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and finally implemented in its current form by [[John Warnock]] et al. after he and [[Chuck Geschke]] founded [[Adobe Systems]], Inc. in [[1982]].
The [[Apple Computer|Apple]] <nowiki>LaserWriter</nowiki> used <nowiki>PostScript</nowiki>
Many applications can transform a document into a <nowiki>PostScript</nowiki> program whose execution will result in the original document. This program can be sent to a interpreter in a printer (resulting 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''.
The combination of technical merits and widespread availability made <nowiki>PostScript</nowiki> a language of choice for graphical output for printing applications. Once the de-facto standard for electronic distribution of final documents, [[Portable Document Format|PDF]] has effectively succeeded <nowiki>PostScript</nowiki> in this area. By [[2001]] there are also less and less printer models which come with support for <nowiki>PostScript</nowiki>.
▲<nowiki>PostScript</nowiki> is an unusually powerful printer language because it is a full [[programming language]], rather than a series of low-level [[escape sequence|escape sequences]]. In this way it parallels [[Emacs]], which exploited a similar insight about editing tasks. It is also noteworthy for implementing on-the fly [[rasterisation]], from [[Bezier curve]] descriptions, of high-quality fonts at low resolution (early laser printers rendered at 300 dpi for example); it was formerly believed that hand-tuned bitmap fonts were required for this task.
=== Usage as a Windowing System ===
In the early 1990s, [[Sun MicroSystems]] developed an entire windowing system based on <nowiki>PostScript</nowiki>, NeWS stands for Network Windowing Systems. Sun also developed TNT (the <nowiki>NeWS</nowiki> toolkit) for [[OpenLook]] programming. There were only a few companies committed to this unproven technology. The <nowiki>OpenLook</nowiki> version of [[FrameMaker]] by [[Frame Technology Inc.]] at the time was one of the few products successfully run on <nowiki>NeWS</nowiki>. After <nowiki>OpenLook</nowiki> was defeated by Motif in the GUI war, and after Adobe acquired <nowiki>FrameMaker</nowiki>, products on <nowiki>NeWS</nowiki> simply vanished. Most [[UNIX]] workstations, including Sun's run <nowiki>NeWS's</nowiki> competitor, the [[X Window System]].▼
"The <nowiki>PostScript</nowiki> Language Reference Manual" is the de facto defining work, known as "The Red Book" on account of its covers. The first edition covered <nowiki>PostScript</nowiki> Level 1, the second edition covered a greatly expanded language known as <nowiki>PostScript</nowiki> Level 2. The third edition covers <nowiki>PostScript</nowiki> 3 (with this version, Adobe dropped "level" from the name).▼
Around the same time, [[NeXT Computer]]
"<nowiki>PostScript</nowiki> Language Tutoral and Cookbook" is the corresponding introductory text, known as "The Blue Book" on account of its covers. It's an unusally good tutorial book, with lots of excellent examples.▼
▲In the early 1990s, [[Sun MicroSystems]] developed an entire windowing system based on <nowiki>PostScript</nowiki>, NeWS stands for Network Windowing Systems. Sun also developed TNT (the <nowiki>NeWS</nowiki> toolkit) for [[OpenLook]] programming. There were only a few companies committed to this unproven technology. The <nowiki>OpenLook</nowiki> version of [[FrameMaker]] by [[Frame Technology Inc.]] at the time was one of the few products successfully run on <nowiki>NeWS</nowiki>. After <nowiki>OpenLook</nowiki> was defeated by Motif in the GUI war, and after Adobe acquired <nowiki>FrameMaker</nowiki>, products on <nowiki>NeWS</nowiki> simply vanished.
Another interesting point is that the windowing servers in these systems ''execute'' user interface elements (''widgets''), because <nowiki>PostScript</nowiki> is a programming language. Part of the behaviour of a widget can be specified in its code. For example, a toggle button's display routine can query the button's state (pressed or not) and select the according visualization. The button can also react to mouse clicks by changing its state from pressed to not pressed and vice versa. All this can happen in the windowing server without interaction with the client program. Compare this to a "dumb" X windows server, which can only report "mouse clicked on button" events to a client, which then has to switch the state, and finally instruct the server to display the new state. If client and server are not on the same machine, these interactions will have to travel over the network, slowing the feedback loop down unnecessarily.
▲Around the same time, [[NeXT Computer]] developed [[Display PostScript]] as a windowing system which was later adopted by other companies such as [[IBM]] for their RS6000 workstations and [[SGI]] for their graphic workstations.
=== The Language ===
<nowiki>PostScript</nowiki> is an interpreted, [[stack]]-based language similar to [[Forth programming language|Forth]]. The language syntax uses [[Reverse Polish notation]] which makes parantheses unnecessary, 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.
Example:
3 4 add 5 1 sub mul
will compute (3 + 4) * (5 - 1). Let's take a look at what happens in detail:
3 and 4 are both literals, so will push themselves onto the stack. So after these two instructions, the stack will look like this:
4
3
'''add''' is an operand, taking the two top-most elements from the stack (3 and 4 in our example), adds them together, and pushes the result onto the stack:
7
Next come two literals again, which will make the stack look like this (note that action is usually constrained to the top of the stack, leaving lower elements unchanged):
1
5
7
Another operand, '''sub''', takes two elements from the top, subtracts the first (higher one) from the second, and pushes the result onto the stack:
4
7
It should be obvious that '''mul''' works like the other operators, taking its two arguments from the stack, and pushing their product:
28
But this did nothing more than an old RPN calculator. Of course, <nowiki>PostScript</nowiki> has variables. In detail, it has a ''dictionary'' where everything that is not a literal is looked up; on a match, the current value stored under the name is pushed; mismatches will result in an error. To place something in the dictionary we need the ''def'' operator, which takes a name and a value as its arguments. Names are constructed by prefixing (or ''quoting'') with a slash. So
/x1 15 def
will first push the name "x1" on the stack, then the value 15, then execute '''def''' which will take both from the stack, and write 15 into the dictionary under the name "x1". Later occurances of "x1" (not to be confused with "/x1") will push 15 onto the stack as long as the variable is unchanged. This code will increment the content of x1 by 2:
/x1 x1 2 add def
Some real programming language power is offered by '''{''' and '''}'''. The opening brace puts the interpreter in ''deferred execution'' mode, so that everything is just placed on the stack, even operators and other executable objects. The one exception is the closing brace, which takes everything put on the stack since the opening brace, bundles it up into an (anonymous) procedure, and places that on the stack.
This construct is used in various ways, for subroutine definition (the anonymous procedure is assigned to a variable), loops, conditionals, etc. Example:
x1 0 eq { 0 } { 1 x1 div } ifelse
This code first uses the '''eq''' operator to test whether the value of x1 is equal to 0, depending on the outcome '''eq''' will push '''true''' or '''false''' onto the stack. After that, two procedures are pushed onto the stack. Then '''ifelse''' is executed, which takes three arguments from the stack, and will execute either the second (if the third is '''true''') or first (if the third is '''false'''). In summary, 0 results if x1 is 0, 1/x1 is the result for all other cases.
/inc3 { 3 add } def
Here '''def''' is used to place something in the dictionary, only this time it is a procedure instead of a simple integer. This works because the values coming from the dictionary are ''executed'' not just pushed (as simplistically stated above). Since executing a literal amounts to pushing it, that did not make a difference before. Now executing "inc3" will first look it up in the dictionary, find the procedure object representing "{ 3 add }" and execute that. One value must reside on the stack for this to work, since '''add''' needs to arguments only one of which is given in the procedure itself. Naturally, one passes arguments to procedures by placing them on the stack, so we can simply view "inc3" as a procedure that takes one argument. Example call:
71 inc3
will put 71 on the stack, which inc3 will increment by three, for a final result of 74.
=== Further Reading ===
▲"The <nowiki>PostScript</nowiki> Language Reference Manual" is the de facto defining work, known as "The Red Book" on account of its covers. The first edition covered <nowiki>PostScript</nowiki> Level 1, the second edition covered a greatly expanded language known as <nowiki>PostScript</nowiki> Level 2. The third edition covers <nowiki>PostScript</nowiki> 3 (with this version, Adobe dropped "level" from the name).
▲"<nowiki>PostScript</nowiki> Language Tutoral and Cookbook" is the corresponding introductory text, known as "The Blue Book" on account of its covers. It's an unusally good tutorial book, with lots of excellent examples.
▲One of the most attractive feature of <nowiki>PostScript</nowiki> as windowing system was to allow infinitely, smoothly scalable graphics and fonts on highend computers back in those days when bitmap were the norm. Nowaday such capability is taken for granted by computer users. One of the major competititors in the font technology was [[BitStream]]. The main font technologies used today are the <nowiki>PostScript</nowiki> Adobe Type 1 technology, and TrueType (developed jointly by Apple and Microsoft.)
''See also:''
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*[[Portable Document Format]]
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''Article based on [http://foldoc.doc.ic.ac.uk/foldoc/foldoc.cgi?query=postscript PostScript] at [http://www.foldoc.org FOLDOC], used with [[Public Domain Resources/Foldoc license|permission]].''
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