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Line 3: Display PostScript is a fairly limited expansion on the original [[PostScript]] language. 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 (~~colours~~colors, brush settings, scale, etc.). This required a modification to the system to allow it to keep several "contexts" (sets of data) active, one for each process (window). ''~~encoding~~encoded names'': many of the ~~proceedures~~procedures and data structures in PostScript are looked up by name. In DPS these names could be replaced by small numbers, which are much ~~easier and~~ faster for a computer to find. ''interaction support'': a number of ~~proceedures~~procedures were defined to handle interaction, including hit detection ''halftone phase'': [[halftone]]s need to be matched to the device they are printed on in order to avoid [[moiré pattern]]s. Since the display can be moved and changed on the fly, DPS added the ability for the device to change the halftoning to match. ''incremental update'': in printing applications the PS code is interpreted until it gets a <code>showpage</code> at which point it is actually printed out. This is not suitable for a display situtuation where you need a large number of minor updates all the time. Line 23: [[Apple Computer\|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 primatives, font handling and measurements, and in many cases looks and feels like DPS. It is not entirely clear why this happened, but speculation suggests that Adobe was asking for a high licencing fee. '''See also:''' :[[Portable Document Format]] ''~~External links~~Links:'' :[http://partners.adobe.com/asn/developer/pdfs/tn/psrefman.pdf PostScript Language Reference Manual] <br>- includes details on DPS in Chapter 7

Display PostScript: Difference between revisions