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<h2>17. Interface To [gt]roff
<a name="17. Interface To [gt]roff"></a>
</h2>
<p style="margin-top: 1em"><font color="#000000">The output
of <b>pic</b> is <b>[gt]roff</b> drawing commands. The GNU
<i>gpic</i>(1) command warns that it relies on drawing
extensions present in <i>groff</i>(1) that are not present
in <i>troff</i>(1).</font></p>
<h3>17.1. Scaling Arguments
<a name="17.1. Scaling Arguments"></a>
</h3>
<p style="margin-top: 1em"><font color="#000000">The DWB
<i>pic</i>(1) program accepts one or two arguments to
<b>.PS</b>, which is interpreted as a width and height in
inches to which the results of <i>pic</i>(1) should be
scaled (width and height scale independently). If there is
only one argument, it is interpreted as a width to scale the
picture to, and height is scaled by the same
proportion.</font></p>
<p style="margin-top: 1em"><font color="#000000">GNU
<b>gpic</b> is less general; it accepts a single width to
scale to, or a zero width and a maximum height to scale to.
With two non-zero arguments, it scales to the maximum
height.</font></p>
<h3>17.2. How Scaling is Handled
<a name="17.2. How Scaling is Handled"></a>
</h3>
<p style="margin-top: 1em"><font color="#000000">When
<b>pic</b> processes a picture description on input, it
passes <b>.PS</b> and <b>.PE</b> through to the
postprocessor. The <b>.PS</b> gets decorated with two
numeric arguments which are the X and Y dimensions of
the picture in inches. The post-processor can use these to
reserve space for the picture and center it.</font></p>
<p style="margin-top: 1em"><font color="#000000">The GNU
incarnation of the <b>ms</b> macro package, for example,
includes the following definitions:</font></p>
<pre style="margin-left:10%; margin-top: 1em">.de PS
.br
.sp \\n[DD]u
.ie \\n[.$]<2 .@error bad arguments to PS (not preprocessed with pic?)
.el \{\
. ds@need (u;\\$1)+1v
. in +(u;\\n[.l]-\\n[.i]-\\$2/2>?0)
.\}
..
.de PE
.par@reset
.sp \\n[DD]u+.5m
..</pre>
<p style="margin-top: 1em"><font color="#000000">Equivalent
definition is supplied by GNU <i>pic</i>(1) if you use the
−mpic option; this should make it usable with macro
pages other than <i>ms</i>(1).</font></p>
<p style="margin-top: 1em"><font color="#000000">If
<b>.PF</b> is used instead of <b>.PE</b>, the <b>troff</b>
position is restored to what it was at the picture start
(Kernighan notes that the F stands for
“flyback”).</font></p>
<p style="margin-top: 1em"><font color="#000000">The
invocation</font></p>
<p style="margin-left:10%; margin-top: 1em"><font color="#000000"><b>.PS
<</b><i>file</i></font></p>
<p style="margin-top: 1em"><font color="#000000">causes the
contents of <i>file</i> to replace the <b>.PS</b> line. This
feature is deprecated; use ‘<b>copy</b>
<i>file</i>’ instead).</font></p>
<h3>17.3. PIC and [gt]roff commands
<a name="17.3. PIC and [gt]roff commands"></a>
</h3>
<p style="margin-top: 1em"><font color="#000000">By
default, input lines that begin with a period are passed to
the postprocessor, embedded at the corresponding point in
the output. Messing with horizontal or vertical spacing is
an obvious recipe for bugs, but point size and font changes
are usually safe.</font></p>
<p style="margin-top: 1em"><font color="#000000">Point
sizes and font changes are also safe within text strings, as
long as they are undone before the end of string.</font></p>
<p style="margin-top: 1em"><font color="#000000">The state
of <b>[gt]roff</b>’s fill mode is preserved across
pictures.</font></p>
<h3>17.4. PIC and EQN
<a name="17.4. PIC and EQN"></a>
</h3>
<p style="margin-top: 1em"><font color="#000000">The
Kernighan paper notes that there is a subtle problem with
complicated equations inside <b>pic</b> pictures; they come
out wrong if <i>eqn</i>(1) has to leave extra vertical space
for the equation. If your equation involves more than
subscripts and superscripts, you must add to the beginning
of each equation the extra information <b>space 0</b>.
He gives the following example:</font></p>
<pre style="margin-left:10%; margin-top: 1em">arrow
box "$space 0 {H( omega )} over {1 - H( omega )}$"
arrow</pre>
<p align="center" style="margin-top: 1em"><font color="#000000"><img src="img/pic47.png" alt="Image img/pic47.png"></font></p>
<p align="center" style="margin-top: 1em"><font color="#000000">Figure
17-1: Equations within pictures</font></p>
<h3>17.5. Absolute Positioning of Pictures
<a name="17.5. Absolute Positioning of Pictures"></a>
</h3>
<p style="margin-top: 1em"><font color="#000000">A
<b>pic</b> picture is positioned vertically by troff at the
current position. The topmost position possible on a page is
not the paper edge but a position which is one baseline
lower so that the first row of glyphs is visible. To make a
picture really start at the paper edge you have to make the
baseline-to-baseline distance zero, this is, you must set
the vertical spacing to 0 (using <b>.vs</b>) before
starting the picture.</font></p>
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