PostScript

PostScript (or PS) is a page description language used primarily in the electronic and desktop publishing areas.

History

The concepts of the PostScript language were seeded in 1976 when John Warnock was working at Evans and Sutherland, a famous computer graphics company. At that time John Gaffney was developing an interpreter for a large three-dimensonal graphics database of New York harbour. Gaffney conceived the Design System language to process the graphics, very similar to the Forth programming language.

In 1978 Evans and Sutherland asked Warnock to move from the San Francisco bay area to their main headqaurters in Utah, but he was not interested in moving. He then joined Xerox PARC to work with Martin Newell. They rewrote Design System to create JaM (John and Martin) which was used for VLSI design and the investigation of type and graphics printing. This work later evolved into an expanded system known as Interpress.

After watching Xerox sit on Interpress, as they had with many of their other technologies, Warnock left with Chuck Geschke and founded Adobe Systems in December 1982. They started selling a simpler version of Interpress as PostScript, which went on the market in 1985. At about this time they were visited by Steve Jobs, who urged them to adapt PostScript to be used as the language for driving laser printers, which was added to a Canon printing engine to create the LaserWriter.

The Apple LaserWriter was the first printer to popularize PostScript, sparking the desktop publishing (DTP) revolution in the mid-1980s. The combination of technical merits and widespread availability made PostScript a language of choice for graphical output for printing applications. For a time an interpreter for this language was an ubiquitous component of laser printers, into the 1990s.

Once the de facto standard for electronic distribution of final documents, PostScript has effectively been succeeded by PDF in this area. By 2001 there were fewer printer models which came with support for PostScript, largely as a result of the increasing power of the built-in printing systems supplied with most operating systems.

Usage in Printing

Traditional Printing

Prior to the introduction of 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 characters 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, the proper dots to use defined as 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 the user to download their own custom character graphics 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.

"Real" graphics printing was left to special-purpose devices, called plotters. Plotters did share 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

PostScript broke tradition by combining the best features of both printers and plotters. Like plotters, PostScript offered a high quality line art and a single control language that could be used across any brand of printer. Like dot-matrix printers, PostScript offered simple ways to generate pages of text and raster graphics. Unlike either, PostScript could place all of these types of media on a single page, which offered far more flexibility than any printer or plotter previously had.

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 whose execution will result 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 also noteworthy for implementing on-the fly rasterisation; everything, even text, is specified in terms of straight lines and 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.

Font Handling

Almost as complex as PostScript itself was PS's handling of fonts. The rich font system used the PS graphics primitives to draw characters as line art, which could then be rendered at any resolution. This might sound like a reasonably straightforward concept, but there are a number of typographic issues that had to be considered.

One is that fonts do not actually scale linearly at small sizes; features of the characters will become proportionally too large or small and they start to "look wrong". PostScript avoided this problem with the inclusion of hints which could be saved along with the font outlines. Basically they are additional information in horizontal or vertical bands that 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 was formerly 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. 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 at a very high cost to those wanting to add hints to their own fonts. Those who were happy without hints, or didn't want to spend the money, were left with the so-called Type 3 Font. Type 3 fonts allowed for all the sophistication of the PostScript language, but without the standardized approach to hinting. Other differences further added to the confusion.

The cost of the licensing was considered by many to be too high, and Adobe continued to stonewall on more attractive rates. It was this issue that led Apple and Microsoft to collaborate on their own system, TrueType, around 1991. Immediately following the announcement of TrueType, Adobe published the specification for Type 1 fonts. Retail tools such as Altsys Fontographer (now owned by Macromedia) 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 as a result.

Other Implementations

Adobe derived most of their licensing fees from their implementation of PostScript for printers, known as a Raster Image Processor or RIP. These were fairly expensive, and typically ran on a limited selection of hardware. With the introduction of a number of RISC-based platforms in the early 1980s, Adobe always seemed to be a step behind in supporting the new machines.

Third-party implementations of PostScript interpreters became quite common as a result. These tended to be found either in low-cost laser printers, or in high-end typesetting equipment. Some of these third-party solutions are still widely used in the typesetting world, particularly the one that is standard in all black-and-white Hewlett-Packard laser printers (as of 2003).

However, many printers don't support any RIP in their basic versions. For these, a free PostScript interpreter called Ghostscript is available. It prints PostScript documents on non-PostScript printers using the CPU of the host computer to do the rasterization, sending the result as a single large bitmap to the printer. It can also be used to preview PostScript documents on a computer monitor.

Usage as a Display System

With PostScript becoming a de-facto standard for printed output, it was natural to consider using the same language for describing the screen output as well. The rapid increase in CPU power in the late 1980s, combined with an interest in windowing systems, led to several attempts to create a display system that used PostScript as its primary display technology.

There are a number of advantages to using PS as the display system. One is that the fonts on other systems required the user to keep not only bitmaps for the screen, but also Type 1 for the printer. Using PS on the display would eliminate this and require only one set. Another advantage is that it allows for the "dumbing down" of printers. When the LaserWriter was released it was the most powerful (and expensive) machine in Apple's lineup, a result of needing considerable processing power and memory to render the page at a "high" resolution of 300 dpi in a reasonable amount of time. In contrast, the 400-dpi printer that shipped with the NeXT platform contained no CPU at all, instead using the computer's CPU to do the rendering and passing off the rendered page as a bitmap to the printer.

But the main advantage in using PostScript as a windowing system is that it allows one to write desktop publishing (DTP) and other graphically-intensive applications with a single set of graphics routines. The same code that is drawing to the window can be used to draw to the printer without any translation. DTP applications on traditional systems require the programmer to construct the GUI editor in the platform's own graphics system (for example, QuickDraw on the Macintosh, or GDI on Microsoft Windows) and then write additional code to translate the graphics into proper PostScript for printing. This often takes up the majority of the programming effort on such projects and is a major source of bugs.

The two main examples of PostScript as a display technology are Display PostScript (DPS) and NeWS. They differed dramatically in terms of where the display logic was applied; in DPS the view system was left to the hosting OS, whereas under NeWS the entire display was written in PS and ran in a single complex interpreter. While certainly very interesting, it's not clear that NeWS is in fact a better approach.

The Language

PostScript is a full-fledged programming language. Typically, PostScript programs are not produced by humans, but by other programs. However, it is perfectly possible to produce graphics or to perform calculations by hand-crafting PostScript programs.

PostScript is an interpreted, stack-based language similar to Forth. The language syntax uses Reverse Polish Notation, which makes parentheses 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).

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, PostScript 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 one needs 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 occurrences 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 two 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.

To produce graphics, PostScript uses an ordinary cartesian coordinate system.

 100 200 moveto 300 400 lineto stroke

moves the "cursor" to the point with coordinates (100, 200) and then draws a line to the point (300, 400).

 50 70 moveto 100 200 50 80 100 100 curve stroke

produces a Bézier curve from (50, 70) to (100, 100) with control points (100, 200) and (50, 80).

 250 250 moveto (Wikipedia) show

will create the text "Wikipedia" at coordinate location (250, 250), rendered in the default font.

Graphics are initially produced in the "user coordinate system" and may then optionally be rotated, scaled or skewed before being copied to the "device coordinate system" specifying the final output.

  200 300 translate 45 rotate

will translate the contents of the user coordinate system 200 points upwards, 300 points to the right and rotate it 45 degrees before they are copied to the device coordinate system.

The character "%" is used to introduce comments in PostScript programs. As a general convention, every PostScript program should start with the characters "%!" so that all devices will properly interpret it as PostScript.

See also:

• Adobe Systems
• Ghostscript
• Portable Document Format
• Scene description language[?]
• Vector graphics
• Font

External links

• PostScript Language Reference, third edition (http://partners.adobe.com/asn/developer/pdfs/tn/PLRM.pdf) is the de facto defining work, known as "The Red Book" on account of its covers. The first edition covered PostScript Level 1, the second edition covered a greatly expanded language known as PostScript Level 2, and includes documentation for Display PostScript as well. The third edition covers PostScript 3 (with this version, Adobe dropped "level" from the name) but no longer includes DPS.
• PostScript Language Tutorial and Cookbook (http://www-cdf.fnal.gov/offline/PostScript/BLUEBOOK.PDF) 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.
• First Guide to PostScript (http://www.cs.indiana.edu/docproject/programming/postscript/postscript) is an introduction to the PostScript system.

PostScript hacks:

• Life in PostScript (http://www.tjhsst.edu/~edanaher/pslife/) - a PostScript version of Conway's Game of Life
• PS-HTTPD - a webserver written in PostScript (http://www.pugo.org:8080/)
• PostScript fractals (http://www.pvv.ntnu.no/~andersr/fractal/PostScript)
• A PostScript raytracer (http://www.3dspot.com/rtnews/rtnews4a#art7), from Ray Tracing News, September 5, 1988, Volume 1, Number 8 .

This article (or an earlier version of it) contains material from FOLDOC's article on PostScript (http://wombat.doc.ic.ac.uk/foldoc/foldoc.cgi?PostScript), used with permission.