Sustainability of Digital Formats: Planning for Library of Congress Collections

PostScript is primarily known as a page description language used in the electronic publishing and desktop publishing business. However, it is also a general-purpose programming language with a focus on support for two-dimensional graphic design. As a language, PostScript has a structure that supports efficient direct interpretation of the language at runtime rather than requiring a separate compilation stage before the code can be executed. Soon after the release of the PostScript language by Adobe Systems Incorporated in 1984, Steve Jobs of Apple persuaded Adobe to adapt it to drive laser printers. In March 1985, the Apple LaserWriter was the first printer to have an embedded interpreter for the PostScript language. Many laser printers were based on PostScript through the 1990s. The different aspects of PostScript are highlighted in the Basic Ideas chapter of the PostScript Language Reference Manual, "Obtaining a complete understanding of the PostScript language requires considering it from several points of view:

• As a general-purpose programming language with powerful built-in graphics primitives
• As a page description language that includes programming features
• As an interactive system for controlling raster output devices (printers and displays)
• As an application- and device-independent interchange format for page descriptions."

This format description focuses on the fourth aspect of PostScript, as an interchange format. The PostScript Language Document Structuring Conventions Specification (DSC) (link through Internet Archive - originally published as Appendix G of the PostScript Language Reference Manual and later published separately) supports the use of PostScript as an application-independent file format for documents. DSC provides conventions for exposing the structure of the file and its characteristics through PostScript comments, which begin with "%". A PostScript page-description file is recognized by its first line, which begins "%!PS-Adobe-" and continues with a version number. Other DSC comment lines begin with two percent characters ("%%") and a keyword. Some keywords are followed immediately by a colon (":") and intended to convey options or arguments. For example, each page should begin with "%%Page:" followed by two arguments, a page label and a sequential page number. Such formalized comments provide an interoperable machine-readable mechanism to convey information to systems other than PostScript interpreters.

The PostScript language is proprietary to Adobe, but fully documented in the PostScript Language Reference Manual, most recently in version 3, published in 1999. Adobe staff have indicated in user forums, e.g., in PostScript verses PDF, a thread starting in September 2007 in the PrintPlanet Forum, that Adobe has no intention of producing a fourth version of the language. Enhancements to Adobe's imaging model, including support for layers and transparency, have been made to the PDF format and not to the PostScript language. In most page-description contexts, members of the PDF Format Family have superseded PostScript. See PostScript vs. PDF for an explanation from Adobe which states, "When you hear people say that PDF is a 'replacement' for PostScript, what they most likely mean is that PDF is a replacement for saved PostScript files and EPS files. PDF is not a replacement for the PostScript language or the PostScript processors that live inside of printers, imagesetters, and platesetters."

The usual file extension for a PostScript document is .ps. The PRONOM database at the UK National Archives identifies five chronological versions of .ps file, based on the DSC version declared in the first line of the file: versions 1.0, 2.0, 2.1, 3.0, 3.1. Each version introduced a number of important structuring comments.

A special subtype of PostScript file is an Encapsulated PostScript File (EPS). An EPS file employs the PostScript language to represent a single rectangular graphic. As stated in the PostScript Language Reference Manual, "An encapsulated PostScript (EPS) file is a PostScript program describing at most a single page in a form that can be imported by other applications to embed within a containing document." An illustration encoded as an EPS file can be embedded in a PostScript document file.

An additional use of PostScript is referred to as Display PostScript (DPS). DPS employed a graphics engine that uses the PostScript imaging model and language to generate on-screen graphics. The NeXT computer used DPS throughout its history (1985-1997) and DPS ran on Unix workstations during the same period.

PostScript is a proprietary but publicly documented programming language and page-description language from Adobe Systems Incorporated.

The latest version of the language specification is PostScript Language Reference, Third Edition, published in 1999 (link via Internet Archive). This manual makes clear which PostScript operators and features were added in which version (aka Language Level). The previous version, PostScript Language Reference, Second Edition was published in 1990 (link via Internet Archive). This manual included a description of Display Postscript, omitted in the the third edition.

The latest published edition, version 3.0, of the Document Structuring Conventions Specification was published in 1992 as Technical Note #5001. This was current as of 1999, when the third edition of the PostScript Language Reference was published. This manual identifies which structuring comments were introduced in this and earlier versions. The PostScript Technology Center (link via Internet Archives) provides access to these and other documents supporting use of PostScript.

PostScript has been widely used in commercial printing systems since the mid-1980s. Laser printers used with personal computers almost always employed PostScript through the 1990s. The Wikipedia entry for PostScript, indicates, "By 2001, few lower-end printer models came with support for PostScript, largely due to growing competition from much cheaper non-PostScript ink jet printers, and new software-based methods to render PostScript images on the computer, making them suitable for any printer; PDF, a descendant of PostScript, provides one such method, and has largely replaced PostScript as de facto standard for electronic document distribution." In 2017, the Macintosh operating system has built-in support for rendering PostScript files in the Preview application, and for saving print output as a PostScript file. For Windows, Microsoft supplies the Microsoft PostScript Printer Driver but this driver is not shipped with Windows by default.

The use of PostScript as an interchange format for documents has been most common for scientific articles. See the explanation of PostScript Variants available as output from the arXiv repository of physics preprints. In most contexts, direct use of PostScript as an interchange format for documents has been superseded by use of PDF.

A key resource that supported the use and adoption of PostScript is Ghostscript, an interpreter for the PostScript language. According to the Wikipedia entry on Ghostscript, Ghostscript was originally written by L. Peter Deutsch for the GNU Project (an early example of a large-scale collaborative effort for building free software), and released under the GNU General Public License in 1986. Among other functions, Ghostscript can be used as a raster image processor (RIP) for converting PostScript page descriptions for transmission to raster printers without embedded PostScript support. Ghostscript can also be used to convert PostScript documents to PDF. Ghostscript was ported to many operating systems, including Unix-like systems, classic Mac OS, OpenVMS, Plan 9, MS-DOS, FreeDOS, OS/2, Atari TOS, AmigaOS, and Microsoft Windows.

Page Description Languages | PostScript lists many resources related to PostScript, including software tools. An important tool is Adobe Acrobat Distiller, which can convert PostScript documents to PDF. The Wikipedia entry for Encapsulated PostScript (EPS) lists applications that support EPS.

There appear to be no licensing problems associated with creating or editing PostScript files. However, in practice, use of a PostScript interpreter, usually embedded in a software application or printer system, is essential for display, printing, or other use. Adobe embeds a PostScript interpreter into its own products and has made a PostScript Software Development Kit as well as a print engine available for license by commercial partners. GhostScript has been available since 1986 as an open source PostScript interpreter, but in 2017, commercial distribution requires a license, according to the Wikipedia entry for GhostScript.

In common with other programming languages, PostScript files are plain text files. It is readable by humans.  However, since PostScript is a fully functional programming language, and can be used to describe graphics in very complex ways, in practice, a PostScript interpreter is essential to render the file for viewing.

Certain pre-defined metadata elements can be supplied as PostScript comments, as specified in DSC (PostScript Language Document Structuring Conventions Specifications). These are intended primarily for use by PostScript printing applications to manage workflow. Three of the elements provide contextual metadata of broader use: %%Creator, to identify the creating application; %%Title, for a text title for the document; %%CreationDate, for a text date (in no particular form).

Neither the PostScript language nor the DSC specification provides a standard mechanism for incorporating rich descriptive or contextual metadata in an externally specified format.

An important characteristic of the PostScript language and a key reason for its adoption for desktop publishing was its innovative capability for handling fonts. As stated in Adobe's Adobe Type and type technology resources page, "Type 1 fonts are a specialized form of PostScript program and are the original file format used for type display on all PostScript printers. The PostScript language was later extended to provide support for the later TrueType and OpenType font standards. Any new Adobe PostScript language device made today supports all three font standards."

As a programming language, PostScript is concatenative and stack-oriented. The Wikipedia article on stack-oriented programming language includes some examples of PostScript code. PS Files the PostScript format for Graphics and Text provides examples of PostScript together with corresponding bitmapped representations.

Although PostScript is not usually used as a general-purpose programming language, it has been used for complex mathematical computations. The TeX community has a collection of macros known as PSTricks, which are routines written in PostScript that can be embedded in TeX documents. One example is PST-ODE, a module for solving initial value problems for sets of ordinary differential equations.

Version 3.0 of DSC provided an extension mechanism, known as Open Structuring Conventions, using a registry for comment prefixes that could be applied for by vendors. The compilers of this resource are not aware of the level of use of this feature. Comments welcome.

A Jan 2017 post from The Register, We don't want to alarm you, but PostScript makes your printer an attack vector, publicizes the vulnerability of networked PostScript printers. The potential for damaging exploits based on communication between computers and PostScript printers has been understood for years, but was not a serious threat before PostScript printers were networked and accessible over the Internet. See also PostScript page from Hacking Printers.

Introduced in 1985, the PostScript language has been extended twice to adapt to new imaging technologies and system environments. While these extensions introduced significant new functionality and flexibility to the language, the basic imaging model remains unchanged. The result is an organization into three Language Levels: Level 1 (the 1985 version); Level 2 (1990); and Level 3 (1999). Each Language Level encompasses all features of previous Language Levels as well as new features. Thus, for example, a feature identified in the reference manual as included in "Language Level 2" is expected to be available in all Language Level 3 implementations as well.

More detail is provided in The history of PostScript from Prepressure.com and History of PostScript from rgbcmyk, a blog by Eduardo Garcia on digital imagery and its representation.

In a discussion thread on PostScript verses PDF starting in September 2007 in the PrintPlanet Forum, Dov Isaacs of Adobe said, " there is not a PostScript Language Level 4 sitting on disk somewhere waiting for marketing to give a signal. Beginning with PDF 1.4, all additions to the Adobe imaging model went into PDF and not PostScript. To be very clear, Adobe will continue to license PostScript technology through our EOM partners as long as there is a demand for it from their customers. We will also continue support a gateway from PostScript to PDF via our Distiller technology in Acrobat. Adobe will continue to support EPS as a 'legacy' graphics format for import of non-color managed, opaque graphical data into Adobe applications (such as InDesign and Illustrator). Although we certain do not recommend that new graphical content be stored in EPS format (except to satisfy the need to import data into page layout programs that aren't quite PDF-centric -- no need to mention names here!), our user base should feel comfortable that there is no need to worry about a need to convert their very sizable libraries of EPS-based graphic assets."