Display PostScript

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.

While early versions of PostScript display systems were developed at Adobe Systems, the full implementation of Display PostScript was developed for Sun Microsystems' NeWS in 1986. NeXT also 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 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. In DPS these names could be replaced by small numbers, 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 halftones would 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 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 300dpi, not the 96dpi that a NeXT screen used. This required hand-built fonts for reasonable output.

• 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. This greatly limited the popularity of DPS when any alternative was available.

Display PostScript at NeXT

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 bitmaps, 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 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.

References

• The most recent PDF specification, version 1.7
• PostScript Language Reference, Second Edition, which includes Display PostScript information
• Display PostScript reference documents

External links

• Description at C2 Wiki
• GNU/BackBone