Although groff provides most functions needed to format a
document, some operations would be unwieldy (e.g. to draw pictures).
Therefore, programs called preprocessors were written which
understand their own language and produce the necessary groff
operations. These preprocessors are able to differentiate their own
input from the rest of the document via markers.
To use a preprocessor, UNIX pipes are used to feed the output
from the preprocessor into groff. Any number of preprocessors
may be used on a given document; in this case, the preprocessors are
linked together into one pipeline. However, with groff, the user
does not need to construct the pipe, but only tell groff what
preprocessors to use.
groff currently has preprocessors for producing tables
(tbl), typesetting equations (eqn), drawing pictures
(pic and grn), processing bibliographies
(refer), and drawing chemical structures (chem). An
associated program which is useful when dealing with preprocessors is
soelim.
A free implementation of grap, a preprocessor for drawing graphs,
can be obtained as an extra package; groff can use grap
also.
Unique to groff is the preconv preprocessor which enables
groff to handle documents in various input encodings.
There are other preprocessors in existence, but, unfortunately, no free
implementations are available. Among them is a preprocessor for drawing
mathematical pictures (ideal).