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= PostScript =
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Introduction
======================================================================
PostScript (PS) is a page description language and dynamically typed,
stack-based programming language. It is most commonly used in the
electronic publishing and desktop publishing realm, but as a Turing
complete programming language, it can be used for many other purposes
as well. PostScript was created at Adobe Systems by John Warnock,
Charles Geschke, Doug Brotz, Ed Taft and Bill Paxton from 1982 to
1984. The most recent version, PostScript 3, was released in
1997.
History
======================================================================
The concepts of the PostScript language were seeded in 1976 by John
Gaffney at Evans & Sutherland, a computer graphics company. At
that time, Gaffney and John Warnock were developing an interpreter for
a large three-dimensional graphics database of New York Harbor.
Concurrently, researchers at Xerox PARC had developed the first laser
printer and had recognized the need for a standard means of defining
page images. In 1975-76 Bob Sproull and William Newman developed the
Press format, which was eventually used in the Xerox Star system to
drive laser printers. But Press, a data format rather than a language,
lacked flexibility, and PARC mounted the Interpress effort to create a
successor.
In 1978, John Gaffney and Martin Newell then at Xerox PARC wrote J
& M or JaM (for "John and Martin") which was used for VLSI design
and the investigation of type and graphics printing. This work later
evolved and expanded into the Interpress language.
Warnock left with Chuck Geschke and founded Adobe Systems in December
1982. They, together with Doug Brotz, Ed Taft and Bill Paxton created
a simpler language, similar to Interpress, called PostScript, which
went on the market in 1984.
Meanwhile, in the spring of 1983, Steve Jobs came to visit Adobe and
was dazzled by PostScript's potential, especially for the new
Macintosh computer he was developing at Apple. To John Sculley's
frustration, Jobs licensed the PostScript technology from Adobe by
offering a $1.5 million advance against PostScript royalties, plus
$2.5 million in exchange for 20 percent of Adobe shares. During a
series of meetings in 1983, Jobs also repeatedly offered for Apple to
buy Adobe outright, but the founders kept turning him down. In
December 1983, the two companies finally signed off on the PostScript
licensing deal, and Adobe had to shift focus immediately from
high-end, high-resolution printing devices to the consumer-oriented
Apple LaserWriter laser printer.
At that time, the 300-dpi Canon laser printing engine to be used in
LaserWriters was seen as good enough only for proof printing (i.e.,
for crude rough drafts of material whose final drafts would be sent to
professional high-resolution devices), but Jobs presented Adobe with
the challenge of making PostScript render high-quality output to such
a low-resolution device (which for most consumers would be their only
printing device). In response, Warnock and Brotz solved the so-called
"appearance problem" of making the stem width of letters scale
properly so that they look good at all resolutions. Their
breakthrough was so important that Adobe never patented the
technology, in order to keep its details concealed as a trade secret.
Many years later, Warnock revealed the trade secret in a 2010 lecture,
and in 2022, Adobe publicly released an early version of PostScript's
original source code from February 1984. Paxton worked on several
other related improvements, such as font hinting. This work occurred
after February 1984, meaning that Paxton's improvements were not
included in the source code release and are still protected as trade
secrets. Adobe was also responsible for porting PostScript to the
Canon's Motorola 68000 chip.
Apple and Adobe announced the LaserWriter at Apple's annual
stockholder meeting on January 23, 1985. It was the first printer to
ship with PostScript, sparking the desktop publishing (DTP) revolution
in the mid-1980s. The original PostScript royalty was five percent of
the list price for each laser printer sold, which was $350 of the
original LaserWriter list price of $6,995, and such royalties provided
nearly all of Adobe's income during its early years. (Apple later
renegotiated the contract to pay a licensing fee based on volume of
printers shipped.) The combination of technical merits and widespread
availability made PostScript the language of choice for graphical
output for printing applications. An interpreter (sometimes referred
to as a RIP for Raster Image Processor) for the PostScript language
was a common component of laser printers during the 1980s and 1990s.
However, the cost of implementation was high; computers output raw PS
code that would be interpreted by the printer into a raster image at
the printer's natural resolution. This required high-performance
microprocessors and ample memory. The LaserWriter used a 12 MHz
Motorola 68000, making it faster than any of the Macintosh computers
to which it was attached. When the laser printer engines themselves
cost over a thousand dollars, the added cost of PS was marginal. But,
as printer mechanisms fell in price, the cost of implementing PS
became too great a fraction of overall printer cost. In addition, with
desktop computers becoming more powerful during the 1990s than their
attached printers, it no longer made sense to offload the
rasterization work onto the resource-constrained printer. By 2001, few
low-end printer models came with onboard support for PostScript,
largely due to growing competition from much cheaper non-PostScript
inkjet printers, and new software-based methods to render PostScript
images on computers, making them suitable for any printer. PDF, a
descendant of PostScript, provides one such method, and has largely
replaced PostScript as the 'de facto' standard for electronic document
distribution.
On high-end printers, PostScript processors remain common, and their
use can dramatically reduce the CPU work involved in printing
documents, transferring the work of rendering PostScript images from
the computer to the printer.
PostScript Level 1
====================
The first version of the PostScript language was released to the
market in 1984. The qualifier 'Level 1' was added when 'Level 2' was
introduced.
PostScript Level 2
====================
PostScript Level 2 was introduced in 1991, and included several
improvements: improved speed and reliability, support for in-Raster
Image Processing (RIP) separations, image decompression (for example,
JPEG images could be rendered by a PostScript program), support for
composite fonts, and the form mechanism for caching reusable content.
PostScript 3
==============
PostScript 3 (Adobe dropped the "level" terminology in favor of simple
versioning) came at the end of 1997, and along with many new
dictionary-based versions of older operators, introduced better color
handling and new filters (which allow in-program
compression/decompression, program chunking, and advanced error
handling).
PostScript 3 was significant in terms of replacing the existing
proprietary color electronic prepress systems, then widely used for
magazine production, through the introduction of smooth shading
operations with up to 4096 shades of grey (rather than the 256
available in PostScript Level 2), as well as DeviceN, a color space
that allowed the addition of additional ink colors (called spot
colors) into composite color pages.
Before PostScript
===================
Prior to the introduction of Interpress and PostScript, printers were
designed to print character output given the text—typically in
ASCII—as input. There were a number of technologies for this task, but
most shared the property that the glyphs were physically difficult to
change, as they were stamped onto typewriter keys, bands of metal, or
optical plates.
This changed to some degree with the increasing popularity of dot
matrix printers. The characters on these systems were drawn as a
series of dots, as defined by a font table inside the printer. As they
grew in sophistication, dot matrix printers started including several
built-in fonts from which the user could select, and some models
allowed users to upload their own custom glyphs into the printer.
Dot matrix printers also introduced the ability to print raster
graphics. The graphics were interpreted by the computer and sent as a
series of dots to the printer using a series of escape sequences.
These printer control languages varied from printer to printer,
requiring program authors to create numerous drivers.
Vector graphics printing was left to special-purpose devices, called
plotters. Almost all plotters shared a common command language, HPGL,
but were of limited use for anything other than printing graphics. In
addition, they tended to be expensive and slow, and thus rare.
PostScript printing
=====================
Laser printers combine the best features of both printers and
plotters. Like plotters, laser printers offer high-quality line art,
and like dot-matrix printers, they are able to generate pages of text
and raster graphics. Unlike either printers or plotters, a laser
printer makes it possible to position high-quality graphics and text
on the same page. PostScript made it possible to fully exploit these
characteristics by offering a single control language that could be
used on any brand of printer.
PostScript went beyond the typical printer control language and was a
complete programming language of its own. Many applications can
transform a document into a PostScript program, the execution of which
results in the original document. This program can be sent to an
interpreter in a printer, which results in a printed document, or to
one inside another application, which will display the document
on-screen. Since the document-program is the same regardless of its
destination, it is called 'device-independent'.
PostScript is noteworthy for implementing on-the-fly rasterization in
which everything, even text, is specified in terms of straight lines
and cubic Bézier curves (previously found only in CAD applications),
which allows arbitrary scaling, rotating and other transformations.
When the PostScript program is interpreted, the interpreter converts
these instructions into the dots needed to form the output. For this
reason, PostScript interpreters are occasionally called PostScript
raster image processors, or RIPs.
Font handling
===============
Almost as complex as PostScript itself is its handling of fonts. The
font system uses the PS graphics primitives to draw glyphs as curves,
which can then be rendered at any resolution. A number of typographic
issues had to be considered with this approach.
One issue is that fonts do not scale linearly at small sizes and
features of the glyphs will become proportionally too large or small
and start to look displeasing. PostScript avoided this problem with
the inclusion of font hinting, in which additional information is
provided in horizontal or vertical bands to help identify the features
in each letter that are important for the rasterizer to maintain. The
result was significantly better-looking fonts even at low resolution.
It had formerly been believed that hand-tuned bitmap fonts were
required for this task.
At the time, the technology for including these hints in fonts was
carefully guarded, and the hinted fonts were compressed and encrypted
into what Adobe called a 'Type 1 Font' (also known as 'PostScript Type
1 Font', 'PS1', 'T1' or 'Adobe Type 1'). Type 1 was effectively a
simplification of the PS system to store outline information only, as
opposed to being a complete language (PDF is similar in this regard).
Adobe would then sell licenses to the Type 1 technology to those
wanting to add hints to their own fonts. Those who did not license the
technology were left with the 'Type 3 Font' (also known as 'PostScript
Type 3 Font', 'PS3' or 'T3'). Type 3 fonts allowed for all the
sophistication of the PostScript language, but without the
standardized approach to hinting.
The Type 2 font format was designed to be used with Compact Font
Format (CFF) charstrings, and was implemented to reduce the overall
font file size. The CFF/Type2 format later became the basis for
handling PostScript outlines in OpenType fonts.
The CID-keyed font format was also designed, to solve the problems in
the OCF/Type 0 fonts, for addressing the complex Asian-language (CJK)
encoding and very large character set issues. The CID-keyed font
format can be used with the Type 1 font format for standard CID-keyed
fonts, or Type 2 for CID-keyed OpenType fonts.
To compete with Adobe's system, Apple designed their own system,
TrueType, around 1991. Immediately following the announcement of
TrueType, Adobe published the specification for the Type 1 font
format. Retail tools such as Altsys Fontographer (acquired by
Macromedia in January 1995, owned by FontLab since May 2005) added the
ability to create Type 1 fonts. Since then, many free Type 1 fonts
have been released; for instance, the fonts used with the TeX
typesetting system are available in this format.
In the early 1990s, there were several other systems for storing
outline-based fonts, developed by Bitstream and Metafont for instance,
but none included a general-purpose printing solution and they were
therefore not widely used.
In the late 1990s, Adobe joined Microsoft in developing OpenType,
essentially a functional superset of the Type 1 and TrueType formats.
When printed to a PostScript output device, the unneeded parts of the
OpenType font are omitted, and what is sent to the device by the
driver is the same as it would be for a TrueType or Type 1 font,
depending on which kind of outlines were present in the OpenType font.
Adobe supported Type 1 fonts in its products until January 2023, when
it fully removed support in favor of OpenType fonts.
Other implementations
=======================
Both computer printers and high-end typesetters such as Linotronic
implemented PostScript. In the 1980s, Adobe drew most of its revenue
from the licensing fees for implementations, known as a raster image
processor or 'RIP'. As a number of new RISC-based platforms became
available in the mid-1980s, some found Adobe's support of the new
machines to be lacking.
This and issues of cost led to third-party implementations of
PostScript becoming common, particularly in low-cost printers (where
the licensing fee was the sticking point) or in high-end typesetting
equipment (where the quest for speed demanded support for new
platforms faster than Adobe could provide). At one point, Microsoft
licensed to Apple a PostScript-compatible interpreter it had bought
called TrueImage, and Apple licensed to Microsoft its new font format,
TrueType. Apple ended up reaching an accord with Adobe and licensed
genuine PostScript for its printers, but TrueType became the standard
outline font technology for both Windows and the Macintosh.
Today, third-party PostScript-compatible interpreters are widely used
in printers and multifunction peripherals (MFPs). For example, CSR
plc's IPS PS3 interpreter, formerly known as PhoenixPage, is standard
in many printers and MFPs, including those developed by
Hewlett-Packard and sold under the LaserJet and Color LaserJet lines.
Other third-party PostScript solutions used by print and MFP
manufacturers include Jaws and the Harlequin RIP, both by Global
Graphics. A free software version, with several other applications,
is Ghostscript. Several compatible interpreters are listed on the
Undocumented Printing Wiki.
Some basic, inexpensive laser printers do not support PostScript,
instead coming with drivers that simply rasterize the platform's
native graphics formats rather than converting them to PostScript
first. When PostScript support is needed for such a printer,
Ghostscript can be used. There are also a number of commercial
PostScript interpreters, such as TeleType Co.'s 'T-Script' or
Brother's 'BR-Script3'.
Use as a display system
======================================================================
PostScript became commercially successful due to the introduction of
the graphical user interface (GUI), allowing designers to directly lay
out pages for eventual output on laser printers. However, the GUIs'
own graphics systems were generally much less sophisticated than
PostScript; Apple's QuickDraw, for instance, supported only basic
lines and arcs, not the complex B-splines and advanced region filling
options of PostScript. In order to take full advantage of PostScript
printing, applications on the computers had to re-implement those
features using the host platform's own graphics system. This led to
numerous issues where the on-screen layout would not exactly match the
printed output, due to differences in the implementation of these
features.
As computer power grew, it became possible to host the PS system in
the computer rather than the printer. This led to the natural
evolution of PS from a printing system to one that could also be used
as the host's own graphics language. There were numerous advantages to
this approach; not only did it help eliminate the possibility of
different output on screen and printer, but it also provided a
powerful graphics system for the computer, and allowed the printers to
be "dumb" at a time when the cost of the laser engines was falling. In
a production setting, using PostScript as a display system meant that
the host computer could render low-resolution to the screen, higher
resolution to the printer, or simply send the PS code to a smart
printer for offboard printing.
However, PostScript was written with printing in mind, and had
numerous features that made it unsuitable for direct use in an
interactive display system. In particular, PS was based on the idea of
collecting up PS commands until the showpage command was seen, at
which point all of the commands read up to that point were interpreted
and output. In an interactive system, this was clearly not
appropriate, nor did PS have any sort of interactivity built in; for
example, supporting hit detection for mouse interactivity obviously
did not apply when PS was being used on a printer.
When Steve Jobs left Apple and started NeXT, he pitched Adobe on the
idea of using PS as the display system for his new workstation
computers. The result was Display PostScript, or DPS. DPS added basic
functionality to improve performance by changing many string lookups
into 32 bit integers, adding support for direct output with every
command, and adding functions to allow the GUI to inspect the diagram.
Additionally, a set of "bindings" was provided to allow PS code to be
called directly from the C programming language. NeXT used these
bindings in their NeXTStep system to provide an object oriented
graphics system. Although DPS was written in conjunction with NeXT,
Adobe sold it commercially and it was a common feature of most Unix
workstations in the 1990s.
Sun Microsystems took another approach, creating NeWS. Instead of
DPS's concept of allowing PS to interact with C programs, NeWS instead
extended PS into a language suitable for running the entire GUI of a
computer. Sun added a number of new commands for timers, mouse
control, interrupts and other systems needed for interactivity, and
added data structures and language elements to allow it to be
completely object oriented internally. A complete GUI, three in fact,
were written in NeWS and provided for a time on their workstations.
However, the ongoing efforts to standardize the X11 system led to its
introduction and widespread use on Sun systems, and NeWS never became
widely used.
Portable Document Format
======================================================================
PDF and PostScript share the same imaging model, and both documents
are mutually convertible to each other. Both documents produce the
same result when printed. The difference between PDF and PostScript is
that PDF lacks the general-purpose programming language framework of
the PostScript language. A PDF document is a static data structure
made for efficient access and embeds navigational information suitable
for interactive viewing.
The language
======================================================================
PostScript is a Turing-complete programming language, belonging to the
concatenative group of programming languages. It is an interpreted,
stack-based language similar to Forth but with strong dynamic typing,
data structures inspired by those found in Lisp, scoped memory and,
since language level 2, garbage collection. The language syntax uses
reverse Polish notation, which makes the order of operations
unambiguous, but reading a program requires some practice, because one
has to keep the layout of the stack in mind. Most 'operators' (what
other languages term 'functions') take their arguments from the stack,
and place their results onto the stack. 'Literals' (for example,
numbers) have the effect of placing a copy of themselves on the stack.
Sophisticated data structures can be built on the 'array' and
'dictionary' types, but cannot be declared to the type system, which
sees them all only as arrays and dictionaries, so any further typing
discipline to be applied to such user-defined "types" is left to the
code that implements them.
The character "%" is used to introduce comments in PostScript
programs. As a convention, every PostScript program should start with
the characters "%!PS" as an interpreter directive so that all devices
will properly interpret it as PostScript.
PostScript programs are typically divided into two parts,
conventionally called the 'prolog' and the 'script'. The prolog
contains procedures and is written by a programmer. The script passes
data to those procedures. The script is often generated automatically,
using a programming language other than PostScript.
"Hello world"
===============
A Hello World program, the customary way to show a small example of a
complete program in a given language, might look like this in
PostScript (level 2):
%!PS
/Courier % name the desired font
20 selectfont % choose the size in points and establish
% the font as the current one
72 500 moveto % position the current point at
% coordinates 72, 500 (the origin is at the
% lower-left corner of the page)
(Hello world!) show % paint the text in parentheses
showpage % print all on the page
or if the output device has a console
%!PS
(Hello world!) =
Units of length
=================
PostScript uses the point as its unit of length. However, unlike some
of the other versions of the point, PostScript uses exactly 72 points
to the inch. Thus:
:
For example, in order to draw a vertical line of 4 cm length, it is
sufficient to type:
0 0 moveto
0 113.385827 rlineto stroke
More readably and idiomatically, one might use the following
equivalent, which demonstrates a simple procedure definition and the
use of the mathematical operators mul and div:
/cm {72 mul 2.54 div} def % 1 inch = 2.54 cm exactly
0 0 moveto
0 4 cm rlineto stroke
(Technically, most printers have a construction-implied unprintable
margin around the physical borders of the sheet, and the 0 0
coordinates are calibrated to its corner, so you might have to use a
different starting point to actually see something.)
Most implementations of PostScript use single-precision reals (24-bit
mantissa), so it is not meaningful to use more than 9 decimal digits
to specify a real number, and performing calculations may produce
unacceptable round-off errors.
Software
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Software which can be used to render PostScript documents:
* Ghostscript
* pstoedit
See also
======================================================================
* PostScript fonts
* Adobe StandardEncoding (PostScript character set)
* Document Structuring Conventions
* Encapsulated PostScript (EPS)
* PostScript Printer Description (PPD)
* Printer Command Language (PCL)
* Forth (programming language)
Further reading
======================================================================
* (NB. This book ('PLR3') together with the is the 'de facto'
defining work on PostScript 3 and is informally called "red book" due
to its red cover.)
* (NB. This edition ('PLR2') covers PostScript Level 2 and also
contains a description of Display PostScript, which is no longer
discussed in the third edition.)
* (NB. This edition ('PLR1') covers PostScript Level 1.)
* (NB. This introductory text is informally called "blue book" due to
its blue cover.)
* (NB. This book is informally called "green book" due to its green
cover.)
* (NB. This book is informally called "black book" due to its black
cover.)
* (NB. Official introductory comparison of PS, EPS vs. PDF.)
*
* [
http://www.math.ubc.ca/~cass/graphics/manual/]
* (NB. A thorough tutorial available online courtesy of the author.)
External links
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*
[
https://computerhistory.org/blog/postscript-a-digital-printing-press/
Computer History Museum: article about early development of
PostScript]
License
=========
All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/PostScript
