PC World Komputer 1996 February/PCWK0296.iso/sharewar/dos/inne/gs300ini/devarch.doc

Text File | 1994-08-02 | 13.3 KB | 342 lines

   Copyright (C) 1993, 1994 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, 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.

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

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.

    - 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 an OutputDevice is the
contents of the current page.  However, OutputDevices 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, 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.

<int> .indexdevicename <outdevname>

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

<outdevname> .selectdevice

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

- .currentdevice <device>

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

<outdevname> .getoutputparams <mark> <name1> <value1> ... <nameN> <valueN>

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

<device> .getoutputparams <mark> <name1> <value1> ... <nameN> <valueN>

    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 documentation) is not supported.

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

    OutputDevice - per later 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 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:

    .HWBitsPerPixel - 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.

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

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

Operators
---------

<device> .setdevice -

    Sets the current device without doing 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 <ff 00>.

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 configuration 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.