Editing Troff (section)

==Preprocessors==
As ''troff'' evolved, since there are several things which cannot be done easily in ''troff'', several [[preprocessor]]s were developed. These programs transform certain parts of a document into ''troff'' input, fitting naturally into the use of "pipelines" in Unix — sending the output of one program as the input to another (see [[pipes and filters]]).  Typically, each preprocessor translates only sections of the input file that are specially marked, passing the rest of the file through unchanged.  The embedded preprocessing instructions are written in a simple application-specific programming language, which provides a high degree of power and flexibility.

* [[eqn (software)|eqn]] preprocessor allows mathematical formulae to be specified in simple and intuitive manner.<ref>B. W. Kernighan and Lorinda L. Cherry.  ''A System for Typesetting Mathematics''. CSTR #17, Bell Labs, May 1974.</ref>
* tbl is a preprocessor for formatting tables.
* [[refer (software)|refer]] (and the similar program ''bib'') processes citations in a document according to a [[bibliographic]] database.

Three preprocessors provide ''troff'' with drawing capabilities by defining a [[domain-specific language]] for describing the picture.
* ''[[pic (software)|pic]]'' is a [[procedural programming language]] providing various drawing functions like ''circle'' and ''box''.<ref>B. W. Kernighan.  ''PIC — A Graphics Language for Typesetting (Revised User Manual)''.  CSTR #116, Bell Labs, December 1984.</ref>
* ''ideal'' allows the drawing of pictures [[Declarative programming|declaratively]], deriving the picture by solving a system of [[simultaneous equations]] based on vectors and transformations described by its input.<ref>C. J. Van Wyk.  ''IDEAL User's Manual''. CSTR #103, Bell Labs, December 1981.</ref>
* ''grn'' describes the pictures through graphical elements drawn at absolute coordinates, based on the ''[[gremlin (file format)|gremlin file format]]'' defined by an early graphics workstation.<ref>grn — groff preprocessor for gremlin files [http://www.unix.com/man-page/All/1/grn/].</ref>

In addition, there is a command [[soelim]] that removes <code>.so</code> inclusion directives from the input text.<ref name="oreilly-quick-svr4"/>

A typical structure of the pipeline might be:

<code>soelim <i>file</i> | refer | ideal | pic | tbl | eqn | troff</code>

Yet more preprocessors allow the drawing of more complex pictures by generating output for ''pic''.
* ''grap'' draws [[chart]]s, like [[scatter plots]] and [[histograms]].<ref>J. L. Bentley and B. W. Kernighan.  ''GRAP — A Language for Typesetting Graphs (Tutorial and User Manual)''. CSTR #114, Bell Labs, August 1984.</ref>
* ''chem'' draws [[structural formula|chemical structure diagrams]].<ref>J. L. Bentley, L. W. Jelinski, and B. W. Kernighan. ''CHEM — A Program for Typesetting Chemical Structure Diagrams''. CSTR #122, Computers and Chemistry, Bell Labs, April 1986.</ref>
* ''dformat'' draws [[row (database)|record-based]] [[data structures]].<ref>J. L. Bentley.  ''DFORMAT — A Program for Typesetting Data Formats''. CSTR #142, Bell Labs, April 1988.</ref>