PostScript: An Overview

Transcription

1 University of Waterloo July 11, 2016

2 1 History and Introduction 2 3 4

3 Before Adobe History and Introduction In 1976, John Warnock worked on a page description language for handling graphics being printed while at Evans and Sutherland 1978, worked with Martin Newell at XEROX PARC to create an improved software known as JaM Leaving with Chuck Geschke, Warnock founded Adobe Systems in 1982

4 Creation of PostScript Began work with Doug Brotz, Ed Taft and Bill Paxton to create a simplified language for greater platform independence which resulted in PostScript and was finished in 1984 After completion, Warnock and co. were visited by Steve Jobs and he urged them to use PostScript as a language for printers In 1985, the Apple LaserWriter was the first printer equipped with PostScript, and started more than 15 years of PostScript being predominant language used by printers.

5 Stages of PostScript History and Introduction PostScript Level 1: Original release of PostScript PostScript Level 2: Released in 1991, aside from standard improvements in speed and reliability, it included functionality for in-rip separations, image decomposition, composite fonts and form mechanism for caching PostScript 3: Released in 1997, added dictionary-based versions of older operators, improved colour handling and extra filters. The improvement in colour handling helped replace older methods with ones more commonly found today PDF: Created in response to demand for printing language to require less space for the program, and improve overall speed. PDF format discards more general-purpose framework using programming languages as a base, for static data structure method while allowing useful features for interactive viewing.

6 Purpose of PostScript Mainly designed to work with raster devices Language made to be general purpose which allows for flexibility Designed to handle conversion of text and images to raster devices Relatively lightweight, yet powerful to suit various needs Includes dynamic fonts for greater amount of options

7 PostScript Interpreter The PostScript interpreter handles the actions of the target output device based on a given PostScript program and produces the desired output on the printer, display, or other raster device The interpreter has three different methods of communication with the output devices in order to achieve its purpose based on the given situation The first method is the conventional method commonly used before the emergence of PostScript, where the PostScript application will make a page description of the output and send it to interpreter to format and send it out to the designated output device immediately, with no further communication. Generally the target of this method will be a printer or sometimes a preview window.

8 PostScript Interpreter The second method is interactive display between the interpreter and some application. The application will pass commands to the interpreter and the interpreter will occasionally send back some data as well. The final method allows the interpreter to interact directly with a user, accepting commands and directly executing them. Unlike in other modes where the interpreter will have or prepare a page description in advance, this mode allows for dynamic interaction between the interpreter and the various output devices.

9 PostScript Interpreter Figure: Visual of Interpreter Models

10 PostScript Language The PostScript language is a complete programming language like basic and c The focus of PostScript language is powerful graphic capabilities and specialises in creating high quality page descriptions for output devices Uses three types of encoding: standard ASCII, binary token and binary object sequences All data and programs created through PostScript language are known as PostScript objects. PostScript allows creation of dictionary entries to allow storage of references to various user defined objects and constructs

11 PostScript Language Structure Overview Management of PostScript program is through five stacks: operand, dictionary, and execution stacks are the standard ones, the other two stacks only concern graphic handling Other than stacks, there is also virtual memory (VM) which is mainly used to store references and values of composite objects, while basic objects have their values self-contained. VM also stores data not designated for immediate use. There are a number of dictionaries defined in PostScript, such as systemdict, globaldict, userdict, errordict, statusdict and FontDirectory to name a few, of these, globaldict and userdict are modifiable an commonly used during program exectuion. FontDirectory can also be modified indirectly through various operations and stores the available fonts and their definitions. Updating this adds new fonts to be available for use.

12 Process of Execution PostScript program is generally divided into parts, the prologue, which handles variables and setting, and the script, which contains the actual commands to be executed to output The program goes through the scanner and the scanner reads the input and immediately exectues it or places it on stack for later. PostScript treats the program like it is temporary and will disappear after input. Data on stack will wait until an appropriate command is issued and only then will it execute the data on stack The interpreter then takes the data and depending on the situation, will perform using one of the three models described earlier.

13 The Seven Sets of Operators There are seven sets of operators used in PostScript to handle graphic design and description, as well as transmission to external devices: Graphic State Operators: These operators have control over the data structure known as graphic state, and are the highest level and have global overview of all graphics operators. Coordinate system and matrix operators: These operators control and manipulate the current transformation matrix (CTM), which is kept within the graphic state and is used for mapping coordinates to output devices. Path construction operators: Controls the current path to allow creation of shapes and lines. These operators rely on the CTM in the background in order to complete these operations.

14 The Seven Sets of Operators Painting operators: Operators that control the painting of graphical elements such as lines, filled areas and images based on the current settings for colour, path, etc. Glyph and font operators: These operators are for choosing and describing glyphs from the available sets of fonts in the FontDirectory Device setup operators: These are used to send data in raster memory to physical output devices Output operators: These are used to transmit completed pages to the output device.

15 Graphic State Table History and Introduction Figure: Elements of the Graphic State Structure (Device-Independent)

16 A Bit More on Graphics Operators On previous slide was a table showing some of graphics state device-independent fields, more include: line cap, line join, miter limit, dash pattern and stroke adjustment. For device dependent, some states include: colour rendering, overprint, black generation, undercolour removal, transfer, halftone, flatness, smoothness and device. For other operator sets, most of their functions have been shown in class, from rotation to line tracing, to filling through rules and examples of PostScript syntax for this.

17 PostScript Colour Generation One of the things PostScript was known for was its high quality colours and the diversity of selections that could be made. PostScript defines the handling of colours as two sections: colour specification, for specifying colours in a manner that is device independent, and colour rendering, which produces the various specified colours for the raster output device by going through a variety of steps Furthermore, PostScript defines several types of colour spaces: CIE-based colour spaces, device colour spaces and special colour spaces

18 Colour Specification Flow Figure: Full Specification for Colours

19 PostScript and Fonts When it was first released and during the time it was actively used, PostScript allowed for highly flexible font choices and high quality rendering of fonts PostScript allowed users to: choose from a large, dynamic library of basic fonts, create their own fonts through use of specialized operators, allowed manipulation of fonts to suit the needs of various users.

20 PostScript vs PDF History and Introduction PostScript and PDF both share many similarities, but there are also some differences between the two PostScript is designed to handle all possible situations concerning generating output to be sent raster output device and provides the user with a large amount of options and power to handle the process through scripts, programs or interactive interfaces. The problem with PostScript is that as printers became cheaper, the ratio between the cost of printers and the cost of PostScript software became worse leading to gradually abandoning the PostScript model for less expensive options

21 PostScript vs PDF History and Introduction PDF was designed as a low cost alternative to PostScript which keeps high the quality output and device independence common in PostScript making it currently the most popular format for document generation The downside is that to achieve this, PDF format abandons the interactive PostScript language, opting for a static data structure format with some interactive features but lacking the diversity offered by PostScript

22 Conclusion History and Introduction Though no longer as popular as in the past, PostScript is still an important part of our society even today Providing high quality interface between user and raster output devices in the form of a powerful and specialized programming language, it greatly improved our ability to create high quality images and documents which formed the basis for our current standards Even though it is not used directly any more, its successor PDF is still the most popular and commonly used format for producing documents today

23 Thanks History and Introduction Thank You For Your Attention

24 Works Cited History and Introduction Adobe Systems Incorporated. PostScript: Language Reference Third Edition. PDF Evans, David. PostScript vs. PDF:Why do we offer two printing technologies? How do they differ?. Adobe PostScript. Wikipedia. June Weingartner, Peter. A First Guide to PostScript. tailrecursive.org. 24 February, 2006.