Encapsulated PostScript

Encapsulated PostScript, or EPS, is a graphics file format. An EPS file is a PostScript file which satisfies additional restrictions. These restrictions are intended to make it easier for software to embed an EPS file within another PostScript document.

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

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 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 files (known as forks) that are logically referred to as one part. 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 invisibily 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.

But neither of these technologies exist on any other operating system. 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) instead. WMF has the potential to provide vector previews. 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; programs concerned with portability should keep the TIFF simple, avoiding more obscure TIFF features.

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 either have been 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. This may mean that no preview is shown, leaving the new technology less useful than the old.

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

External links

• The EPS file format (http://www.prepressure.com/formats/eps/fileformat.htm)
• The Graphics File Formats Page (http://www.dcs.ed.ac.uk/home/mxr/gfx/2d-hi.html) (has the actual EPS standard as a downloadable file)

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