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.

Preview bitmaps

EPS files also frequently include a preview bitmap 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, and EPS was invented to allow libraries of PS clip art[?] to be easily manipulated on these machines. To solve the problem Adobe included a Mac-standard PICT file as the bitmap, stored in the Mac's non-standard resource fork. This had a number of advantages, as the Mac operating system invisibily maintains the resource and data "forks" together as a unit, guarenteeing that the PICT preview in the resource fork would always move with the PS code in the data fork.

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, just for ease of use apparently, they used the WMF (Windows MetaFile) instead.

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.

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)

- has the actual EPS standard as a downloadable file