The interface between Ghostscript and device drivers

## Table of contents

> - Adding a driver
> - Keeping things simple
> - Driver structure
>   - Structure definition
>   - For sophisticated developers only
> - Coordinates and types
>   - Coordinate system
>   - Color definition
>   - Types
> - Coding conventions
>   - Allocating storage
>   - Driver instance allocation
> - Printer drivers
> - Driver procedures
>   - Life cycle
>   - Open, close, sync
>   - Color and alpha mapping
>   - Drawing
>   - Bitmap imaging
>   - Pixmap imaging
>   - Compositing
>     - The source specification S
>     - The texture specification T
>     - The function specification f
>     - Notes
>   - Polygons
>   - High-level drawing
>   - High-level bitmap imaging
>     - Notes
>   - Reading bits back
>   - Parameters
>     - Default color rendering dictionary (CRD) parameters
>   - External fonts
>   - Page devices
>   - Miscellaneous

For other information, see the Ghostscript overview and the documentation on how to build Ghostscript.

# Adding a driver

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.

1. The list of devices, in the section headed "Catalog". Add `smurf` to the list.
2. The section headed "Device drivers".

   Suppose the files containing the smurf driver are called "`joe`" and "`fred`". Then you should add the following lines:

   >     # ------ The SMURF device ------ #

   >     smurf_=joe.$(OBJ) fred.$(OBJ)
   >     smurf.dev: $(smurf_)
   >             $(SETDEV) smurf $(smurf_)

   >     joe.$(OBJ): joe.c
   >     fred.$(OBJ): fred.c

   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:

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

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

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

   or

   >     smurf.dev: $(smurf_) page.dev
   >             $(SETPDEV) smurf $(smurf_)
   >             $(ADDMOD) smurf -lib gorf

# Keeping things simple

If you want to add a simple device (specifically, a 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 shared by all instances of each device;
- parameters that are present in all devices but may be different for each device or instance; and
- device-specific parameters that may be different for each instance.

Normally the procedure structure is defined and initialized at compile time. A prototype of the parameter structure (including both generic and device-specific parameters) is defined and initialized at compile time, but is copied and filled in when an instance of the device is created. Both of these structures should be declared as `const`, but for backward compatibility reasons are not.

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

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

The device structure instance must have the name `gs_smurf_device`, where `smurf` is the device name used in `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, by virtue of the definition

>     #define mdev ((gx_device_memory *)dev)

(This is a cheap version of "object-oriented" programming: in C++, for example, the cast would be unnecessary, and in fact the procedure table would be constructed by the compiler.)

## Structure definition

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

> `const char *dname;` |  | The device name
> `bool is_open;` |  | True if device has been opened
> `gx_device_color_info color_info;` |  | Color information
> `int width;` |  | Width in pixels
> `int height;` |  | Height in pixels

The name in the structure (`dname`) should be the same as the name in `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`.
> - Possibly, the default device implementation in `gdevdflt.c`, `gdevddrw.c`, and `gxcmap.c`.
> - The bounding box device in `gdevbbox.c` (probably just adding `NULL` procedure entries if the new procedures don't produce output).
> - These devices that must have complete (non-defaulted) procedure vectors:
>   - The null device in `gdevnfwd.c`.
>   - The command list "device" in `gxclist.c`. This is not an actual device; it only defines procedures.
>   - The "memory" devices in `gdevmem.h` and `gdevm*.c`.
>   - The halftoning device in `gdevht.c`.
> - The clip list accumulation "device" in `gxacpath.c`.
> - The clipping "devices" `xclip.c`, `gxclip2.c`, and `gxclipm.c`.
> - The pattern accumulation "device" in `gxpcmap.c`.
> - The hit detection "device" in `gdevhit.c`.
> - The generic printer device macros in `gdevprn.h`.
> - The generic printer device code in `gdevprn.c`.
> - The RasterOp source device in `gdevmrop.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

Ghostscript represents colors internally as RGB or CMYK values. In communicating with devices, however, it assumes that each device has a palette of colors identified by integers (to be precise, elements of type `gx_color_index`). Drivers may provide a uniformly spaced gray ramp or color cube for halftoning, or they may do their own color approximation, or both.

The `color_info` member of the device structure defines the color and gray-scale capabilities of the device. Its type is defined as follows:

>     typedef struct gx_device_color_info_s {
>             int num_components;		/* 1 = gray only, 3 = RGB, */
>             				/* 4 = CMYK */
>             int depth;			/* # of bits per pixel */
>             gx_color_value max_gray;	/* # of distinct gray levels -1 */
>             gx_color_value max_rgb;		/* # of distinct color levels -1 */
>             				/* (only relevant if num_comp. > 1) */
>             gx_color_value dither_gray;	/* size of gray ramp for halftoning */
>             gx_color_value dither_rgb;	/* size of color cube ditto */
>             				/* (only relevant if num_comp. > 1) */
>     } gx_device_color_info;

The following macros (in `gxdevice.h`) provide convenient shorthands for initializing this structure for ordinary black-and-white or color devices:

> `#define dci_black_and_white` ...
> `#define dci_color(depth,maxv,dither)` ...

The idea is that a device has a certain number of gray levels (`max_gray`+1) and a certain number of colors (`max_rgb`+1) that it can produce directly. When Ghostscript wants to render a given RGB or CMYK color as a device color, it first tests whether the color is a gray level (if `num_components` is 1, it converts all colors to gray levels), 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.

## Types

Here is a brief explanation of the various types that appear as parameters or results of the drivers.

`gx_color_value` (defined in `gxdevice.h`)
  This is the type used to represent RGB or CMYK color values. It is currently equivalent to unsigned short. However, Ghostscript may use less than the full range of the type to represent color values: `gx_color_value_bits` is the number of bits actually used, and `gx_max_color_value` is the maximum value, equal to (2^`gx_max_color_value_bits`)-1.

`gx_device` (defined in `gxdevice.h`)
  This is the device structure, as explained above.

`gs_matrix` (defined in `gsmatrix.h`)
  This is a 2-D homogeneous coordinate transformation matrix, used by many Ghostscript operators.

`gx_color_index` (defined in `gxdevice.h`)
  This is meant to be whatever the driver uses to represent a device color. For example, it might be an index in a color map, or it might be R, G, and B values packed into a single integer. Ghostscript doesn't ever do any computations with `gx_color_index` values: it gets them from `map_rgb_color` or `map_cmyk_color` and hands them back as arguments to several other procedures. The special value `gx_no_color_index` (defined as `(gx_color_index)(-1)` ) means "transparent" for some of the procedures. The type definition is simply:

  > `typedef unsigned long gx_color_index;`

`gs_param_list` (defined in `gsparam.h`)
  This is a parameter list, which is used to read and set attributes in a device. See the comments in `gsparam.h`, and the description of the `get_params` and `put_params` procedures below, for more detail.

`gx_tile_bitmap` (defined in `gxbitmap.h`)
`gx_strip_bitmap` (defined in `gxbitmap.h`)
  These structure types represent bitmaps to be used as a tile for filling a region (rectangle). `gx_tile_bitmap` is an older type lacking `shift` and `rep_shift`; `gx_strip_bitmap` has superseded it, and it should not be used in new code. Here is a copy of the relevant part of the file:

  >     /*
  >      * Structure for describing stored bitmaps.
  >      * Bitmaps are stored bit-big-endian (i.e., the 2^7 bit of the first
  >      * byte corresponds to x=0), as a sequence of bytes (i.e., you can't
  >      * do word-oriented operations on them if you're on a little-endian
  >      * platform like the Intel 80x86 or VAX).  Each scan line must start on
  >      * a (32-bit) word boundary, and hence is padded to a word boundary,
  >      * although this should rarely be of concern, since the raster and width
  >      * are specified individually.  The first scan line corresponds to y=0
  >      * in whatever coordinate system is relevant.
  >      *
  >      * For bitmaps used as halftone tiles, we may replicate the tile in
  >      * X and/or Y, but it is still valuable to know the true tile dimensions
  >      * (i.e., the dimensions prior to replication).  Requirements:
  >      *      width % rep_width = 0
  >      *      height % rep_height = 0
  >      *
  >      * For halftones at arbitrary angles, we provide for storing the halftone
  >      * data as a strip that must be shifted in X for different values of Y.
  >      * For an ordinary (non-shifted) halftone that has a repetition width of
  >      * W and a repetition height of H, the pixel at coordinate (X,Y)
  >      * corresponds to halftone pixel (X mod W, Y mod H), ignoring phase;
  >      * for a shifted halftone with shift S, the pixel at (X,Y) corresponds
  >      * to halftone pixel ((X + S * floor(Y/H)) mod W, Y mod H).  Requirements:
  >      *      strip_shift < rep_width
  >      *      strip_height % rep_height = 0
  >      *      shift = (strip_shift * (size.y / strip_height)) % rep_width
  >      */
  >     typedef struct gx_strip_bitmap_s {
  >             byte *data;
  >             int raster;                     /* bytes per scan line */
  >             gs_int_point size;              /* width, height */
  >             gx_bitmap_id id;
  >             ushort rep_width, rep_height;   /* true size of tile */
  >             ushort strip_height;
  >             ushort strip_shift;
  >             ushort shift;
  >     } gx_strip_bitmap;

# Coding conventions

All the driver procedures defined below that return `int` results return 0 on success, or an appropriate negative error code in the case of error conditions. The error codes are defined in `gserrors.h`; they correspond directly to the errors defined in the PostScript language reference manuals. The most common ones for drivers are:

> `gs_error_invalidfileaccess`
>   An attempt to open a file failed.
> `gs_error_ioerror`
>   An error occurred in reading or writing a file.
> `gs_error_limitcheck`
>   An otherwise valid parameter value was too large for the implementation.
> `gs_error_rangecheck`
>   A parameter was outside the valid range.
> `gs_error_VMerror`
>   An attempt to allocate memory failed. (If this happens, the procedure should release all memory it allocated before it returns.)

If a driver does return an error, 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).

>     char *gs_malloc(uint num_elements, uint element_size,
>       const char *client_name);

Like `calloc`, but unlike `malloc`, `gs_malloc` takes an element count and an element size. For structures, `num_elements` is 1 and `element_size` is `sizeof` the structure; for byte arrays, `num_elements` is the number of bytes and `element_size` is 1. Unlike `calloc`, `gs_malloc` does not clear the block of storage.

The `client_name` is used for tracing and debugging. It must be a real string, not `NULL`. Normally it is the name of the procedure in which the call occurs.

>     void gs_free(char *data, uint num_elements, uint element_size,
>       const char *client_name);

Unlike `free`, `gs_free` demands that `num_elements` and element_size be supplied. It also requires a client name, like `gs_malloc`.

## Driver instance allocation

All driver instances allocated by Ghostscript's standard allocator must point to a "structure descriptor" that tells the garbage collector how to trace pointers in the structure. For drivers 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. Of the driver procedures defined in the next section, they only need implement two: `map_rgb_color` (or `map_cmyk_color`) and `map_color_rgb`. In addition, they must implement a `print_page` or `print_page_copies` procedure. There are macros in `gdevprn.h` that generate the device structure for such devices, of which the simplest is `prn_device`; for an example, see `gdevbj10.c`. If you are writing a printer driver, we suggest you start by reading `gdevprn.h` and the subsection on "Color mapping" below; you may be able to ignore all the rest of the driver procedures.

The `print_page` procedures are defined as follows:

>     int (*print_page)(P2(gx_device_printer *, FILE *))
>     int (*print_page_copies)(P3(gx_device_printer *, FILE *, int))

This procedure must read out the rendered image from the device and write whatever is appropriate to the file. To read back one or more scan lines of the image, the `print_page` procedure must call one of the following procedures:

>     int gdev_prn_copy_scan_lines(P4(gx_device_printer *pdev, int y, byte *str,
>         uint size)

For this procedure, `str` is where the data should be copied to, and `size` is the size of the buffer starting at `str`. This procedure returns the number of scan lines copied, or <0 for an error. `str` need not be aligned.

>     int gdev_prn_get_bits(gx_device_printer *pdev, int y, byte *str,
>       byte **actual_data)

This procedure reads out exactly one scan line. If the scan line is available in the correct format already, `*actual_data` is set to point to it; otherwise, the scan line is copied to the buffer starting at `str`, and `*actual_data` is set to `str`. This saves a copying step most of the time. `str` need not be aligned; however, if `*actual_data` is set to point to an existing scan line, it will be aligned. (See the description of the `get_bits` procedure below for more details.)

In either case, each row of the image is stored in the form described in the comment under `gx_tile_bitmap` above; each pixel takes the number of bits specified as `color_info.depth` in the device structure, and holds values returned by the device's `map_`{`rgb`,`cmyk`}`_color` procedure.

The `print_page` procedure can determine the number of bytes required to hold a scan line by calling:

>     uint gdev_prn_raster(P1(gx_device_printer *))

For a very simple concrete example, we suggest reading the code in `bit_print_page` in `gdevbit.c`.

If the device provides `print_page`, Ghostscript will call `print_page` the requisite number of times to print the desired number of copies; if the device provides `print_page_copies`, Ghostscript will call `print_page_copies` once per page, passing it the desired number of copies.

# Driver procedures

Most of the procedures that a driver may implement are optional. If a device doesn't supply an optional procedure `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:

> `open_device
> get_initial_matrix
> get_params
> put_params
> get_hardware_params`

When `setdevice` installs a device instance in the graphics state, it checks whether the instance is closed or open. If the instance is closed, `setdevice` calls the open routine, and then sets the state to open. There is currently no user-accessible operation to close a device instance. Device instances are only closed when they are about to be freed, which occurs in three situations:

- when a 'restore' occurs, if the instance was created since the corresponding 'save';
- by the garbage collector, if the instance is no longer accessible; or
- when Ghostscript exits (terminates).

## Open, close, sync

`int (*open_device)(P1(gx_device *))` [OPTIONAL]
  Open the device: do any initialization associated with making the device instance valid. This must be done before any output to the device. The default implementation does nothing.

`void (*get_initial_matrix)(P2(gx_device *, gs_matrix *))` [OPTIONAL]
  Construct the initial transformation matrix mapping user coordinates (nominally 1/72 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)(P1(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)(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 and alpha mapping

A given driver normally implements either `map_rgb_color` or `map_cmyk_color`, but not both. Black-and-white drivers need implement neither. Note that the `map_xxx_color` procedures must not return `gx_no_color_index` (all 1s).

`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 (that is, `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 (that is, `color_info.num_components` > 1), `map_cmyk_color` returns a color index for a gray level (as opposed to a non-gray color) iff cyan = magenta = yellow.

`int (*map_color_rgb)(P3(gx_device *, gx_color_index color, gx_color_value rgb[3]))` [OPTIONAL]
  Map a device color code to RGB values. The default algorithm returns (0 if `color`==0 else `gx_max_color_value`) for all three components.

`gx_color_index (*map_rgb_alpha_color)(P5(gx_device *, gx_color_value red, gx_color_value green, gx_color_value blue, gx_color_value alpha))` [OPTIONAL]
  Map a RGB color and an opacity value to a device color. The range of legal values of the RGB and alpha arguments is 0 to `gx_max_color_value`; `alpha` = 0 means transparent, `alpha` = `gx_max_color_value` means fully opaque. The default is to use the `map_rgb_color` procedure and ignore alpha.

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

`int (*map_color_rgb_alpha)(P3(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 `map_color_rgb`, 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 `map_color_rgb`.

  Note that CMYK devices currently do not support variable opacity; alpha is ignored on such devices.

`typedef enum { go_text, go_graphics } graphic_object_type; int (*get_alpha_bits)(P4(gx_device *dev, graphic_object_type type))` [OPTIONAL]
  Return the number of alpha (opacity) bits that should be used in rendering an object of the given type. The default value is 1; the only values allowed are 1, 2, and 4.

## Drawing

All drawing operations use device coordinates and device color values.

`int (*fill_rectangle)(P6(gx_device *, int x, int y, int width, int height, gx_color_index color))`
  Fill a rectangle with a color. The set of pixels filled is {(px,py) | x <= px < x + width and y <= py < y + height}. In other words, the point (x,y) is included in the rectangle, as are (x+w-1,y), (x,y+h-1), and (x+w-1,y+h-1), but not (x+w,y), (x,y+h), or (x+w,y+h). If `width` <= 0 or height <= 0, `fill_rectangle` should return 0 without drawing anything.

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

`int (*draw_line)(P6(gx_device *, int x0, int y0, int x1, int y1, gx_color_index color))` [OPTIONAL]
  Draw a minimum-thickness line from (x0,y0) to (x1,y1). The precise set of points to be filled is defined as follows. First, if `y1` < `y0`, exchange (x0,y0) and (x1,y1). Then the line includes the point (x0,y0) but not the point (x1,y1). If `x0` = `x1` and `y0` = `y1`, `draw_line` should return 0 without drawing anything.

## Bitmap imaging

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

`int (*copy_mono)(P11(gx_device *, const unsigned char *data, int data_x, int raster, gx_bitmap_id id, int x, int y, int width, int height, gx_color_index color0, gx_color_index color1))` [OPTIONAL]
  Copy a monochrome image (similar to the PostScript image operator). Each scan line is raster bytes wide. Copying begins at (`data_x`,0) and transfers a rectangle of the given width 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)(P10(gx_device *, const gx_tile_bitmap *tile, int x, int y, int width, int height, gx_color_index color0, gx_color_index color1, int phase_x, int phase_y))` [OPTIONAL] [OBSOLETE]
  This procedure is still supported, but has been superseded by `strip_tile_rectangle`. New drivers should implement `strip_tile_rectangle`; if they cannot cope with non-zero shift values, they should test for this explicitly and call the default implementation (`gx_default_strip_tile_rectangle`) if shift != 0. Clients should call `strip_tile_rectangle`, not `tile_rectangle`.

`int (*strip_tile_rectangle)(P10(gx_device *, const gx_strip_bitmap *tile, int x, int y, int width, int height, gx_color_index color0, gx_color_index color1, int phase_x, int phase_y))` [OPTIONAL]
  Tile a rectangle. Tiling consists of doing multiple `copy_mono` operations to fill the rectangle with copies of the tile. The tiles are aligned with the device coordinate system, to avoid "seams". Specifically, the (`phase_x`, `phase_y`) point of the tile is aligned with the origin of the device coordinate system. (Note that this is backwards from the PostScript definition of halftone phase.) `phase_x` and `phase_y` are guaranteed to be in the range [0..`tile->width`) and [0..`tile->height`) respectively.

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

  This operation is the workhorse for halftone filling in Ghostscript, so implementing it efficiently for solid tiles (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, or 32. The pixel values are device color codes (that is, whatever it is that `map_rgb_color` returns).

`int (*copy_color)(P9(gx_device *, const unsigned char *data, int data_x, int raster, gx_bitmap_id id, int x, int y, int width, int height))` [OPTIONAL]
  Copy a color image with multiple bits per pixel. The raster is in bytes, but `x` and `width` are in pixels, not bits. If `id` is different from `gx_no_bitmap_id`, it identifies the bitmap contents unambiguously; a call with the same `id` will always have the same `data`, `raster`, and data contents.

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

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

`int (*create_compositor)(P5(dev_t *dev, gx_device_t **pcdev, const gs_composite_t *pcte, const gs_imager_state *pis, gs_memory_t *memory))` [OPTIONAL]
  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)(P15(gx_device *dev, const byte *sdata, int sourcex, uint sraster, gx_bitmap_id id, const gx_color_index *scolors, const gx_tile_bitmap *texture, const gx_color_index *tcolors, int x, int y, int width, int height, int phase_x, int phase_y, int command))` [OPTIONAL]
  This procedure is still supported, but has been superseded by `strip_copy_rop`. New drivers should implement `strip_copy_rop`; if they cannot cope with non-zero shift values in the texture, they should test for this explicitly and call the default implementation (`gx_default_strip_copy_rop`) if shift != 0. Clients should call `strip_copy_rop`, not `copy_rop`.

`int (*strip_copy_rop)(P15(gx_device *dev, const byte *sdata, int sourcex, uint sraster, gx_bitmap_id id, const gx_color_index *scolors, const gx_strip_bitmap *texture, const gx_color_index *tcolors, int x, int y, int width, int height, int phase_x, int phase_y, int command))` [OPTIONAL]
  Combine an optional source image S (as for `copy_mono` or `copy_color`) and an optional texture T (a tile, as for `tile_rectangle`) with the existing bitmap or pixmap D held by the driver, pixel by pixel, using any 3-input Boolean operation as modified by "transparency" flags: schematically, set D = f(D,S,T), computing f in RGB space rather than using actual device pixel values. S and T may each (independently) be a solid color, a bitmap with "foreground" and "background" colors, or a pixmap. This is a complex (and currently rather slow) operation. The arguments are as follows:

  > `dev` |  | the device, as for all driver procedures
  > `sdata`, `sourcex`, `sraster`, `id`, `scolors` |  | specify S, see below
  > `texture`, `tcolors` |  | specify T, see below
  > `x`, `y`, `width`, `height` |  | as for the other copy and fill procedures
  > `phase_x`, `phase_y` |  | part of T specification, see below
  > `command` |  | see below

### The source specification S

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 H-P 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 H-P 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)

## Polygons

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

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

`int (*fill_trapezoid)(P10(gx_device *dev, fixed fx0, fixed fw0, fixed fy0, fixed fx1, fixed fw1, fixed fh, bool swap_axes, const gx_drawing_color *pdcolor, gs_logical_operation_t lop))` [OPTIONAL]
  Fill a trapezoid whose parallel sides are parallel to a coordinate axis. The corners are (fx0,fy0), (fx0+fw0,fy0), (fx1,fy0+fh), and (fx1+fw1,fy0+fy). We require `fw0` >= 0, `fw1` >= 0, and `fh` >= 0; if `fw0` = 0 or `fw1` = 0, the trapezoid is actually a triangle. If `swap_axes` is set, the meanings of X and Y are interchanged.

`int (*fill_parallelogram)(P9(gx_device *dev, fixed px, fixed py, fixed ax, fixed ay, fixed bx, fixed by, const gx_drawing_color *pdcolor, gs_logical_operation_t lop))` [OPTIONAL]
  Fill a parallelogram whose corners are (px,py), (px+ax,py+ay), (px+bx,py+by), and (px+ax+bx,py+ay+by). There are no constraints on the values of any of the parameters, so the parallelogram may have any orientation relative to the coordinate axes.

`int (*fill_triangle)(P9(gx_device *dev, fixed px, fixed py, fixed ax, fixed ay, fixed bx, fixed by, const gx_drawing_color *pdcolor, gs_logical_operation_t lop))` [OPTIONAL]
  Fill a triangle whose corners are (px,py), (px+ax,py+ay), and (px+bx,py+by).

`int (*draw_thin_line)(P7(gx_device *dev, fixed fx0, fixed fy0, fixed fx1, fixed fy1, const gx_drawing_color *pdcolor, gs_logical_operation_t lop))` [OPTIONAL]
  Draw a one-pixel-wide line from (fx0,fy0) to (fx1,fy1). This replaces the older `draw_line` procedure, which required integer coordinates, a pure color, and no logical operation.

## High-level drawing

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

> Header |  | Defines
> `gxpaint.h` |  | `gx_fill_params`, `gx_stroke_params`
> `gxfixed.h` |  | `fixed`, `gs_fixed_point` (used by `gx_*_params`)
> `gxistate.h` |  | `gs_imager_state` (used by `gx_*_params`)
> `gxline.h` |  | `gx_line_params` (used by `gs_imager_state`)
> `gslparam.h` |  | line cap/join values (used by `gx_line_params`)
> `gxmatrix.h` |  | `gs_matrix_fixed` (used by `gs_imager_state`)
> `g[s,x,z]path.h` |  | `gx_path`
> `g[x,z]cpath.h` |  | `gx_clip_path`

`int (*fill_path)(P6(gx_device *dev, const gx_imager_state *pis, gx_path *ppath, const gx_fill_params *params, const gx_drawing_color *pdcolor, const gx_clip_path *pcpath))` [OPTIONAL]
  Fill the given path, clipped by the given clip path, according to the given parameters, with the given color. The clip path pointer may be `NULL`, meaning do not clip.

`int (*stroke_path)(P6(gx_device *dev, const gx_imager_state *pis, gx_path *ppath, const gx_stroke_params *params, const gx_drawing_color *pdcolor, const gx_clip_path *pcpath))` [OPTIONAL]
  Stroke the given path, clipped by the given clip path, according to the given parameters, with the given color. The clip path pointer may be `NULL`, meaning not to clip.

`int (*fill_mask)(P13(gx_device *dev, const byte *data, int data_x, int raster, gx_bitmap_id id, int x, int y, int width, int height, const gx_drawing_color *pdcolor, int depth, int command, const gx_clip_path *pcpath))` [OPTIONAL]
  Color the 1-bits in the given mask (or according to the alpha values, if `depth` > 1), clipped by the given clip path, with the given color and logical operation. The clip path pointer may be `NULL`, meaning do not clip. The parameters `data`, ..., `height` are as for `copy_mono`; depth is as for `copy_alpha`; command is as for `copy_rop`.

## High-level bitmap imaging

Similar to the high-level interface for fill 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:

>     typedef struct {
>             int ImageType;
>             gs_matrix ImageMatrix;
>     } gs_image_common_t;

Bitmap images that supply data (all image types except `image_type_from_device` (2)) are defined by:

>     #define gs_image_max_components 5
>     typedef struct {
>             << gs_image_common_t >>
>             int Width;
>             int Height;
>             int BitsPerComponent;
>             float Decode[gs_image_max_components * 2];
>             bool Interpolate;
>     } gs_data_image_t;

Images that supply pixel (as opposed to mask) data are defined by:

>     typedef struct {
>             << gs_data_image_t >>
>             const gs_color_space *ColorSpace;
>             bool CombineWithColor;
>     } gs_pixel_image_t;

Ordinary PostScript Level 1 or Level 2 (`ImageType` 1) images are defined by:

>     typedef struct {
>             << gs_pixel_image_t >>
>             bool ImageMask;
>             bool adjust;
>             bool HasAlpha;		/* DPNEXT configurations only */
>     } gs_image1_t;
>     typedef gs_image1_t gs_image_t;

>     typedef enum {
>             gs_image_format_chunky = 0,
>             gs_image_format_component_planar = 1
>     } gs_image_format_t;

For more details, consult the source code in `gsiparam.h`, defines parameters for an image. The `begin[_typed_]image` driver procedures return 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;
>             gx_image_enum_procs_t procs;
>             int num_planes;
>             int plane_depths[gs_image_max_components];  /* [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(P4(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.  We keep this procedure last,
>              * so that we can detect obsolete static initializers.
>              */
>     #define image_enum_proc_end_image(proc)\
>       int proc(P3(gx_device *dev,\
>         gx_image_enum_common_t *info, bool draw_last))

>             image_enum_proc_end_image((*end_image));

>     } 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:

>     typedef struct gx_image_plane_s {
>       const byte *data;
>       int data_x;
>       uint raster;
>     } gx_image_plane_t;

`int (*begin_image)(P9(gx_device *dev, const gs_imager_state *pis, const gs_image_t *pim, gs_image_format_t format, gs_int_rect *prect, const gx_drawing_color *pdcolor, const gx_clip_path *pcpath, gs_memory_t *memory, gx_image_enum_common_t **pinfo))` [OPTIONAL]
  Begin the transmission of an image. Zero or more calls of `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 `plane_data`. See the description of `plane_data` below
  > `prect` |  | if not `NULL`, defines a subrectangle of the image; only the data for this subrectangle will be passed to `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.

`int (*begin_typed_image)(P9(gx_device *dev, const gs_imager_state *pis, const gs_matrix *pmat, const gs_image_common_t *pim, gs_int_rect *prect, const gx_drawing_color *pdcolor, const gx_clip_path *pcpath, gs_memory_t *memory, gx_image_enum_common_t **pinfo))` [OPTIONAL]
  This has exactly the same function as `begin_image`, except
  - 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)(P4(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 `plane_data` passes complete pixels: for example, for a chunky image with 24 bits per pixel, each call of `plane_data` passes 3N bytes of data (specifically, 3 × Width × height).

  The interpretation of planes depends on the format argument of `begin_image`:

  - If format is `gs_image_format_chunky`, `planes[0].data` points to data in "chunky" format, in which the components follow each other (for instance, RGBRGBRGB....)
  - If 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. Someday we may add `gs_image_format_bit_planar`, specifying that each plane would contain only a single bit of the pixel value, but this is not currently implemented.

  If, as a result of this call, `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.

  `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(P3(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.]

## Reading bits back

`int (*get_bits_rectangle)(P4(gx_device *dev, const gs_int_rect *prect, gs_get_bits_params_t *params, gs_int_rect **unread))` [OPTIONAL]

> UNDER DEVELOPMENT. USE AT YOUR OWN RISK.

  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.

  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 d`ev`->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)(P4(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

  >     (*get_bits_rectangle)
  >       (dev, {0, y, dev->width, y+1},
  >        {(align_any | (return_copy|return_pointer) | offset_0 |
  >          raster_standard | format_chunky | colors_native | alpha_none),
  >         {data}})

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

`int (*get_hardware_params)(P2(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)(P2(gx_device *dev, gs_param_list *plist))` [OPTIONAL]
  Set the parameters of the device from the parameter list at `plist`, using the `param_read_`* macros/procedures defined in `gsparam.h`. All `put_params` procedures must use a "two-phase commit" algorithm; see `gsparam.h` for details.

### 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:

`xfont_procs *(*get_xfont_procs)(P1(gx_device *dev))` [OPTIONAL]
  Return a structure of procedures for handling external fonts and text display. A `NULL` value means that this driver doesn't provide this capability.

For technical reasons, a second procedure is also needed:

`gx_device *(*get_xfont_device)(P1(gx_device *dev))` [OPTIONAL]
  Return the device that implements `get_xfont_procs` in a non-default way for this device, if any. Except for certain special internal devices, this is always the device argument.

## Page devices

`gx_device *(*get_page_device)(P1(gx_device *dev))` [OPTIONAL]
  According to the Adobe specifications, some devices are "page devices" and some are not. This procedure returns `NULL` if the device is not a page device, or the device itself if it is a page device. In the case of forwarding devices, `get_page_device` returns the underlying page device (or `NULL` if the underlying device is not a page device).

## Miscellaneous

`int (*get_band)(P3(gx_device *dev, int y, int *band_start))` [OPTIONAL]
  If the device is a band device, this procedure stores in `*band_start` the scan line (device Y coordinate) of the band that includes the given Y coordinate, and returns the number of scan lines in the band. If the device is not a band device, this procedure returns 0. The latter is the default implementation.

`void (*get_clipping_box)(P2(gx_device *dev, gs_fixed_rect *pbox))` [OPTIONAL]
  Stores in `*pbox` a rectangle that defines the device's clipping region. For all but a few specialized devices, this is ((0,0),(width,height)).

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

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

Ghostscript version 5.50, 16 September 1998