Encapsulated PostScript

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Encapsulated PostScript
Filename extension .eps
Type code EPSF
Uniform Type Identifier com.adobe.encapsulated-​postscript
Extended from PostScript

Encapsulated PostScript, or EPS, is a DSC-conforming PostScript document with additional restrictions intended to make EPS files usable as a graphics file format. In other words, EPS files are more-or-less self-contained, reasonably predictable PostScript documents that describe an image or drawing, that can be placed within another PostScript document.

At a minimum, an EPS file contains a BoundingBox DSC comment, describing the rectangle containing the image described by the EPS file. Applications can use this information to lay out the page, even if they are unable to directly render the PostScript inside.

EPS, together with DSC's Open Structuring Conventions, form the basis of early versions of the Adobe Illustrator Artwork file format.


[edit] EPS previews

EPS files also frequently include a preview picture of the content, for on-screen display. The idea is to allow a simple preview of the final output in any application that can draw a bitmap. Without this preview the applications would have to directly render the PostScript (PS) data inside the EPS, which was beyond the capabilities of most machines until recently.

When EPS was first implemented, the only machines widely using PostScript were Apple Macintoshes. These machines could not directly render the PostScript, which presented Adobe with the problem of how to provide a preview image while also including the actual PS version for the printer. On the Mac this turned out to be easy to solve, as the Mac file system includes two parts (known as forks) that are logically referred to as one file. By placing the PostScript in the data fork and a standard Mac PICT resource in the resource fork, both images could be moved about together invisibly as if they were one file. While a PICT preview often contains a bitmap it could also contain a vector representation of the whole image, providing very high quality previews.

Neither of these technologies exist on any other operating system, however.[dubious ] When faced with the same problems on Microsoft Windows-based versions of their programs, Adobe chose to instead include a TIFF file encoded into the header section of the PostScript. Sometimes, though more rarely, they used the WMF (Windows Metafile) format instead. WMF has the potential to provide vector previews, similar to PICT on the Mac. Both of these PC format EPS files have a particular disadvantage: because the PostScript data, header and preview are all in the same file, they will cause printing errors if a program does not understand the format well enough to extract only the PostScript data.

A fourth format known as a EPSI includes an ASCII-encoded preview bitmap. This format allows for black-and-white previews only. It is mainly used on UNIX systems.

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.

The most widely supported kind of preview is a Windows format preview with a TIFF.

[edit] Beyond previews

In recent years, applications have started appearing which ignore the preview portion of an EPS file, yet still show the preview on screen. They do this by interpreting the PostScript to get their own preview. This has become possible with the increased processing power of modern computers; when EPS was first designed this would have been either beyond the abilities of the typical computer, or too slow to have been acceptable.

The application might retain the PostScript portion for PostScript printing, or it might discard it, using only its converted data. This has a problem in that device-specific tricks that might have been hidden in the EPS may be lost.

Unfortunately, in many cases the PostScript interpreter is of poorer quality, or is limited, compared to the one in a printer.[citation needed] This may mean that no preview is shown, leaving the new technology less useful than the old.[who?]

Current versions of Microsoft Office (starting with Office 2003) include this new method.

[edit] Identifying EPS files

Because of the different ways in which EPS previews are handled, there is no one way to identify an EPS file.

  • A Windows-format EPS file containing a TIFF or WMF preview must start with the four bytes containing, in hexadecimal, C5 D0 D3 C6. Bear in mind these files are widespread on all platforms.
  • In all other cases an EPS file must start with a line %!PS-Adobe-a.b EPSF-c.d where a, b, c and d are all single digit numbers.
    • A Mac-format EPS file is accompanied by a resource fork. The preview is a PICT resource with ID 256. An EPS file on the Mac is expected to have a file type code of "EPSF", whether or not it has a preview.
    • An EPSI file will contain a line starting %%BeginPreview: in the DSC prolog.
    • In many cases no preview is present at all.

[edit] Saving to EPS file format

A number of programs will save or convert text and vector art to EPS format, including:

Many image converter programs can create EPS files containing the pixels of the image.

An EPS file is a stream of generic PostScript printing commands. Thus many PostScript printer drivers have an option to save as EPS, or to add EPS DSC information to their output which you can "print to file". Saving as EPS was a feature of Microsoft's PSCRIPT.DRV Windows printer driver and Adobe's ADOBEPS.DRV Windows printer driver for Windows versions prior to Windows 2000.[1]

[edit] See also

[edit] References

  • Adobe Developer Support, 1992. Encapsulated PostScript — File Format Specification, Version 3.0, Tech Note #5002, Adobe Systems Incorporated, San Jose, CA, USA, 34 pp. PDF

[edit] Notes

  1. ^ Making an EPS file, Quite Software, November 11, 2002; retrieved October 9, 2008
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