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PostScript

File format and programming language

  • 18 min read
  • 6 chapters

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History

The concepts of the PostScript language were seeded in 1976 by John Gaffney at Evans & Sutherland, a computer graphics company. At that time, Gaffney and John Warnock were developing an interpreter for a large three-dimensional graphics database of New York Harbor.

Concurrently, researchers at Xerox PARC had developed the first laser printer and had recognized the need for a standard means of defining page images. In 1975–76 Bob Sproull and William Newman developed the Press format, which was eventually used in the Xerox Star system to drive laser printers. But Press, a data format rather than a language, lacked flexibility, and PARC mounted the Interpress effort to create a successor.

In 1978, John Gaffney and Martin Newell then at Xerox PARC wrote J & M or JaM (for "John and Martin") which was used for VLSI design and the investigation of type and graphics printing. This work later evolved and expanded into the Interpress language.

Warnock left with Chuck Geschke and founded Adobe Systems in December 1982. They, together with Doug Brotz, Ed Taft and Bill Paxton created a simpler language, similar to Interpress, called PostScript, which went on the market in 1984.

Meanwhile, in the spring of 1983, Steve Jobs came to visit Adobe and was dazzled by PostScript's potential, especially for the new Macintosh computer he was developing at Apple. To John Sculley's frustration, Jobs licensed the PostScript technology from Adobe by offering a $1.5 million advance against PostScript royalties, plus $2.5 million in exchange for 20 percent of Adobe shares. During a series of meetings in 1983, Jobs also repeatedly offered for Apple to buy Adobe outright, but the founders kept turning him down. In December 1983, the two companies finally signed off on the PostScript licensing deal, and Adobe had to shift focus immediately from high-end, high-resolution printing devices to the consumer-oriented Apple LaserWriter laser printer.

At that time, the 300-dpi Canon laser printing engine to be used in LaserWriters was seen as good enough only for proof printing (i.e., for crude rough drafts of material whose final drafts would be sent to professional high-resolution devices), but Jobs presented Adobe with the challenge of making PostScript render high-quality output to such a low-resolution device (which for most consumers would be their only printing device). In response, Warnock and Brotz solved the so-called "appearance problem" of making the stem width of letters scale properly so that they look good at all resolutions. Their breakthrough was so important that Adobe never patented the technology, in order to keep its details concealed as a trade secret. Many years later, Warnock revealed the trade secret in a 2010 lecture, and in 2022, Adobe publicly released an early version of PostScript's original source code from February 1984. Paxton worked on several other related improvements, such as font hinting. This work occurred after February 1984, meaning that Paxton's improvements were not included in the source code release and are still protected as trade secrets. Adobe was also responsible for porting PostScript to the Canon's Motorola 68000 chip.

Apple and Adobe announced the LaserWriter at Apple's annual stockholder meeting on January 23, 1985. It was the first printer to ship with PostScript, sparking the desktop publishing (DTP) revolution in the mid-1980s. The original PostScript royalty was five percent of the list price for each laser printer sold, which was $350 of the original LaserWriter list price of $6,995, and such royalties provided nearly all of Adobe's income during its early years. (Apple later renegotiated the contract to pay a licensing fee based on volume of printers shipped.) The combination of technical merits and widespread availability made PostScript the language of choice for graphical output for printing applications. An interpreter (sometimes referred to as a RIP for Raster Image Processor) for the PostScript language was a common component of laser printers during the 1980s and 1990s.

However, the cost of implementation was high; computers output raw PS code that would be interpreted by the printer into a raster image at the printer's natural resolution. This required high-performance microprocessors and ample memory. The LaserWriter used a 12 MHz Motorola 68000, making it faster than any of the Macintosh computers to which it was attached. By 1987, the cost of licensing PS and fonts from Adobe was $400 to $900 per printer. When the laser printer engines themselves cost over a thousand dollars, the added cost of PS was marginal. But, as printer mechanisms fell in price, the cost of implementing PS became too great a fraction of overall printer cost. In addition, with desktop computers becoming more powerful during the 1990s than their attached printers, it no longer made sense to offload the rasterization work onto the resource-constrained printer. By 2001, few low-end printer models came with onboard support for PostScript, largely due to growing competition from much cheaper non-PostScript inkjet printers, and new software-based methods to render PostScript images on computers, making them suitable for any printer. PDF, a descendant of PostScript, provides one such method, and has largely replaced PostScript as the de facto standard for electronic document distribution.

On high-end printers, PostScript processors remain common, and their use can dramatically reduce the CPU work involved in printing documents, transferring the work of rendering PostScript images from the computer to the printer.

PostScript Level 1

The first version of the PostScript language was released to the market in 1984. The qualifier Level 1 was added when Level 2 was introduced.

PostScript Level 2

PostScript Level 2 was introduced in 1991, and included several improvements: improved speed and reliability, support for in-Raster Image Processing (RIP) separations, image decompression (for example, JPEG images could be rendered by a PostScript program), support for composite fonts, and the form mechanism for caching reusable content.

PostScript 3

PostScript 3 (Adobe dropped the "level" terminology in favor of simple versioning) came at the end of 1997, and along with many new dictionary-based versions of older operators, introduced better color handling and new filters (which allow in-program compression/decompression, program chunking, and advanced error handling).

PostScript 3 was significant in terms of replacing the existing proprietary color electronic prepress systems, then widely used for magazine production, through the introduction of smooth shading operations with up to 4096 shades of grey (rather than the 256 available in PostScript Level 2), as well as DeviceN, a color space that allowed the addition of additional ink colors (called spot colors) into composite color pages.

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