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Encapsulated PostScript (EPS) is a standard file format for importing and exporting PostScript files. It is usually a single page PostScript program that describes an illustration or entire page. The purpose of an EPS file is to be included in other pages. Sometimes EPS files are called EPSF files. EPSF simply stands for Encapsulated PostScript Format. |
An EPS file can contain any combination of text, graphics and images. Since it is actually a PostScript file, it is the most versatile file format that is available. To avoid the need for a PostScript interpreter, EPS-files usually contain a small preview image that is used to visualize its content.
EPS files can be generated by all drawing applications as well as most layout applications. Image manipulation programs like Adobe PhotoShop can also save bitmap images as EPS-files. Some printer drivers are also capable of generating EPS-files as well as PostScript files.
An EPS file must conform to the Adobe Document Structuring Conventions (DSC). These are a set of rules that define how PostScript data should be organized.
At a minimum, it must include a header comment, %!PS-Adobe-3.0 EPSF-3.0, and a bounding box comment, %%BoundingBox: llx lly urx ury, that describes the bounds of the illustration. (The specification does not require the EPSF version, but many programs will reject a file that does not have it.)
The EPS program must not use operators that initialize or permanently change the state of the machine in a manner that cannot be undone by the enclosing application's use of save and restore (e.g.. the operators starting with "init'' like initgraphics). As a special case, the EPS program may use the showpage operator. The importing application is responsible for disabling the normal effects of showpage. The EPS program should make no environment-sensitive decisions (the importing application may be trying to attain some special effect, and the EPS program shouldn't screw this up), although it can use some device-dependent tricks to improve appearance such as a snap-to-pixel algorithm.
There are some operators that should not be used within an EPS file: banddevice, cleardictstack, copypage, erasepage, exitserver, framedevice, grestoreall, initclip, initgraphics, initmatrix, quit, renderbands, setglobal, setpagedevice, setshared and startjob. These also include operators from statusdict and userdict operators like legal, letter, a4, b5, etc. There are some operators that should be carefully used: nulldevice, setgstate, sethalftone, setmatrix, setscreen, settransfer and undefinefont.
EPS files can be encoded using 7-bits (ASCII, like PostScript data usually are) as well as 8-bits (binary, which is virtually always done on Macintosh because it decreases the size of the file significantly). 8 bit EPS-files cannot be handled properly by all operating systems or applications.
The image preview
EPS files can optionally contain a bitmapped image preview, so that systems that can't render PostScript directly can at least display a crude representation of what the graphic will look like.
There are 4 preview formats:
PICT, mainly used in files generated on Macs. The PICT file is stored in the resource fork of the EPS file, while the actual PostScript data are stored in the data fork. PICT is the default file format of QuickDraw, the graphics model that is used by all Mac applications to generate the screen display.
TIFF: Most EPS files created by Windows applications contain a TIFF file for preview purposes.
Metafile: Some EPS files originating on PC contain a Windows Metafile preview. WMF or Windows Metafile is the PC equivalent of the Macintosh PICT file format.
EPSI which is an EPS file with a platform device independent preview. EPSI is an all ASCII (no binary data or headers) version of EPS. EPSI provides for a hexadecimal encoded preview representation of the image that will be displayed or printed. EPSI files were documented by Adobe as a means of providing a preview for EPS files which would be cross-platform. In reality though DOS machines and Windows favour embedding TIFF or even Windows Metafiles in the PostScript. EPSI is mainly used on Unix systems.
It is also possible to have an EPS file without a preview though. In this case the imported file is usually displayed as a greyed out box or a box with diagonal lines running through it.
The preview image has a fixed resolution, which is usually 72 dpi. If you enlarge an EPS file in a document, the preview image is stretched and may become 'blocky' and lacking of detail. This does not necessarily mean that the EPS-data themselves will degrade in quality. As long as the EPS-file only contains text and vector graphics, scaling it does not affect its quality.
If you print a file containing an EPS-image on a non-PostScript printer, it is usually the preview image that gets printed. The preview image is ignored when you print to a PostScript device.
Although an EPS file contains PostScript data, you cannot always sent it straight to a printer to have it printed. Some interpreters cannot handle the preview data that may be included in the EPS file. Others don't output the file because the 'showpage" operator is missing. It can also happen that the printer does process the job but outputs a blank page because the content of the EPS-file was located outside the printable area.
EPS-files can contain PostScript level 2 operators that make it impossible to output the file on an older PostScript level 1 device.
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