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.
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 is commonly used on any operating system, however.
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 (WindowsMetafile) 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.
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.
