Document Structuring Conventions (DSC) is a set of standards for
PostScript
PostScript (PS) is a page description language and dynamically typed, stack-based programming language. It is most commonly used in the electronic publishing and desktop publishing realm, but as a Turing complete programming language, it c ...
, based on the use of
comments, that specifies a way to structure a PostScript file and a way to expose that structure in a machine-readable way. A PostScript file that conforms to DSC is called a ''conforming document''.
The need for a structuring convention arises since PostScript is a
Turing-complete
In computability theory, a system of data-manipulation rules (such as a model of computation, a computer's instruction set, a programming language, or a cellular automaton) is said to be Turing-complete or computationally universal if it can be ...
programming language. There is thus no guaranteed method — short of actually printing the document — to do things like determining how many pages long a given document is or how large a given page is, or how to skip to a particular page. The addition of structure, with DSC comments exposing that structure, helps provide a way for, e.g., an intelligent
print spooler to have the ability to rearrange the pages for printing, or for a
page layout program to find the
bounding box of a PostScript file used as a graphic image. Collectively, any such program that takes PostScript files as input data is called a ''document manager''.
In order for a PostScript print file to properly distill to
PDF
Portable document format (PDF), standardized as ISO 32000, is a file format developed by Adobe Inc., Adobe in 1992 to present documents, including text formatting and images, in a manner independent of application software, computer hardware, ...
using Adobe tools, it should conform to basic DSC standards.
Some DSC comments serve a second function, specifying a way to tell the document manager to do certain things, like inserting a font or other PostScript code (collectively called ''resources'') into the file. DSC comments that serve this second function are more akin to
preprocessing directives and are not purely comments. Documents using those kinds of DSC comments require a functioning document manager to come out as intended; sending them directly to a printer will not work.
DSC is the basis for
Encapsulated PostScript (EPS): EPS files are documents that conform to the DSC standards with further restrictions.
The set of DSC comments can be expanded by a mechanism called the Open Structuring Conventions, which, together with the EPS specification, form the basis of early versions of the
Adobe Illustrator Artwork
Adobe Illustrator is a vector graphics editor and Computer-aided design, design software developed and marketed by Adobe Inc., Adobe. Originally designed for the Apple Inc., Apple Mac (computer), Macintosh, development of Adobe Illustrator began ...
file format.
DSC at a glance
The basic premise of DSC is the separation of ''prolog'' (static definitions) and ''script'' (
code
In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communicati ...
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
argument
An argument is a series of sentences, statements, or propositions some of which are called premises and one is the conclusion. The purpose of an argument is to give reasons for one's conclusion via justification, explanation, and/or persu ...
s.
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.
Example
A DSC-conforming document (this one generated by
dvips) might begin:
%!PS-Adobe-2.0
%%Creator: dvips(k) 5.95a Copyright 2005 Radical Eye Software
%%Title: texput.dvi
%%Pages: 1
%%PageOrder: Ascend
%%BoundingBox: 0 0 612 792
%%DocumentPaperSizes: Letter
%%EndComments
which has the following meaning:
# marks the document as conforming to version 2.0 of the DSC
# identifies the PostScript-producing program as dvips 5.95a
# identifies the document title
# tells the document manager that the document consists of one page
# tells the document manager that pages are independent (i.e., not in Special ordering) and appear in ascending order in the document; in this example, since the document only consists of one page, this information is not usually relevant, but will be needed if additional pages are to be inserted by a document manager
# tells the document manager the coordinates, measured in PostScript
points, of the bounding box for all the pages taken together; 0 0 612 792 is the coordinates of a
US Letter–sized page
# tells the document manager what kind of paper sizes are used in the whole document; in this example only one size is used, namely the US Letter size
# marks the end of the prolog
See also
*
Adobe Systems
Adobe Inc. ( ), formerly Adobe Systems Incorporated, is an American software, computer software company based in San Jose, California. It offers a wide range of programs from web design tools, photo manipulation and vector creation, through to ...
*
Encapsulated PostScript
*
PostScript
PostScript (PS) is a page description language and dynamically typed, stack-based programming language. It is most commonly used in the electronic publishing and desktop publishing realm, but as a Turing complete programming language, it c ...
External links
PostScript Language Document Structuring Conventions Specification(25 September 1992)
{{Wikibooks, PostScript FAQ
PostScript