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Copyright (C) 1989, 1996 Aladdin Enterprises. All rights reserved. This file is part of Aladdin Ghostscript. Aladdin Ghostscript is distributed with NO WARRANTY OF ANY KIND. No author or distributor accepts any responsibility for the consequences of using it, or for whether it serves any particular purpose or works at all, unless he or she says so in writing. Refer to the Aladdin Ghostscript Free Public License (the "License") for full details. Every copy of Aladdin Ghostscript must include a copy of the License, normally in a plain ASCII text file named PUBLIC. The License grants you the right to copy, modify and redistribute Aladdin Ghostscript, but only under certain conditions described in the License. Among other things, the License requires that the copyright notice and this notice be preserved on all copies. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - This file, drivers.txt, describes the interface between Ghostscript and device drivers. For an overview of Ghostscript and a list of the documentation files, see README. ******** ******** Adding a driver ******** ******** To add a driver to Ghostscript, all you need to do is edit devs.mak in two places. The first is the list of devices, in the section headed # -------------------------------- Catalog ------------------------------- # Pick a name for your device, say smurf, and add smurf to the list. (Device names must be 1 to 8 characters, consisting of only letters, digits, and underscores, of which the first character must be a letter. Case is significant: all current device names are lower case.) The second is the section headed # ---------------------------- Device drivers ---------------------------- # Suppose the files containing the smurf driver are called joe and fred. Then you should add the following lines: # ------ The SMURF device ------ # smurf_=joe.$(OBJ) fred.$(OBJ) smurf.dev: $(smurf_) $(SETDEV) smurf $(smurf_) joe.$(OBJ): joe.c ...and whatever it depends on fred.$(OBJ): fred.c ...and whatever it depends on If the smurf driver also needs special libraries, e.g., a library named gorf, then the entry should look like this: smurf.dev: $(smurf_) $(SETDEV) smurf $(smurf_) $(ADDMOD) smurf -lib gorf If, as will usually be the case, your driver is a printer driver (as discussed below), the device entry should look like this: smurf.dev: $(smurf_) page.dev $(SETPDEV) smurf $(smurf_) or smurf.dev: $(smurf_) page.dev $(SETPDEV) smurf $(smurf_) $(ADDMOD) smurf -lib gorf ******** ******** Keeping things simple ******** If you want to add a simple device (specifically, a black-and-white printer), you probably don't need to read the rest of this document; just use the code in an existing driver as a guide. The Epson and BubbleJet drivers (gdevepsn.c and gdevbj10.c) are good models for dot-matrix printers, which require presenting the data for many scan lines at once; the DeskJet/LaserJet drivers (gdevdjet.c) are good models for laser printers, which take a single scan line at a time but support data compression. (For color printers, there are unfortunately no good models: the two major color inkjet printer drivers, gdevcdj.c and gdevstc.c, are far too complex to read.) On the other hand, if you're writing a driver for some more esoteric device, you probably do need at least some of the information in the rest of this document. It might be a good idea for you to read it in conjunction with one of the existing drivers. Duplication of code, and sheer code volume, is a serious maintenance and distribution problem for Ghostscript. If your device is similar to an existing one, try to implement your driver by adding some parameterization to an existing driver rather than copying code. gdevepsn.c and gdevdjet.c are good examples of this approach. ******** ******** Driver structure ******** ******** A device is represented by a structure divided into three parts: - procedures that are shared by all instances of each device; - parameters that are present in all devices but may be different for each device or instance; and - device-specific parameters that may be different for each instance. Normally, the procedure structure is defined and initialized at compile time. A prototype of the parameter structure (including both generic and device-specific parameters) is defined and initialized at compile time, but is copied and filled in when an instance of the device is created. Both of these structures should be declared as const, but for backward compatibility reasons are not. The gx_device_common macro defines the common structure elements, with the intent that devices define and export a structure along the following lines. Do not fill in the individual generic parameter values in the usual way for C structures: use the macros defined for this purpose in gxdevice.h or, if applicable, gdevprn.h. typedef struct smurf_device_s { gx_device_common; ... device-specific parameters ... } smurf_device; smurf_device far_data gs_smurf_device = { ... macro for generic parameter values ..., { ... procedures ... }, /* std_procs */ ... device-specific parameter values if any ... }; The device structure instance *must* have the name gs_smurf_device, where smurf is the device name used in devs.mak. All the device procedures are called with the device as the first argument. Since each device type is actually a different structure type, the device procedures must be declared as taking a gx_device * as their first argument, and must cast it to smurf_device * internally. For example, in the code for the "memory" device, the first argument to all routines is called dev, but the routines actually use mdev to reference elements of the full structure, by virtue of the definition #define mdev ((gx_device_memory *)dev) (This is a cheap version of "object-oriented" programming: in C++, for example, the cast would be unnecessary, and in fact the procedure table would be constructed by the compiler.) Structure definition -------------------- You should consult the definition of struct gx_device_s in gxdevice.h for the complete details of the generic device structure. Some of the most important members of this structure for ordinary drivers are: const char *dname; /* the device name */ bool is_open; /* true if device has been opened */ gx_device_color_info color_info; /* color information */ int width; /* width in pixels */ int height; /* height in pixels */ The name in the structure (dname) should be the same as the name in devs.mak. gx_device_common is a macro consisting of just the element definitions. For sophisticated developers only --------------------------------- If for any reason you need to change the definition of the basic device structure, or add procedures, you must change the following places: - This document and NEWS (if you want to keep the documentation up to date). - The definition of gx_device_common and/or the procedures in gxdevice.h. - Possibly, the default forwarding procedures declared in gxdevice.h and implemented in gdevnfwd.c. - The device procedure record completion routines in gdevdflt.c. - Possibly, the default device implementation in gdevdflt.c and gdevddrw.c. - If you are adding procedures that produce output, the bounding box device in gdevbbox.c. - The following devices that must have complete (non-defaulted) procedure vectors: - The null device in gdevnfwd.c. - The command list "device" in gxclist.c. This is not an actual device; it only defines procedures. - The "memory" devices in gdevmem.h and gdevm*.c. - The halftoning device in gdevht.c. - The clip list accumulation "device" in gxacpath.c. - The clipping "devices" in gxcpath.c and gxclip2.c. - The Pattern accumulation "device" in gxpcmap.c. - The hit detection "device" in zupath.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 and/or gx_default_put_params (in gsdparam.c). You should not have to change any of the real devices in the standard Ghostscript distribution (listed in devs.mak) or any of your own devices, because all of them are supposed to use the macros in gxdevice.h or gdevprn.h to define and initialize their state. ******** ******** Types and coordinates ******** ******** Coordinate system ----------------- Since each driver specifies the initial transformation from user to device coordinates, the driver can use any coordinate system it wants, as long as a device coordinate will fit in an int. (This is only an issue on MS-DOS systems, where ints are only 16 bits. User coordinates are represented as floats.) Most current drivers use a coordinate system with (0,0) in the upper left corner, with X increasing to the right and Y increasing toward the bottom. However, there is supposed to be nothing in the rest of Ghostscript that assumes this, and indeed some drivers use a coordinate system with (0,0) in the lower left corner. Drivers must check (and, if necessary, clip) the coordinate parameters given to them: they should not assume the coordinates will be in bounds. The fit_fill and fit_copy macros in gxdevice.h are very helpful in doing this. Color definition ---------------- Ghostscript represents colors internally as RGB or CMYK values. In communicating with devices, however, it assumes that each device has a palette of colors identified by integers (to be precise, elements of type gx_color_index). Drivers may provide a uniformly spaced gray ramp or color cube for halftoning, or they may do their own color approximation, or both. The color_info member of the device structure defines the color and gray-scale capabilities of the device. Its type is defined as follows: typedef struct gx_device_color_info_s { int num_components; /* 1 = gray only, 3 = RGB, */ /* 4 = CMYK */ int depth; /* # of bits per pixel */ gx_color_value max_gray; /* # of distinct gray levels -1 */ gx_color_value max_rgb; /* # of distinct color levels -1 */ /* (only relevant if num_comp. > 1) */ gx_color_value dither_gray; /* size of gray ramp for halftoning */ gx_color_value dither_rgb; /* size of color cube ditto */ /* (only relevant if num_comp. > 1) */ } gx_device_color_info; The following macros (in gxdevice.h) provide convenient shorthands for initializing this structure for ordinary black-and-white or color devices: #define dci_black_and_white ... #define dci_color(depth,maxv,dither) ... The idea is that a device has a certain number of gray levels (max_gray +1) and a certain number of colors (max_rgb +1) that it can produce directly. When Ghostscript wants to render a given RGB or CMYK color as a device color, it first tests whether the color is a gray level. (If num_components is 1, it converts all colors to gray levels.) If so: - If max_gray is large (>= 31), Ghostscript asks the device to approximate the gray level directly. If the device returns a valid gx_color_index, Ghostscript uses it. Otherwise, Ghostscript assumes that the device can represent dither_gray distinct gray levels, equally spaced along the diagonal of the color cube, and uses the two nearest ones to the desired color for halftoning. If the color is not a gray level: - If max_rgb is large (>= 31), Ghostscript asks the device to approximate the color directly. If the device returns a valid gx_color_index, Ghostscript uses it. Otherwise, Ghostscript assumes that the device can represent dither_rgb * dither_rgb * dither_rgb distinct colors, equally spaced throughout the color cube, and uses two of the nearest ones to the desired color for halftoning. Types ----- Here is a brief explanation of the various types that appear as parameters or results of the drivers. gx_color_value (defined in gxdevice.h) This is the type used to represent RGB or CMYK color values. It is currently equivalent to unsigned short. However, Ghostscript may use less than the full range of the type to represent color values: gx_color_value_bits is the number of bits actually used, and gx_max_color_value is the maximum value (equal to 2^gx_max_color_value_bits - 1). gx_device (defined in gxdevice.h) This is the device structure, as explained above. gs_matrix (defined in gsmatrix.h) This is a 2-D 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, it should use the return_error macro rather than a simple return statement, e.g., return_error(gs_error_VMerror); This macro is defined in gx.h, which is automatically included by gdevprn.h but not by gserrors.h. Allocating storage ------------------ While most drivers (especially printer drivers) follow a very similar template, there is one important coding convention that is not obvious from reading the code for existing drivers: Driver procedures must not use malloc to allocate any storage that stays around after the procedure returns. Instead, they must use gs_malloc and gs_free, which have slightly different calling conventions. (The prototypes for these are in gsmemory.h, which is included in gx.h, which is included in gdevprn.h.) This is necessary so that Ghostscript can clean up all allocated memory before exiting, which is essential in environments that provide only single-address-space multi-tasking (some versions of Microsoft Windows). char *gs_malloc(uint num_elements, uint element_size, const char *client_name); Like calloc, but unlike malloc, gs_malloc takes an element count and an element size. For structures, num_elements is 1 and element_size is sizeof the structure; for byte arrays, num_elements is the number of bytes and element_size is 1. Unlike calloc, gs_malloc does NOT clear the block of storage. The client_name is used for tracing and debugging. It must be a real string, not NULL. Normally it is the name of the procedure in which the call occurs. void gs_free(char *data, uint num_elements, uint element_size, const char *client_name); Unlike free, gs_free demands that num_elements and element_size be supplied. It also requires a client name, like gs_malloc. Driver instance allocation -------------------------- All driver instances allocated by Ghostscript's standard allocator must point to a "structure descriptor" that tells the garbage collector how to trace pointers in the structure. For drivers that are registered in the normal way (using the makefile approach described above), no special care is needed as long as instances are only created by calling the gs_copydevice procedure defined in gsdevice.h. If you have a need to define devices that are not registered in this way, you must fill in the stype member in any dynamically allocated instances with a pointer to the same structure descriptor used to allocate the instance. For more information about structure descriptors, see gsmemory.h and gsstruct.h. ******** ******** Printer drivers ******** ******** Printer drivers (which include drivers that write some kind of raster file) are especially simple to implement. Of the driver procedures defined in the next section, they only need implement two: map_rgb_color (or map_cmyk_color) and map_color_rgb. In addition, they must implement a print_page or print_page_copies procedure. There are a set of macros in gdevprn.h that generate the device structure for such devices, of which the simplest is prn_device; for an example, see gdevbj10.c. If you are writing a printer driver, we suggest you start by reading gdevprn.h and the subsection on "Color mapping" below; you may be able to ignore all the rest of the driver procedures. The print_page procedures are defined as follows: int (*print_page)(P2(gx_device_printer *, FILE *)) int (*print_page_copies)(P3(gx_device_printer *, FILE *, int)) This procedure must read out the rendered image from the device and write whatever is appropriate to the file. To read back one or more scan lines of the image, the print_page procedure must call one of the following procedures: int gdev_prn_copy_scan_lines(P4(gx_device_printer *pdev, int y, byte *str, uint size) For this procedure, str is where the data should be copied to, and size is the size of the buffer starting at str. This procedure returns the number of scan lines copied, or <0 for an error. str need not be aligned. int gdev_prn_get_bits(gx_device_printer *pdev, int y, byte *str, byte **actual_data) This procedure reads out exactly one scan line. If the scan line is available in the correct format already, *actual_data is set to point to it; otherwise, the scan line is copied to the buffer starting at str, and *actual_data is set to str. This saves a copying step most of the time. str need not be aligned; however, if *actual_data is set to point to an existing scan line, it will be aligned. (See the description of the get_bits procedure below for more details.) In either case, each row of the image is stored in the form described in the comment under gx_tile_bitmap above; each pixel takes the number of bits specified as color_info.depth in the device structure, and holds values returned by the device's map_{rgb,cmyk}_color procedure. The print_page procedure can determine the number of bytes required to hold a scan line by calling: uint gdev_prn_raster(P1(gx_device_printer *)) For a very simple concrete example, we suggest reading the code in bit_print_page in gdevbit.c. If the device provides print_page, Ghostscript will call print_page the requisite number of times to print the desired number of copies; if the device provides print_page_copies, Ghostscript will call print_page_copies once per page, passing it the desired number of copies. ******** ******** Driver procedures ******** ******** Most of the procedures that a driver may implement are optional. If a device doesn't supply an optional procedure <proc>, the entry in the procedure structure may be either gx_default_<proc>, e.g. gx_default_tile_rectangle, or NULL or 0. (The device procedure must also call the gx_default_ procedure if it doesn't implement the function for particular values of the arguments.) Since C compilers supply 0 as the value for omitted structure elements, this convention means that statically initialized procedure structures will continue to work even if new (optional) members are added. Life cycle ---------- A device instance start out life in a closed state. In this state, no output operations will occur. Only the following procedures may be called: open_device get_initial_matrix get_params put_params When setdevice installs a device instance in the graphics state, it checks whether the instance is closed or open. If the instance is closed, setdevice calls the open routine, and then sets the state to open. There is currently no user-accessible operation to close a device instance. Device instances are only closed when they are about to be freed, which occurs in three situations: - When a 'restore' occurs, if the instance was created since the corresponding 'save'; - By the garbage collector, if the instance is no longer accessible; - When Ghostscript exits (terminates). Open/close/sync --------------- int (*open_device)(P1(gx_device *)) [OPTIONAL] Open the device: do any initialization associated with making the device instance valid. This must be done before any output to the device. The default implementation does nothing. void (*get_initial_matrix)(P2(gx_device *, gs_matrix *)) [OPTIONAL] Construct the initial transformation matrix mapping user coordinates (nominally 1/72" per unit) to device coordinates. The default procedure computes this from width, height, and x/y_pixels_per_inch on the assumption that the origin is in the upper left corner, i.e. xx = x_pixels_per_inch/72, xy = 0, yx = 0, yy = -y_pixels_per_inch/72, tx = 0, ty = height. int (*sync_output)(P1(gx_device *)) [OPTIONAL] Synchronize the device. If any output to the device has been buffered, send / write it now. Note that this may be called several times in the process of constructing a page, so printer drivers should NOT implement this by printing the page. The default implementation does nothing. int (*output_page)(P3(gx_device *, int num_copies, int flush)) [OPTIONAL] Output a fully composed page to the device. The num_copies argument is the number of copies that should be produced for a hardcopy device. (This may be ignored if the driver has some other way to specify the number of copies.) The flush argument is true for showpage, false for copypage. The default definition just calls sync_output. Printer drivers should implement this by printing and ejecting the page. int (*close_device)(P1(gx_device *)) [OPTIONAL] Close the device: release any associated resources. After this, output to the device is no longer allowed. The default implementation does nothing. Color/alpha mapping ------------------- A given driver normally will implement either map_rgb_color or map_cmyk_color, but not both. Black-and-white drivers do not need to implement either one. Note that the map_xxx_color procedures must not return gx_no_color_index (all 1's). gx_color_index (*map_rgb_color)(P4(gx_device *, gx_color_value red, gx_color_value green, gx_color_value blue)) [OPTIONAL] Map a RGB color to a device color. The range of legal values of the RGB arguments is 0 to gx_max_color_value. The default algorithm uses the map_cmyk_color procedure if the driver supplies one, otherwise returns 1 if any of the values exceeds gx_max_color_value/2, 0 otherwise. Ghostscript assumes that for devices that have color capability (i.e., color_info.num_components > 1), map_rgb_color returns a color index for a gray level (as opposed to a non-gray color) iff red = green = blue. gx_color_index (*map_cmyk_color)(P5(gx_device *, gx_color_value cyan, gx_color_value magenta, gx_color_value yellow, gx_color_value black)) [OPTIONAL] Map a CMYK color to a device color. The range of legal values of the CMYK arguments is 0 to gx_max_color_value. The default algorithm calls the map_rgb_color procedure, with suitably transformed arguments. Ghostscript assumes that for devices that have color capability (i.e., color_info.num_components > 1), map_cmyk_color returns a color index for a gray level (as opposed to a non-gray color) iff cyan = magenta = yellow. int (*map_color_rgb)(P3(gx_device *, gx_color_index color, gx_color_value rgb[3])) [OPTIONAL] Map a device color code to RGB values. The default algorithm returns (0 if color==0 else gx_max_color_value) for all three components. gx_color_index (*map_rgb_alpha_color)(P5(gx_device *, gx_color_value red, gx_color_value green, gx_color_value blue, gx_color_value alpha)) [OPTIONAL] Map a RGB color and an opacity value to a device color. The range of legal values of the RGB and alpha arguments is 0 to gx_max_color_value; alpha = 0 means transparent, alpha = gx_max_color_value means fully opaque. The default is to use the map_rgb_color procedure and ignore alpha. Note that if a driver implements map_rgb_alpha_color, it must also implement map_rgb_color, and must implement them in such a way that map_rgb_alpha_color(dev, r, g, b, gx_max_color_value) returns the same value as map_rgb_color(dev, r, g, b). Note that there is no map_cmyk_alpha_color procedure. CMYK devices currently do not support variable opacity; alpha is ignored on such devices. typedef enum { go_text, go_graphics } graphic_object_type; int (*get_alpha_bits)(P4(gx_device *dev, graphic_object_type type)) [OPTIONAL] Return the number of alpha (opacity) bits that should be used in rendering an object of the given type. The default value is 1; the only values allowed are 1, 2, and 4. Drawing ------- All drawing operations use device coordinates and device color values. int (*fill_rectangle)(P6(gx_device *, int x, int y, int width, int height, gx_color_index color)) Fill a rectangle with a color. The set of pixels filled is {(px,py) | x <= px < x + width and y <= py < y + height}. In other words, the point (x,y) is included in the rectangle, as are (x+w-1,y), (x,y+h-1), and (x+w-1,y+h-1), but *not* (x+w,y), (x,y+h), or (x+w,y+h). If width <= 0 or height <= 0, fill_rectangle should return 0 without drawing anything. Note that fill_rectangle is the only non-optional procedure in the driver interface. int (*draw_line)(P6(gx_device *, int x0, int y0, int x1, int y1, gx_color_index color)) [OPTIONAL] Draw a minimum-thickness line from (x0,y0) to (x1,y1). The precise set of points to be filled is defined as follows. First, if y1 < y0, swap (x0,y0) and (x1,y1). Then the line includes the point (x0,y0) but not the point (x1,y1). If x0=x1 and y0=y1, draw_line should return 0 without drawing anything. Bitmap imaging -------------- Bitmap (or pixmap) images are stored in memory in a nearly standard way. The first byte corresponds to (0,0) in the image coordinate system: bits (or polybit color values) are packed into it left-to-right. There may be padding at the end of each scan line: the distance from one scan line to the next is always passed as an explicit argument. int (*copy_mono)(P11(gx_device *, const unsigned char *data, int data_x, int raster, gx_bitmap_id id, int x, int y, int width, int height, gx_color_index color0, gx_color_index color1)) [OPTIONAL] Copy a monochrome image (similar to the PostScript image operator). Each scan line is raster bytes wide. Copying begins at (data_x,0) and transfers a rectangle of the given width 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 (i.e., where either color0 and color1 are both gx_no_color_index, for colored halftones, or neither one is gx_no_color_index, for monochrome halftones) is very important. Pixmap imaging -------------- Pixmaps are just like bitmaps, except that each pixel occupies more than one bit. All the bits for each pixel are grouped together (this is sometimes called "chunky" or "Z" format). For copy_color, the number of bits per pixel is given by the color_info.depth parameter in the device structure: the legal values are 1, 2, 4, 8, 16, 24, or 32. The pixel values are device color codes (i.e., whatever it is that map_rgb_color returns). int (*copy_color)(P9(gx_device *, const unsigned char *data, int data_x, int raster, gx_bitmap_id id, int x, int y, int width, int height)) [OPTIONAL] Copy a color image with multiple bits per pixel. The raster is in bytes, but x and width are in pixels, not bits. If id is different from gx_no_bitmap_id, it identifies the bitmap contents unambiguously; a call with the same id will always have the same data, raster, and data contents. We do not provide a separate procedure for tiling with a pixmap; instead, tile_rectangle can also take colored tiles. This is indicated by the color0 and color1 arguments both being gx_no_color_index. In this case, as for copy_color, the raster and height in the "bitmap" are interpreted as for real bitmaps, but the x and width are in pixels, not bits. int (*copy_alpha)(P11(gx_device *dev, const unsigned char *data, int data_x, int raster, gx_bitmap_id id, int x, int y, int width, int height, gx_color_index color, int depth)) [OPTIONAL] Fill a given region with a given color modified by an individual alpha value for each pixel. depth, the number of bits per pixel, is either 2 or 4, and in any case is always a value returned by a previous call on the get_alpha_bits procedure. Note that if get_alpha_bits always returns 1, this procedure will never be called. Bitmap/pixmap imaging --------------------- int (*copy_rop)(P15(gx_device *dev, const byte *sdata, int sourcex, uint sraster, gx_bitmap_id id, const gx_color_index *scolors, const gx_tile_bitmap *texture, const gx_color_index *tcolors, int x, int y, int width, int height, int phase_x, int phase_y, int command)) [OPTIONAL] This procedure is still supported, but has been superseded by strip_copy_rop. New drivers should implement strip_copy_rop; if they cannot cope with non-zero shift values in the texture, they should test for this explicitly and call the default implementation (gx_default_strip_copy_rop) if shift != 0. Clients should call strip_copy_rop, not copy_rop. int (*strip_copy_rop)(P15(gx_device *dev, const byte *sdata, int sourcex, uint sraster, gx_bitmap_id id, const gx_color_index *scolors, const gx_strip_bitmap *texture, const gx_color_index *tcolors, int x, int y, int width, int height, int phase_x, int phase_y, int command)) [OPTIONAL] Combine an optional source image S (as for copy_mono or copy_color) and an optional texture T (a tile, as for tile_rectangle) with the existing bitmap or pixmap D held by the driver, pixel by pixel, using any 3-input Boolean operation as modified by "transparency" flags: schematically, set D = f(D,S,T), computing f in RGB space rather than using actual device pixel values. S and T may each (independently) be a solid color, a bitmap with "foreground" and "background" colors, or a pixmap. This is a complex (and currently rather slow) operation. The arguments are as follows: dev - the device, as for all driver procedures sdata, sourcex, sraster, id, scolors - specify S, see below texture, tcolors - specify T, see below x, y, width, height - as for the other copy/fill procedures phase_x, phase_y - part of T specification, see below command - see below S specification ............... As noted above, the source (S) may be a solid color, a bitmap, or a pixmap. If S is a solid color: - sdata, sourcex, sraster, and id are irrelevant. - scolors points to two gx_color_index values; scolors[0] = scolors[1] = the color. If S is a bitmap: - sdata, sourcex, sraster, and id arguments are as for copy_mono or copy_color (data, data_x, raster, id), and specify a source bitmap. - scolors points to two gx_color_index values; scolors[0] is the background color (the color corresponding to 0-bits in the bitmap), scolors[1] is the foreground color (the color corresponding to 1-bits in the bitmap). If S is a pixmap: - sdata, sourcex, sraster, and id arguments are as for copy_mono or copy_color (data, data_x, raster, id), and specify a source pixmap whose depth is the same as the depth of the destination. - scolors is NULL. Note that if the source is a bitmap with background=0 and foreground=1, and the destination is 1 bit deep, then the source can be treated as a pixmap (scolors=NULL). T specification ............... Similar to the source, the texture (T) may be a solid color, a bitmap, or a pixmap. If T is a solid color: - The texture pointer is irrelevant. - tcolors points to two gx_color_index values; tcolors[0] = tcolors[1] = the color. If T is a bitmap: - The texture argument points to a gx_tile_bitmap, as for the tile_rectangle procedure. Similarly, phase_x and phase_y specify the offset of the texture relative to the device coordinate system origin, again as for tile_rectangle. The tile is a bitmap (1 bit per pixel). - tcolors points to two gx_color_index values; tcolors[0] is the background color (the color corresponding to 0-bits in the bitmap), tcolors[1] is the foreground color (the color corresponding to 1-bits in the bitmap). If T is a pixmap: - The texture argument 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 and/or clipped as needed. f specification ............... Command indicates the raster operation and transparency as follows: bits 7-0: raster op bit 8: 0 if source opaque, 1 if source transparent bit 9: 0 if texture opaque, 1 if texture transparent bits ?-10: unused, must be 0 The raster operation follows the Microsoft and H-P specification. It is an 8-element truth table that specifies the output value for each of the possible 2x2x2 input values as follows: Bit Texture Source Destination 7 1 1 1 6 1 1 0 5 1 0 1 4 1 0 0 3 0 1 1 2 0 1 0 1 0 0 1 0 0 0 0 Transparency affects the output in the following way. A source or texture pixel is considered transparent if its value is all 1's (e.g., 1 for bitmaps, 0xffffff for 24-bit RGB pixmaps) *and* the corresponding transparency bit is set in the command. For each pixel, the result of the Boolean operation is written into the destination iff neither the source nor the texture pixel is transparent. (Note that the H-P RasterOp specification, on which this is based, specifies that if the source and texture are both all 1's and the command specifies transparent source and opaque texture, the result *should* be written in the output. We think this is an error in the documentation.) Notes ..... Note that copy_rop is defined to operate on pixels in RGB space, again following the H-P and Microsoft specification. For devices that don't use RGB (or gray-scale with black=0, white=all 1's) as their native color representation, the implementation of copy_rop must convert to RGB or gray space, do the operation, and convert back (or do the equivalent of this). Here are the copy_rop equivalents of the most important previous imaging calls. Note that rop3_S may be replaced by any other Boolean operation. For monobit devices, we assume that black = 1. We assume the following declaration: static const gx_color_index white2[2] = { 1, 1 }; /* For all devices: */ (*fill_rectangle)(dev, x, y, w, h, color) ==> { gx_color_index colors[2]; colors[0] = colors[1] = color; (*dev_proc(dev, copy_rop))(dev, NULL, 0, 0, gx_no_bitmap_id, colors, NULL, colors /*irrelevant*/, x, y, w, h, 0, 0, rop3_S); } /* For black-and-white devices only: */ (*copy_mono)(dev, base, sourcex, sraster, id, x, y, w, h, (gx_color_index)0, (gx_color_index)1) ==> (*dev_proc(dev, copy_rop))(dev, base, sourcex, sraster, id, NULL, NULL, white2 /*irrelevant*/, x, y, w, h, 0, 0, rop3_S); /* For color devices, where neither color0 nor color1 is gx_no_color_index: */ (*copy_mono)(dev, base, sourcex, sraster, id, x, y, w, h, color0, color1) ==> { gx_color_index colors[2]; colors[0] = color0, colors[1] = color1; (*dev_proc(dev, copy_rop))(dev, base, sourcex, sraster, id, colors, NULL, white2 /*irrelevant*/, x, y, w, h, 0, 0, rop3_S); } /* For black-and-white devices only: */ (*copy_mono)(dev, base, sourcex, sraster, id, x, y, w, h, gx_no_color_index, (gx_color_index)1) ==> (*dev_proc(dev, copy_rop))(dev, base, sourcex, sraster, id, NULL, NULL, white2 /*irrelevant*/, x, y, w, h, 0, 0, rop3_S | lop_S_transparent); /* For all devices: */ (*copy_color)(dev, base, sourcex, sraster, id, x, y, w, h) ==> [same as first copy_mono above] /* For black-and-white devices only: */ (*tile_rectangle)(dev, tile, x, y, w, h, (gx_color_index)0, (gx_color_index)1, px, py) ==> (*dev_proc(dev, copy_rop))(dev, NULL, 0, 0, gx_no_bitmap_id, white2 /*irrelevant*/, tile, NULL, x, y, w, h, px, py, rop3_T) Polygons -------- In addition to the lowest-level drawing operations that take integer device coordinates and pure device colors, the driver interface includes higher-level operations that draw polygons using fixed-point coordinates, possibly halftoned colors, and possibly a non-default logical operation. The fill_* drawing operations all use the center-of-pixel rule: a pixel is colored iff its center falls within the polygonal region being filled. If a pixel center (X+0.5,Y+0.5) falls exactly on the boundary, the pixel is filled iff the boundary is horizontal and the filled region is above it, or the boundary is not horizontal and the filled region is to the right of it. int (*fill_trapezoid)(P10(gx_device *dev, fixed fx0, fixed fw0, fixed fy0, fixed fx1, fixed fw1, fixed fh, bool swap_axes, const gx_drawing_color *pdcolor, gs_logical_operation_t lop)) [OPTIONAL] Fill a trapezoid whose parallel sides are parallel to a coordinate axis. The corners are (fx0,fy0), (fx0+fw0,fy0), (fx1,fy0+fh), and (fx1+fw1,fy0+fy). We require fw0 >= 0, fw1 >= 0, and fh >= 0; if fw0 = 0 or fw1 = 0, the trapezoid is actually a triangle. If swap_axes is set, the meanings of X and Y are interchanged. int (*fill_parallelogram)(P9(gx_device *dev, fixed px, fixed py, fixed ax, fixed ay, fixed bx, fixed by, const gx_drawing_color *pdcolor, gs_logical_operation_t lop)) [OPTIONAL] Fill a parallelogram whose corners are (px,py), (px+ax,py+ay), (px+bx,py+by), and (px+ax+bx,py+ay+by). There are no constraints on the values of any of the parameters, so the parallelogram may have any orientation relative to the coordinate axes. int (*fill_triangle)(P9(gx_device *dev, fixed px, fixed py, fixed ax, fixed ay, fixed bx, fixed by, const gx_drawing_color *pdcolor, gs_logical_operation_t lop)) [OPTIONAL] Fill a triangle whose corners are (px,py), (px+ax,py+ay), and (px+bx,py+by). int (*draw_thin_line)(P7(gx_device *dev, fixed fx0, fixed fy0, fixed fx1, fixed fy1, const gx_drawing_color *pdcolor, gs_logical_operation_t lop)) [OPTIONAL] Draw a one-pixel-wide line from (fx0,fy0) to (fx1,fy1). This replaces the older draw_line procedure, which required integer coordinates, a pure color, and no logical operation. High-level drawing ------------------ In addition to the low-level drawing operations described above, the driver interface provides a set of high-level operations. Normally these will have their default implementation, which converts the high-level operation to the low-level ones just described; however, drivers that generate high-level output formats such as CGM, or communicate with devices that have firmware for higher-level operations such as polygon fills, may implement these high-level operations directly. For more details, please consult the source code, specifically: gxpaint.h - defines gx_fill_params, gx_stroke_params gxfixed.h - defines fixed, gs_fixed_point (used by gx_*_params) gxistate.h - defines gs_imager_state (used by gx_*_params) gxline.h - defines gx_line_params (used by gs_imager_state) gslparam.h - defines line cap/join values (used by gx_line_params) gxmatrix.h - defines gs_matrix_fixed (used by gs_imager_state) g[s,x,z]path.h - defines gx_path g[x,z]cpath.h - defines gx_clip_path int (*fill_path)(P6(gx_device *dev, const gx_imager_state *pis, gx_path *ppath, const gx_fill_params *params, const gx_drawing_color *pdcolor, const gx_clip_path *pcpath)) [OPTIONAL] Fill the given path, clipped by the given clip path, according to the given parameters, with the given color. The clip path pointer may be NULL, meaning do not clip. int (*stroke_path)(P6(gx_device *dev, const gx_imager_state *pis, gx_path *ppath, const gx_stroke_params *params, const gx_drawing_color *pdcolor, const gx_clip_path *pcpath)) [OPTIONAL] Stroke the given path, clipped by the given clip path, according to the given parameters, with the given color. The clip path pointer may be NULL, meaning do not clip. int (*fill_mask)(P13(gx_device *dev, const byte *data, int data_x, int raster, gx_bitmap_id id, int x, int y, int width, int height, const gx_drawing_color *pdcolor, int depth, int command, const gx_clip_path *pcpath)) [OPTIONAL] Color the 1-bits in the given mask (or according to the alpha values, if depth > 1), clipped by the given clip path, with the given color and logical operation. The clip path pointer may be NULL, meaning do not clip. The parameters data, ..., height are as for copy_mono; depth is as for copy_alpha; command is as for copy_rop. High-level bitmap imaging ------------------------- Similar to the high-level interface for fill/stroke graphics, a high-level interface exists for bitmap images. All of the procedures in this part of the interface are optional; however, if begin_image is defined, image_data and end_image also must be defined. A bitmap image is defined by a gs_image_t structure. We only summarize this structure here. typedef struct gs_image_s { int Width; /* Width of source image in pixels */ int Height; /* Height ditto */ gs_matrix ImageMatrix; /* transformation from 1x1 image space to */ /* user space defined by imager CTM */ int BitsPerComponent; const gs_color_space *ColorSpace; float Decode[8]; /* linear remapping of input values */ bool Interpolate; /* smooth image if true */ bool ImageMask; /* true = mask, false = solid image */ bool adjust; /* expand bits if mask */ bool CombineWithColor; /* use drawing color as RasterOp texture */ } gs_image_t; typedef enum { gs_image_format_chunky = 0, gs_image_format_component_planar = 1 } gs_image_format_t; For more details, consult the source code in gsiparam.h - defines parameters for an image int (*begin_image)(P9(gx_device *dev, const gs_imager_state *pis, const gs_image_t *pim, gs_image_format_t format, gs_int_rect *prect, const gx_drawing_color *pdcolor, const gx_clip_path *pcpath, gs_memory_t *memory, void **pinfo)) [OPTIONAL] Begin the transmission of an image. Zero or more calls of image_data will follow, and then a call of end_image. The parameters of begin_image are as follows: pis - pointer to an imager state. The only relevant elements of the imager state are the CTM (coordinate transformation matrix), the logical operation (RasterOp/transparency), and the color rendering information. pim - pointer to the gs_image_t structure that defines the image parameters. format - defines how pixels are represented for image_data. See the description of image_data below. prect - if not NULL, defines a subrectangle of the image; only the data for this subrectangle will be passed to image_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 bookkeeping structure here. begin_image is expected to allocate a structure for its bookkeeping needs, using the allocator defined by the memory parameter, and return it in *pinfo. begin_image should not assume that the structures in *pim, *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 (*image_data)(P6(gx_device *dev, void *info, const byte **planes, int data_x, uint raster, int height)) [OPTIONAL] 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 raster (number of bytes per row) may include some padding at the end of each row. Note that for non-mask images, the input data may be in any color space and may have any number of bits per component (1, 2, 4, 8, 12); currently mask images always have 1 bit per component, but in the future, they might allow multiple bits of alpha. Note also that each call of image_data passes complete pixels: e.g., for a chunky image with 24 bits per pixel, each call of image_data passes 3N bytes of data (specifically, 3 * Width * height). The interpretation of planes depends on the format argument of begin_image: -- If format is gs_image_format_chunky, planes[0] points to data in "chunky" format, in which the components follow each other (e.g., RGBRGBRGB....) -- If format is gs_image_format_component_planar, planes[0 .. N-1] point to data for the N components (e.g., N=3 for RGB data); each plane contains samples for a single component, e.g., RR..., GG..., BB.... Note that the planes are divided by component, not by bit: for example, for 24-bit RGB data, N=3, with 8-bit values in each plane of data. Someday we may add gs_image_format_bit_planar, specifying that each plane would contain only a single bit of the pixel value, but this is not currently implemented. If, as a result of this call, image_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_data, unlike most other driver procedures that take bitmaps as arguments, does not require the data to be aligned in any way. int end_image(P3(gx_device *dev, void *info, bool draw_last)) [OPTIONAL] Finish processing an image, either because all data have been supplied or because the caller has decided to abandon this image. end_image may be called at any time after begin_image. It should free the info structure and any subsidiary structures. If draw_last is true, it should finish drawing any buffered lines of the image. While there will almost never be more than one image enumeration in progress -- i.e., after a begin_image, end_image will almost always be called before the next begin_image -- driver code should not rely on this property; in particular, it should store all information regarding the image in the info structure, not in the driver structure. Note that if begin_image saves its parameters in the info structure, it can decide on each call whether to use its own algorithms or to use the default implementation. (It may need to call gx_default_begin/end_image partway through.) [A later revision of this document may include an example here.] Reading bits back ----------------- int (*get_bits)(P4(gx_device *, int y, byte *str, byte **actual_data)) [OPTIONAL] Read one scan line of bits back from the device into the area starting at str, namely, scan line y. If the bits cannot be read back (e.g., from a printer), return -1; otherwise return 0. The contents of the bits beyond the last valid bit in the scan line (as defined by the device width) are unpredictable. str need not be aligned in any particular way. If actual_data is NULL, the bits are always returned at str. If actual_data is not NULL, get_bits may either copy the bits to str and set *actual_data = str, or it may leave the bits where they are and return a pointer to them in *actual_data. In the latter case, the bits are guaranteed to start on a 32-bit boundary and to be padded to a multiple of 32 bits; also in this case, the bits are not guaranteed to still be there after the next call on get_bits. Parameters ---------- Devices may have an open-ended set of parameters, which are simply pairs consisting of a name and a value. The value may be of various types: integer (int or long), boolean, float, string, name, null, array of integer, or array of float. For example, the Name of a device is a string; the Margins of a device is an array of 2 floats. See gsparam.h for more details. If a device has parameters other than the ones applicable to all devices (or, in the case of printer devices, all printer devices), it must provide get_params and put_params procedures. If your device has parameters beyond those of a straightforward display or printer, we strongly advise using the _get_params and _put_params procedures in an existing device (for example, gdevcdj.c or gdevbit.c) as a model for your own code. int (*get_params)(P2(gx_device *dev, gs_param_list *plist)) [OPTIONAL] Read the parameters of the device into the parameter list at plist, using the param_write_* macros/procedures defined in gsparam.h. int (*put_params)(P2(gx_device *dev, gs_param_list *plist)) [OPTIONAL] Set the parameters of the device from the parameter list at plist, using the param_read_* macros/procedures defined in gsparam.h. All put_params procedures must use a "two-phase commit" algorithm; see gsparam.h for details. External fonts -------------- Drivers may include the ability to display text. More precisely, they may supply a set of procedures that in turn implement some font and text handling capabilities. These procedures are documented in another file, xfonts.txt. The link between the two is the driver procedure that supplies the font/text procedures: xfont_procs *(*get_xfont_procs)(P1(gx_device *dev)) [OPTIONAL] Return a structure of procedures for handling external fonts and text display. A NULL value means that this driver doesn't provide this capability. For technical reasons, a second procedure is also needed: gx_device *(*get_xfont_device)(P1(gx_device *dev)) [OPTIONAL] Return the device that implements get_xfont_procs in a non-default way for this device, if any. Except for certain special internal devices, this is always the device argument. Page devices ------------ gx_device *(*get_page_device)(P1(gx_device *dev)) [OPTIONAL] According to the Adobe specifications, some devices are "page devices" and some are not. This procedure returns NULL if the device is not a page device, or the device itself if it is a page device. In the case of forwarding devices, get_page_device returns the underlying page device (or NULL if the underlying device is not a page device). Miscellaneous ------------- int (*get_band)(P3(gx_device *dev, int y, int *band_start)) [OPTIONAL] If the device is a band device, this procedure stores in *band_start the scan line (device Y coordinate) of the band that includes the given Y coordinate, and returns the number of scan lines in the band. If the device is not a band device, this procedure returns 0. The latter is the default implementation. 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)).