Page description language

from Wikipedia, the free encyclopedia

A page description language (PDL) describes the exact structure of a page , how it should look later in a special output program or device, such as a printer . These are output formats that are usually not intended for further processing.

With the help of page description languages, a data stream is generated for a job to be printed , which is sent to the printer. Depending on the language variant and print content, this data stream consists of:

Demarcation

A distinction is generally made between page description languages ​​and printer languages , which usually contain hardware-specific commands for controlling printer components.

Page description languages ​​work on a higher ( abstraction ) level than simple raster graphics , for example by describing pages with vectors . As a result, a better quality result is generally achieved for the output, despite lower memory consumption.

Page description languages ​​primarily define the fixed graphic representation of content on one or more pages of fixed size and are sometimes used like graphic formats . In contrast to this, with markup languages such as HTML or XSL-FO, the exact graphical representation must first be calculated by a program (e.g. an HTML renderer ), with different graphical representations being possible for different output media with different properties. A distinction can be made between print and screen output and the dimensions of the output medium can be taken into account. The raster image processor (RIP) is used to convert it to the rasterized print output on the printer .

Page description languages ​​also provide an interface to the source code of a document (e.g. notated in a markup language) or to programs that manage the source code ( editors ) or format the document (typesetting programs, many word processing programs do both). This can lead to an “abstraction” in the form that rather semantic information ( semantic markup or assignment to format templates ) is lost that would allow or make it easier to format the document differently, e.g. B. Change the font sizes of headings and running text paragraphs.

use

PostScript and other page description languages ​​were there for a long time to send their code from computers to ("PostScript-capable") printers, so that the conversion into control commands (RIP) took place in the latter. Many programming interfaces of the hardware abstraction layer such as GDI or OpenGL can accordingly make their output in page description languages. Alternatively, a typesetting program stores the page description code of a document in a file. These can be viewed as a preview on the screen and printed out with a file viewer such as Ghostview (Postscript), Adobe Reader (PDF) or YAP (for DVI under Windows) or xdvi (for DVI under Linux - see DVI viewer ) . The printer does not have to be able to process the page description language itself; instead, the viewer can address the printer via the computer's hardware abstraction layer. Under the Common Unix Printing System , the standard printer control under Linux, the PostScript and PDF interpreter ghostscript functions as a RIP in the computer. Such page description files are also advantageous for the electronic exchange of documents or their dissemination ( online publication ) compared to the source formats, since they save the recipient from having to recreate the view version of the document (which can even fail) (" exchange formats ").

The page description language PDF extends the functionality of the “preview” on the screen with hypertext functionality , so that you don't have to memorize page numbers when reading in order to follow references from the table of contents and cross-references, but instead click on links. Nor do you have to type or copy a URL into the address line of a web browser for links from the document to the Internet ; rather, the (standard) browser opens the target page when you click on a link in the displayed text. The URL doesn't even have to be displayed / printed. See also PDF # Navigation on the screen .

Individual page description languages

history

The first page description languages ​​emerged when laser printers that could output graphics in high quality became available and became cheaper and cheaper. In 1973 the first commercially usable laser printer (EARS) was completed in Xerox 's Palo Alto Research Center (PARC), which could be controlled with the PC Xerox Alto via a graphical user interface with mouse operation (→ laser printer # history ). Also at Xerox PARC, John Warnock and Charles Geschke developed the page description language Interpress , which Xerox did not want to market. Warnock and Geschke left Xerox in 1982, founded Adobe Inc. and marketed the advanced InterPress under the name PostScript . In 1984, however, Xerox published InterPress.

A review article from 1988 already cited eight characterizations of page description languages ​​from earlier publications, e.g. as an interface between computer and printer, which allows the merging of text and graphics on one page with graphic effects such as font selection, rotation and scaling regardless of the output device. Due to rotation , lines of text can run in different directions on the same page (in addition to horizontally, also vertically / diagonally). Output devices at that time could be printers, screens and photo typesetting machines . Device independence means that a file that represents a document using the page description language provides the same appearance on all output devices except for scaling (" zooming " in the screen display or reducing it so that a page fits on a DIN A4 sheet), provided the different ones Image resolutions and colors (different printers or the screen) allow this. This made the preview possible in such a way that the page composition ( page composition in the article mentioned) can be checked and revised on the computer screen without further (slow and costly) test printouts. This is the basis of desktop publishing, which in its present form , based on PostScript, implemented on the Apple LaserWriter , and the layout program PageMaker was introduced in 1985.

Certain characteristics of PostScript were viewed as design flaws, Adobe also demanded high license fees for them and the display driver was expensive, so that competing products (for printers from other manufacturers than Apple) soon emerged.

Independently of this, the Device Independent File Format (DVI) has been developed since 1979 as the target format for the TeX typesetting program, which is used for scientific publications in which mathematical formulas play a major role. However, this only shows the positions of horizontal text lines and horizontal or vertical bars . To take graphics into account, TeX can keep the required fields on the page free from text. Colors are also not supported natively. Diagrams can be represented by character sets for constituent parts or by joining a large number of tiny dots. Otherwise, information for graphics programs is embedded in the DVI file so that graphic effects can be displayed on the screen and on the printer with the aid of device drivers . In particular, the format is still used today (2015) with embedded PostScript commands in order to generate a representation of the document in PostScript format using the dvips driver, for example . However, the DVI viewers available today can also display and print a large part of the graphic effects directly from the DVI file (on the screen). As a page description language, DVI only satisfies the criterion of device independence with regard to horizontally placed text lines or characters and vertical / horizontal bars (also scalable on the screen), less the graphic criteria. It is practically only used as an intermediate format in order to combine the typographically highest demands (especially for mathematical formulas) with TeX typesetting with the capabilities of graphics software.

For very similar purposes as TeX, troff was previously developed by Joseph Ossanna . It could not control the new laser printer, so Ossanna started a new version with device-independent output, which, like TeX, is also called di troff because of its device-independent nature. The output format is also called ditroff . Ossanna died in 1977 before the completion of the edition-independent troff, it was developed from 1979 by Brian Kernighan .

The literature does not include the “requirement” hypertext functionality , which was implemented from 1989 in HTML (variable line breaks) and then in the page description language Portable Document Format (PDF; again by Adobe Inc. ), which is now a standard exchange format . With this page description language, the "preview" on the screen is no longer just an aid to avoid test printouts, but offers convenient functions that are not possible in the printout: you do not have to scroll to the target for cross-references and literature references , but can click on the Jump reference directly to the target page. Navigation within the document is also facilitated by page icons or the display of structure trees (according to the table of contents). If you click on external links, the target document is displayed in the web browser . However, these functions must be "built in" by the authoring software (for example with LaTeX and the LaTeX extension package, see PDF # TeX / DVI ) and implemented by the viewer . PDF became an open standard in 2008 as ISO 32000-1: 2008 .

See also

literature

  • Nenad Marovac: Page description languages . Concepts and implementations. In: Rae Earnshaw (Ed.): Workstations and Publication Systems . Springer, New York 1987, ISBN 1-4612-9148-8 , pp. 14–26 , doi : 10.1007 / 978-1-4612-4770-8_2 ( limited preview in Google Book Search [accessed April 27, 2016]).
  • AL Oakley, AC Norris: Page description languages: development, implementation and standardization . In: Electronic Publishing: Origination, Dissemination & Design . tape 1 , no. 2 . John Wiley & Sons, Chichester UK September 1988, p. 79–96 ( cs.nott.ac.uk [PDF; 122 kB ; accessed on August 3, 2015] on pp. 79f. 8 characterizations of page description language from previous publications are cited and summarized. The section Schemes for the description of printed pages from p. 89 to p. 92 describes relationships between page description languages ​​and [other] markup languages . With bibliometrics in the ACM Digital Library ).
  • Jürgen Schönhut: Page Description Languages ​​(PDLs) . In: Werner Purgathofer, Mr. Schönhut (ed.): Advances in Computer Graphics V . Springer, Berlin / Heidelberg 1989, ISBN 3-642-64798-7 , pp. 155–180 , doi : 10.1007 / 978-3-642-61340-1_6 ( individual pages of the chapter are displayed from the Google search for springer link page description languages laser).

Web links

Individual evidence

  1. ↑ Oakley and Norris ( #Literature ), pp. 79, 82 and scattered throughout the article.
  2. ↑ See Schönhut ( #Literature ), p. 155.
  3. ^ Oakley and Norris ( # literature ) p. 86.
  4. ↑ Oakley and Norris ( #Literature ) pp. 79f.
  5. ↑ Oakley and Norris ( #Literature ) pp. 86ff.
  6. ↑ PiCTeX on CTAN.
  7. ^ Frank Mittelbach, Michel Goossens a. a .: The LaTeX Companion, Second Edition . 4th, revised edition. Addison-Wesley, Boston, MA a. a. 2005, ISBN 0-201-36299-6 , pp. 593 f . (first two pages of chapter 10, also in other editions).
  8. ^ The history of troff. In: www.troff.org. 2012, accessed on August 9, 2015 .
  9. ^ The 8 most popular document formats on the web. In: duff-johnson.com. Retrieved August 7, 2015 .
  10. ↑ hyperref