PostScript

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 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 (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.

Prior to the introduction of Interpress and 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 shared 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.

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, 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: the execution of which results 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 in which 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.

Almost as complex as PostScript itself is its handling of fonts. The font system uses the PS graphics primitives to draw glyphs as curves, which can then be rendered at any resolution. A number of typographic issues had to be considered with this approach.

One issue is that fonts do not scale linearly at small sizes and features of the glyphs will become proportionally too large or small and start to look displeasing. PostScript avoided this problem with the inclusion of font hinting, in which additional information is provided in horizontal or vertical bands to help identify the features in each letter that are important for the rasterizer to maintain. The result was significantly better-looking fonts even at low resolution. It had formerly been believed that hand-tuned bitmap fonts were required for this task.

At the time, the technology for including these hints in fonts was carefully guarded, and the hinted fonts were compressed and encrypted into what Adobe called a Type 1 Font (also known as PostScript Type 1 Font, PS1, T1 or Adobe Type 1). Type 1 was effectively a simplification of the PS system to store outline information only, as opposed to being a complete language (PDF is similar in this regard). Adobe would then sell licenses to the Type 1 technology to those wanting to add hints to their own fonts. Those who did not license the technology were left with the Type 3 Font (also known as PostScript Type 3 Font, PS3 or T3). Type 3 fonts allowed for all the sophistication of the PostScript language, but without the standardized approach to hinting.

The Type 2 font format was designed to be used with Compact Font Format (CFF) charstrings, and was implemented to reduce the overall font file size. The CFF/Type2 format later became the basis for handling PostScript outlines in OpenType fonts.

The CID-keyed font format was also designed, to solve the problems in the OCF/Type 0 fonts, for addressing the complex Asian-language (CJK) encoding and very large character set issues. The CID-keyed font format can be used with the Type 1 font format for standard CID-keyed fonts, or Type 2 for CID-keyed OpenType fonts.

To compete with Adobe's system, Apple designed their own system, TrueType, around 1991. Immediately following the announcement of TrueType, Adobe published the specification for the Type 1 font format. Retail tools such as Altsys Fontographer (acquired by Macromedia in January 1995, owned by FontLab since May 2005) added the ability to create Type 1 fonts. Since then, many free Type 1 fonts have been released; for instance, the fonts used with the TeX typesetting system are available in this format.

In the early 1990s, there were several other systems for storing outline-based fonts, developed by Bitstream and Metafont for instance, but none included a general-purpose printing solution and they were therefore not widely used.

In the late 1990s, Adobe joined Microsoft in developing OpenType, essentially a functional superset of the Type 1 and TrueType formats. When printed to a PostScript output device, the unneeded parts of the OpenType font are omitted, and what is sent to the device by the driver is the same as it would be for a TrueType or Type 1 font, depending on which kind of outlines were present in the OpenType font.

In the 1980s, Adobe drew most of its revenue from the licensing fees for their implementation of PostScript for printers, known as a raster image processor or RIP. As a number of new RISC-based platforms became available in the mid-1980s, some found Adobe's support of the new machines to be lacking.

This and issues of cost led to third-party implementations of PostScript becoming common, particularly in low-cost printers (where the licensing fee was the sticking point) or in high-end typesetting equipment (where the quest for speed demanded support for new platforms faster than Adobe could provide). At one point, Microsoft licensed to Apple a PostScript-compatible interpreter it had bought called TrueImage, and Apple licensed to Microsoft its new font format, TrueType. Apple ended up reaching an accord with Adobe and licensed genuine PostScript for its printers, but TrueType became the standard outline font technology for both Windows and the Macintosh.

Today, third-party PostScript-compatible interpreters are widely used in printers and multifunction peripherals (MFPs). For example, CSR plc's IPS PS3[6] interpreter, formerly known as PhoenixPage, is standard in many printers and MFPs, including those developed by Hewlett-Packard and sold under the LaserJet and Color LaserJet lines. Other third-party PostScript solutions used by print and MFP manufacturers include Jaws[7] and the Harlequin RIP, both by Global Graphics. A free software version, with several other applications, is Ghostscript. Several compatible interpreters are listed on the Undocumented Printing Wiki.[8]

Some basic, inexpensive laser printers do not support PostScript, instead coming with drivers that simply rasterize the platform's native graphics formats rather than converting them to PostScript first. When PostScript support is needed for such a printer, Ghostscript can be used. There are also a number of commercial PostScript interpreters, such as TeleType Co.'s T-Script.

A Hello World program, the customary way to show a small example of a complete program in a given language, might look like this in PostScript (level 2):

 %!PS
 /Courier             % name the desired font
 20 selectfont        % choose the size in points and establish 
                      % the font as the current one
 72 500 moveto        % position the current point at 
                      % coordinates 72, 500 (the origin is at the 
                      % lower-left corner of the page)
 (Hello world!) show  % stroke the text in parentheses
 showpage             % print all on the page

or if the output device has a console

 %!PS
 (Hello world!) =

PostScript uses the point as its unit of length. However, unlike some of the other versions of the point, PostScript uses exactly 72 points to the inch. Thus:

1 point = 1/72 inch = 25.4/72 mm = 127/360 mm = 352.777… micrometers

For example, in order to draw a vertical line of 4 cm length, it is sufficient to type:

0 0 moveto 
0 113.385827 lineto stroke

More readably and idiomatically, one might use the following equivalent, which demonstrates a simple procedure definition and the use of the mathematical operators mul and div:

/cm {72 mul 2.54 div} def               % 1 inch = 2.54 cm exactly
 0 0 moveto
 0 4 cm lineto stroke

Most implementations of PostScript use single-precision reals (24-bit mantissa), so it is not meaningful to use more than 9 decimal digits to specify a real number, and performing calculations may produce unacceptable round-off errors.