Sustainability of Digital Formats: Planning for Library of Congress Collections

The Encapsulated PostScript File Format, usually abbreviated as EPS and sometimes as EPSF, was developed in the late 1980s by Adobe Systems Incorporated to facilitate the incorporation of illustrations into textual documents for printing. An EPS file employs Adobe's 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." The EPS format is particularly suited to vector graphics or to graphics that combine vector graphics with raster data, sometimes referred to as "metafiles." For such images it is still widely used in scientific publications in 2017; vector graphics are often recommended because they are scalable. It has also been used for raster images, usually using the TIFF format, to be embedded in articles, but since raster images are not inherently scalable, and most publication processes now provide direct support for raster formats such as TIFF, PNG, or JPEG, this usage is now less common. In many contexts, EPS has been superseded by PDF.

EPS is a proprietary but publicly documented format. The format specification was originally published by Adobe in the late 1980s; version 3 was first released in 1990 and published as a separate document in 1992. The specification uses EPSF as the acronym for the format, but describes files conforming to the specification as EPS files. This description uses EPS in both contexts, as more consistent with later usage. The EPS format uses a primarily textual file based on Adobe's PostScript language, defined in the PostScript Language Reference Manual; an EPS file is also expected to comply with the PostScript Language Document Structuring Conventions Specification (DSC), which provides conventions for using PostScript comments to convey document characteristics and printing instructions. An EPS file is constrained to represent a single rectangular area. Although the primary purpose of an EPS file is for an illustration to be included in other pages, it is also used for layouts of complete pages. EPS has been considered a good choice of format for vector graphic illustrations intended for high-resolution or large-scale printing and commonly used for printing to PostScript printers and imagesetters. EPS files are typically created and edited in illustration programs such as Adobe Illustrator or CorelDRAW. As vector graphics, EPS files have been particularly useful for illustrations intended for use at different scales, such as logos and advertisements. They have also been widely used in scientific publishing for graphs and diagrams to be embedded in articles or books.

EPS Version 3.0 is the latest published version of the specification. See History Note below for more detail on the early chronology. Despite acknowledged shortcomings compared with more recent formats, its widespread adoption by certain industry segments and in important workflows, means that it is still in common use. Several aspects of the format reflect the technology environment of the late 1980s. In particular, the specification defines variations specific to the primary personal computer operating systems in use at the time, Macintosh and DOS.

A typical EPS file contains not only the PostScript code that defines the image in a form that can be transmitted to a PostScript printer without information loss, but also a "preview" image in a format intended for convenient use in a workflow that involves several systems or applications.  The intent of a preview is to have an image in a format that most graphics applications can render; a preview is usually of lower resolution, in pixel dimensions and/or in bit-depth.  The preview file can be in one of a number of formats.  The specification for EPS_3 lists three "device-specific" preview formats: for the Apple Macintosh, a PICT image as used by the QuickDraw application; for DOS computers, a TIFF bitmap or Windows Metafile. PICT and Windows Metafile can incorporate both bitmap data and vector graphics. In addition, the specification defines a very simple device-independent representation for an embedded bitmapped preview image. This representation is known as Encapsulated PostScript Interchange Format, or EPSI. An EPSI preview is a bitmap represented as ASCII hexadecimal, wrapped between a few PostScript comments for identification and intended to be simple and easily transportable. In order to distinguish EPS files with the different preview formats, different DOS file extensions and Macintosh file types were recommended in the EPS specification. See Notes below for more detail. The Wikipedia entry for Encapsulated PostScript states that the DOS/Windows format with a TIFF preview is the most widely supported variant. In the Macintosh version of some Adobe graphics applications, the options for saving or exporting EPS files include the use of PICT previews. The compilers of this resource have not been able to determine whether the other preview variants are still in use. Comments welcome. An attempt to open an EPS file in many graphics applications will present the preview without any indication that the rendered image is not the main image.

EPS files use lines of 255 or fewer ASCII characters. The specification recommends 7-bit ASCII. Lines may be terminated by any of the new line character combinations: CR (hex 0D), LF (hex 0A), CR LF, or LF CR.

The PostScript code that specifies an EPS image is subject to constraints, specified in the PostScript Language Document Structuring Conventions Specification (DSC). DSC is a set of conventions for PostScript documents, based on the use of comments. The structure of the file and its characteristics are exposed through comments introduced with '%' characters. The comments provide a standard way expose the structure to systems other than PostScript interpreters in a machine-readable way. Most importantly, an EPS file must include two required DSC header comments:

• %!PS-Adobe-3.0 EPSF-3.0
  This provides a magic number for identifying an EPS 3.0 file. For version 3.1 the string is "%!PS-Adobe-3.1 EPSF-3.0".
• '%%BoundingBox: llx lly urx ury'
  The four arguments of the bounding box comment correspond to the lower-left (llx, lly) and upper-right (urx, ury) corners of the bounding box.

In addition to these mandatory comment lines, a compliant EPS file uses additional comments to indicate structure and features. For example, specific comments are used to indicate that certain PostScript language versions or extensions must be present in the interpreter used for printing.

The EPS specification is brief and its effective use requires an understanding of the structure and context of the underlying PostScript Language and of the technological environment of the late 1980s. The EPS file format from Prepressure.com provides a more informal introduction and more current contextual information about the format together with practical guidance.

Adobe has stated that there will be no further versions of the PostScript Language or the EPS Specification, but Adobe's existing PostScript technology will continue to be available to license to commercial partners whose customers require it. See History Notes below for a specific statement that discusses EPS. As enhancements to the imaging model have been needed, Adobe has made them in PDF and not in the PostScript language.

In 2017, security threats were identified and exploited in EPS files. As a result, Microsoft disabled its import filter for EPS in distributions of its Office products; customers requiring EPS support must make registry changes in their Windows operating system. See Notes and Useful References below for more details on security issues related to the EPS format and networked PostScript printers.

Version 3 of Encapsulated PostScript (EPS) is a proprietary but publicly documented vector graphics format from Adobe Systems Incorporated.

EPS has been widely used as a vector graphics format since its introduction in the late 1980s, particularly for images intended for placement in other documents, such as drawings, charts, diagrams, when high-resolution printing was required. EPS is not supported for viewing in browsers. Help for saving artwork in Adobe's Illustrator application states that EPS is one of five basic file formats that are considered "native" in that they can preserve all Illustrator data if appropriate options are chosen. A File Formats page for Corel partners states, "An industry standard, EPS is perhaps the most commonly used file format when dealing with graphic design and professional output." (The original information from this page was no longer available when checked in July 2021.) The Wikipedia entry for Encapsulated PostScript lists over 30 applications that can create EPS files. EPS is included in the list of File Formats supported by Vector Magic, which indicates that EPS is "perhaps the most common vector image format. It is the standard interchange format in the print industry. It is widely supported as an export format, but due to the complexity of the full format specification, not all programs that claim to support EPS are able to import all variants of it."

EPS has been part of well established workflows and without additional reasons for change, has continued as a preferred format in those workflows, despite shortcomings. Particular shortcomings in the PostScript image model, on which EPS is built, are the lack of support for layers and for transparency. See, for example, Cannot open EPS files in illustrator, particularly the technical responses by Willi Adelberger. Color maintenance is not supported in PostScript or in a single standard way in EPS.

Meanwhile, recommendations can be found in forum posts and guidance from print service providers for continued use of EPS for certain situations. DigiCopy, a print service provider describes EPS as "the best choice of graphics format for high resolution printing of illustrations" and "preferred for large signs and banners" in its list of Print File Formats. In a 2011 post in InDesignSecrets (now CreativePro.com), TIFF vs PSD vs EPS vs PDF vs…, David Blatner states, "EPS is a dying format. There is virtually no reason for you to ever save anything yourself as EPS." He provides two "good reasons to use an EPS file": for re-use of an old vector graphic; or when using special-purpose software (e.g., for barcodes) that makes only EPS graphics. What unique benefits does the EPS format provide?, a 2015 thread on Graphic Design Stack Exchange, provides similar reasons for its continued use.

Use of EPS is still common for scientific articles. Scientific publishers such as IEEE and AAAS argue for use of vector graphics for figures and often recommend EPS. Additionally, the popular LaTeX document preparation system has expected use of EPS for graphics, although recent LaTeX variants support other image formats, particularly PDF, PNG, and JPEG. See Useful References below.

There appear to be no licensing problems associated with creating or editing EPS 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.

The core of an EPS file is in textual form using the PostScript programming language. 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). These comments are strongly recommended in the EPS specification.

The EPS specification provides no standard mechanism for incorporating rich descriptive or contextual metadata in an externally specified format. However, some EPS files do contain XMP metadata.

Accessibility Features

Accessibility for still image content is often supported by the use of alt text when displayed on the web. The carriage of this data is typically not embedded in the file itself but rather in the HTML code. See W3C's Text alternatives for non-text content for more information.

Being a legacy format, EPS has no direct support for accessibility features. A common path is conversion to PDF or other formats with greater support options.

File extensions and Macintosh file types: Because the image type for the preview image that may be stored in an EPS file is significant to operating systems and applications, several different extensions and Macintosh file types have been used. When the EPS specification was written, file extensions in DOS were limited to 3 characters. The type of a Macintosh file was identified by a 4-character code; file names often had no extension component. Since Mac OS X 10.6, the 4-character type codes have been ignored, but they may still be present in files.

• DOS/Windows: The recommended file extension for DOS (and now Windows) was .EPS. However, for EPS files with an EPSI preview, the recommended extension was .EPI.
• Macintosh: The Macintosh file type for application-created PostScript language files was EPSF. A file of type EPSF should contain a PICT resource in the resource fork of the file containing a screen preview image of the EPS file. The EPS data (the image itself) is in the data fork. The file name may follow any naming convention as long as the file type is EPSF. Files of type TEXT were also allowed so users could create EPS files with standard text editors. If the file type was TEXT, the extensions .epsf, and .epsi were to be used for EPS files with Macintosh-specific (typically PICT) and device-independent preview images, e.g., EPSI (Encapsulated PostScript Interchange format), respectively.

The compilers of this resource have not been able to determine whether all the extensions and preview types are still in use. The .eps extension and the TIFF preview appear to be most common. Some Macintosh applications offer the option for a preview using the Macintosh PICT metafile format. Comments welcome.

Interoperability issues: Judging from information found in help files for graphics software and associated user forums, interoperability problems are common with EPS files. The Wikipedia entry for Encapsulated PostScript states "Unfortunately, with several different ways of representing the preview, they have limited portability. An application which is unable to interpret an EPS file's preview will typically show an empty box on screen, but it will be able to print the file correctly." Even across Adobe products, the EPS files created by different applications (for example, Illustrator, InDesign, Photoshop) have different structures and may or may not support different preview types. See a selection of Adobe help files in Useful References below.

Graphics applications have taken advantage of the fact that the EPS format provides a mechanism for embedding arbitrary data. This can generate other compatibility problems. According to a 2006 post, What's in a file? from the RWIllustrator blog, a file saved from Adobe Illustrator CS2 as an Illustrator EPS (.eps) would contain: EPS content (flattened, i.e., without layers or transparency) and embedded Native Illustrator CS2 content (unflattened). The former is for use in other applications, the latter for re-opening in the same version of Adobe Illustrator. A note points out that choosing to save for a different version of Illustrator can cause compatibility problems because the embedded Illustrator content is not recognized. This compatibility issue is highlighted in a screen capture from Adobe Illustrator in EPS – the Zombie among file formats, which shows options for saving as older Illustrator EPS formats. (When checked in July 2021, this link required an Adobe login. The content may be available at an alternate site: https://www.vektorgarten.de/eps-file-format-zombie.html.) Different types of EPS in Photoshop, a 2015 thread on Graphic Design Stack Exchange, explains reasons for EPS options in Photoshop.

Security Issues: In early 2017, Microsoft took steps to prevent exploits based on Word's import "filter" for EPS. Based on the assessment that "EPS files are a legacy format that has largely fallen out of favor in today’s ecosystem", Microsoft disabled the import filter by default in its April 2017 security update for Office. According to PPTools post I can no longer insert EPS graphics into PowerPoint, this update applied to Office 2010, 2013, and 2016/365. Import of EPS files into Office 2007 has been disabled since 2015. The stand-alone Office for Mac is not affected.

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.

The first version of EPS was released by Adobe sometime between 1985 and 1988. Version 2.0 of the EPS specification was published in January 1989. A specification for version 3.0 of EPS was published in December 1990 as Appendix H of the second edition of the PostScript Language Reference Manual. The specification for version 3.0 of EPS was published as a separate document in 1992; this is the latest published version.

Although the last published specification for EPS is for version 3.0, EPS files sometimes contain "EPSF-3.1" in the first line of the file. It has been hypothesized, e.g., in a 2008 discussion thread in an Adobe Forum, that this reflects the potential use of structural comments defined in the 1999 Adobe Technical Note 5644: PostScript Language Document Comment Extensions for Page Layout.

EPS, together with the Open Structuring Conventions extension mechanism described in clause 9 of Adobe's PostScript Language Document Structuring Conventions Specification, formed the basis of early versions of the Adobe Illustrator Artwork file format. See Adobe's draft Adobe Illustrator File Format Specification from 1992.

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."

For more on the history of EPS, the PostScript Language, and associated printing technology, see The History of PostScript from Prepressure.com and History of PostScript from rgbcmyk, a blog by Eduardo Garcia on digital imagery and its representation.