Prior to the introduction of PostScript, printers were designed to print character output given the text—typically in ASCII—as input. There were a number of technologies for this task, but most shared the property that the glyphs were physically difficult to change, as they were stamped onto typewriter keys, bands of metal, or optical plates.
This changed to some degree with the increasing popularity of dot matrix printers. The characters on these systems were drawn as a series of dots, as defined by a font table inside the printer. As they grew in sophistication, dot matrix printers started including several built-in fonts from which the user could select, and some models allowed users to upload their own custom glyphs into the printer.
Dot matrix printers also introduced the ability to print raster graphics. The graphics were interpreted by the computer and sent as a series of dots to the printer using a series of escape sequences. These printer control languages varied from printer to printer, requiring program authors to create numerous drivers.
Vector graphics printing was left to special-purpose devices, called plotters. Almost all plotters did share a common command language, HPGL, but were of limited use for anything other than printing graphics. In addition, they tended to be expensive and slow, and thus rare.
PostScript printing
Laser printers combine the best features of both printers and plotters. Like plotters, laser printers offer high quality line art, and like dot-matrix printers, they are able to generate pages of text and raster graphics. Unlike either printers or plotters, however, a laser printer makes it possible to position high-quality graphics and text on the same page. PostScript made it possible to fully exploit these characteristics, by offering a single control language that could be used on any brand of printer.
PostScript went beyond the typical printer control language and was a complete programming language of its own. Many applications can transform a document into a PostScript program whose execution will result in the original document. This program can be sent to an interpreter in a printer, which results in a printed document, or to one inside another application, which will display the document on-screen. Since the document-program is the same regardless of its destination, it is called device-independent.
PostScript is noteworthy for implementing on-the fly rasterization; everything, even text, is specified in terms of straight lines and cubic Bézier curves (previously found only in CAD applications), which allows arbitrary scaling, rotating and other transformations. When the PostScript program is interpreted, the interpreter converts these instructions into the dots needed to form the output. For this reason PostScript interpreters are occasionally called PostScript raster image processors, or RIPs.