Display PostScript (or DPS) is an on-screen display system. As the name implies, DPS uses the PostScript (PS) imaging model and language to generate on-screen graphics.
NeXT Computer Inc. designed DPS as a display system for their series of Unix-based personal computers starting in 1987. While early versions of PostScript display systems were developed at Adobe Systems, the full implementation of Display PostScript was developed by NeXT in cooperation with Adobe, and made an official Adobe product with its own standards, documents and licensing requirements.
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In order to support interactive, on-screen use with reasonable performance, changes were needed:
showpage at 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.pswrap", which allowed developers to wrap PostScript code into a C language 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, 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.
The developers of NeXT 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 pswraps, which were in turn wrapped in objects and presented to the programmer in object form.
Apple's Mac OS X operating system uses a central window server (created entirely by Apple) that caches window graphics as bitmaps, instead of storing and executing PostScript code. A graphics library called Quartz 2D provides PostScript-style imaging using the PDF graphics primitives (a superset, plus tweaks, of the PostScript model), but this is used by application frameworks—there is no PostScript or PDF 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.
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