Display PostScript

Display PostScript (or DPS) is a 2D graphics engine system for computers which uses the PostScript (PS) imaging model and language (originally developed for computer printing) to generate on-screen graphics. To the basic PS system, DPS adds a number of features intended to ease working with bitmapped displays and improve performance of some common tasks.

Early versions of PostScript display systems were developed at Adobe Systems. During development of the NeXT computers, NeXT and Adobe collaborated to produce the official DPS system, which was released in 1987. NeXT used DPS throughout its history, while versions from Adobe were popular on Unix workstations for a time during the 1980s and 1990s.

Design

In order to support interactive, on-screen use with reasonable performance, changes were needed:

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.

History

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.

Modern derivatives

Apple'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 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.

See also

External links

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