Ghostscript and the PostScript language
Table of contents
Ghostscript’s capabilities in relation to PostScript
The Ghostscript interpreter, except as noted below, is intended to execute properly any source program written in the (LanguageLevel 3) PostScript language as defined in the PostScript Language Reference, Third Edition (ISBN 0-201-37922-8) published by Addison-Wesley in mid-1999. 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 (see the documentation on building Ghostscript for how to configure it) 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 available only if the color, dps, or level2 feature was selected when Ghostscript was built.
- 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 available only if the dps feature or the level2 feature was selected when Ghostscript was built.
- 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 available only if the compfont feature or the level2 feature was selected when Ghostscript was built.
- The ability to load TrueType fonts and to handle PostScript Type 42 (encapsulated TrueType) fonts. These facilities are available only if the ttfont feature was selected when Ghostscript was built.
- The PostScript Level 2 “filter” facilities except the DCTEncode and DCTDecode filters. These facilities are available only if the filter, dps, or level2 feature was selected when Ghostscript was built.
- The PostScript Level 2 DCTEncode and DCTDecode filters. These facilities are available only if the dct or level2 feature was selected when Ghostscript was built.
- 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 available only if the level2 feature was selected when Ghostscript was built.
- All PostScript LanguageLevel 3 operators and facilities listed in the Third Edition, except as noted below. These facilities are available only if the psl3 feature was selected when Ghostscript was built.
- The ability to recognize DOS EPSF files and process only the PostScript part, ignoring bitmap previews or other information. This facility is available only if the epsf feature was selected when Ghostscript was built.
Ghostscript currently does not implement the following PostScript LanguageLevel 3 facilities:
- Native Separation and DeviceN color spaces — the alternate space is always used.
- Settable ProcessColorModel for page devices, except for a very few special devices.
- IODevices other than %stdin, %stdout, %stderr, %lineedit, %statementedit, %os%, and (if configured) %pipe%.
Ghostscript can also interpret files in the Portable Document Format (PDF) 1.3 format defined in the Portable Document Format Reference Manual Version 1.3 of March 11, 1999, distributed by Adobe Systems Incorporated, except as noted below. This facility is available only if the pdf feature was selected when Ghostscript was built.
Ghostscript currently does not implement the following PDF 1.3 facilities:
- Native Separation and DeviceN color spaces, as noted above for PostScript.
- Native ICCBased color spaces — these too always use the alternate space.
Ghostscript also includes a number of additional operators defined below that are not in the PostScript language defined by Adobe.
The implementation limits show here correspond to those in Tables B.1 and B.2 of the Second and Third Editions, which describe the quantities fully. Where Ghostscript’s limits are different from those of Adobe’s implementations (as shown in the Third Edition), Adobe’s limits are also shown.
* The limit on the length of a file name is 128 characters if the name starts with a %. % IODevice designation, or 124 characters if it does not.
Typical memory limits in LanguageLevel 1
for1-, 2-, 4-, or 8-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.
Additional operators in Ghostscript
Graphics and text operators
Ghostscript provides a set of operators for implementing the transparency and compositing facilities of PDF 1.4. These are defined only if the transpar option was selected when Ghostscript was built. We do not attempt to explain the underlying graphics model here: for details, see Adobe Technical Note #5407, “Transparency in PDF”. Note, however, that Ghostscript’s model generalizes that of PDF 1.4 in that Ghostscript maintains separate alpha and mask values for opacity and shape, rather than a single value with a Boolean that says whether it represents opacity or shape. EVERYTHING IN THIS SECTION IS SUBJECT TO CHANGE.
Graphics state operators
Rendering stack operators
The interpreter state is extended to include a (per-context) rendering stack for handling transparency groups and masks (generically, “layers”). Groups accumulate a full value for each pixel (paint plus transparency); masks accumulate only a coverage value. Layers must be properly nested, i.e., the ‘end’ or ‘discard’ operator must match the corresponding ‘begin’ operator.
Beginning and ending layers must nest properly with respect to save and restore: save and restore do not save and restore the layer stack. Currently, layers are not required to nest with respect to gsave and grestore, except that the device that is current in the graphics state when ending a layer must be the same as the device that was current when beginning the layer. THIS AREA IS SUBJECT TO CHANGE. <paramdict> <llx> <lly> <urx> <ury> .begintransparencygroup - Begins a new transparency group. The ll/ur coordinates are the bounding box of the group in the current user coordinate system. paramdict has the following keys: /Isolated (optional) Boolean; default value = false. /Knockout (optional) Boolean; default value = false. - .discardtransparencygroup - Ends and discards the current transparency group. - .endtransparencygroup - Ends the current transparency group, compositing the group being ended onto the group that now becomes current. <paramdict> <llx> <lly> <urx> <ury> .begintransparencymask - Begins a new transparency mask. The ll/ur coordinates are the bounding box of the mask in the current user coordinate system. paramdict has the following keys: /Subtype (required) Name, either /Alpha or /Luminosity. /Background (optional) Array of number. /TransferFunction (optional) Function object (produced by applying .buildfunction to a Function dictionary). - .discardtransparencymask - Ends and discards the current transparency mask. <masknum> .endtransparencymask - Ends the current transparency mask, installing it as the current opacity (masknum = 0) or shape (masknum = 1) mask in the graphics state. <masknum> .inittransparencymask - Resets the current opacity (masknum = 0) or shape (masknum = 1) mask to an infinite mask with alpha = 1 everywhere.
The transparency extension defines a new ImageType 103, similar to ImageType 3 with the following differences:
- The required MaskDict is replaced by two optional dictionaries, OpacityMaskDict and ShapeMaskDict. If present, these dictionaries must have a BitsPerComponent entry, whose value may be greater than 1. Note that in contrast to ImageType 3, where any non-zero chunky mask value is equivalent to 1, ImageType 103 simply takes the low-order bits of chunky mask values.
- A Matte entry may be present in one or both mask dictionaries, indicating premultiplication of the data values. If both MaskDicts have a Matte entry and the values of the two Matte entries are different, a rangecheck error occurs.
- InterleaveType appears in the MaskDicts, not the DataDict, because each mask has its own InterleaveType. InterleaveType 2 (interlaced scan lines) is not supported.
Other graphics state operators
Ghostscript supports an experimental extension of the PostScript imaging model to include RasterOp and some related facilities. This extension is available only 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.
This creates a path whose pathbbox is the bbox of the string.
- If <bool> is false, the equivalent of: p1x p1y moveto
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 valid only in the context of a show operator — that is, 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 the initialization file 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.
String and name operators
It is intended that in some future version of Ghostscript, files opened with .tempfile will be closed and deleted automatically when Ghostscript exits; however, this is not currently the case (version 7.01). <file> <integer> .unread - Pushes back the last-read character onto the front of the file. If the file is open only 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. This operator is now deprecated: use .peekstring in new code.
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 supported only on operating systems that provide popen (primarily Unix systems, and not all of those).
Virtual memory operators
except that it doesn’t actually create the array. <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 operating system error. - .oserrorstring <string> Returns the error string for the most recent operating system error.
Alternatively, palette can be 16, 24, 32, or null (equivalent to 24). These are interpreted as:
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. It is an 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 section on device parameters 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.
In its usual configuration, Ghostscript supports all the standard PostScript LanguageLevel 3 filters, both encoding and decoding, except that it does not currently support:
- the EarlyChange key in the LZWEncode filter.
Ghostscript also supports additional keys in the optional dictionary operands for some filters. For the LZWDecode filter: InitialCodeLength <integer> (default 8) 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. FirstBitLowOrder <boolean> (default false) If true, codes appear with their low-order bit first. BlockData <boolean> (default false) If true, the data is broken into blocks in the manner specified for the GIF file format.
For the CCITTFaxEncode and CCITTFaxDecode filters: DecodedByteAlign <integer> (default 1) 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.
In addition to the standard PostScript LanguageLevel 3 filters, Ghostscript supports the following non-standard filters. Many of these filters are used internally to implement standard filters or facilities; they are almost certain to remain, in their present form or a backward-compatible one, in future Ghostscript releases. <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 encrypted format described in the Adobe Type 1 Font Format documentation. The seed_integer must be 55665 for the eexec section of a font, or 4330 for a CharString. Note that for the eexec section of a font, 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 encrypted as described in the Adobe Type 1 Font Format documentation. The seed_integer must be 55665 or 4330 as described just above. Recognized dictionary keys are: seed <16-bit integer> (required)
lenIV <non-negative integer> (default=4) <target> /MD5Encode filter <file> Creates a filter that produces the 16-byte MD5 digest of the input. Note that no output is produced until the filter is closed. <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 to 4, default=1)
BitsPerComponent <integer> (1, 2, 4, or 8, default=8)
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 the PNG specification http://www.w3.org/TR/WD-png-960128.html <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 data compression method variously known as ‘zlib’ (the name of a popular library that implements it), ‘Deflate’ (as in RFC 1951, which is a detailed specification for the method), ‘gzip’ (the name of a popular compression application that uses it), or ‘Flate’ (Adobe’s name). Note that the PostScript Flate filters are actually a combination of this filter with an optional predictor filter.
Some versions of Ghostscript may also support other non-standard filters for experimental purposes. The current version includes the following such filters, which are not documented further. No code should assume that these filters will exist in compatible form, or at all, in future versions. <target/source> <string> ByteTranslateEncode/Decode filter <file> string must be a string of exactly 256 bytes. Creates a filter that converts each input byte b to string[b]. Note that the Encode and Decode filters operate identically: the client must provide a string for the Decode filter that is the inverse mapping of the string for the Encode filter. <target/source> <dict> BoundedHuffmanEncode/Decode filter <file> These filters encode and decode data using Huffman codes. Since these filters aren’t used anywhere, we don’t document them further, except to note the recognized dictionary keys, which must be set identically for encoding and decoding: FirstBitLowOrder <bool> (default=false)
Tables <int_array> <target/source> <dict> BWBlockSortEncode/Decode filter <file> This filter implements the Burroughs-Wheeler block sorting compression method, which we’ve heard is also used in the popular bzip2 compression application. See http://sources.redhat.com/bzip2/ for more information. The only recognized dictionary key is: BlockSize <integer> (default=16384) <target/source> MoveToFrontEncode/Decode filter <file> The Encode filter starts by initializing an internal 256-byte array a to the values 0 .. 255. This array will always hold a permutation of these values. Then for each input byte b, the filter outputs the index i such that a[i] = b, and moves that element to the front (element 0) of a, moving elements 0 .. i-1 to positions 1 .. i. The Decode filter inverts this process.
In addition, the following are defined per Adobe’s documentation for the setpagedevice operator: Duplex (if supported)
NumCopies (for printers only)
ProcessColorModel (usually read-only)
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 these are also defined: BufferSpace <integer> Buffer space for band lists, if the bitmap is too big to fit in memory. MaxBitmap <integer> Maximum space for a full bitmap in memory. OutputFile <string> An empty string means “send to printer directly”, otherwise specifies the file name for output; %d is replaced by the page number; on Unix systems %pipe%command writes to a pipe. (|command also writes to a pipe, but is now deprecated.) 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 and 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.
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This file is part of AFPL Ghostscript. See the Aladdin Free Public License (the “License”) for full details of the terms of using, copying, modifying, and redistributing AFPL Ghostscript.
Ghostscript version 7.03, 20 October 2001