   Copyright (C) 1989, 1995, 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, 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.

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

Ghostscript also includes the optional ability to interpret files in the PDF
1.1 format defined in the June 1993 printing of the Portable Document Format
Reference manual published by Addison-Wesley (ISBN 0-201-62628-4) as amended
by Adobe Technical Note #5156, except for the encryption features.  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.

Ghostscript-specific additions
==============================

Miscellaneous
-------------

^D and ^Z are self-delimiting tokens, like [ and ].  They are initially
defined as empty procedures so that they will be ignored in the input
stream.

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.

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).

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

	<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 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.

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 garbage bytes.

	<source> <seed_integer> /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.

	<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> /zlibEncode 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.

	<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 all the properties of a device.  Currently defined
		  names and values for all devices are:
			BitsPerPixel <integer>
				Usually read-only.  Number of bits per pixel.
			.HWMargins [<4 floats>]
				Size of non-imageable regions around edges
				  of 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
				As for the 'deviceinfo' operator of
				  Display PostScript.
			TextAlphaBits, GraphicsAlphaBits
				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.  Read-only for
				  almost all devices.
		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].
		Red/Green/Blue/ColorValues are only defined if Colors > 1.
		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.

	<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.

Ghostscript supports the following page device parameters in addition to the
ones documented by Adobe and the ones described under getdeviceprops above:

	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.

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
			- If <bool> is false, the equivalent of:
				p1x p1y moveto
				p3x p3y lineto
		  In either case, this creates a 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.

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

PostScript is a trademark of Adobe Systems, Incorporated.
