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Postscript

Language type:

T - Threaded or stack-based

Description:

PostScript is a threaded interpreted language with stack-based semantics. While usable for general-purpose computation, PostScript was specially designed to drive graphic devices, initially printers, and has a large set of operators for rendering onto a page. In terms of syntax and semantics, PostScript resembles Forth: operators use data from a stack, and place their results back on the stack. PostScript data types include floating-point numbers, strings, fonts, files, vectors, and symbols. Defined procedures are simply code sequences stored in dictionaries.
The essence of PostScript is its model of execution. In a threaded interpreted language, a program simply consists of a series of tokens. The interpreter processes each token in turn, with no look-ahead or complex parsing needed. Supporting this style requires storage for intermediate results, PostScript uses several stacks: a code stack, a data stack, and a dictionary stack. (There is also a special stack just for graphical contexts, as needed for the rendering model, making a total of four stacks.) Threaded interpreted languages can be difficult to use, but they offer the advantage that interpreters for them can be very compact and fast. As PostScript was originally designed to execute inside a printer or similar device with very little memory, compactness was an important design criterion.
Several implementations of PostScript exist, but the two main ones are the commercial implementations from Adobe Systems, and the free Ghostscript implementation from Aladdin Enterprises.
The current version of PostScript language is Level 3. PostScript level 2 is also still very widely used in printers and other devices. Implementations of PostScript for GUI systems use a language dialect called "Display PostScript," that includes special operators and slightly different semantics to accomodate user interaction.

Origin:

John Warnock et al, Adobe Systems, 1982.

See Also:

Forth    Logo   

Remarks:

PostScript is a simple language, in terms of syntax and structure. The complexity, when learning it, comes from the richness of its graphics model and the large set of built-in operators.
Adobe Systems, the company that owns PostScript, publishes a fine set of books on learning the language. These first editions of these books, published around 1985, were originally blue for the tutorial, red for the reference, and green for the cookbook. The current edition of the PostScript reference manual is no longer red, but it is still sometimes referred to as "the red book."
The first mass-market printer to support PostScript was the Apple LaserWriter. By 1993, most high-end laser printers and many typesetters supported the language.
An interesting thing about PostScript is that most programmers who learn it don't do so because they want to write PostScript programs, they do it because they want to write programs to generate PostScript code to draw graphics.
PostScript is a descendant of an older graphics language, also co-invented by John Warnock, called JaM.

Links:

Ghostscript and related free utilities

First Guide to Postscript

Postscript Speed Test

Date:

Last updated 12/16/97

Sample code:

%!PS-Adobe-2.0
% Draw a string at an angle at a point
/printat {    % str x y angle => -
	gsave
	4 1 roll translate
        exch rotate 0 0 moveto show
	grestore
} def

% find the center of the page
/pagecenter {  % - => cx cy
        clippath pathbbox 4 1 roll exch sub 2. div
        3 1 roll sub 2. div
} def

% set up font and constants
/Times-Bold findfont 36 scalefont setfont
pagecenter /cy exch def /cx exch def
/steps 9 def
/basegray 0.75 def
/incrgray basegray steps div def
/baseangle 360 steps div def
/incrangle 360 steps div def

% draw a string as a rosette
steps { 
basegray setgray
(PostScript) cx cy baseangle printat
/basegray basegray incrgray sub def
/baseangle baseangle incrangle add def
} repeat

% done with this page
showpage

Descriptions in this dictionary are ©1997-99 Neal Ziring. Some examples copyright of their respective authors. Some technologies and languages are trademarked. Permission to copy descriptions is granted as long as authorship credit is preserved.

Comments on this dictionary, corrections and suggestions, are all welcome. Please use email, the address is ziring@home.com