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An Introduction to PostScript
PostScript, for those who don’t know, is a "page description language" developed by a company called Adobe Systems. It is typically used for printing high-quality text and graphics on laser printers, imagesetters, and sometimes computers equipped with a "PostScript interpreter." Adobe and Apple co-developed the first Adobe PostScript interpreter and released it in the original LaserWriter printer back in 1985.
Since that time, the PostScript language has thrived and evolved. Several other vendors now market their own "non-Adobe" versions of PostScript, and PostScript printers can be used with virtually every brand of computer. Adobe and dozens of other developers sell PostScript fonts and PostScript-compatible drawing programs and clip art. But PostScript printers typically cost about twice as much as non-PostScript units. What’s the deal?
Although Windows and/or DOS-based PCs, Macintoshes, Amigas and other computers all have their own programs that are "platform specific" (i.e., the software only runs on one type of computer), their printer drivers can all create PostScript files that will print on any PostScript output device. Thus, any of the major operating systems can produce a file that can be printed on any other operating system, as long as it, or the printer it is connected to, has a PostScript interpreter. Well, in theory, at least. It is not difficult to produce a file that will choke even the best PostScript interpreter. Programs which automatically trace bitmaps to produce "outline art" (CorelTrace, Adobe Streamline, etc.) are notorious in this regard.
If you open a PostScript file with a word processor, you will see that it is just plain text -- in computer parlance, an ASCII file. A PostScript file could therefore be written with any word processor, but typically you would let a drawing or desktop publishing program do the work of creating a complex page description and then let the printer driver for your computer output the file as PostScript.
When you select a PostScript printer driver to print from your program, the driver does the job of converting the objects and characters on the screen to the language that PostScript understands.
The printer driver can send this info directly to the printer, although in many cases, the operating system can be set up to intercept the print-out command and "spools" the file in the background, so that you can continue to work as pages print. The checkbox labeled "Use Print Manager" in Windows does this, for example, as does the "Background Printing" button in the Macintosh’s Chooser.
Alternatively, you may choose to print PostScript to disk -- again, usually via an option in the Print dialog box. You can then send this file to a service bureau or someone else’s PostScript printer and it will print correctly, even if they don’t have the fonts or program(s) you used to create the file. This is a great benefit when sending a file to a service bureau. You can proof the page on your 300-dots-per-inch PostScript printer at home and then send the printed-to-disk file to have it output to an imagesetter at resolutions as high as 3750 dpi.
The PostScript language itself is said to be similar to that of the Forth programming language: procedural and stack-based. People refer to this kind of language as "RPN," for Reverse Polish Notation. Pocket calculators sometimes use RPN, too; typically, those with advanced functions.
Part of the PostScript language specifically applies to fonts, while other parts are more generalized, and can be used to create a variety of graphical effects. The two main font formats are known as Type 1 and Type 3. The former used to be a jealously guarded secret by Adobe, but the company responded to the announcement of TrueType by Apple and Microsoft by publishing the Type 1 specification -- presumably in an effort to legitimize the PostScript Type 1 "standard."
Type 3 fonts have largely faded from public view since the release of the Type 1 spec, although Type 3 fonts are capable of certain graphic effects (including tints and stroked lines) that Type 1 fonts are not.
Today, the Type 1 font format is most commonly used in professional publishing applications. TrueType has taken its place as a consumer standard, popular on Windows PCs and Macs running System 7. Still, however, most service bureaus will only accept PC files containing TrueType fonts if they have been printed to disk as a ".PRN" file. At the high end, at least, PostScript still reigns supreme.
One of the most valuable features of PostScript -- and, indeed, the main reason it was developed -- is its device independence. This means that a PostScript file output by any Postscript-compatible device at any resolution will resemble as closely as possible that of any other PostScript device. This means that although your 300 dot-per-inch printer may not be able to print details as fine as those from a 2540-dpi imagesetter, PostScript ensures that the letter- and word-spacing will be consistent.
This device independence was one of the most often touted benefits of the Display PostScript system of the NeXTstep operating system.
PostScript promises a solution to the dilemma of how to print a document on somebody else’s printer, especially if they don’t have the same fonts as you. As DTP pioneers can tell you, this promise was often unfulfilled in reality. New innovations such as PageMaker 5.0’s Panose font substitution system and SuperATM go a long way to solving these problems.
There are printers and other devices with PostScript interpreters built in, there are PostScript add-ons and options for many printers (most of Hewlett-Packard’s LaserJets, for example) and there are software programs you can run on your computer that give it the ability to interpret, display and/or print PostScript to otherwise "non-PostScript" output devices.
There are quite a few such products for the PC. Some of the most popular are GoScript from LaserGo (selected as the "Readers’ Choice" by WordPerfect magazine), Freedom of the Press by Color Age, PMW’s UltraScript (also marketed in Canada by Roland DG as RavenScript), and the public-domain GhostScript. Originally developed for Unix machines, GhostScript is available from many BBSes and other PD sources.
Freedom of the Press, UltraScript and the PD GhostScript are also available for the Mac, along with several others, notably Teletypesetting’s T-Script and Techpool Software’s Transverter Pro. For the Amiga, the most popular PostScript interpreter is Post, another PD title. For Atari computers, CompoScript is the best choice, or that ubiquitous PD standby, GhostScript.
Of course, the advantage of a software PostScript interpreter (especially a free one) is its low cost, compared to the $1000 or so a PostScript option usually adds to the cost of a laser printer. The disadvantage of "doing it in software" is speed -- or lack of it. PostScript is no speed demon at the best of times, and a software PostScript interpreter can tax the patience of even the most budget-conscious user.
This is an upgrade?
Why would you want PostScript if it is so expensive and slows down the printer? If you use graphics extensively, you will find these drawbacks minor compared to the dramatic improvement in your output -- especially if your graphics or DTP software takes advantage of PostScript-specific features.
For users who primarily print text, the benefits are less concrete. In fact, users who primarily output plain text (word processor files without a lot of fancy effects, etc.) will be better off with a non-PostScript printer, in terms of speed. If you think you might someday explore a pro-level DTP or graphics application (PageMaker or CorelDRAW), you should at least get a printer with the option to add PostScript.
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