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GNU Ghostscript
   Copyright (C) 1993, 1994 Aladdin Enterprises.  All rights reserved.
  
  This file is part of GNU Ghostscript.
  
  GNU Ghostscript is distributed in the hope that it will be useful, but
  WITHOUT ANY WARRANTY.  No author or distributor accepts responsibility to
  anyone for the consequences of using it or for whether it serves any
  particular purpose or works at all, unless he says so in writing.  Refer
  to the GNU Ghostscript General Public License for full details.
  

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

This file, devarch.doc, contains notes about the architecture of
device handling in Ghostscript.  It is intended only for developers
with a fairly deep understanding of Level 2 PostScript.

For an overview of Ghostscript and a list of the documentation files, see
README.

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

****** MUCH OF THE MATERIAL IN THIS DOCUMENT IS UNRELIABLE.  DON'T
TRUST IT. ******

Background
==========

PostScript language
-------------------

Adobe's Level 1 PostScript specification ignored, except for a few
now-obsolete operators, the possibility of a single PostScript system that
had multiple hardware devices attached to it; Level 1 also did not deal, in
a systematic way, with control of printing features such as page size and
output finishing.  In contrast, the available Level 2 specifications (the
PostScript Language Reference Manual, Second Edition, and subsequent
documentation obtained from the Adobe file server describing Adobe
PostScript products through PostScript version 2013) include extensive
facilities in this area.

Adobe's documentation refers explicitly to three concepts of "device":

	- A physical device such as a file system, a communication port, or
a printing engine.  The IODevice resource enumerates these devices without
providing any other information about them.  Each IODevice may have a set
of state variables specific to it; the setdevparams and currentdevparams
operators control the state of individual IODevices.  (In the rest of this
document, we are not concerned with IODevices.)

	- A class of destination for rendering.  The OutputDevice resource
enumerates these, and gives some further information (available page sizes
and resolutions).  OutputDevices do not have state per se, although in
Adobe's model, they typically correspond 1-for-1 with those IODevices that
can serve as destinations for rendering.

	- An actual destination for rendering.  This may be a special
destination like the null device or a cache device, a display (for which
the documentation specifies no further details), or an instance of an
OutputDevice.  OutputDevice instances are called "page devices".  The only
directly visible, program-controlled state of a page device is the
contents of the current page.  However, page devices also have a very
large set of optional parameters that may be set when setpagedevice
creates the instance, and read by currentpagedevice; some of these
parameters may change from time to time as a result of external events or
conditions.  The parameters control both external hardware (e.g., duplex
or collation) and internally implemented facilities such as portrait vs.
landscape orientation.

Ghostscript
-----------

Ghostscript generalizes the Adobe model in several areas at the level of
the PostScript program.

	- In addition to hard copy devices, Ghostscript can produce output
on physical displays, window systems, raster files, or RAM; a typical
Ghostscript configuration supports more than one of these.  Ghostscript
models this by providing new OutputDevices, and a new concept called an
OutputDevice family for grouping them.

	- Ghostscript normally includes multiple drivers that can write to
the same physical device (for example, multiple raster file format
generators, or multiple printer drivers).  Ghostscript models this by
making each driver a different OutputDevice.

	- Ghostscript explicitly makes visible the objects that represent
OutputDevice instances (page devices and their display, file, and RAM
analogues) as new PostScript objects of type /devicetype.

In the remainder of this document, we describe Ghostscript's handling of
devices from four viewpoints: that of an ordinary PostScript program, that
of the Ghostscript implementation at the PostScript level, that of the
Ghostscript implementation at the C level, and that of the Ghostscript
device driver.

PostScript programs
===================

OutputDevice resource
---------------------

The OutputDevice resource is described in Adobe documentation as having
been added in Adobe interpreter version 2011.  Briefly, the resource names
are the same as the allowed values of the /OutputDevice page device
parameter, and the resource values are dictionaries giving properties of
the output device.  The defined properties as of version 2012 are
HWResolution, ManualSize, and PageSize.

Ghostscript provides one OutputDevice for each available driver.  This
includes file formats (e.g., /gifmono) and display or window systems
(e.g., /x11) as well as printer types (e.g., /ljet3).  It provides a
special OutputDevice called /RAM whose instances simply accumulate a
bitmap in RAM for later readout.  In addition, if the available drivers
include any printer, fax, or display driver, Ghostscript provides
OutputDevice entries called /Printer, /Fax, /RasterFile, and /Display
respectively that are aliases for the default driver in the respective
family.  (There is nothing special about these three names.  Any driver
can declare that it belongs to a particular family, and the driver of any
given family that appears earliest in the build list becomes the default
driver with that family name.)  Finally, Ghostscript always provides an
OutputDevice called /Default that is an alias for the default OutputDevice
(the one used if setpagedevice is invoked with no current page device and
no OutputDevice key in the parameter dictionary).

Operators
---------

Ghostscript provides the setpagedevice and currentpagedevice operators, as
required by the Level 2 PostScript specifications.  Ghostscript also
provides a few other operators that implement increased functionality.

 .indexdevicename 

	Returns the name of the int'th driver in the build list.  The first
driver is numbered 0.

 .selectdevice

	Creates and makes current a default instance of a given
OutputDevice.

- .currentdevice 

	Returns the current device, which may be a page device or a special
device.

 .getoutputparams    ...  

	Returns the parameters of a default instance of the given
OutputDevice.  Also returns the information for the OutputDevice resource
dictionary.

 .getoutputparams    ...  

	Returns the parameters of a given device.

Page device parameters
----------------------

Most of the page device parameters documented by Adobe (either in the New
Red Book or in the post-Book notes) are specific to particular devices.
Ghostscript leaves these up to the device driver to process, and only
provides the Policies and PolicyReport mechanism to handle those that are
unknown or out of range.  Here are the parameters documented by Adobe that
Ghostscript knows about specifically:

	BeginPage, EndPage, Install - per Red Book documentation.

	Policies - per Red Book documentation.  The PageSize policy value
	of 7 (in later Adobe documentation) is not supported.

	ImagingBBox, HWResolution, Margins, PageSize - per Red Book
	documentation.  Margins are given in device-specific units, per
	later Adobe documentation.

	OutputDevice - per later Adobe documentation.

	InputAttributes, OutputAttributes - per Red Book documentation.
	The feature whereby if InputAttributes is null, the interpreter
	consults the driver directly for media selection (per later
	documentation) is not implemented.

	Orientation - per Red Book documentation, also supported for RAM
	and display devices.  We aren't sure yet how Orientation will
	affect raster file output devices.

	ProcessColorModel - per later Adobe documentation, but read-only
	for all but RAM devices.  For RAM devices, only DeviceGray,
	DeviceRGB, and DeviceCMYK are supported.

	InsertSheet, OutputPage, SeparationColorNames, SeparationOrder -
	not currently supported.

	Collate and the concept of a "set" - not currently supported.

Ghostscript also implements the following parameters that are not
documented by Adobe:

	BitsPerPixel - the number of bits per pixel, an integer between 1
	and 32.  This is read-only for all but RAM devices and a few file
	formats.

	GrayValues, RedValues, GreenValues, BlueValues, ColorValues -
	as defined by Adobe for the deviceinfo operator.  These are
	read-only for all but RAM devices and a few file formats.

	.HWGrayMap, .HWRedMap, .HWGreenMap, .HWBlueMap, .HWColorMap -
	the maps of the realizable device colors.  Each map is a string of
	M x xxxValues bytes; each entry is M bytes, where M = 1 for the
	first 4 maps and 1, 3, or 4 bytes according to the
	ProcessColorModel for HWColorMap.  (I.e., the maps are like the
	lookup table for an Indexed color space.)  If a map is missing, the
	realizable colors are 0, 1/N, ..., 1, where N is xxxValues-1.
	The Map parameters are read-only for all but RAM devices and a few
	file formats.  Also, the strings returned are themselves always
	read-only; the only way to change a Map is to set the parameter.

	MaxRasterMemory - the maximum amount of RAM that can be allocated
	for raster memory.  This is a constraint in addition to that
	imposed by the MaxRasterMemory system parameter.

	.PageCount - the number of times that the EndPage routine of this
	page device returned true.  Normally, this is the same as the
	number of executions of showpage or copypage.

	.ShowpageCount - the number of times that showpage was executed
	with this page device; this is the value supplied to the BeginPage
	and EndPage procedures.

	.HWMargins - the widths of the unprintable areas at the left,
	bottom, right, and top edges of the page, expressed in 1/72"
	units.  If ImagingBBox has not been specified, the default
	ImagingBBox is the PageSize inset by the .HWMargins.

All other documented page device parameters, including Details
dictionaries, are simply passed to the driver.

Implementation - PostScript level
=================================

Operators
---------

 .setdevice -

	Sets the current device without doing any of the things required
by setpagedevice.

Implementation - C level
========================

Driver interface
================

Appendix: Relationship to older Ghostscript versions
====================================================

Operators
---------

Previous Ghostscript versions provided a set of device handling operators
that were designed before the newer Adobe specifications were available.
We now indicate how these map to newer facilities.

.getdevice
	This low-level operator, renamed .indexdevice, still exists to
allow enumerating the output devices.  The standard way to enumerate
OutputDevices is now (*) {...} /OutputDevice resourceforall.

selectdevice
	This has been renamed .selectdevice, in keeping with Ghostscript
practice of preceding all Ghostscript-specific operators with a dot.
/outdevname selectdevice is essentially the same as << /OutputDevice
/outdevname >> setpagedevice.

currentdevice
	This has been renamed .currentdevice.

setdevice
	We aren't sure what will happen to this.

copydevice
	This is essentially equivalent to
		gsave .setdevice << >> setpagedevice
		  .currentdevice grestore
since setpagedevice always creates a new device.

getdeviceprops
	Renamed .getoutputparams.

devicename
	This can be implemented using .getoutputparams.

deviceinitialmatrix
	This can be determined by selecting the device and then reading out
the defaultmatrix.

putdeviceprops
	This has been renamed .putoutputparams.  Its status in the new
scheme of things, like the status of setdevice, is unclear.

makeimagedevice
	Replaced by setpagedevice with << /OutputDevice /RAM >>.  The
matrix, width, height, and palette arguments are replaced by parameters in
the device dictionary.  The default matrix is the identity matrix, the
default width and height are 0, the default depth is 1, and the default
palette is .

copyscanlines
	We aren't sure whether to retain this or to replace it with an
.imagefilter operator that provides a stream to read out the bits of a
memory device.

Page device parameters
----------------------

Previous Ghostscript versions had a concept of "device properties", which
correspond to page device parameters in the current architecture.  Here we
list the correspondence between former Ghostscript device properties and
current Ghostscript parameters.

	Prior Ghostscript		Current Ghostscript
	-----------------		-------------------

Size and shape:

	InitialMatrix			Margins, Orientation, Install

Color:

	Colors				[color]Values

	HWColorMap			HW[color]Map

Other:

	Name				OutputDevice

	BufferSpace, MaxBitmap		MaxRasterMemory

Orientation, HWResolution, and Margins, together with the ability of the
Install procedure to set the default CTM, are sufficient to specify all
possible legal values of the initial transformation matrix.  ****** Not
sufficient to handle an inverted Y axis.


 

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