Encapsulated PostScript

What is Encapsulated PostScript?

At some point, you may want to include some nice PostScript image into a document. There are a number of problems associated with this, but the main one is that your page layout program needs to know how big the image is, and how to move it to the correct place on the page. Encapsulated PostScript (EPS) is that part of Adobe’s Document Structuring Convention that provides this information.

What Is the Document Structuring Convention?

The DSC is a special file format for PostScript documents. The full details for the DSC can (and should) be gotten from Adobe. If you are writing a PostScript printer driver or other utility which will be used by a large number of people to create or manipulate PostScript documents, do not even think about writing it without making it DSC-compliant. You will save yourself and your users a lot of headaches.

Although the full DSC is beyond the scope of this guide, the most basic rules can be explained. A DSC-compliant document is an ordinary PostScript document with a number of comments added. These comments provide information to any post-processors which work with the files. Some comments strictly provide information, others are used to structure the document into sections, which may be shuffled or processed in other ways by the post-processor.

Every DSC-compliant document is indicated by having the comment %!PS-Adobe-3.0 as the first line. This comment is a flag to indicate that the document is compliant. You should never use this comment unless your document really is DSC compliant. There are many other parts to proper DSC. A document which follows the DSC can be manipulated in many ways. In particular, post-processors can shuffle the pages, print two or more pages on a side, and so on. The printer drivers from some notable companies do not follow the DSC, and their PostScript documents are, therefore, impossible to work with once they’ve been generated.

Now, What About EPS?

An EPS file is a PostScript file which follows the DSC and which follows a couple of other rules. These rules can be summarized as follows:

• The first line must be %!PS-Adobe-3.0 EPSF-3.0
• The file must make use of the BoundingBox comment
• The file should be a single page image (in DSC terms, the %%Pages comment must have a value of 0 or 1).
• The file should not use any operators which affect the global state.
• Finally, the EPS file should not use showpage. Actually, Adobe says that it is fine to use showpage in your EPS files. Officially, it is the responsibility of the importing application to redefine showpage so that the EPS file does not actually eject the page. Still, in creating EPS files, it would be wise not to use this operator.

BoundingBox

The BoundingBox comment is used in DSC to indicate where the actual image will be on a page. The comment describes a rectangle which completely encloses the image. The form of the comment is: %%BoundingBox: llx lly urx ury. For instance, suppose I have an image which extends from x=72 to x=144 and from y=150 to y=170. The BoundingBox comment in the document should then be: %%BoundingBox: 72 150 144 170.