Editing Encapsulated PostScript (section)

== Previews ==
EPS files 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 that used PostScript.

When EPS was first implemented, the only machines widely using PostScript were [[Apple Macintosh]]es. These machines could not directly render the PostScript, which presented [[Adobe Systems|Adobe]] with the dilemma of how to provide a preview image for the designer while also including the 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 is commonly used on any other operating system, however. When faced with the same problems on [[Microsoft Windows]]-based versions of their programs, Adobe chose to instead add a [[TIFF]] file encoded into the header section of the PostScript. Sometimes, although more rarely, they used the [[Windows Metafile]] (WMF) 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 provides only black-and-white previews. It is mainly used on [[Unix-like]] systems. With several different ways of representing the preview, EPSI files have limited portability. An application that 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.