MacWorld 1998 March/Macworld (1998-03) (Disk 1).dmg/Shareware World/Info/For Developers/GhostScript 5.10/MacGS-510/doc/language.txt

Text File | 1997-11-20 | 30KB | 769 lines

   Copyright (C) 1989, 1995, 1996, 1997 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, language.txt, describes the relationship between the Ghostscript
interpreter and the PostScript language.

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

The Ghostscript interpreter, except as noted below, is intended to execute
properly any source program written in the (Level 2) PostScript language as
defined in the December 1990 printing of the PostScript Language Reference
Manual (Second Edition) published by Addison-Wesley (ISBN 0-201-18127-4).
However, the interpreter is configurable in ways that can restrict it to
various subsets of this language.  Specifically, the base interpreter
accepts the Level 1 subset of the PostScript language, as defined in the
first edition of the PostScript Language Reference Manual, ISBN
0-201-10174-2, Addison-Wesley, 1985, plus the file system, version 25.0
language, and miscellaneous additions listed in sections A.1.6, A.1.7, and
A.1.8 of the Second Edition respectively, including allowing a string
operand for the 'status' operator.  The base interpreter may be configured
by adding any combination of the following:

    - The ability to process PostScript Type 1 fonts.  This facility is
normally included in the interpreter.

    - The CMYK color extensions listed in section A.1.4 of the Second
Edition (including colorimage).  These facilities are only available if the
color, dps, or level2 feature was selected at the time that Ghostscript was
compiled and linked.

    - The Display PostScript extensions listed in section A.1.3
of the Second Edition, but excluding the operators listed in section
A.1.2.  These facilities are only available if the dps feature or the
level2 feature was selected at the time that Ghostscript was compiled
and linked.

    - The composite font extensions listed in section A.1.5 of the
Second Edition, and the ability to handle Type 0 fonts.  These facilities
are only available if the compfont feature or the level2 feature was
selected at the time that Ghostscript was compiled and linked.

    - The ability to load TrueType fonts and to handle PostScript Type
42 (encapsulated TrueType) fonts.  These facilities are only available if
the ttfont feature was selected at the time that Ghostscript was compiled
and linked.

    - The PostScript Level 2 "filter" facilities aside from DCTEncode
and DCTDecode filters.  These facilities are only available if the filter,
dps, or level2 feature was selected at the time that Ghostscript was
compiled and linked.

    - The PostScript Level 2 DCTEncode and DCTDecode filters.  These
facilities are only available if the dct or level2 feature was selected at
the time that Ghostscript was compiled and linked.

    - All the other PostScript Level 2 operators and facilities listed
in section A.1.1 of the Second Edition and not listed in any of the other
A.1.n sections.  These facilities are only available if the level2 feature
was selected at the time that Ghostscript was compiled and linked.

    - The ability to recognize MS-DOS EPSF files and process only the
PostScript part, ignoring bitmap previews or other information.  This
facility is only available if the epsf feature was selected at the time that
Ghostscript was compiled and linked.

Adding all of these produces a full Level 2 PostScript language
interpreter.

Ghostscript also includes the optional ability to interpret files in the PDF
1.2 format defined in the Portable Document Format Reference Manual, Version
1.2 (November 12, 1996) distributed by Adobe Systems Incorporated.  This
facility is only available if the 'pdf' feature was selected at the time
that Ghostscript was compiled and linked.

Ghostscript also includes a number of operators defined below that are not
in the PostScript language.

Implementation limits
=====================

The following implementation limits correspond to those in Table B.1 and
B.2 of the Second Edition.  Those marked with * are different from the ones
in the Second Edition.

Architectural limits
--------------------

integer        32-bit two's complement integer
real        single-precision IEEE float
*array
    On 16-bit systems:    8191 elements
    On 32-bit systems:    65535 elements
*dictionary
    On 16-bit systems:    8190 elements
    On 32-bit systems:    65534 elements
*string        65535 characters
*name        16383 characters
filename    100 characters
*save level    none (capacity of memory)
*gsave level    none (capacity of memory)

Typical memory limits in Level 1
--------------------------------

userdict        200
FontDirectory        100
*operand stack        800
dictionary stack    20
execution stack        250
*interpreter level    none (capacity of memory)
*path            none (capacity of memory)
dash            11
*VM            capacity of memory
*file            determined by operating system
*image            65535 values (samples x components) for 1, 2, 4,
              or 8-bit samples; 32767 values for 12-bit samples

Other differences in VM consumption
-----------------------------------

Packed array elements occupy either 2 bytes or 8 bytes.  The average
element size is probably about 5 bytes.

Names occupy 12 bytes plus the space for the string.

Graphics/text operator additions
================================

Graphics state operators
------------------------

    <bool> .setaccuratecurves -
        Sets a graphics state flag that determines whether curves
          and arcs, when flattened, always start and end with a
          line that a segment of the tangent; this also causes
          butt and square caps to be properly perpendicular to
          the tangent.  initgraphics sets this flag to false,
          to match other PostScript implementations.

    - .currentaccuratecurves <bool>
        Returns the current value of the accurate curves flag.

    <bool> .setclipoutside -
        Sets a graphics state flag that determines whether the
          effective clipping region is the inside (false) or the
          outside (true) of the region defined by the clipping
          operators (clip, rectclip, etc.)  initclip sets this
          flag to false.

    - .currentclipoutside <bool>
        Returns the current value of the outside clipping flag.

    <bool> .setdashadapt -
        Sets a graphics state flag that determines whether dash
          patterns do (true) or do not (false) automatically scale
          themselves so that each line segment consists of an
          integral number of pattern repetitions.  initgraphics sets
          this flag to false.

    - .currentdashadapt <bool>
        Returns the current value of the dash adaptation flag.

    <matrix> .setdefaultmatrix -
        Sets the default matrix that is returned by defaultmatrix
          and installed by initmatrix.  Ordinary programs should
          not use this operator.

    <num> <bool> .setdotlength -
        Sets a graphics state parameter that determines the handling
          of zero-length lines (dots).  If the dot length is zero,
          dots are painted as circles if round line caps are in
          effect, otherwise they are not painted at all.  If the dot
          length is non-zero, dots are treated exactly like lines of
          the given length: the length is specified in user
          coordinates (like line width) if bool is false, or in
          default user coordinates (1/72" units) if bool is true.
          Dots occurring as part of dash patterns will be oriented
          correctly; isolated dots will be oriented as though they
          were part of a vertical line.  initgraphics sets the dot
          length to zero.

    - .currentdotlength <num> <bool>
        Returns the current dot length and dot length mode.

    <dx> <dy> .setfilladjust2 -
        Sets graphics state parameters that cause all filled and
          stroked regions to be "fattened" by the given amount
          relative to an algorithm that only paints pixels whose
          centers fall within the region to be painted.
          dx and dy are numbers between 0 and 0.5, measured in
          device space.  The only two values that are likely to
          be useful are 0, which gives a pure center-of-pixel
          rule, and 0.5, which gives Adobe's any-part-of-pixel
          rule.  (0.5 is treated slightly specially in order to
          create half-open pixels per Adobe's specification.)

    - .currentfilladjust2 <dx> <dy>
        Returns the current fill adjustment values.

Path operators
--------------

    - .dashpath -
        If there is no current dash pattern, does nothing.
          Otherwise, does the equivalent of flattenpath and then
          chops up the path as determined by the dash pattern.

    <x> <y> <width> <height> .rectappend -
    <numarray> .rectappend -
    <numstring> .rectappend -
        Appends a rectangle or rectangles to the current path, in
          the same manner as rectfill, rectclip, etc.  Only
          defined if the dps and/or level2 option is selected.

Painting operators
------------------

Ghostscript supports an experimental extension of the PostScript imaging
model to include RasterOp and some related facilities.  This extension is
only available if the rasterop option was selected when building
Ghostscript.

With the RasterOp extension, imaging operations compute a function D =
f(D,S,T) in RGB space, where f is an arbitrary 3-input Boolean function, D
is the destination (frame buffer or print buffer), S is the source
(described below), and T is the texture (the current PostScript color, which
may be a pattern).  The source and texture depend on the PostScript imaging
operation:

    - For fill and stroke, the source is solid black, covering the
    region to be painted; the texture is the current PostScript color.

    - For show and imagemask, the source is solid black, covering the
    pixels to be painted; the texture is the current PostScript color.

    - For image and colorimage, the source is the image data; the
    texture depends on an optional Boolean parameter, CombineWithColor,
    in the image dictionary.  If CombineWithColor is false (the
    default), the texture is solid black.  If CombineWithColor is true,
    the texture is the current color.  For the non-dictionary form of
    the image operator, CombineWithColor is considered to be false.

The rasterop option adds the following operators:

    <int8> .setrasterop -
        Sets the RasterOp function in the graphics state.
          The default function is 252, Source | Texture.

    - .currentrasterop <int8>
        Returns the current RasterOp function.

    <bool> .setsourcetransparent -
        Sets source transparency in the graphics state.  When source
          transparency is true, white source pixels prevent storing
          into the destination, regardless of what the RasterOp
          function returns.  The default source transparency is
          false.

    - .currentsourcetransparent <bool> -
        Returns the current source transparency.

    <bool> .settexturetransparent -
        Sets texture transparency in the graphics state.  When texture
          transparency is true, white texture pixels prevent storing
          into the destination, regardless of what the RasterOp
          function returns.  The default texture transparency is
          false.

    - .currenttexturetransparent <bool> -
        Returns the current texture transparency.

For more information on RasterOp and transparency, please consult chapter 5
of the "PCL 5 Color Technical Reference Manual", Hewlett-Packard Manual Part
No. 5961-0635.

Character operators
-------------------

    <string> <bool> .charboxpath -
        For each character C in the rendering of <string>, let the
          bounding box of C *in device space* be the four
          *user-space* points p1x/y, p2x/y, p3x/y, and p4x/y.  For
          each character in order, .charboxpath appends the
          following to the current path:
            - If <bool> is true, the equivalent of:
                p1x p1y moveto
                p2x p2y lineto
                p3x p3y lineto
                p4x p4y lineto
                closepath
          This creates a path whose pathbbox is the bbox of the
          string.
            - If <bool> is false, the equivalent of:
                p1x p1y moveto
                p3x p3y lineto
          If the CTM is well-behaved (consists only of reflection,
          scaling, and rotation by multiples of 90 degrees), this
          too creates a (simpler) path whose pathbbox is the bbox of
          the string.

    <font> <charname|charcode> <charname> <charstring> .type1execchar -
        Does all the work for rendering a Type 1 outline.  This
          operator, like setcharwidth and setcachedevice, is
          only valid in the context of a show operator -- i.e.,
          it must only be called from within a BuildChar or
          BuildGlyph procedure.

    <font> <charcode> %Type1BuildChar -
        This is not a new operator: rather, it is a name known
          specially to the interpreter.  Whenever the interpreter
          needs to render a character (during a ...show,
          stringwidth, or charpath), it looks up the name
          BuildChar in the font dictionary to find a procedure to
          run.  If it does not find this name, and if the FontType
          is 1, the interpreter instead uses the value (looked up
          on the dictionary stack in the usual way) of the name
          %Type1BuildChar.
        The standard definition of %Type1BuildChar is in gs_type1.ps.
          Users should not need to redefine %Type1BuildChar, except
          perhaps for tracing or debugging.

    <font> <charname> %Type1BuildGlyph -
        Provides the Type 1 implementation of BuildGlyph.

Other operator additions
========================

Mathematical operators
----------------------

    <number> arccos <number>
        Computes the arc cosine of a number between -1 and 1.

    <number> arcsin <number>
        Computes the arc sine of a number between -1 and 1.

String operators
----------------

    <state> <fromString> <toString> .type1encrypt <newState> <toSubstring>
        Encrypts fromString according to the algorithm for Adobe
          Type 1 fonts, writing the result into toString.
          toString must be at least as long as fromString or a
          rangecheck error occurs.  state is the initial state of
          the encryption algorithm (a 16-bit non-negative
          integer); newState is the new state of the algorithm.

    <state> <fromString> <toString> .type1decrypt <newState> <toSubstring>
        Decrypts fromString according to the algorithm for Adobe
          Type 1 fonts, writing the result into toString.  Other
          specifications are as for type1encrypt.

Relational operators
--------------------

    <number|string> <number|string> max <number|string>
        Returns the larger of two numbers or strings.

    <number|string> <number|string> min <number|string>
        Returns the smaller of two numbers or strings.

File operators
--------------

    <string> findlibfile <foundstring> <file> true
    <string> findlibfile <string> false
        Opens the file of the given name for reading, searching
          through directories as described in use.txt.  If the
          search fails, findlibfile simply pushes false on the
          stack and returns, rather than causing an error.

    <file> <integer> unread -
         Pushes back the last-read character onto the front of the
          file.  If the file is only open for writing, or if the
          integer argument is not the same as the last character
          read from the file, causes an ioerror error.  May also
          cause an ioerror if the last operation on the file was not
          a reading operation.

    <file> <device> writeppmfile -
        Writes the contents of the device, which must be an image
          device, onto the file, in Portable PixMap (ppm) format.
          Does not close the file.

Ghostscript also supports the following IODevice in addition to a subset of
those defined in the Adobe documentation: %pipe%command, which opens a pipe
on the given command.  This is only supported on operating systems that
provide popen (primarily Unix systems, and not all of those).

Filters
-------

Ghostscript supports all the standard PostScript Level 2 filters, except
that it does not currently support the EarlyChange key in the LZWEncode
filter.  Ghostscript also supports the as yet undocumented FlateEncode and
FlateDecode filters from PDF 1.2 and (presumably) PostScript Level 3, except
for the Effort key in FlateEncode.  In addition, Ghostscript supports the
following non-standard filters:

    <target> /BCPEncode filter <file>
    <source> /BCPDecode filter <file>
        Create filters that implement the Adobe Binary
          Communications Protocol.  See Adobe documentation for
          details.

    <target> <seed_integer> /eexecEncode filter <file>
        Creates a filter for encrypting data into the
          eexec encrypted format described in the
          Adobe Type 1 Font Format documentation.  The
          seed_integer must be 55665 for proper operation.
          This filter produces binary output and does not
          include the initial 4 (or lenIV) garbage bytes.

    <source> <seed_integer> /eexecDecode filter <file>
    <source> <dict> /eexecDecode filter <file>
        Creates a filter for decrypting data that has been
          encrypted using eexec encryption as described in the
          Adobe Type 1 Font Format documentation.  The
          seed_integer must be 55665 for proper operation.
          Recognized dictionary keys are:
            seed <16-bit integer> (required)
            lenIV <non-negative integer> (default=4)

    <source> /PCXDecode filter <file>
        Creates a filter that decodes data in the run-length
          encoding used in the PCX graphics file format.

    <source> <hex_boolean> /PFBDecode filter <file>
        Creates a filter that decodes data in .PFB format, the
          usual semi-binary representation for Type 1 font files
          on IBM PC and compatible systems.  If hex_boolean is true,
          binary packets are converted to hex; if false, binary
          packets are not converted.

    <target> <dict> /PixelDifferenceEncode filter <file>
    <source> <dict> /PixelDifferenceDecode filter <file>
        Implements the Predictor=2 pixel-differencing option of the
          LZW filters.  Recognized keys are:
            Colors <integer> 1..4 (default=1)
            BitsPerComponent <integer> 1,2,4,8 (default=8)
            Columns <integer> >= 0 (required)
          See the Adobe "Portable Document Format Reference Manual"
          for details.

    <target> <dict> /PNGPredictorEncode filter <file>
    <source> <dict> /PNGPredictorDecode filter <file>
        Implements the "filter" algorithms of the PNG graphics
          format.  Recognized keys are:
            Colors <integer> 1..16 (default=1)
            BitsPerComponent <integer> 1,2,4,8,16 (default=8)
            Columns <integer> >= 0 (default=1)
            Predictor <integer> 10..15 (default=15)
        The Predictor is the PNG algorithm number + 10 for the
          Encoding filter; the Decoding filter ignores Predictor.
          15 means the encoder attempts to optimize the choice of
          algorithm.  For more details, see libpng.mak, which has
          a pointer to the PNG specification.

    <target> /TBCPEncode filter <file>
    <source> /TBCPDecode filter <file>
        Create filters that implement the Adobe Tagged Binary
          Communications Protocol.  See Adobe documentation for
          details.

    <target> /zlibEncode filter <file>
    <source> /zlibDecode filter <file>
        Creates filters that use the zlib data compression method
          (the same method used by the gzip application).
          This filter is only available if the fzlib feature was
          selected when Ghostscript was compiled and linked.

Various versions of Ghostscript may also support other non-standard filters
for experimental purposes.  The current version includes the following
non-standard filters, which are not documented further.  No guarantee is
made that these filters will exist in compatible form, or at all, in future
versions.

    <target/source> <string> ByteTranslateEncode/Decode
    <target> <int> BigStringEncode
    <target/source> <dict> BoundedHuffmanEncode/Decode
        FirstBitLowOrder <bool> false
        MaxCodeLength <int> 16
        EndOfData <bool> true
        EncodeZeroRuns <int> 256
        Tables <int_array>
    <target/source> <dict> BWBlockSortEncode/Decode
        BlockSize <int> 16384
    <target/source> MoveToFrontEncode/Decode

Ghostscript also supports additional keys in the optional dictionary
operands for some filters.  For the LZWDecode filter:

    InitialCodeLength <integer>
        An integer between 2 and 11 specifying the initial number
of data bits per code.  Note that the actual initial code length is 1
greater than this, to allow for the reset and end-of-data code values.
Default value: 8.

    FirstBitLowOrder <boolean>
        If true, codes appear with their low-order bit first.
Default value: false.

    BlockData <boolean>
        If true, the data is broken into blocks in the manner
specified for the GIF file format.  Default value: false.

For the CCITTFaxEncode and CCITTFaxDecode filters:

    DecodedByteAlign <integer>
        An integer N with the value 1, 2, 4, 8, or 16, specifying
that decoded data scan lines are always a multiple of N bytes.  The encoding
filter skips data in each scan line from Columns to the next multiple of N
bytes; the decoding filter pads each scan line to a multiple of N bytes.
Default value: 1.

Virtual memory operators
------------------------

    <save> .forgetsave -
        Cancels the effect of a save -- makes it as though the
          save never happened.

Miscellaneous operators
-----------------------

    <string> getenv <string> true
    <string> getenv false
        Looks up a name in the shell environment.  If the name is
          found, returns the corresponding value and true; if the
          name is not found, returns false.

    <name> <array> .makeoperator <operator>
        Constructs and returns a new operator that is actually the
          given procedure in disguise.  The name is only used for
          printing.  The operator has the executable attribute.
        Operators defined in this way do one other thing besides
          running the procedure: if an error occurs during the
          execution of the procedure, and there has been no net
          reduction in operand or dictionary stack depth, the
          operand or dictionary stack pointer respectively is reset
          to its position at the beginning of the procedure.

    <string> <boolean> .setdebug -
        If the Ghostscript interpreter was built with the DEBUG
          flag set, sets or resets any subset of the debugging
          flags normally controlled by -Z in the command line.
          Has no effect otherwise.

    - .oserrno <errno>
        Returns the error code for the most recent OS error.

    - .oserror <string>
        Returns the error string for the most recent OS error.

Device operators
----------------

    <device> copydevice <device>
        Copies a device.  The copy is writable and installable.

    <index> .getdevice <device>
        Returns a device from the set of devices known to the
          system.  The first device, which is default, is numbered
          0.  If the index is out of range, causes a rangecheck
          error.  This device is actually a prototype, not a
          directly usable device, and is marked read-only;
          it cannot have its parameters changed or be installed as
          the current device.

    <matrix> <width> <height> <palette> makeimagedevice <device>
        Makes a new device that accumulates an image in memory.
          matrix is the initial transformation matrix: it must be
          orthogonal (i.e., [a 0 0 b x y] or [0 a b 0 x y]).
          palette is a string of 2^N or 3*2^N elements, specifying
          how the 2^N possible pixel values will be interpreted.
          Each element is interpreted as a gray value, or as RGB
          values, multiplied by 255.  For example, if you want
          a monochrome image for which 0=white and 1=black, the
          palette should be <ff 00>; if you want a 3-bit deep
          image with just the primary colors and their complements
          (ignoring the fact that 3-bit images are not supported),
          the palette might be <000000 0000ff 00ff00 00ffff
          ff0000 ff00ff ffff00 ffffff>.  At present, the palette
          must contain exactly 2, 4, 16, or 256 entries,
          and must contain an entry for black and an entry
          for white; if it contains any entries that aren't black,
          white, or gray, it must contain at least the six primary
          colors (red, green, blue, and their complements cyan,
          magenta, and yellow); aside from this, its contents are
          arbitrary.
        Alternatively, palette can be 16, 24, 32, or null
          (equivalent to 24).  These are interpreted as:
            16 = 5R, 6G, 5B bits;
            24 = 8R, 8G, 8B bits;
            32 = 8C, 8M, 8Y, 8K bits.
        Note that one can also make an image device (with the same
          palette as an existing image device) by copying a device
          using the copydevice operator.

    <matrix> <width> <height> <palette> <word?> makewordimagedevice
        <device>
        Makes an image device as described above.  word? is a
          Boolean value indicating whether the data should be stored
          in a word-oriented format internally.  No ordinary
          PostScript programs should use this operator.

    <device> <index> <string> copyscanlines <substring>
        Copies one or more scan lines from an image device into a
          string, starting at a given scan line in the image.
          The data is in the same format as for the image
          operator.  Error if the device is not an image device or
          if the string is too small to hold at least one complete
          scan line.  Always copies an integral number of scan
          lines.

    <device> setdevice -
        Sets the current device to the specified device.  Also
          resets the transformation and clipping path to the
          initial values for the device.  Signals an invalidaccess
          error if the device is a prototype.

    - currentdevice <device>
        Gets the current device from the graphics state.

    <device> getdeviceprops <mark> <name1> <value1> ... <namen> <valuen>
        Gets the properties of a device.  See the "device
          parameters" section below for details.

    <mark> <name1> <value1> ... <namen> <valuen> <device>
        putdeviceprops <device>
        Sets properties of a device.  May cause undefined,
          typecheck, rangecheck, or limitcheck errors.

    - flushpage -
        On displays, flushes any buffered output, so that it
          is guaranteed to show up on the screen; on printers,
          has no effect.

Device parameters
=================

Ghostscript supports the concept of device parameters for all devices, not
just page devices.  (For non-page devices, these are accessible through
get/putdeviceprops, as indicated above.)

Here are the currently defined parameters for all devices:

    BitsPerPixel <integer> (usually read-only)
        Number of bits per pixel.

    .HWMargins [<4 floats>]
        Size of non-imageable regions around the edges
          of the page, in 1/72" units.

    HWSize [<integer> <integer>]
        X and Y size in pixels.

    Name <string> (read-only)
        The device name.  Currently, same as OutputDevice.

    Colors, GrayValues, RedValues, GreenValues,
      BlueValues, ColorValues (usually read-only)
        As for the 'deviceinfo' operator of Display PostScript.
          Red/Green/Blue/ColorValues are only defined if Colors > 1.

    TextAlphaBits, GraphicsAlphaBits (usually read-only)
        The number of bits of anti-aliasing
          information for text or graphics
          respectively.  Legal values are 1 (no
          anti-aliasing), 2, or 4; the default value
          for most devices is 1.

In addition, the following are defined per Adobe's documentation for the
setpagedevice operator:
    Duplex (if supported)
    HWResolution
    ImagingBBox
    Margins
    NumCopies (for printers only)
    Orientation (if supported)
    OutputDevice
    PageOffset (write-only)
    PageSize
    ProcessColorModel
Some devices may only allow certain values for HWResolution and PageSize.
The null device ignores attempts to set PageSize; its size is always [0 0].

For printers, the following are also defined:

    BufferSpace <integer>
        Buffer space for band lists, if the bitmap
          is too big to fit in RAM.

    MaxBitmap <integer>
        Maximum space for a full bitmap in RAM.

    OutputFile <string>
        () means send to printer directly,
          otherwise specifies the file name for
          output; a %d is replaced by the page #;
          on Unix systems, (|command) writes to a pipe

    OpenOutputFile <boolean>
        If true, open the device's output file
          when the device is opened, rather than
          waiting until the first page is ready to
          print.

    PageCount <integer> (read-only)
        Counts the number of pages printed on the device.

The following parameters are for use only by very specialized applications
that separate band construction from band rasterization.  Improper use may
cause unpredictable errors.  In particular, if you only want to allocate
more memory for banding, to increase band size and improve performance, use
the BufferSpace parameter, not BandBufferSpace.

    BandHeight <integer>
        The height of bands when banding.  0 means use the largest
          band height that will fit within the BandBufferSpace
          (or BufferSpace, if BandBufferSpace is not specified).

    BandWidth <integer>
        The width of bands in the rasterizing pass, in pixels.  0
          means use the actual page width.

    BandBufferSpace <integer>
        The size of the band buffer in the rasterizing pass, in
          bytes.  0 means use the same buffer size as for the
          interpretation pass.

In addition, Ghostscript supports the following parameter for
setpagedevice/currentpagedevice that is *not* a device parameter per se:

    ViewerPreProcess    procedure
        Specifies a procedure to be applied to the page device
          dictionary before any other processing is done.  The
          procedure may not alter the dictionary, but it may return
          a modified copy.  This "hook" is provided for use by
          viewing programs such as GSview.

Miscellaneous additions
=======================

run can take either a string or a file as its argument.  In the latter
case, it just runs the file, closing it at the end, and trapping errors
just as for the string case.

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