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Code Editor : pic-3.html

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<h2>3. Basic PIC Concepts
<a name="3. Basic PIC Concepts"></a>
</h2>

Pictures are described
procedurally, as collections of objects connected by
motions. Normally, pic tries to string together
objects left-to-right in the sequence they are described,
joining them at visually natural points. Here is an example
illustrating the flow of data in pic processing:

Figure 3-1: Flow
of pic data

This was produced from the
following pic program:

.PS 
ellipse &quot;document&quot;; 
arrow; 
box width 0.6 &quot;\fIgpic\/\fP(1)&quot; 
arrow; 
box width 1.1 &quot;\fIgtbl\/\fP(1) or \fIgeqn\/\fP(1)&quot;
&quot;(optional)&quot; dashed; 
arrow; 
box width 0.6 &quot;\fIgtroff\/\fP(1)&quot;; 
arrow; 
ellipse &quot;PostScript&quot; 
.PE

This little program illustrates
several pic basics. Firstly, we see how to invoke
three object types; ellipses, arrows, and boxes. We see how
to declare text lines to go within an object (and that text
can have font changes in it). We see how to change the line
style of an object from solid to dashed. And we see that a
box can be made wider than its default size to accommodate
more text (we&rsquo;ll discuss this facility in detail in
the next section).

We also get to see
pic&rsquo;s simple syntax. Statements are ended by
newlines or semicolons. String quotes are required around
all text arguments, whether or not they contain spaces. In
general, the order of command arguments and modifiers like
&ldquo;width 1.2&rdquo; or &ldquo;dashed&rdquo;
doesn&rsquo;t matter, except that the order of text
arguments is significant.

Here are all but one of the
basic pic objects at their default sizes:

Figure 3-2: Basic
pic objects

The missing simple object type
is a spline. There is also a way to collect objects
into block composites which allows you to treat the
whole group as a single object (resembling a box) for many
purposes. We&rsquo;ll describe both of these later on.

The box, ellipse, circle, and
block composite objects are closed; lines, arrows,
arcs and splines are open. This distinction is often
important in explaining command modifiers.

Figure 3-2 was produced by the
following pic program, which introduces some more
basic concepts:

.PS 
box &quot;box&quot;; 
move; 
line &quot;line&quot; &quot;&quot;; 
move; 
arrow &quot;arrow&quot; &quot;&quot;; 
move; 
circle &quot;circle&quot;; 
move; 
ellipse &quot;ellipse&quot;; 
move; 
arc; down; move; &quot;arc&quot; 
.PE

The first thing to notice is the
move command, which moves a default distance (1/2
inch) in the current movement direction.

Secondly, see how we can also
decorate lines and arrows with text. The line and arrow
commands each take two arguments here, specifying text to go
above and below the object. If you wonder why one argument
would not do, contemplate the output of arrow
&quot;ow!&quot;:

Figure 3-3: Text
centered on an arrow

When a command takes one text
string, pic tries to place it at the object&rsquo;s
geometric center. As you add more strings, pic treats
them as a vertical block to be centered. The program

line
&quot;1&quot;; 
line &quot;1&quot; &quot;2&quot;; 
line &quot;1&quot; &quot;2&quot; &quot;3&quot;; 
line &quot;1&quot; &quot;2&quot; &quot;3&quot;
&quot;4&quot;; 
line &quot;1&quot; &quot;2&quot; &quot;3&quot; &quot;4&quot;
&quot;5&quot;;

for example, gives you this:

Figure 3-4:
Effects of multiple text arguments

The last line of Figure
3-2&rsquo;s program, &lsquo;arc; down; move;
&quot;arc&quot;&rsquo;, describing the captioned arc,
introduces several new ideas. Firstly, we see how to change
the direction in which objects are joined. Had we written
arc; move; &quot;arc&quot;, omitting down the
caption would have been joined to the top of the arc, like
this:

Figure 3-5:
Result of arc; move;

This is because drawing an arc
changes the default direction to the one its exit end points
at. To reinforce this point, consider:

Figure 3-6:
Result of arc cw; move;

All we&rsquo;ve done differently
here is specify &ldquo;cw&rdquo; for a clockwise arc
(&ldquo;ccw&rdquo; specifies counter-clockwise direction).
Observe how it changes the default direction to down, rather
than up.

Another good way to see this via
with the following program:

line; arc; arc
cw; line

which yields:

Figure 3-7: Result of line; arc; arc
cw; line

Notice that we did not have to
specify &ldquo;up&rdquo; for the second arc to be joined to
the end of the first.

Finally, observe that a string,
alone, is treated as text to be surrounded by an invisible
box of a size either specified by width and height
attributes or by the defaults textwid and
textht. Both are initially zero (because we
don&rsquo;t know the default font size).
<hr>
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