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NeXT Computer Inc. designed Display PostScript (or DPS) as a display system for their series of Unix-based personal computers starting around 1987. Display PostScript was developed with (or given to) Adobe, and made an official Adobe product with its own standards documents and licensing requirements.
As the name implies, DPS uses the PostScript (PS) imaging model and language to generate on-screen graphics. In order to support interactive, on-screen use with reasonable performance, a few changes were needed:
- multiple execution contexts: unlike a printer setting where the PS interpreter had only 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
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 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.
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 now makes use of a similar imaging model to Display PostScript, but does not have the same level of programmability. The new system, known as Quartz, is based on the PDF model in which the source of the image is not the PostScript code itself, but the result of interpreting that code. It keeps the basic graphics primitives, font handling and measurements, and in many cases looks and feels like DPS. It is not entirely clear why this switch happened, but speculation suggests that Adobe was asking for a high licensing fee.
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