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 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. 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.
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.
Preview bitmaps
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