Display PostScript

Display PostScript (or DPS) is a

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:

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 windows 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.^{[citation needed]}
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 halftones 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 showpage, at which point it is 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
anti-aliased fonts on grayscale or colour displays, which significantly improved quality. However, this development was too late to be of much use. Modern displays are still around 100 dpi,^{[needs update}
] but have far superior font quality without using bitmap fonts.

Programming language support: DPS introduced the concept of a "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.^{[citation needed]} This greatly limited the popularity of DPS when any alternative was available.^{[citation needed]}

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 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.

Display PostScript

Design

History

Modern derivatives

See also

References

Further reading

External links