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.\" $FreeBSD: releng/9.2/share/doc/usd/22.trofftut/tt06 96897 2002-05-19 03:37:41Z grog $
.\"
.NH
Local Motions: Drawing lines and characters
.PP
Remember `Area = \(*pr\u2\d' and the big `P'
in the Paternoster.
How are they done?
.UL troff
provides a host of commands for placing characters of any size
at any place.
You can use them to draw special characters 
or to tune your output for a particular appearance.
Most of these commands are straightforward, but messy to read
and tough to type correctly.
.PP
If you won't use 
.UL eqn ,
subscripts and superscripts are most easily done with
the half-line local motions
.BD \eu
and
.BD \ed .
To go back up the page half a point-size, insert a
.BD \eu
at the desired place;
to go down, insert a
.BD \ed .
.BD \eu \& (
and
.BD \ed
should always
be used in pairs, as explained below.)
Thus
.P1
Area = \e(*pr\eu2\ed
.P2
produces
.P1
Area = \(*pr\u2\d
.P2
To make the `2' smaller, bracket it with
.BD \es\-2...\es0 .
Since
.BD \eu
and
.BD \ed
refer to the current point size,
be sure to put them either both inside or both outside
the size changes,
or you will get an unbalanced vertical motion.
.PP
Sometimes the space given by
.BD \eu
and
.BD \ed
isn't the right amount.
The 
.BD \ev
command can be used to request an arbitrary amount of vertical motion.
The in-line command
.P1
\ev'(amount)'
.P2
causes motion up or down the page by the amount specified in
`(amount)'.
For example, to move the `P' down, we used
.P1 2
.ta 1i
^in +0.6i	(move paragraph in)
^ll \-0.3i	(shorten lines)
^ti \-0.3i	(move P back)
\ev'2'\es36P\es0\ev'\-2'ater noster qui est
in caelis ...
.P2
A minus sign causes upward motion, while
no sign or a plus sign means down the page.
Thus
.BD \ev\(fm\-2\(fm
causes an upward vertical motion 
of two line spaces.
.PP
There are many other ways to specify the amount of motion _
.P1
\ev'0.1i'
\ev'3p'
\ev'\-0.5m'
.P2
and so on are all legal.
Notice that the scale specifier
.BD i
or
.BD p
or
.BD m
goes inside the quotes.
Any character can be used in place of the quotes;
this is also true of all other
.UL troff
commands described in this section.
.PP
Since
.UL troff
does not take within-the-line vertical motions into account
when figuring out where it is on the page,
output lines can have unexpected positions
if the left and right ends aren't at the same
vertical position.
Thus
.BD \ev ,
like
.BD \eu
and
.BD \ed ,
should always balance upward vertical motion in a line with
the same amount in the downward direction.
.PP
Arbitrary horizontal motions are also available _
.BD \eh
is quite analogous to
.BD \ev ,
except that the default scale factor is ems instead of line spaces.
As an example,
.P1
\eh'\-0.1i'
.P2
causes a backwards motion of a tenth of an inch.
As a practical matter, consider printing the mathematical symbol
`>>'.
The default spacing is too wide, so
.UL eqn
replaces this by
.P1
>\eh'\-0.3m'>
.P2
to produce >\h'-.3m'>.
.PP
Frequently
.BD \eh
is used with the `width function'
.BD \ew
to generate motions equal to the width
of some character string.
The construction
.P1
\ew'thing'
.P2
is a number equal to the width of `thing' in machine units
(1/432 inch).
All
.UL troff
computations are ultimately done in these units.
To move horizontally the width of an `x',
we can say
.P1
\eh'\ew'x'u'
.P2
As we mentioned above,
the default scale factor for
all horizontal dimensions is
.BD m ,
ems, so here we must have the
.BD u
for machine units,
or the motion produced will be far too large.
.UL troff
is quite happy with the nested quotes, by the way,
so long as you don't leave any out.
.PP
As a live example of this kind of construction,
all of the command names in the text, like
.BD .sp ,
were done by overstriking with a slight offset.
The commands for
.BD .sp
are
.P1
^sp\eh'\-\ew'.sp'u'\eh'1u'.sp
.P2
That is, put out `.sp', move left by the width of `.sp',
move right 1 unit, and print
`.sp' again.
(Of course there is a way to avoid typing that much input
for each command name, which we will discuss in Section 11.)
.WS
.PP
There are also several special-purpose
.UL troff
commands for local motion.
We have already seen
.BD \e0 ,
which is an unpaddable white space
of the same width as a digit.
`Unpaddable' means that it will never be widened
or split across a line by line justification and filling.
There is also
.BD \e (blank),
.tr ^^
which is an unpaddable character the width of a space,
.BD \e| ,
which is half that width,
.BD \e^ ,
which is one quarter of the width of a space,
and
.BD \e& ,
which has zero width.
.tr ^.
(This last one is useful, for example, in entering
a text line which would otherwise begin with a `.'.)
.PP
The command
.BD \eo ,
used like
.P1
\eo'set of characters'
.P2
causes (up to 9)
characters to be overstruck,
centered on the widest.
This is nice for accents, as in
.P1 2
syst\eo"e\e(ga"me t\eo"e\e(aa"l\eo"e\e(aa"phonique
.P2
which makes
.P1
syst\o"e\(ga"me t\o"e\(aa"l\o"e\(aa"phonique
.P2
The accents are
.BD \e(ga
and
.BD \e(aa ,
or
.BD \e\` 
and
.BD \e\' ;
remember that each is just one character to
.UL troff .
.PP
You can make your own overstrikes with another special convention,
.BD \ez ,
the zero-motion command.
.BD \ezx
suppresses the normal horizontal motion
after printing the single character
.BD x ,
so another character can be laid on top of it.
Although sizes can be changed within
.BD \eo ,
it centers the characters on the widest,
and
there can be no horizontal or vertical motions,
so
.BD \ez
may be the only way to get what you want:
.P1
.sp 2
\s8\z\(sq\s14\z\(sq\s22\z\(sq\s36\(sq
.P2
is produced by
.P1
^sp 2
\es8\ez\e(sq\es14\ez\e(sq\es22\ez\e(sq\es36\e(sq
.P2
The
.BD .sp
is needed to leave room for the result.
.PP
As another example, an extra-heavy semicolon
that looks like
.P1
\s+6\z,\v'-0.25m'.\v'0.25m'\s0  instead of  ;  or  \s+6;\s0
.P2
can be constructed with a big comma and a big period above it:
.P1
\es+6\ez,\ev'\(mi0.25m'.\ev'0.25m'\es0 
.P2
`0.25m' is an experimentally-derived constant.
.PP
A more ornate overstrike is given by the bracketing function
.BD \eb ,
which piles up characters vertically,
centered on the current baseline.
Thus we can get big brackets,
constructing them with piled-up smaller pieces:
.P1
.sp
.ne 3
\b'\(lt\(lk\(lb' \b'\(lc\(lf' x \b'\(rc\(rf' \b'\(rt\(rk\(rb'
.sp
.P2
by typing in only this:
.P1 0
\&^sp
\eb\(fm\e(lt\e(lk\e(lb\(fm \eb\(fm\e(lc\e(lf\(fm x \eb\(fm\e(rc\e(rf\(fm \eb\(fm\e(rt\e(rk\e(rb\(fm
.P2
.PP
.UL troff
also provides a convenient facility for drawing horizontal and vertical
lines of arbitrary length with arbitrary characters.
.BD \el\(fm1i\(fm
draws a line one inch long, like this:
\l'1i'\|.
The length can be followed by
the character to use if the \(ru isn't appropriate;
.BD \el\(fm0.5i.\(fm
draws a half-inch line of dots: \l'.5i.'.
The construction
.BD \eL
is entirely analogous,
except that it draws a vertical line instead of horizontal.
