Document Structuring Conventions - DSC at A Glance

DSC At A Glance

The basic premise of DSC is the separation of prolog (static definitions) and script (code that affects job-specific printed output), plus the disallowing of certain PostScript operators deemed inappropriate for page descriptions. This ensures a basic level of predictability in the PostScript code, thus forming the basis of document manageability.

An optional, additional layer of document manageability is provided by separating the script into a document setup section, zero or more functionally independent pages, and an optional trailer (cleanup code). (“Zero pages” in DSC usually means “one page without the use of the PostScript ‘showpage’ operator.) The functional independence between pages, plus the disallowing of more PostScript operators in the pages section, form the basis for page independence, which allows pages to be reordered, and independently and randomly accessed.

This imposed structure is then exposed by delimiting the PostScript file with DSC comments, which normally begin with two percent signs followed by a keyword. Some keywords need to be followed by a colon, an optional space character, and then a series of arguments.

Finally, the document is marked as conforming by starting it with a comment starting with “%!PS-Adobe-” followed by the DSC version number.

Sections of reusable PostScript code can be modularized into procsets (procedure sets, corresponding to function libraries in other programming languages), in order to ease the generation of PostScript code. Procsets and other PostScript resources (for example, fonts) can be omitted from the PostScript file itself, and externally referenced by a directive-like DSC comment; such external referencing, however, can only work with a document manager that understands such DSC comments.

DSC version 3.0 was released on September 25, 1992. The specification states, "Even though the DSC comments are a layer of communication beyond the PostScript language and do not affect the final output, their use is considered to be good PostScript language programming style." Thus, most PostScript-producing programs output DSC-conformant comments along with the code, although some such programs do not actually produce conforming documents.