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The Art of Unix Programming
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Unix Programming - Applying Minilanguages - Case Study: PostScript

Case Study: PostScript

PostScript is a minilanguage specialized for describing typeset text and graphics to imaging devices. It is an import into Unix, based on design work done at the legendary Xerox Palo Alto Research Center along with the earliest laser printers. For years after its first commercial release in 1984, it was available only as a proprietary product from Adobe, Inc., and was primarily associated with Apple computers. It was cloned under license terms very close to open-source in 1988, and has since become the de-facto standard for printer control under Unix. A fully open-source version is shipped with most most modern Unixes.[91] A good technical introduction to PostScript is also available.[92]

PostScript bears some functional resemblance to troff markup; both are intended to control printers and other imaging devices, and both are normally generated by programs or macro packages rather than being hand-written by humans. But where troff requests are a jumped-up set of format-control codes with some language features tacked on as an afterthought, PostScript was designed from the ground up as a language and is far more expressive and powerful. The main thing that makes Postscript useful is that algorithmic descriptions of images written in it are far smaller than the bitmaps they render to, and so take up less storage and communication bandwidth.

PostScript is explicitly Turing-complete, supporting conditionals and loops and recursion and named procedures. The ontology of types includes integers, reals, strings, and arrays (each element of an array may be of any type) but no equivalent of structures. Technically, PostScript is a stack-based language; arguments of PostScript's primitive procedures (operators) are normally popped off a push-down stack of arguments, and the result(s) are pushed back onto it.

There are about 40 basic operators out of a total of around 400. The one that does most of the work is show, which draws a string onto the page. Others set the current font, change the gray level or color, draw lines or arcs or Bezier curves, fill closed regions, set clipping regions, etc. A PostScript interpreter is supposed to be able to interpret these commands into bitmaps to be thrown on a display or print medium.

Other PostScript operators implement arithmetic, control structures, and procedures. These allow repetitive or stereotyped images (such as text, which is composed of repeated letterforms) to be expressed as programs that combine images. Part of the utility of PostScript comes from the fact that PostScript programs to print text or simple vector graphics are much less bulky than the bitmaps the text or vectors render to, are device-resolution independent, and travel more quickly over a network cable or serial line.

Historically, PostScript's stack-based interpretation resembles a language called FORTH, originally designed to control telescope motors in real time, which was briefly popular in the 1980s. Stack-based languages are famous for supporting extremely tight, economical coding and infamous for being difficult to read. PostScript shares both traits.

PostScript is often implemented as firmware built into a printer. The open-source implementation Ghostscript can translate PostScript to various graphics formats and (weaker) printer-control languages. Most other software treats PostScript as a final output format, meant to be handed to a PostScript-capable imaging device but not edited or eyeballed.

PostScript (either in the original or the trivial variant EPSF, with a bounding box declared around it so it can be embedded in other graphics) is a very well designed example of a special-purpose control language and deserves careful study as a model. It is a component of other standards such as PDF, the Portable Document Format.

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The Art of Unix Programming
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