Making Pictures With GNU PIC
Eric S. Raymond
<esr@snark.thyrsus.com>
ABSTRACT
The pic language is a troff extension that
makes it easy to create and alter box-and-arrow
diagrams of the kind frequently used in technical
papers and textbooks. This paper is both an in-
troduction to and reference for gpic(1), the im-
plementation distributed by the Free Software
Foundation for use with groff(1). It also cata-
logs other implementations and explains the dif-
ferences among them.
1. Introduction to PIC
1.1. Why PIC?
The pic language provides an easy way to write proce-
dural box-and-arrow diagrams to be included in troff docu-
ments. The language is sufficiently flexible to be quite
useful for state charts, Petri-net diagrams, flow charts,
simple circuit schematics, jumper layouts, and other kinds
of illustration involving repetitive uses of simple geomet-
ric forms and splines. Because these descriptions are pro-
cedural and object-based, they are both compact and easy to
modify.
The phrase "GNU pic" may refer to either of two pic im-
plementations distributed by the Free Software Foundation
and intended to accept the same input language. The gpic(1)
implementation is for use with the groff(1) implementation
of troff. The pic2plot(1) implementation runs standalone
and is part of the plotutils package. Because both imple-
mentations are widely available in source form for free,
they are good bets for writing very portable documentation.
1.2. PIC Versions
The original 1984 pre-ditroff(1) version of pic is long
obsolete. The rewritten 1991 version is still available as
part of the Documenter's Work Bench module of System V.
Where differences between Documenter's Work Bench
(1991) pic and GNU pic need to be described, original pic is
-2-
referred to as "DWB pic". Details on the history of the
program are given at the end of this document.
The pic2plot program does not require the rest of the
groff(1) toolchain to render graphics. It can display pic
diagrams in an X window, or generate output plots in a large
number of other formats. These formats include: PNG, PBM,
PGM, PPM, GIF, SVG, Adobe Illustrator format, idraw-editable
Postscript, the WebCGM format for Web-based vector graphics,
the format used by the xfig drawing editor, the Hewlett-
Packard PCL 5 printer language, the Hewlett-Packard Graphics
Language (by default, HP-GL/2), the ReGIS (remote graphics
instruction set) format developed by DEC, Tektronix format,
and device-independent GNU graphics metafile format.
In this document, gpic(1) and pic2plot(1) extensions
are marked as such.
2. Invoking PIC
Every pic description is a little program describing
drawing actions. The [gtn]roff-dependent versions compile
the program by pic(1) into gtroff(1) macros; the pic2plot(1)
implementation uses a plotting library to draw the picture
directly. Programs that process or display gtroff(1) output
need not know or care that parts of the image began life as
pic descriptions.
The pic(1) program tries to translate anything between
.PS and .PE markers, and passes through everything else.
The normal definitions of .PS and .PE in the ms macro pack-
age and elsewhere have also the side-effect of centering the
pic output on the page.
2.1. PIC Error Messages
If you make a pic syntax error, gpic(1) issues an error
message in the standard gcc(1)-like syntax. A typical error
message looks like this
pic:pic.ms:<nnn>: parse error before `<token>'
pic:pic.ms:<nnn>: giving up on this picture
where <nnn> is a line number, and <token> is a token near
(usually just after) the error location.
3. Basic PIC Concepts
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 de-
scribed, joining them at visually natural points. Here is
an example illustrating the flow of data in pic processing:
-3-
+-----+ ++++++++++++ +-----+
----++ +-gtbl(1) or geqn(1)++ +---+-
document gpic(1) +(optional)+ gtroff(1) PostScript
+-----+ + ++++++++++ +-----+
Figure 3-1: Flow of pic data
This was produced from the following pic program:
.PS
ellipse "document";
arrow;
box width 0.6 "\fIgpic\/\fP(1)"
arrow;
box width 1.1 "\fIgtbl\/\fP(1) or \fIgeqn\/\fP(1)" "(optional)" dashed;
arrow;
box width 0.6 "\fIgtroff\/\fP(1)";
arrow;
ellipse "PostScript"
.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'll dis-
cuss this facility in detail in the next section).
We also get to see pic's simple syntax. Statements are
ended by newlines or semicolons. String quotes are required
around all text arguments, whether or not they contain spa-
ces. In general, the order of command arguments and modi-
fiers like "width 1.2" or "dashed" doesn'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:
+------+
| | -line- arrow-
| box | circle ellipse
+------+ arc
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 (re-
sembling a box) for many purposes. We'll describe both of
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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 modi-
fiers.
Figure 3-2 was produced by the following pic program,
which introduces some more basic concepts:
.PS
box "box";
move;
line "line" "";
move;
arrow "arrow" "";
move;
circle "circle";
move;
ellipse "ellipse";
move;
arc; down; move; "arc"
.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 argu-
ments here, specifying text to go above and below the ob-
ject. If you wonder why one argument would not do, contem-
plate the output of arrow "ow!":
-ow!+-
Figure 3-3: Text centered on an arrow
When a command takes one text string, pic tries to
place it at the object's geometric center. As you add more
strings, pic treats them as a vertical block to be centered.
The program
line "1";
line "1" "2";
line "1" "2" "3";
line "1" "2" "3" "4";
line "1" "2" "3" "4" "5";
for example, gives you this:
-5-
1
1 1 2
--1----1----2----2----3---
2 3 3 4
4 5
Figure 3-4: Effects of multiple text arguments
The last line of Figure 3-2's program, `arc; down;
move; "arc"', describing the captioned arc, introduces sev-
eral new ideas. Firstly, we see how to change the direction
in which objects are joined. Had we written arc; move;
"arc", omitting down the caption would have been joined to
the top of the arc, like this:
arc
Figure 3-5: Result of arc; move;
This is because drawing an arc changes the default di-
rection to the one its exit end points at. To reinforce
this point, consider:
arc
Figure 3-6: Result of arc cw; move;
All we've done differently here is specify "cw" for a
clockwise arc ("ccw" 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:
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------
------
Figure 3-7: Result of line; arc; arc cw; line
Notice that we did not have to specify "up" 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't know the default font size).
4. Sizes and Spacing
Sizes are specified in inches. If you don't like
inches, it's possible to set a global style variable scale
that changes the unit. Setting scale = 2.54 effectively
changes the internal unit to centimeters (all other size
variable values are scaled correspondingly).
4.1. Default Sizes of Objects
Here are the default sizes for pic objects:
|
Object | Default Size
--------+-------------------------
box | 0.75" wide by 0.5" high
circle | 0.5" diameter
ellipse | 0.75" wide by 0.5" high
arc | 0.5" radius
line | 0.5" long
arrow | 0.5" long
--------+-------------------------
The simplest way to think about these defaults is that
they make the other basic objects fit snugly into a default-
sized box.
pic2plot(1) does not necessarily emit a physical inch
for each virtual inch in its drawing coordinate system. In-
stead, it draws on a canvas 8 virtual inches by 8 virtual
inches wide. If its output page size is "letter", these
virtual inches will map to real ones. Specifying a differ-
ent page size (such as, say, "a4") will scale virtual inches
so they are output as one eighth of the page width. Also,
pic2plot(1) centers all images by default, though the -n op-
tion can be used to prevent this.
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4.2. Objects Do Not Stretch!
Text is rendered in the current font with normal troff
line spacing. Boxes, circles, and ellipses do not automati-
cally resize to fit enclosed text. Thus, if you say box
"this text far too long for a default box" you'll get this:
+------+
| |
this text is far too long|for a default box
+------+
Figure 4-1: Boxes do not automatically resize
which is probably not the effect you want.
4.3. Resizing Boxes
To change the box size, you can specify a box width
with the "width" modifier:
+-----------------------------+
| |
this text is far too long for a default box
+-----------------------------+
Figure 4-2: Result of box width 3
This modifier takes a dimension in inches. There is
also a "height" modifier that changes a box's height. The
width keyword may be abbreviated to wid; the height keyword
to ht.
4.4. Resizing Other Object Types
To change the size of a circle, give it a rad[ius] or
diam[eter] modifier; this changes the radius or diameter of
the circle, according to the numeric argument that follows.
0.2 0.3
0.1
Figure 4-3: Circles with increasing radii
The move command can also take a dimension, which just
tells it how many inches to move in the current direction.
-8-
Ellipses are sized to fit in the rectangular box de-
fined by their axes, and can be resized with width and
height like boxes.
You can also change the radius of curvature of an arc
with rad[ius] (which specifies the radius of the circle of
which the arc is a segment). Larger values yield flatter
arcs.
0.1 0.2 0.3
Figure 4-4: arc rad with increasing radii
Observe that because an arc is defined as a quarter
circle, increasing the radius also increases the size of the
arc's bounding box.
4.5. The `same' Keyword
In place of a dimension specification, you can use the
keyword same. This gives the object the same size as the
previous one of its type. As an example, the program
.PS
box; box wid 1 ht 1; box same; box
.PE
gives you
+---------+---------+
+------+ | |+------+
| | | || |
| | | || |
+------+ | |+------+
| | |
+---------+---------+
Figure 4-5: The same keyword
5. Generalized Lines and Splines
5.1. Diagonal Lines
It is possible to specify diagonal lines or arrows by
adding multiple up, down, left, and right modifiers to the
line object. Any of these can have a multiplier. To under-
stand the effects, think of the drawing area as being grid-
ded with standard-sized boxes.
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++++++ +++++++++++ ++++++++++++++++ +++++++++++++++++++++
+ + + + + + + + + + + + + +
+ + + + + + + + + + + + + +
++++++ +++++++++++ ++++++++++++++++ +++++++++++++++++++++
arrow up left arrow up left 1 arrow up left 1.5 arrow up left 2
Figure 5-1: Diagonal arrows (dotted boxes show the implied
0.5-inch grid)
5.2. Multi-Segment Line Objects
A "line" or "arrow" object may actually be a path con-
sisting of any number of segments of varying lengths and di-
rections. To describe a path, connect several line or arrow
commands with the keyword then.
-----------
-----------
Figure 5-2: line right 1 then down .5 left 1 then right 1
If a path starts with then, the first segment is as-
sumed to be into the current direction, using the default
length.
5.3. Spline Objects
If you start a path with the spline keyword, the path
vertices are treated as control points for a spline curve
fit.
1 ++++++++++2
++
++++
3 ++++++++++4
The spline c...with tangents displayed
Figure 5-3: spline right 1 then down .5 left 1 then right 1
You can describe many natural-looking but irregular
curves this way. For example:
-10-
+
| +-
spline right then up then leftsplinedleft-then up right then down right ->;
Figure 5-4: Two more spline examples
Note the arrow decorations. Arrowheads can be applied natu-
rally to any path-based object, line or spline. We'll see
how in the next section.
6. Decorating Objects
6.1. Text Special Effects
All pic implementations support the following font-
styling escapes within text objects:
\fR, \f1
Set Roman style (the default)
\fI, \f2
Set Italic style
\fB, \f3
Set Bold style
\fP
Revert to previous style; only works one level deep,
does not stack.
In the pic implementations that are preprocessors for a
toolchain that include [gtn]roff, text objects may also con-
tain [gtn]roff vertical- and horizontal-motion escapes such
as \h or \v. Troff special glyphs are also available. All
\-escapes will be passed through to the postprocessing stage
and have their normal effects. The base font family is set
by the [gtn]roff environment at the time the picture is ren-
dered.
pic2plot replaces [gtn]roff horizontal- and vertical-
motion escapes with \-escapes of its own. Troff special
glyphs are not available, but in most back ends Latin-1 spe-
cial characters and a square-root radical will be. See the
pic2plot documentation for full details.
6.2. Dashed Objects
We've already seen that the modifier dashed can change
the line style of an object from solid to dashed. GNU gpic
permits you to dot or dash ellipses, circles, and arcs (and
splines in TeX mode only); some versions of DWB may only
-11-
permit dashing of lines and boxes. It's possible to change
the dash interval by specifying a number after the modifier.
++++++++ +++++++++ +------+ |-- -- -+ +-- --+
+ + + + | | | | | |
default+ + 0.05 + | 0.1 | | 0.15 | | 0.2 |
++++++++ +++++++++ +------+ +- -- --| +-- --+
Figure 6-1: Dashed objects
6.3. Dotted Objects
Another available qualifier is dotted. GNU gpic per-
mits you to dot or dash ellipses, circles, and arcs (and
splines in TeX mode only); some versions of DWB may only
permit dashing of lines and boxes. It too can be suffixed
with a number to specify the interval between dots:
++++++++ +++++++++ ++++++++ + ++ ++ + + + + ++
+ + + + + + + + + +
default+ + 0.05 + + 0.1 + + 0.15 + + 0.2 +
++++++++ +++++++++ ++++++++ + ++ ++ + + + + ++
Figure 6-2: Dotted objects
6.4. Rounding Box Corners
It is also possible, in GNU gpic only, to modify a box
so it has rounded corners:
-------| |------ ----- ---- ---
| | | | | | | | + +
rad 0.05 |rad 0.1| rad 0.15 |rad 0.2| rad 0.25
|------- ------| ----- ---- ---
Figure 6-3: box rad with increasing radius values
Radius values higher than half the minimum box dimen-
sion are silently truncated to that value.
6.5. Slanted Boxes
GNU gpic supports slanted boxes:
-12-
xslanted -0.2
xslanted 0.1 yslanted -0.1 yslanted 0.1
Figure 6-4: Various slanted boxes.
The xslanted and yslanted attributes specify the x and
y offset, respectively, of the box's upper right corner from
its default position.
6.6. Arrowheads
Lines and arcs can be decorated as well. Any line or
arc (and any spline as well) can be decorated with arrow-
heads by adding one or more as modifiers:
-+--+-
Figure 6-5: Double-headed line made with line <- ->
In fact, the arrow command is just shorthand for line
->. And there is a double-head modifier <->, so the figure
above could have been made with line <->.
Arrowheads have a width attribute, the distance across
the rear; and a height attribute, the length of the arrow-
head along the shaft.
Arrowhead style is controlled by the style variable ar-
rowhead. The DWB and GNU versions interpret it differently.
DWB defaults to open arrowheads and an arrowhead value of 2;
the Kernighan paper says a value of 7 makes solid arrow-
heads. GNU gpic defaults to solid arrowheads and an arrow-
head value of 1; a value of 0 produces open arrowheads.
Note that solid arrowheads are always filled with the cur-
rent outline color.
6.7. Line Thickness
It's also possible to change the line thickness of an
object (this is a GNU extension, DWB pic doesn't support
it). The default thickness of the lines used to draw ob-
jects is controlled by the linethick variable. This gives
the thickness of lines in points. A negative value means
use the default thickness: in TeX output mode, this means
use a thickness of 8 milliinches; in TeX output mode with
the -c option, this means use the line thickness specified
by .ps lines; in troff output mode, this means use a thick-
ness proportional to the pointsize. A zero value means draw
the thinnest possible line supported by the output device.
Initially it has a value of -1. There is also a thickness
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attribute (which can be abbreviated to thick). For example,
circle thickness 1.5 would draw a circle using a line with a
thickness of 1.5 points. The thickness of lines is not af-
fected by the value of the scale variable, nor by any width
or height given in the .PS line.
6.8. Invisible Objects
The modifier invis[ible] makes an object entirely in-
visible. This used to be useful for positioning text in an
invisible object that is properly joined to neighboring
ones. Newer DWB versions and GNU pic treat stand-alone text
in exactly this way.
6.9. Filled Objects
It is possible to fill boxes, circles, and ellipses.
The modifier fill[ed] accomplishes this. You can suffix it
with a fill value; the default is given by the style vari-
able fillval.
DWB pic and gpic have opposite conventions for fill
values and different defaults. DWB fillval defaults to 0.3
and smaller values are darker; GNU fillval uses 0 for white
and 1 for black.
Figure 6-6: circle fill; move; circle fill 0.4; move; circle
fill 0.9;
GNU gpic makes some additional guarantees. A fill
value greater than 1 can also be used: this means fill with
the shade of gray that is currently being used for text and
lines. Normally this is black, but output devices may pro-
vide a mechanism for changing this. The invisible attribute
does not affect the filling of objects. Any text associated
with a filled object is added after the object has been
filled, so that the text is not obscured by the filling.
The closed-object modifier solid is equivalent to fill
with the darkest fill value (DWB pic had this capability but
mentioned it only in a reference section).
6.10. Colored Objects
As a GNU extension, three additional modifiers are
available to specify colored objects. outline sets the
color of the outline, shaded the fill color, and color sets
both. All three keywords expect a suffix specifying the
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color. Example:
+------+
| +---+-
| |
+------+
Figure 6-7: box color "yellow"; arrow color "cyan"; circle
shaded "green" outline "black";
Alternative spellings are colour, colored, coloured,
and outlined.
Predefined color names for [gtn]roff-based pic imple-
mentations are defined in the device macro files, for exam-
ple ps.tmac; additional colors can be defined with the .def-
color request (see the manual page of GNU troff(1) for more
details). Currently, color support is not available at all
in TeX mode.
The pic2plot(1) carries with its own set of color
names, essentially those recognized by the X window system
with "grey" accepted as a variant of "gray".
pic assumes that at the beginning of a picture both
glyph and fill color are set to the default value.
7. More About Text Placement
By default, text is centered at the geometric center of
the object it is associated with. The modifier ljust causes
the left end to be at the specified point (which means that
the text lies to the right of the specified place!), the
modifier rjust puts the right end at the place. The modi-
fiers above and below center the text one half line space in
the given direction.
Text attributes can be combined:
| |
| | -----ljust-above--------+-
ljustrjust text| rjust below
| |
Figure 7-1: Text attributes
What actually happens is that n text strings are cen-
tered in a box that is textwid wide by textht high. Both
these variables are initially zero (that is pic's way of not
making assumptions about [tg]roff(1)'s default point size).
-15-
In GNU gpic, objects can have an aligned attribute.
This only works if the postprocessor is grops or gropdf.
Any text associated with an object having the aligned attri-
bute is rotated about the center of the object so that it is
aligned in the direction from the start point to the end
point of the object. Note that this attribute has no effect
for objects whose start and end points are coincident.
8. More About Direction Changes
We've already seen how to change the direction in which
objects are composed from rightwards to downwards. Here are
some more illustrative examples:
right; box; arrow; circle; arrow; ellipse
+------+
| |
| +---+- ----+-
+------+
left; box; arrow; circle; arrow; ellipse
+------+
| |
-+---- -+----| |
+------+
Figure 8-1: Effects of different motion directions (right
and left)
down; box; arrow; circle; arrup; box;parrow; circle; arrow; ellipse;
+------+
| |
| |
+---+--+ |
| +
+ |
| |
| |
| +
+ |
| +--+---+
| |
| |
+------+
Figure 8-2: Effects of different motion directions (up and down)
Something that may appear surprising happens if you
change directions in the obvious way:
-16-
+------+
| +---+- |
| | |
+------+ +
|
Figure 8-3: box; arrow; circle; down; arrow; ellipse
You might have expected that program to yield this:
+------+
| +---+-
| |
+------+ |
|
+
|
Figure 8-4: More intuitive?
But, in fact, to get Figure 8.3 you have to do this:
.PS
box;
arrow;
circle;
move to last circle .s;
down;
arrow;
ellipse
.PE
Why is this? Because the exit point for the current direc-
tion is already set when you draw the object. The second
arrow in Figure 8.2 dropped downwards from the circle's at-
tachment point for an object to be joined to the right.
The meaning of the command move to last circle .s
should be obvious. In order to see how it generalizes,
we'll need to go into detail on two important topics; loca-
tions and object names.
-17-
9. Naming Objects
The most natural way to name locations in pic is rela-
tive to objects. In order to do this, you have to be able
to name objects. The pic language has rich facilities for
this that try to emulate the syntax of English.
9.1. Naming Objects By Order Of Drawing
The simplest (and generally the most useful) way to
name an object is with a last clause. It needs to be fol-
lowed by an object type name; box, circle, ellipse, line,
arrow, spline, "", or [] (the last type refers to a compos-
ite object which we'll discuss later). So, for example, the
last circle clause in the program attached to Figure 9.1.3
refers to the last circle drawn.
More generally, objects of a given type are implicitly
numbered (starting from 1). You can refer to (say) the
third ellipse in the current picture with 3rd ellipse, or to
the first box as 1st box, or to the fifth text string (which
isn't an attribute to another object) as 5th "".
Objects are also numbered backwards by type from the
last one. You can say 2nd last box to get the second-to-
last box, or 3rd last ellipse to get the third-to-last el-
lipse.
In places where nth is allowed, `expr'th is also al-
lowed. Note that 'th is a single token: no space is allowed
between the ' and the th. For example,
for i = 1 to 4 do {
line from `i'th box.nw to `i+1'th box.se
}
9.2. Naming Objects With Labels
You can also specify an object by referring to a label.
A label is a word (which must begin with a capital letter)
followed by a colon; you declare it by placing it immedi-
ately before the object drawing command. For example, the
program
-18-
.PS
A: box "first" "object"
move;
B: ellipse "second" "object"
move;
arrow right at A .r;
.PE
declares labels A and B for its first and second objects.
Here's what that looks like:
+------+
|first +---+-second
|object| object
+------+
Figure 9-1: Example of label use
The at statement in the fourth line uses the label A (the
behavior of at is explained in the next section). We'll see
later on that labels are most useful for referring to block
composite objects.
Labels are not constants but variables (you can view
colon as a sort of assignment). You can say something like
A: A + (1,0); and the effect is to reassign the label A to
designate a position one inch to the right of its old value.
10. Describing locations
The location of points can be described in many differ-
ent ways. All these forms are interchangeable as for as the
pic language syntax is concerned; where you can use one, any
of the others that would make semantic sense are allowed.
The special label Here always refers to the current po-
sition.
10.1. Absolute Coordinates
The simplest is absolute coordinates in inches; pic
uses a Cartesian system with (0,0) at the lower left corner
of the virtual drawing surface for each picture (that is,
X increases to the right and Y increases upwards). An abso-
lute location may always be written in the conventional form
as two comma-separated numbers surrounded by parentheses
(and this is recommended for clarity). In contexts where it
creates no ambiguity, the pair of X and Y coordinates suf-
fices without parentheses.
It is a good idea to avoid absolute coordinates, how-
ever. They tend to make picture descriptions difficult to
understand and modify. Instead, there are quite a number of
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ways to specify locations relative to pic objects and previ-
ous locations.
Another possibility of surprise is the fact that pic
crops the picture to the smallest bounding box before writ-
ing it out. For example, if you have a picture consisting
of a small box with its lower left corner at (2,2) and an-
other small box with its upper right corner at (5,5), the
width and height of the image are both 3 units and not 5.
To get the origin at (0,0) included, simply add an invisible
object to the picture, positioning the object's left corner
at (0,0).
10.2. Locations Relative to Objects
The symbol Here always refers to the position of the
last object drawn or the destination of the last move.
Alone and unqualified, a last circle or any other way
of specifying a closed-object or arc location refers as a
position to the geometric center of the object. Unquali-
fied, the name of a line or spline object refers to the po-
sition of the object start.
Also, pic objects have quite a few named locations as-
sociated with them. One of these is the object center,
which can be indicated (redundantly) with the suffix .center
(or just .c). Thus, last circle .center is equivalent to
last circle.
10.2.1. Locations Relative to Closed Objects
Every closed object (box, circle, ellipse, or block
composite) also has eight compass points associated with it;
.nw------.n-----.ne .n .n
| | .nw .ne .nw .ne
| |
.w | .c |.e.w .c ..w .c .e
| |
| |
+--------------+ .sw .se .sw .se
.sw .s .se .s .s
Figure 10-1: Compass points
these are the locations where eight compass rays from the
geometric center would intersect the figure. So when we say
last circle .s we are referring to the south compass point
of the last circle drawn. The explanation of Figure 8-3's
program is now complete.
(In case you dislike compass points, the names .top,
.bottom, .left and .right are synonyms for .n, .s, .e, and
-20-
.w respectively; they can even be abbreviated to .t, .b, .l
and .r).
The names center, top, bottom, left, right, north,
south, east, and west can also be used (without the leading
dot) in a prefix form marked by of; thus, center of last
circle and top of 2nd last ellipse are both valid object
references. Finally, the names left and right can be pre-
fixed with upper and lower which both have the obvious mean-
ing.
Arc objects also have compass points; they are the com-
pass points of the implied circle.
Non-closed objects (line, arrow, or spline) have com-
pass points too, but the locations of them are completely
arbitrary. In particular, different pic implementations re-
turn different locations.
10.2.2. Locations Relative to Open Objects
Every open object (line, arrow, arc, or spline) has
three named points: .start, .center (or .c), and .end. They
can also be used without leading dots in the of prefix form.
The center of an arc is the center of its circle, but the
center of a line, path, or spline is halfway between its
endpoints.
.end .start
.center
.start .center .end
.start ----------- .end
.center .center
-----.end-- .start
Figure 10-2: Special points on open objects
10.3. Ways of Composing Positions
Once you have two positions to work with, there are
several ways to combine them to specify new positions.
10.3.1. Vector Sums and Displacements
Positions may be added or subtracted to yield a new po-
sition (to be more precise, you can only add a position and
an expression pair; the latter must be on the right side of
-21-
the addition or subtraction sign). The result is the con-
ventional vector sum or difference of coordinates. For ex-
ample, last box .ne + (0.1, 0) is a valid position. This
example illustrates a common use, to define a position
slightly offset from a named one (say, for captioning pur-
poses).
10.3.2. Interpolation Between Positions
A position may be interpolated between any two posi-
tions. The syntax is `fraction of the way between position1
and position2'. For example, you can say 1/3 of the way be-
tween Here and last ellipse .ne. The fraction may be in nu-
merator/denominator form or may be an ordinary number (val-
ues are not restricted to [0,1]). As an alternative to this
verbose syntax, you can say `fraction <position1 , posi-
tion2>'; thus, the example could also be written as 1/3
<Here, last ellipse>.
|
+
P
Figure 10-3: P: 1/3 of the way between last arrow .start and
last arrow .end
This facility can be used, for example, to draw double
connections.
+------+ +------+
| +---+-| |
| yin ++----|yang |
+------+ +------+
Figure 10-4: Doubled arrows
You can get Figure 10-4 from the following program:
.PS
A: box "yin"; move;
B: box "yang";
arrow right at 1/4 <A.e,A.ne>;
arrow left at 1/4 <B.w,B.sw>;
.PE
Note the use of the short form for interpolating points.
-22-
10.3.3. Projections of Points
Given two positions p and q, the position (p, q) has
the X coordinate of p and the Y coordinate of q. This can
be helpful in placing an object at one of the corners of the
virtual box defined by two other objects.
A (B,A) is here
(A,B) is here B
Figure 10-5: Using (x, y) composition
10.4. Using Locations
There are four ways to use locations; at, from, to, and
with. All four are object modifiers; that is, you use them
as suffixes to a drawing command.
The at modifier says to draw a closed object or arc
with its center at the following location, or to draw a
line/spline/arrow starting at the following location.
The to modifier can be used alone to specify a move
destination. The from modifier can be used alone in the
same way as at.
The from and to modifiers can be used with a line or
arc command to specify start and end points of the object.
In conjunction with named locations, this offers a very
flexible mechanism for connecting objects. For example, the
following program
.PS
box "from"
move 0.75;
ellipse "to"
arc cw from 1/3 of the way \
between last box .n and last box .ne to last ellipse .n;
.PE
yields:
-23-
+------+
| |
| from | to
+------+
Figure 10-6: A tricky connection specified with English-like syntax
The with modifier allows you to identify a named at-
tachment point of an object (or a position within the ob-
ject) with another point. This is very useful for connect-
ing objects in a natural way. For an example, consider
these two programs:
+------+
+----+| |
+----+------+ | || |
| | | | || |
| | | +----++------+
+----+ |
+------+
box wid 0.5 ht 0.5;
box wid 0.5 ht 0.5; box wbox0wid 0.75.ht 0.75 with .sw at last box .se;
Figure 10-7: Using the with modifier for attachments
10.5. The `chop' Modifier
When drawing lines between circles that don't intersect
them at a compass point, it is useful to be able to shorten
a line by the radius of the circle at either or both ends.
Consider the following program:
.PS
circle "x"
circle "y" at 1st circle - (0.4, 0.6)
circle "z" at 1st circle + (0.4, -0.6)
arrow from 1st circle to 2nd circle chop
arrow from 2nd circle to 3rd circle chop
arrow from 3rd circle to 1st circle chop
.PE
It yields the following:
-24-
x
+ -+
|
y --+- z
Figure 10-8: The chop modifier
Notice that the chop attribute moves arrowheads rather than
stepping on them. By default, the chop modifier shortens
both ends of the line by circlerad. By suffixing it with a
number you can change the amount of chopping.
If you say line ... chop r1 chop r2 with r1 and r2 both
numbers, you can vary the amount of chopping at both ends.
You can use this in combination with trigonometric functions
to write code that deals with more complex intersections.
11. Object Groups
There are two different ways to group objects in pic;
brace grouping and block composites.
11.1. Brace Grouping
The simpler method is simply to group a set of objects
within curly bracket or brace characters. On exit from this
grouping, the current position and direction are restored to
their value when the opening brace was encountered.
11.2. Block Composites
A block composite object is created a series of com-
mands enclosed by square brackets. The composite can be
treated for most purposes like a single closed object, with
the size and shape of its bounding box. Here is an example.
The program fragment
A: [
circle;
line up 1 at last circle .n;
line down 1 at last circle .s;
line right 1 at last circle .e;
line left 1 at last circle .w;
box dashed with .nw at last circle .se + (0.2, -0.2);
Caption: center of last box;
]
-25-
yields the block in figure 11-1, which we show both with and
without its attachment points. The block's location becomes
the value of A.
| .nw .n .ne
| |
| |
| |
| |
| |
| |
----------- ----------- .w ----------- .c----------.e
| |
| +++++++++ | +++++++++
| + + | + +
| + + | + +
| +++++++++ | +++++++++
| |
| |
.sw .s .se
Figure 11-1: A sample composite object
To refer to one of the composite's attachment points, you
can say (for example) A .s. For purposes of object naming,
composites are a class. You could write last [] .s as an
equivalent reference, usable anywhere a location is needed.
This construction is very important for putting together
large, multi-part diagrams.
Blocks are also a variable-scoping mechanism, like a
groff(1) environment. All variable assignments done inside
a block are undone at the end of it. To get at values
within a block, write a name of the block followed by a dot,
followed by the label you want. For example, we could refer
the center of the box in the above composite as last []
.Caption or A.Caption.
This kind of reference to a label can be used in any
way any other location can be. For example, if we added
"Hi!" at A.Caption the result would look like this:
-26-
|
|
|
|
|
|
|
----------- -----------
|
| +++++++++
| + Hi! +
| + +
| +++++++++
|
|
Figure 11-2: Adding a caption using interior labeling
You can also use interior labels in either part of a
with modifier. This means that the example composite could
be placed relative to its caption box by a command contain-
ing with A.Caption at.
Note that both width and height of the block composite
object are always positive:
+------+ +------+
+----+ | +----+| |
| | | | || |
| | | | || |
+----+-+ +----++------+
box wid -0.5 ht 0.5; box wid [box wid0-0.5 ht 0.5]; box wid 0.75 ht 0.75
Figure 11-3: Composite block objects always have positive
width and height
Blocks may be nested. This means you can use block at-
tachment points to build up complex diagrams hierarchically,
from the inside out. Note that last and the other sequen-
tial naming mechanisms don't look inside blocks, so if you
have a program that looks like
-27-
.PS
P: [box "foo"; ellipse "bar"];
Q: [
[box "baz"; ellipse "quxx"]
"random text";
]
arrow from 2nd last [];
.PE
the arrow in the last line is attached to object P, not ob-
ject Q.
In DWB pic, only references one level deep into en-
closed blocks were permitted. GNU gpic removes this re-
striction.
The combination of block variable scoping, assignabil-
ity of labels and the macro facility that we'll describe
later on can be used to simulate functions with local vari-
ables (just wrap the macro body in block braces).
12. Style Variables
There are a number of global style variables in pic
that can be used to change its overall behavior. We've men-
tioned several of them in previous sections. They're all
described here. For each variable, the default is given.
| |
Style Variable | Default | What It Does
---------------+---------+-----------------------------------------------
boxht | 0.5 | Default height of a box
boxwid | 0.75 | Default width of a box
lineht | 0.5 | Default length of vertical line
linewid | 0.75 | Default length of horizontal line
linethick | -1 | Default line thickness
arcrad | 0.25 | Default radius of an arc
circlerad | 0.25 | Default radius of a circle
ellipseht | 0.5 | Default height of an ellipse
ellipsewid | 0.75 | Default width of an ellipse
moveht | 0.5 | Default length of vertical move
movewid | 0.75 | Default length of horizontal move
textht | 0 | Default height of box enclosing a text object
textwid | 0 | Default width of box enclosing a text object
arrowht | 0.1 | Length of arrowhead along shaft
arrowwid | 0.05 | Width of rear of arrowhead
arrowhead | 1 | Enable/disable arrowhead filling
dashwid | 0.05 | Interval for dashed lines
maxpswid | 8.5 | Maximum width of picture
maxpsht | 11 | Maximum height of picture
scale | 1 | Unit scale factor
fillval | 0.5 | Default fill value
---------------+---------+-----------------------------------------------
-28-
Any of these variables can be set with a simple assignment
statement. For example:
+--+ +--+ +--+ +--+
| | | | | | | |
| | | | | | | |
| | | | | | | |
| | | | | | | |
| | | | | | | |
+--+ +--+ +--+ +--+
Figure 12-1: boxht=1; boxwid=0.3; movewid=0.2; box; move;
box; move; box; move; box;
In GNU pic, setting the scale variable re-scales all
size-related state variables so that their values remain
equivalent in the new units.
The command reset resets all style variables to their
defaults. You can give it a list of variable names as argu-
ments (optionally separated by commas), in which case it re-
sets only those.
State variables retain their values across pictures un-
til reset.
13. Expressions, Variables, and Assignment
A number is a valid expression, of course (all numbers
are stored internally as floating-point). Decimal-point no-
tation is acceptable; in GNU gpic, scientific notation in
C's `e' format (like 5e-2) is accepted.
Anywhere a number is expected, the language also ac-
cepts a variable. Variables may be the built-in style vari-
able described in the last section, or new variables created
by assignment.
DWB pic supports only the ordinary assignment via =,
which defines the variable (on the left side of the equal
sign) in the current block if it is not already defined
there, and then changes the value (on the right side) in the
current block. The variable is not visible outside of the
block. This is similar to the C programming language where
a variable within a block shadows a variable with the same
name outside of the block.
GNU gpic supports an alternate form of assignment using
:=. The variable must already be defined, and the value is
assigned to that variable without creating a variable local
to the current block. For example, this
-29-
x=5
y=5
[
x:=3
y=3
]
print x " " y
prints 3 5.
You can use the height, width, radius, and x and y co-
ordinates of any object or corner in expressions. If A is
an object label or name, all the following are valid:
A.x # x coordinate of the center of A
A.ne.y # y coordinate of the northeast corner of A
A.wid # the width of A
A.ht # and its height
2nd last circle.rad # the radius of the 2nd last circle
Note the second expression, showing how to extract a corner
coordinate.
Basic arithmetic resembling those of C operators are
available; +, *, -, /, and %. So is ^ for exponentiation.
Grouping is permitted in the usual way using parentheses.
GNU gpic allows logical operators to appear in expressions;
! (logical negation, not factorial), &&, ||, ==, !=, >=, <=,
<, >.
Various built-in functions are supported: sin(x),
cos(x), log(x), exp(x), sqrt(x), max(x,y), atan2(x,y),
min(x,y), int(x), rand(), and srand(). Both exp and log are
base 10; int does integer truncation; rand() returns a ran-
dom number in [0-1), and srand() sets the seed for a new se-
quence of pseudo-random numbers to be returned by rand()
(srand() is a GNU extension).
GNU gpic also documents a one-argument form or rand,
rand(x), which returns a random number between 1 and x, but
this is deprecated and may be removed in a future version.
The function sprintf() behaves like a C sprintf(3)
function that only takes %, %e, %f, and %g format strings.
14. Macros
You can define macros in pic, with up to 32 arguments
(up to 16 on EBCDIC platforms). This is useful for diagrams
with repetitive parts. In conjunction with the scope rules
for block composites, it effectively gives you the ability
to write functions.
-30-
The syntax is
define name { replacement text }
This defines name as a macro to be replaced by the replace-
ment text (not including the braces). The macro may be
called as
name(arg1, arg2, ... argn)
The arguments (if any) are substituted for tokens $1, $2 ...
$n appearing in the replacement text.
As an example of macro use, consider this:
-31-
.PS
# Plot a single jumper in a box, $1 is the on-off state.
define jumper { [
shrinkfactor = 0.8;
Outer: box invis wid 0.45 ht 1;
# Count on end ] to reset these
boxwid = Outer.wid * shrinkfactor / 2;
boxht = Outer.ht * shrinkfactor / 2;
box fill (!$1) with .s at center of Outer;
box fill ($1) with .n at center of Outer;
] }
# Plot a block of six jumpers.
define jumperblock {
jumper($1);
jumper($2);
jumper($3);
jumper($4);
jumper($5);
jumper($6);
jwidth = last [].Outer.wid;
jheight = last [].Outer.ht;
box with .nw at 6th last [].nw wid 6*jwidth ht jheight;
# Use {} to avoid changing position from last box draw.
# This is necessary so move in any direction works as expected
{"Jumpers in state $1$2$3$4$5$6" at last box .s + (0,-0.2);}
}
# Sample macro invocations.
jumperblock(1,1,0,0,1,0);
move;
jumperblock(1,0,1,0,1,1);
.PE
It yields the following:
+--------------------------+ +--------------------------+
|+-+ +-+ +-+ +-+ +-+ +-+| | +-+ +-+ +-+ +-+ +-+ +-+|
|| | | | | | | | | | | || | | | | | | | | | | | | ||
|+-+ +-+ +-+ +-+ +-+ +-+| | +-+ +-+ +-+ +-+ +-+ +-+|
|| | | | | | | | | | | || | | | | | | | | | | | | ||
|+-+ +-+ +-+ +-+ +-+ +-+| | +-+ +-+ +-+ +-+ +-+ +-+|
+--------------------------+ +--------------------------+
Jumpers in state 110010 Jumpers in state 101011
Figure 14-1: Sample use of a macro
-32-
This macro example illustrates how you can combine [], brace
grouping, and variable assignment to write true functions.
One detail the example above does not illustrate is the
fact that macro argument parsing is not token-oriented. If
you call jumper( 1 ), the value of $1 is " 1 ". You could
even call jumper(big string) to give $1 the value
"big string".
If you want to pass in a coordinate pair, you can avoid
getting tripped up by the comma by wrapping the pair in
parentheses.
Macros persist through pictures. To undefine a macro,
say undef name; for example,
undef jumper
undef jumperblock
would undefine the two macros in the jumper block example.
15. Import/Export Commands
Commands that import or export data between pic and its
environment are described here.
15.1. File and Table Insertion
The statement
copy filename
inserts the contents of filename in the pic input stream.
Any .PS/.PE pair in the file is ignored. You can use this
to include pre-generated images.
A variant of this statement replicates the copy thru
feature of grap(1). The call
copy filename thru macro
calls macro (which may be either a name or replacement text)
on the arguments obtained by breaking each line of the file
into blank-separated fields. The macro may have up to 9 ar-
guments. The replacement text may be delimited by braces or
by a pair of instances of any character not appearing in the
rest of the text.
If you write
-33-
copy thru macro
omitting the filename, lines to be parsed are taken from the
input source up to the next .PE.
In either of the last two copy commands, GNU gpic per-
mits a trailing `until word' clause to be added which termi-
nates the copy when the first word matches the argument (the
default behavior is therefore equivalent to until .PE).
Accordingly, the command
.PS
copy thru % circle at ($1,$2) % until "END"
1 2
3 4
5 6
END
box
.PE
is equivalent to
.PS
circle at (1,2)
circle at (3,4)
circle at (5,6)
box
.PE
15.2. Debug Messages
The command print accepts any number of arguments, con-
catenates their output forms, and writes the result to stan-
dard error. Each argument must be an expression, a posi-
tion, or a text string.
15.3. Escape to Post-Processor
If you write
command arg...
pic concatenates the arguments and pass them through as a
line to troff or TeX. Each arg must be an expression, a po-
sition, or text. This has a similar effect to a line begin-
ning with . or \, but allows the values of variables to be
passed through.
For example,
-34-
.PS
x = 14
command ".ds string x is " x "."
.PE
\*[string]
prints
x is 14.
15.4. Executing Shell Commands
The command
sh { anything... }
macro-expands the text in braces, then executes it as a
shell command. This could be used to generate images or
data tables for later inclusion. The delimiters shown as {}
here may also be two copies of any one character not present
in the shell command text. In either case, the body may
contain balanced {} pairs. Strings in the body may contain
balanced or unbalanced braces in any case.
16. Control-flow constructs
The pic language provides conditionals and looping.
For example,
pi = atan2(0,-1);
for i = 0 to 2 * pi by 0.1 do {
"-" at (i/2, 0);
"." at (i/2, sin(i)/2);
":" at (i/2, cos(i)/2);
}
which yields this:
::: ..... :::
::: ... :::
.. :: ... ::
.-----::------..------::------.
:: .. :: ...
:: ::: ...
::::::: .......
Figure 16-1: Plotting with a for loop
-35-
The syntax of the for statement is:
for variable = expr1 to expr2 [by [*]expr3] do X body X
The semantics are as follows: Set variable to expr1. While
the value of variable is less than or equal to expr2, do
body and increment variable by expr3; if by is not given,
increment variable by 1. If expr3 is prefixed by * then
variable is multiplied instead by expr3. The value of expr3
can be negative for the additive case; variable is then
tested whether it is greater than or equal to expr2. For
the multiplicative case, expr3 must be greater than zero.
If the constraints aren't met, the loop isn't executed. X
can be any character not occurring in body; or the two Xs
may be paired braces (as in the sh command).
The syntax of the if statement is as follows:
if expr then X if-true X [else Y if-false Y]
Its semantics are as follows: Evaluate expr; if it is non-
zero then do if-true, otherwise do if-false. X can be any
character not occurring in if-true. Y can be any character
not occurring in if-false.
Eithe or both of the X or Y pairs may instead be bal-
anced pairs of braces ({ and }) as in the sh command. In
either case, the if-true may contain balanced pairs of
braces. None of these delimiters are seen inside strings.
All the usual relational operators my be used in condi-
tional expressions; ! (logical negation, not factorial), &&,
||, ==, !=, >=, <=, <, >.
String comparison is also supported using == and !=.
String comparisons may need to be parenthesized to avoid
syntactic ambiguities.
17. Interface To [gt]roff
The output of pic is [gt]roff drawing commands. The
GNU gpic(1) command warns that it relies on drawing exten-
sions present in groff(1) that are not present in troff(1).
17.1. Scaling Arguments
The DWB pic(1) program accepts one or two arguments to
.PS, which is interpreted as a width and height in inches to
which the results of pic(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.
-36-
GNU gpic 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.
17.2. How Scaling is Handled
When pic processes a picture description on input, it
passes .PS and .PE through to the postprocessor. The .PS
gets decorated with two numeric arguments which are the X
and Y dimensions of the picture in inches. The post-proces-
sor can use these to reserve space for the picture and cen-
ter it.
The GNU incarnation of the ms macro package, for exam-
ple, includes the following definitions:
.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
..
Equivalent definition is supplied by GNU pic(1) if you use
the -mpic option; this should make it usable with macro
pages other than ms(1).
If .PF is used instead of .PE, the troff position is
restored to what it was at the picture start (Kernighan
notes that the F stands for "flyback").
The invocation
.PS <file
causes the contents of file to replace the .PS line. This
feature is deprecated; use `copy file' instead).
17.3. PIC and [gt]roff commands
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
-37-
font changes are usually safe.
Point sizes and font changes are also safe within text
strings, as long as they are undone before the end of
string.
The state of [gt]roff's fill mode is preserved across
pictures.
17.4. PIC and EQN
The Kernighan paper notes that there is a subtle prob-
lem with complicated equations inside pic pictures; they
come out wrong if eqn(1) has to leave extra vertical space
for the equation. If your equation involves more than sub-
scripts and superscripts, you must add to the beginning of
each equation the extra information space 0. He gives the
following example:
arrow
box "$space 0 {H( omega )} over {1 - H( omega )}$"
arrow
+------+
----++ +---+-
|1-H() |
+------+
Figure 17-1: Equations within pictures
17.5. Absolute Positioning of Pictures
A pic 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 .vs) before starting the
picture.
18. Interface to TeX
TeX mode is enabled by the -t option. In TeX mode, pic
defines a vbox called \graph for each picture; the name can
be changed with the pseudo-variable figname (which is actu-
ally a specially parsed command). You must yourself print
that vbox using, for example, the command
\centerline{\box\graph}
-38-
Actually, since the vbox has a height of zero (it is defined
with \vtop) this produces slightly more vertical space above
the picture than below it;
\centerline{\raise 1em\box\graph}
would avoid this.
To make the vbox having a positive height and a depth
of zero (as used e.g. by LaTeX's graphics.sty), define the
following macro in your document:
\def\gpicbox#1{%
\vbox{\unvbox\csname #1\endcsname\kern 0pt}}
Now you can simply say \gpicbox{graph} instead of
\box\graph.
You must use a TeX driver that supports the tpic spe-
cials, version 2.
Lines beginning with \ are passed through transpar-
ently; a % is added to the end of the line to avoid unwanted
spaces. You can safely use this feature to change fonts or
to change the value of \baselineskip. Anything else may
well produce undesirable results; use at your own risk.
Lines beginning with a period are not given any special
treatment.
The TeX mode of pic(1) does not translate troff font
and size changes included in text strings!
Here an example how to use figname.
.PS
figname = foo;
...
.PE
.PS
figname = bar;
...
.PE
\centerline{\box\foo \hss \box\bar}
Use this feature sparsingly and only if really needed: A
different name means a new box register in TeX, and the max-
imum number of box registers is only 256. Also be careful
not to use a predefined TeX or LaTeX macro name as an argu-
ment to figname since this inevitably causes an error.
-39-
19. Obsolete Commands
GNU gpic(1) has a command
plot expr ["text"]
This is a text object which is constructed by using text as
a format string for sprintf with an argument of expr. If
text is omitted a format string of "%g" is used. Attributes
can be specified in the same way as for a normal text ob-
ject. Be very careful that you specify an appropriate for-
mat string; pic does only very limited checking of the
string. This is deprecated in favour of sprintf.
20. Some Larger Examples
Here are a few larger examples, with complete source
code. One of our earlier examples is generated in an in-
structive way using a for loop:
.PS
# Draw a demonstration up left arrow with grid box overlay
define gridarrow
{
move right 0.1
[
{arrow up left $1;}
box wid 0.5 ht 0.5 dotted with .nw at last arrow .end;
for i = 2 to ($1 / 0.5) do
{
box wid 0.5 ht 0.5 dotted with .sw at last box .se;
}
move down from last arrow .center;
[
sprintf("\fBarrow up left %g\fP", $1)
]
]
move right 0.1 from last [] .e;
}
gridarrow(0.5);
gridarrow(1);
gridarrow(1.5);
gridarrow(2);
undef gridarrow
.PE
-40-
++++++ +++++++++++ ++++++++++++++++ +++++++++++++++++++++
+ + + + + + + + + + + + + +
+ + + + + + + + + + + + + +
++++++ +++++++++++ ++++++++++++++++ +++++++++++++++++++++
arrow up left arrow up left 1 arrow up left 1.5 arrow up left 2
Figure 20-1: Diagonal arrows (dotted boxes show the implied
0.5-inch grid)
Here's an example concocted to demonstrate layout of a
large, multiple-part pattern:
-41-
.PS
define filter {box ht 0.25 rad 0.125}
lineht = 0.25;
Top: [
right;
box "\fBms\fR" "sources";
move;
box "\fBHTML\fR" "sources";
move;
box "\fBlinuxdoc-sgml\fP" "sources" wid 1.5;
move;
box "\fBTexinfo\fP" "sources";
line down from 1st box .s lineht;
A: line down;
line down from 2nd box .s; filter "\fBhtml2ms\fP";
B: line down;
line down from 3rd box .s; filter "\fBformat\fP";
C: line down;
line down from 4th box .s; filter "\fBtexi2roff\fP";
D: line down;
]
move down 1 from last [] .s;
Anchor: box wid 1 ht 0.75 "\fBms\fR" "intermediate" "form";
arrow from Top.A.end to Anchor.nw;
arrow from Top.B.end to 1/3 of the way between Anchor.nw and Anchor.ne;
arrow from Top.C.end to 2/3 of the way between Anchor.nw and Anchor.ne;
arrow from Top.D.end to Anchor.ne
{
# PostScript column
move to Anchor .sw;
line down left then down ->;
filter "\fBpic\fP";
arrow;
filter "\fBeqn\fP";
arrow;
filter "\fBtbl\fP";
arrow;
filter "\fBgroff\fP";
arrow;
box "PostScript";
# HTML column
move to Anchor .se;
line down right then down ->;
A: filter dotted "\fBpic2img\fP";
arrow;
B: filter dotted "\fBeqn2html\fP";
arrow;
C: filter dotted "\fBtbl2html\fP";
arrow;
filter "\fBms2html\fP";
arrow;
box "HTML";
-42-
# Nonexistence caption
box dashed wid 1 at B + (2,0) "These tools" "don't yet exist";
line chop 0 chop 0.1 dashed from last box .nw to A.e ->;
line chop 0 chop 0.1 dashed from last box .w to B.e ->;
line chop 0 chop 0.1 dashed from last box .sw to C.e ->;
}
.PE
+------+ +------+ +--------------+ +------+
| ms | |HTML | |linuxdoc-sgml | Texinfo|
sources| sources| | sources | sources|
+---+--+ +--+---+ +------+-------+ +---+--+
| --+--- --+--- ---+--
| +html2ms+ format + texi2roff
| --+--- --+--- ---+--
| | | |
+
+- +- | -+
+---------+
| ms |
intermediate
| form |
+---------+
| |
+ +
--+--- ++++++++
+ -pic--+ +pic2img+-++++
+ + ++++++++++++++++
--+--- ++++++++ These tools
+ -eqn--+ eqn2html+-+++++++don't yet exist
+ + ++++++++ ++++
--+--- ++++++++ +++++++
+ -tbl--+ tbl2html+-+
+ +
--+--- --+---
+ groff-+ +ms2html+
+ +
+--+---+ +--+---+
| | | |
PostScript |HTML |
+------+ +------+
Figure 20-2: Hypothetical production flow for dual-mode publishing
-43-
+----+
+---------+ |
| Master | |
| 1 Slave|
+---------+ |
| |
+----+
Figure 20-3: Three-dimensional Boxes
Here the source code for figure 20-3:
.PS
# a three-dimensional block
#
# tblock(<width>, <height>, <text>)
define tblock { [
box ht $2 wid $1 \
color "gold" outlined "black" \
xslanted 0 yslanted 0 \
$3;
box ht .1 wid $1 \
color "yellow" outlined "black" \
xslanted .1 yslanted 0 \
with .sw at last box .nw;
box ht $2 wid .1 \
color "goldenrod" outlined "black" \
xslanted 0 yslanted .1 \
with .nw at 2nd last box .ne;
] }
tblock(1, .5, "Master" "1");
move -.1
tblock(.5, 1, "Slave");
.PE
21. PIC Reference
This is an annotated grammar of pic.
21.1. Lexical Items
In general, pic is a free-format, token-oriented lan-
guage that ignores whitespace outside strings. But certain
lines and contructs are specially interpreted at the lexical
level:
A comment begins with # and continues to \n (comments
may also follow text in a line). A line beginning with a
period or backslash may be interpreted as text to be passed
-44-
through to the post-processor, depending on command-line op-
tions. An end-of-line backslash is interpreted as a request
to continue the line; the backslash and following newline
are ignored.
Here are the grammar terminals:
INT A positive integer.
NUMBER
A floating point numeric constant. May contain a
decimal point or be expressed in scientific nota-
tion in the style of printf(3)'s %e escape. A
trailing `i' or `I' (indicating the unit `inch')
is ignored.
TEXT A string enclosed in double quotes. A double
quote within TEXT must be preceded by a backslash.
Instead of TEXT you can use
sprintf ( TEXT [, <expr> ...] )
except after the `until' and `last' keywords, and
after all ordinal keywords (`th' and friends).
VARIABLE
A string starting with a character from the set
[a-z], optionally followed by one or more charac-
ters of the set [a-zA-Z0-9_]. (Values of vari-
ables are preserved across pictures.)
LABEL
A string starting with a character from the set
[A-Z], optionally followed by one or more charac-
ters of the set [a-zA-Z0-9_].
COMMAND-LINE
A line starting with a command character (`.' in
groff mode, `\' in TeX mode).
BALANCED-TEXT
A string either enclosed by `{' and `}' or with X
and X, where X doesn't occur in the string.
BALANCED-BODY
Delimiters as in BALANCED-TEXT; the body is inter-
preted as `<command>...'.
FILENAME
The name of a file. This has the same semantics
as TEXT.
-45-
MACRONAME
Either VARIABLE or LABEL.
21.2. Semi-Formal Grammar
Tokens not enclosed in <> are literals, except:
1. \n is a newline.
2. Three dots is a suffix meaning `replace with 0 or more
repetitions of the preceding element(s).
3. An enclosure in square brackets has its usual meaning
of `this clause is optional'.
4. Square-bracket-enclosed portions within tokens are op-
tional. Thus, `h[eigh]t' matches either `height' or
`ht'.
If one of these special tokens has to be referred to liter-
ally, it is surrounded with single quotes.
The top-level pic object is a picture.
<picture> ::=
.PS [NUMBER [NUMBER]]\n
<statement> ...
.PE \n
The arguments, if present, represent the width and
height of the picture, causing pic to attempt to scale it to
the given dimensions in inches. In no case, however, the X
and Y dimensions of the picture exceed the values of the
style variables maxpswid and maxpsheight (which default to
the normal 8.5i by 11i page size).
If the ending `.PE' is replaced by `.PF', the page ver-
tical position is restored to its value at the time `.PS'
was encountered. Another alternate form of invocation is
`.PS <FILENAME', which replaces the `.PS' line with a file
to be interpreted by pic (but this feature is deprecated).
The `.PS', `.PE', and `.PF' macros to perform centering
and scaling are normally supplied by the post-processor.
In the following, either `|' or a new line starts an
alternative.
<statement> ::=
<command> ;
<command> \n
-46-
<command> ::=
<primitive> [<attribute>]
LABEL : [;] <command>
LABEL : [;] <command> [<position>]
{ <command> ... }
VARIABLE [:] = <any-expr>
figname = MACRONAME
up | down | left | right
COMMAND-LINE
command <print-arg> ...
print <print-arg> ...
sh BALANCED-TEXT
copy FILENAME
copy [FILENAME] thru MACRONAME [until TEXT]
copy [FILENAME] thru BALANCED-BODY [until TEXT]
for VARIABLE = <expr> to <expr> [by [*] <expr>] do BALANCED-BODY
if <any-expr> then BALANCED-BODY [else BALANCED-BODY]
reset [VARIABLE [[,] VARIABLE ...]]
<print-arg> ::=
TEXT
<expr>
<position>
The current position and direction are saved on entry
to a `{ ... }' construction and restored on exit from it.
Note that in `if' constructions, newlines can only oc-
cur in BALANCED-BODY. This means that
if
{ ... }
else
{ ... }
fails. You have to use the braces on the same line as
the keywords:
if {
...
} else {
...
}
This restriction doesn't hold for the body after the
`do' in a `for' construction.
At the beginning of each picture, `figname' is reset to
the vbox name `graph'; this command has only a meaning in
TeX mode. While the grammar rules allow digits and the
-47-
underscore in the value of `figname', TeX normally accepts
uppercase and lowercase letters only as box names (you have
to use `\csname' if you really need to circumvent this limi-
tation).
<any-expr> ::=
<expr>
<text-expr>
<any-expr> <logical-op> <any-expr>
! <any-expr>
<logical-op> ::=
== | != | && | '||'
<text-expr> ::=
TEXT == TEXT
TEXT != TEXT
Logical operators are handled specially by pic since
they can deal with text strings also. pic uses strcmp(3) to
test for equality of strings; an empty string is considered
as `false' for `&&' and `||'.
<primitive> ::=
box # closed object -- rectangle
circle # closed object -- circle
ellipse # closed object -- ellipse
arc # open object -- quarter-circle
line # open object -- line
arrow # open object -- line with arrowhead
spline # open object -- spline curve
move
TEXT TEXT ... # text within invisible box
plot <expr> TEXT # formatted text
'[' <command> ... ']'
Drawn objects within `[ ... ]' are treated as a single
composite object with a rectangular shape (that of the
bounding box of all the elements). Variable and label as-
signments within a block are local to the block. Current
direction of motion is restored to the value at start of
block upon exit. Position is not restored (unlike `{ }');
instead, the current position becomes the exit position for
the current direction on the block's bounding box.
-48-
<attribute> ::=
h[eigh]t <expr> # set height of closed figure
wid[th] <expr> # set width of closed figure
rad[ius] <expr> # set radius of circle/arc
diam[eter] <expr> # set diameter of circle/arc
up [<expr>] # move up
down [<expr>] # move down
left [<expr>] # move left
right [<expr>] # move right
from <position> # set from position of open figure
to <position> # set to position of open figure
at <position> # set center of open figure
with <path> # fix corner/named point at specified location
with <position> # fix position of object at specified location
by <expr-pair> # set object's attachment point
then # sequential segment composition
dotted [<expr>] # set dotted line style
dashed [<expr>] # set dashed line style
thick[ness] <expr> # set thickness of lines
chop [<expr>] # chop end(s) of segment
'->' | '<-' | '<->' # decorate with arrows
invis[ible] # make primitive invisible
solid # make closed figure solid
fill[ed] [<expr>] # set fill density for figure
xscaled <expr> # slant box into x direction
yscaled <expr> # slant box into y direction
colo[u]r[ed] TEXT # set fill and outline color for figure
outline[d] TEXT # set outline color for figure
shaded TEXT # set fill color for figure
same # copy size of previous object
cw | ccw # set orientation of curves
ljust | rjust # adjust text horizontally
above | below # adjust text vertically
aligned # align parallel to object
TEXT TEXT ... # text within object
<expr> # motion in the current direction
Missing attributes are supplied from defaults; inappro-
priate ones are silently ignored. For lines, splines, and
arcs, height and width refer to arrowhead size.
The `at' primitive sets the center of the current ob-
ject. The `with' attribute fixes the specified feature of
the given object to a specified location. (Note that `with'
is incorrectly described in the Kernighan paper.)
The `by' primitive is not documented in the tutorial
portion of the Kernighan paper, and should probably be con-
sidered unreliable.
The primitive `arrow' is a synonym for `line ->'.
-49-
Text is normally an attribute of some object, in which
case successive strings are vertically stacked and centered
on the object's center by default. Standalone text is
treated as though placed in an invisible box.
A text item consists of a string or sprintf-expression,
optionally followed by positioning information. Text (or
strings specified with `sprintf') may contain font changes,
size changes, and local motions, provided those changes are
undone before the end of the current item. Text may also
contain \-escapes denoting special characters. The base
font and specific set of escapes supported is implementation
dependent, but supported escapes always include the follow-
ing:
\fR, \f1
Set Roman style (the default)
\fI, \f2
Set Italic style
\fB, \f3
Set Bold style
\fP
Revert to previous style; only works one level deep,
does not stack.
Color names are dependent on the pic implementation,
but in all modern versions color names recognized by the
X window system are supported.
A position is an (x,y) coordinate pair. There are lots
of different ways to specify positions:
<position> ::=
<position-not-place>
<place>
( <position> )
<position-not-place> ::=
<expr-pair>
<position> + <expr-pair>
<position> - <expr-pair>
( <position> , <position> )
<expr> [of the way] between <position> and <position>
<expr> '<' <position> , <position> '>'
<expr-pair> ::=
<expr> , <expr>
( expr-pair )
-50-
<place> ::=
<label>
<label> <corner>
<corner> [of] <label>
Here
<label> ::=
LABEL [. LABEL ...]
<nth-primitive>
<corner> ::=
.n | .e | .w | .s
.ne | .se | .nw | .sw
.c[enter] | .start | .end
.t[op] | .b[ot[tom]] | .l[eft] | .r[ight]
left | right | <top-of> | <bottom-of>
<north-of> | <south-of> | <east-of> | <west-of>
<center-of> | <start-of> | <end-of>
upper left | lower left | upper right | lower right
<xxx-of> ::=
xxx # followed by `of'
<nth-primitive> ::=
<ordinal> <object-type>
[<ordinal>] last <object-type>
<ordinal> ::=
INT th
INT st | INT nd | INT rd
` <any-expr> 'th
<object-type> ::=
box
circle
ellipse
arc
line
arrow
spline
'[]'
TEXT
As Kernighan notes, "since barbarisms like 1th and 3th
are barbaric, synonyms like 1st and 3rd are accepted as
well." Objects of a given type are numbered from 1 upwards
in order of declaration; the last modifier counts backwards.
-51-
The "'th" form (which allows you to select a previous
object with an expression, as opposed to a numeric literal)
is not documented in DWB's pic(1).
The <xxx-of> rule is special: The lexical parser checks
whether xxx is followed by the token `of' without eliminat-
ing it so that the grammar parser can still see `of'. Valid
examples of specifying a place with corner and label are
thus
A .n
.n of A
.n A
north of A
while
north A
A north
both cause a syntax error. (DWB pic also allows the weird
form `A north of'.)
Here the special rules for the `with' keyword using a
path:
<path> ::=
<relative-path>
( <relative-path> , <relative-path> )
<relative-path> ::=
<corner>
. LABEL [. LABEL ...] [<corner>]
The following style variables control output:
| |
Style Variable | Default | What It Does
---------------+---------+-----------------------------------------------
boxht | 0.5 | Default height of a box
boxwid | 0.75 | Default width of a box
lineht | 0.5 | Default length of vertical line
linewid | 0.75 | Default length of horizontal line
arcrad | 0.25 | Default radius of an arc
circlerad | 0.25 | Default radius of a circle
ellipseht | 0.5 | Default height of an ellipse
ellipsewid | 0.75 | Default width of an ellipse
moveht | 0.5 | Default length of vertical move
movewid | 0.75 | Default length of horizontal move
textht | 0 | Default height of box enclosing a text object
textwid | 0 | Default width of box enclosing a text object
-52-
| |
Style Variable | Default | What It Does
---------------+---------+-----------------------------------------------
arrowht | 0.1 | Length of arrowhead along shaft
arrowwid | 0.05 | Width of rear of arrowhead
arrowhead | 1 | Enable/disable arrowhead filling
dashwid | 0.05 | Interval for dashed lines
maxpswid | 8.5 | Maximum width of picture
maxpsht | 11 | Maximum height of picture
scale | 1 | Unit scale factor
fillval | 0.5 | Default fill value
---------------+---------+-----------------------------------------------
Any of these can be set by assignment, or reset using the
reset statement. Style variables assigned within `[ ]'
blocks are restored to their beginning-of-block value on
exit; top-level assignments persist across pictures. Dimen-
sions are divided by scale on output.
All pic expressions are evaluated in floating point;
units are always inches (a trailing `i' or `I' is ignored).
Expressions have the following simple grammar, with seman-
tics very similar to C expressions:
<expr> ::=
VARIABLE
NUMBER
<place> <place-attribute>
<expr> <op> <expr>
- <expr>
( <any-expr> )
! <expr>
<func1> ( <any-expr> )
<func2> ( <any-expr> , <any-expr> )
rand ( )
<place-attribute>
.x | .y | .h[eigh]t | .wid[th] | .rad
<op> ::=
+ | - | * | / | % | ^ | '<' | '>' | '<=' | '>='
<func1> ::=
sin | cos | log | exp | sqrt | int | rand | srand
<func2> ::=
atan2 | max | min
Both exp and log are base 10; int does integer truncation;
and rand() returns a random number in [0-1).
-53-
There are define and undef statements which are not
part of the grammar (they behave as pre-processor macros to
the language). These may be used to define pseudo-func-
tions.
define name { replacement-text }
This defines name as a macro to be replaced by the replace-
ment text (not including the braces). The macro may be
called as
name(arg1, arg2, ..., argn)
The arguments (if any) are substituted for tokens $1, $2 ...
$n appearing in the replacement text. To undefine a macro,
say undef name, specifying the name to be undefined.
22. History and Acknowledgements
Original pic was written to go with Joseph Ossanna's
original troff(1) by Brian Kernighan, and later re-written
by Kernighan with substantial enhancements (apparently as
part of the evolution of troff(1) into ditroff(1) to gener-
ate device-independent output).
The language had been inspired by some earlier graphics
languages including ideal and grap. Kernighan credits Chris
van Wyk (the designer of ideal) with many of the ideas that
went into pic.
The pic language was originally described by Brian
Kernighan in Bell Labs Computing Science Technical Report
#116 (you can obtain a PostScript copy of the revised ver-
sion, [1], by sending a mail message to netlib@re-
search.att.com with a body of `send 116 from re-
search/cstr'). There have been two revisions, in 1984 and
1991.
The document you are reading effectively subsumes
Kernighan's description; it was written to fill in lacunae
in the exposition and integrate in descriptions of the GNU
gpic(1) and pic2plot(1) features.
The GNU gpic implementation was written by James Clark
<jjc@jclark.com>.
The GNU pic2plot implementation is based on James
Clark's parser code and maintained by Robert Maier, princi-
pal author of plotutils.
-54-
23. Bibliography
1. Kernighan, B. W. PIC -- A Graphics Language for Type-
setting (Revised User Manual). Bell Labs Computing
Science Technical Report #116, December 1991.
2. Van Wyk, C. J. A high-level language for specifying
pictures. ACM Transactions On Graphics 1,2 (1982)
163-182.
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