e2

.NH
Photocomposition
.PP
Photocomposition techniques
can be used to solve some of the problems of typesetting mathematics.
A phototypesetter is a device which exposes
a piece of photographic paper or film, placing characters
wherever they are wanted.
The Graphic Systems phototypesetter[2] on the
.UC UNIX
operating
system[3] works by shining light through a character stencil.
The character is made the right size by lenses,
and the light beam directed by fiber optics
to the desired place on a piece of photographic paper.
The exposed paper is developed and typically used
in some form of photo-offset reproduction.
.PP
On
.UC UNIX ,
the phototypesetter is driven by a formatting program called
.UC TROFF
[4].
.UC TROFF
was designed for setting running text.
It also provides all of the facilities that one needs for
doing mathematics, such as
arbitrary horizontal and vertical motions,
line-drawing, size changing,
but the syntax for describing these special operations is
difficult to learn,
and difficult even for experienced users to type correctly.
.PP
For this reason we decided to use
.UC TROFF
as an ``assembly language,''
by
designing a language for describing mathematical
expressions,
and compiling it into
.UC TROFF .