Display PostScript
Encyclopedia
Display PostScript is an on-screen display system. As the name implies, DPS uses the PostScript
(PS) imaging model and language to generate on-screen graphics. To the basic PS system, DPS adds a number of features intended to ease working with bitmap
ped displays and improve performance of some common tasks.
Early versions of PostScript display systems were developed at Adobe Systems
. In 1986, Sun Microsystems
developed a totally independent implementation of Adobe's PostScript page description language called NeWS
, which rendered on multiple overlapping arbitrarily shaped windows on the display. Unlike Adobe's Display PostScript, NeWS had extensions for handling input devices, events, light weight threads, locks, networking and object oriented programming. NeXT
developed a Display PostScript system in cooperation with Adobe in 1987. It was an official Adobe product with its own standards, documents and licensing requirements.
DPS did not, however, add a windowing system. That was left to the implementation to provide, and DPS was meant to be used in conjunction with an existing windowing engine. This was often the X Window System
, and in this form Display PostScript was later adopted by companies such as IBM and SGI
for their workstations. Often the code needed to get from an X window to a DPS context was much more complicated than the entire rest of the DPS interface. This greatly limited the popularity of DPS when any alternative was available.
wrote a completely new windowing engine to take full advantage of NeXT's object-oriented operating system
. A number of commands were added to DPS to actually create the windows and to react to events, similar to but simpler than NeWS
. The single API made programming at higher levels much easier and made NeXT one of the few systems to extensively use DPS. The user-space windowing system library Nextstep
used PostScript to draw items like titlebars and scrollers. This, in turn, made extensive use of
's Mac OS X
operating system uses a central window server (created entirely by Apple) that caches window graphics as PDF
, instead of storing and executing PostScript code. A graphics library called Quartz 2D
provides PostScript-style imaging using the PDF rendering model (a subset, plus tweaks, of the PostScript model), but this is used by application frameworks—there is no PostScript present in the Mac OS X window server. Apple chose to use this model for a variety of reasons, including the avoidance of high Adobe-imposed licensing fees for DPS, and more efficient support of legacy Carbon
and Classic
code; QuickDraw
-based applications use bitmapped drawing exclusively. Adobe's copyright stipulations for the PDF standard are much less restrictive, granting conditional copyright permission to anyone to use the format in software applications, free of charge.
PostScript
PostScript is a dynamically typed concatenative programming language created by John Warnock and Charles Geschke in 1982. It is best known for its use as a page description language in the electronic and desktop publishing areas. Adobe PostScript 3 is also the worldwide printing and imaging...
(PS) imaging model and language to generate on-screen graphics. To the basic PS system, DPS adds a number of features intended to ease working with bitmap
Bitmap
In computer graphics, a bitmap or pixmap is a type of memory organization or image file format used to store digital images. The term bitmap comes from the computer programming terminology, meaning just a map of bits, a spatially mapped array of bits. Now, along with pixmap, it commonly refers to...
ped displays and improve performance of some common tasks.
Early versions of PostScript display systems were developed at Adobe Systems
Adobe Systems
Adobe Systems Incorporated is an American computer software company founded in 1982 and headquartered in San Jose, California, United States...
. In 1986, Sun Microsystems
Sun Microsystems
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...
developed a totally independent implementation of Adobe's PostScript page description language called NeWS
NeWS
NeWS was a windowing system developed by Sun Microsystems in the mid 1980s. Originally known as "SunDew", its primary authors were James Gosling and David S. H. Rosenthal...
, which rendered on multiple overlapping arbitrarily shaped windows on the display. Unlike Adobe's Display PostScript, NeWS had extensions for handling input devices, events, light weight threads, locks, networking and object oriented programming. NeXT
NeXT
Next, Inc. was an American computer company headquartered in Redwood City, California, that developed and manufactured a series of computer workstations intended for the higher education and business markets...
developed a Display PostScript system in cooperation with Adobe in 1987. It was an official Adobe product with its own standards, documents and licensing requirements.
Changes from Ordinary PostScript
In order to support interactive, on-screen use with reasonable performance, changes were needed:- Multiple execution contexts: Unlike a printer environment where a PS interpreter processes one job at a time, DPS would be used in a number of windowWindow (computing)In computing, a window is a visual area containing some kind of user interface. It usually has a rectangular shape that can overlap with the area of other windows...
s at the same time, each with their own settings (colors, brush settings, scale, etc.). This required a modification to the system to allow it to keep several "contexts" (sets of state data) active, one for each process (window).
- Encoded names: Many of the procedures and data structures in PostScript are looked up by name, string identifier. In DPS these names could be replaced by integers, which are much faster for a computer to find.
- Interaction support: A number of procedures were defined to handle interaction, including hit detection.
- Halftone phase: In order to improve scrolling performance, DPS only drew the small portion of the window that became visible, shifting the rest of the image instead of re-drawing it. However this meant that the halftoneHalftoneHalftone is the reprographic technique that simulates continuous tone imagery through the use of dots, varying either in size, in shape or in spacing...
s might not line up, producing visible lines and boxes in the display of graphics. DPS included additional code to properly handle these cases. Modern full-color displays with no halftones have made this idea mostly obsolete.
- Incremental updates: In printing applications the PS code is interpreted until it gets a
showpageat which point it is actually printed out. This is not suitable for a display situation where a large number of minor updates are needed all the time. DPS included modes to allow semi-realtime display as the instructions were received from the user programs.
- Bitmap font support: DPS added the ability to map PS fonts onto hand-drawn bitmap fonts and change from one to the other on the fly. While PS's ability to display fonts on "low resolution" devices was good, "low resolution" meant something on the order of 300 dpiDots per inchDots per inch is a measure of spatial printing or video dot density, in particular the number of individual dots that can be placed in a line within the span of 1 inch . The DPI value tends to correlate with image resolution, but is related only indirectly.- DPI measurement in monitor...
, not the 96 dpi that a NeXT screen used. This resolution required hand-built bitmap fonts to provide reasonable quality.
- Programming language support: DPS introduced the concept of a "
pswrap", which allowed developersSoftware developerA software developer is a person concerned with facets of the software development process. Their work includes researching, designing, developing, and testing software. A software developer may take part in design, computer programming, or software project management...
to wrap PostScript code into a C languageC (programming language)C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
function which could then be called from an application.
DPS did not, however, add a windowing system. That was left to the implementation to provide, and DPS was meant to be used in conjunction with an existing windowing engine. This was often the X Window System
X Window System
The X window system is a computer software system and network protocol that provides a basis for graphical user interfaces and rich input device capability for networked computers...
, and in this form Display PostScript was later adopted by companies such as IBM and SGI
Silicon Graphics
Silicon Graphics, Inc. was a manufacturer of high-performance computing solutions, including computer hardware and software, founded in 1981 by Jim Clark...
for their workstations. Often the code needed to get from an X window to a DPS context was much more complicated than the entire rest of the DPS interface. This greatly limited the popularity of DPS when any alternative was available.
Display PostScript at NeXT
The developers of NeXTNeXT
Next, Inc. was an American computer company headquartered in Redwood City, California, that developed and manufactured a series of computer workstations intended for the higher education and business markets...
wrote a completely new windowing engine to take full advantage of NeXT's object-oriented operating system
Object-oriented operating system
An object-oriented operating system is an operating system which internally uses object-oriented methodologies.An object-oriented operating system is in contrast to an object-oriented user interface or programming framework, which can be placed above a non-object-oriented operating system like DOS,...
. A number of commands were added to DPS to actually create the windows and to react to events, similar to but simpler than NeWS
NeWS
NeWS was a windowing system developed by Sun Microsystems in the mid 1980s. Originally known as "SunDew", its primary authors were James Gosling and David S. H. Rosenthal...
. The single API made programming at higher levels much easier and made NeXT one of the few systems to extensively use DPS. The user-space windowing system library Nextstep
NEXTSTEP
NeXTSTEP was the object-oriented, multitasking operating system developed by NeXT Computer to run on its range of proprietary workstation computers, such as the NeXTcube...
used PostScript to draw items like titlebars and scrollers. This, in turn, made extensive use of
pswraps, which were in turn wrapped in objects and presented to the programmer in object form.Modern Derivatives
AppleApple Computer
Apple Inc. is an American multinational corporation that designs and markets consumer electronics, computer software, and personal computers. The company's best-known hardware products include the Macintosh line of computers, the iPod, the iPhone and the iPad...
's Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
operating system uses a central window server (created entirely by Apple) that caches window graphics as PDF
Portable Document Format
Portable Document Format is an open standard for document exchange. This file format, created by Adobe Systems in 1993, is used for representing documents in a manner independent of application software, hardware, and operating systems....
, instead of storing and executing PostScript code. A graphics library called Quartz 2D
Quartz 2D
Quartz 2D is the primary two-dimensional graphics rendering API for Mac OS X, part of the Core Graphics framework.-Overview:Quartz 2D is available to all Mac OS X application environments, and provides resolution-independent and device-independent rendering of bitmap graphics, text, and vectors...
provides PostScript-style imaging using the PDF rendering model (a subset, plus tweaks, of the PostScript model), but this is used by application frameworks—there is no PostScript present in the Mac OS X window server. Apple chose to use this model for a variety of reasons, including the avoidance of high Adobe-imposed licensing fees for DPS, and more efficient support of legacy Carbon
Carbon (API)
Carbon is one of Apple Inc.'s procedural application programming interfaces for the Macintosh operating system. It provides C programming language access to Macintosh system services...
and Classic
Classic (Mac OS X)
Classic, or Classic Environment, was a hardware and software abstraction layer in Mac OS X that allowed applications compatible with Mac OS 9 to run on the Mac OS X operating system...
code; QuickDraw
QuickDraw
QuickDraw is the 2D graphics library and associated Application Programming Interface which is a core part of the classic Apple Macintosh operating system. It was initially written by Bill Atkinson and Andy Hertzfeld. QuickDraw still exists as part of the libraries of Mac OS X, but has been...
-based applications use bitmapped drawing exclusively. Adobe's copyright stipulations for the PDF standard are much less restrictive, granting conditional copyright permission to anyone to use the format in software applications, free of charge.