enum_common_t *info)
image_enum_proc_flush((*flush));
} gx_image_enum_procs_t;
In other words, begin[_typed]_image sets up an
enumeration structure that contains the procedures that will process the
image data, together with all variables needed to maintain the state of the
process. Since this is somewhat tricky to get right, if you plan to create
one of your own you should probably read an existing implementation of
begin[_typed]_image, such as the one in gdevbbox.c or gdevps.c.
The data passed at each call of image_plane_data consists of
one or more planes, as appropriate for the type of image.
begin[_typed]_image must initialize the
plane_depths array in the enumeration structure with the
depths (bits per element) of the planes. The array of
gx_image_plane_t structures passed to each call of
image_plane_data then defines where the data are stored, as
follows:
Begin the transmission of an image. Zero or more calls of
image_plane_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 or
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_plane_data. See the description of
image_plane_data below
prect
if not NULL, defines a subrectangle of the
image; only the data for this subrectangle will be passed to
image_plane_data, and only this subrectangle should
be drawn
pdcolor
defines a drawing color, only needed for masks or if
CombineWithColor is true
pcpath
if not NULL, defines an optional clipping path
memory
defines the allocator to be used for allocating bookkeeping
information
pinfo
the implementation should return a pointer to its state
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, *prect, 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.
begin_image returns 0 normally, or 1 if the image does not
need any data. In the latter case, begin_image does not
allocate an enumeration structure.
The image may be of any ImageType, not only
image_type_simple (1);
The image format is included in the image structure, not supplied as a
separate argument;
The optional pmat argument provides a matrix that
substitutes for the one in the imager state;
For mask images, if pmat is not NULL
and the color is pure, pis may be NULL.
The actual transmission of data uses the procedures in the enumeration
structure, not driver procedures, since the handling of the data usually
depends on the image type and parameters rather than the device. These
procedures are specified as follows.
int (*image_plane_data)(gx_device *dev,
gx_image_enum_common_t *info,
const gx_image_plane_t *planes, int height)
This call provides more of the image source data: specifically,
height rows, with Width pixels supplied for
each row.
The data for each row are packed big-endian within each byte, as for
copy_color. The data_x (starting X position
within the row) and raster (number of bytes per row) are
specified separately for each plane, and may include some padding at the
beginning or 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_plane_data passes complete pixels: for example, for
a chunky image with 24 bits per pixel, each call of
image_plane_data passes 3N bytes of data (specifically,
3 × Width × height).
The interpretation of planes depends on the format member of
the gs_image[_common]_t structure:
If the format is gs_image_format_chunky,
planes[0].data points to data in "chunky" format, in which
the components follow each other (for instance, RGBRGBRGB....)
If the format is gs_image_format_component_planar,
planes[0 .. N-1].data point to data for the
N components (for example, N=3 for RGB
data); each plane contains samples for a single component, for instance,
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.
If the format is gs_image_format_bit_planar,
planes[0 .. N*B-1].data point to data for the
N components of B bits each (for example,
N=3 and B=4 for RGB data with 4 bits per
component); each plane contains samples for a single bit, for instance, R0
R1 R2 R3 G0 G1 G2 G3 B0 B1 B2 B3. Note that the most significant bit of
each plane comes first.
If, as a result of this call, image_plane_data has been called with all
the data for the (sub-)image, it returns 1; otherwise, it returns 0 or an
error code as usual.
image_plane_data, unlike most other procedures that take bitmaps as
arguments, does not require the data to be aligned in any way.
Note that for some image types, different planes may have different
numbers of bits per pixel, as defined in the plane_depths array.
int (*end_image)(gx_device *dev, void *info,
bool draw_last)
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.
Notes
While there will almost never be more than one image enumeration in
progress -- that is, 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_[typed_]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.]
Text
The third high-level interface handles text. As for images, the interface
is based on creating an enumerator which then may execute the operation in
multiple steps. As for the other high-level interfaces, the procedures are
optional.
Begin processing text, by creating a state structure and storing it in
*ppte. The parameters of text_begin are as
follows:
dev
The usual pointer to the device.
pis
A pointer to an imager state. All elements may be relevant,
depending on how the text is rendered.
text
A pointer to the structure that defines the text operation
and parameters. See gstext.h for details.
font
Defines the font for drawing.
path
Defines the path where the character outline will be appended
(if the text operation includes TEXT_DO_...PATH),
and whose current point indicates where drawing should occur
and will be updated by the string width (unless the text
operation includes TEXT_DO_NONE).
pdcolor
Defines the drawing color for the text. Only relevant if
the text operation includes TEXT_DO_DRAW.
pcpath
If not NULL, defines an optional clipping path.
Only relevant if the text operation includes
TEXT_DO_DRAW.
memory
Defines the allocator to be used for allocating bookkeeping
information.
ppte
The implementation should return a pointer to its state
structure here.
text_begin must allocate a structure for its bookkeeping
needs, using the allocator defined by the memory parameter,
and return it in *ppte. text_begin may
assume that the structures passed as parameters will survive until text
processing is complete.
Clients should not call the driver text_begin procedure
directly. Instead, they should call gx_device_text_begin,
which takes the same parameters and also initializes certain common elements
of the text enumeration structure, or gs_text_begin, which
takes many of the parameters from a graphics state structure. For details,
see gstext.h.
The actual processing of text uses the procedures in the enumeration
structure, not driver procedures, since the handling of the text may depend
on the font and parameters rather than the device. Text processing may also
require the client to take action between characters, either because the
client requested it (TEXT_INTERVENE in the operation) or
because rendering a character requires suspending text processing to call an
external package such as the PostScript interpreter. (It is a deliberate
design decision to handle this by returning to the client, rather than
calling out of the text renderer, in order to avoid potentially unknown
stack requirements.) Specifically, the client must call the following
procedures, which in turn call the procedures in the text enumerator.
int gs_text_process(gs_text_enum_t *pte)
Continue processing text. This procedure may return 0 or a negative
error code as usual, or one of the following values (see
gstext.h for details).
TEXT_PROCESS_RENDER
The client must cause the current character to be rendered.
This currently only is used for PostScript Type 0-4 fonts
and their CID-keyed relatives.
TEXT_PROCESS_INTERVENE
The client has asked to intervene between characters.
This is used for cshow and kshow.
int gs_text_release(gs_text_enum_t *pte,
client_name_t cname)
Finish processing text and release
all associated structures. Clients must call this procedure after
gs_text_process returns 0 or an error, and may call it at
any time.
There are numerous other procedures that clients may call during text
processing. See gstext.h for details.
Notes
Note that unlike many other optional procedures, the default implementation
of text_begin cannot simply return: like the default
implementation of begin[_typed]_image, it must create and
return an enumerator. Furthermore, the implementation of the
process procedure (in the enumerator structure, called by
gs_text_process) cannot simply return without doing
anything, even if it doesn't want to draw anything on the output. See the
comments in gxtext.h for details.
Reading bits back
int (*get_bits_rectangle)(gx_device *dev,
const gs_int_rect *prect, gs_get_bits_params_t *params,
gs_int_rect **unread)[OPTIONAL]
Read a rectangle of bits back from the device. The params
structure consists of:
options
the allowable formats for returning the data
data[32]
pointers to the returned data
x_offset
the X offset of the first returned pixel in data
raster
the distance between scan lines in the returned data
options is a bit mask specifying what formats the client is
willing to accept. (If the client has more flexibility, the implementation
may be able to return the data more efficiently, by avoiding representation
conversions.) The options are divided into groups.
alignment
Specifies whether the returned data must be aligned in the normal
manner for bitmaps, or whether unaligned data are acceptable.
pointer or copy
Specifies whether the data may be copied into storage provided by the
client and/or returned as pointers to existing storage. (Note that if
copying is not allowed, it is much more likely that the implementation will
return an error, since this requires that the client accept the data in the
implementation's internal format.)
X offset
Specifies whether the returned data must have a specific X offset
(usually zero, but possibly other values to avoid skew at some later stage
of processing) or whether it may have any X offset (which may avoid skew in
the get_bits_rectangle operation itself).
raster
Specifies whether the raster (distance between returned scan lines)
must have its standard value, must have some other specific value, or may
have any value. The standard value for the raster is the device width
padded out to the alignment modulus when using pointers, or the minimum
raster to accommodate the X offset + width when copying (padded out to the
alignment modulus if standard alignment is required).
format
Specifies whether the data are returned in chunky (all components of a
single pixel together), component-planar (each component has its own scan
lines), or bit-planar (each bit has its own scan lines) format.
color space
Specifies whether the data are returned as native device pixels, or in
a standard color space. Currently the only supported standard space is
RGB.
standard component depth
Specifies the number of bits per component if the data are returned in
the standard color space. (Native device pixels use
dev->color_info.depth bits per pixel.)
alpha
Specifies whether alpha channel information should be returned as the
first component, the last component, or not at all. Note that for devices
that have no alpha capability, the returned alpha values will be all 1s.
The client may set more than one option in each of the above groups; the
implementation will choose one of the selected options in each group to
determine the actual form of the returned data, and will update
params[].options to indicate the form. The returned
params[].options will normally have only one option set per
group.
For further details on params, see gxgetbit.h. For further details on
options, see gxbitfmt.h.
Define w = prect->q.x - prect->p.x, h
= prect->q.y - prect->p.y. If the
bits cannot be read back (for example, from a printer), return
gs_error_unknownerror; if raster bytes is not enough space
to hold offset_x + w pixels, or if the source rectangle
goes outside the device dimensions (p.x < 0 || p.y < 0 || q.x >
dev->width || q.y > dev->height),
return gs_error_rangecheck; if any regions could not be
read, return gs_error_ioerror if unpainted is
NULL, otherwise the number of rectangles (see below);
otherwise return 0.
The caller supplies a buffer of raster × h
bytes starting at data[0] for the returned data in chunky
format, or N buffers of
raster × h bytes starting at
data[0] through
data[N-1] in planar format
where N is the number of components or bits. The contents
of the bits beyond the last valid bit in each scan line (as defined by w)
are unpredictable. data need not be aligned in any way. If
x_offset is non-zero, the bits before the first valid bit
in each scan line are undefined. If the implementation returns pointers to
the data, it stores them into data[0] or
data[0..N-1].
If not all the source data are available (for example, because the source
was a partially obscured window and backing store was not available or not
used), or if the rectangle does not fall completely within the device's
coordinate system, any unread bits are undefined, and the value returned
depends on whether unread is NULL. If unread is
NULL, return gs_error_ioerror; in this case,
some bits may or may not have been read. If unread is not
NULL, allocate (using dev->memory) and
fill in a list of rectangles that could not be read, store the pointer to
the list in *unread, and return the number of rectangles; in
this case, all bits not listed in the rectangle list have been read back
properly. The list is not sorted in any particular order, but the
rectangles do not overlap. Note that the rectangle list may cover a
superset of the region actually obscured: for example, a lazy implementation
could return a single rectangle that was the bounding box of the region.
int (*get_bits)(gx_device *dev, int y,
byte *data, byte **actual_data)[OPTIONAL]
Read scan line y of bits back from the device into the
area starting at data. This call is functionally equivalent to
with the returned value of
params->data[0] stored in
*actual_data, and will in fact be implemented this way if
the device defines a get_bits_rectangle procedure and does
not define one for get_bits. (If
actual_data is NULL,
GB_RETURN_POINTER is omitted from the options.)
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, array of float, or arrays or dictionaries of mixed types.
For example, the Name of a device is a string; the
Margins of a device is an array of two 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)(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 or procedures defined in gsparam.h.
int (*get_hardware_params)(gx_device *dev,
gs_param_list *plist)[OPTIONAL]
Read the hardware-related parameters of the device into the parameter
list at plist. These are any parameters whose values are under control of
external forces rather than the program -- for example, front panel
switches, paper jam or tray empty sensors, etc. If a parameter involves
significant delay or hardware action, the driver should only determine the
value of the parameter if it is "requested" by the
gs_param_list [param_requested(plist,
key_name)]. This function may cause the asynchronous
rendering pipeline (if enabled) to be drained, so it should be used
sparingly.
int (*put_params)(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.
Default color rendering
dictionary (CRD) parameters
Drivers that want to provide one or more default CIE color rendering
dictionaries (CRDs) can do so through get_params. To do
this, they create the CRD in the usual way (normally using the
gs_cie_render1_build and _initialize
procedures defined in gscrd.h), and then write
it as a parameter using param_write_cie_render1 defined in
gscrdp.h. However, the TransformPQR procedure
requires special handling. If the CRD uses a TransformPQR procedure
different from the default (identity), the driver must do the following:
The TransformPQR element of the CRD must include a
proc_name, and optionally proc_data. The
proc_name is an arbitrary name chosen by the driver to
designate the particular TransformPQR function. It must not be the same as
any device parameter name; we strongly suggest it include the device name,
for instance, "bitTPQRDefault".
For each such named TransformPQR procedure, the driver's
get_param procedure must provide a parameter of the same
name. The parameter value must be a string whose bytes are the actual
procedure address.
For a complete example, see the bit_get_params procedure in
gdevbit.c. Note that it is essential that
the driver return the CRD or the procedure address only if specifically
requested (param_requested(...) > 0); otherwise, errors
will occur.
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, described in a separate
document. The link between the two is the driver procedure that
supplies the font and text procedures:
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.
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)(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.
This software is provided AS-IS with no warranty, either express or
implied.
This software is distributed under license and may not be copied,
modified or distributed except as expressly authorized under the terms
of the license contained in the file LICENSE in this distribution.
For more information about licensing, please refer to
http://www.ghostscript.com/licensing/. For information on
commercial licensing, go to http://www.artifex.com/licensing/ or
contact Artifex Software, Inc., 101 Lucas Valley Road #110,
San Rafael, CA 94903, U.S.A., +1(415)492-9861.