groff_out(5) File Formats Manual groff_out(5)
Name
groff_out - GNU roff intermediate output format
Description
The fundamental operation of the troff(1) formatter is the translation
of the groff(7) input language into a series of instructions concerned
primarily with placing glyphs or geometric objects at specific
positions on a rectangular page. In the following discussion, the term
command refers to this intermediate output language, never to the
groff(7) language intended for use by document authors. Intermediate
output commands comprise several categories: glyph output; font, color,
and text size selection; motion of the printing position; page
advancement; drawing of geometric primitives; and device control
commands, a catch-all for other operations. The last includes
directives to start and stop output, identify the intended output
device, and embed URL hyperlinks in supported output formats.
Because the front-end command groff(1) is a wrapper that normally runs
the troff formatter to generate intermediate output and an output
driver ("postprocessor") to consume it, users normally do not encounter
this language. The groff program's -Z option inhibits postprocessing
such that this intermediate output is sent to the standard output
stream as when troff is run manually.
groff's intermediate output facilitates the development of output
drivers and other postprocessors by offering a common programming
interface. It is an extension of the page description language
developed by Brian Kernighan for AT&T device-independent troff circa
1980. Where a distinction is necessary, we will say "troff output" to
describe the output of GNU troff, and "intermediate output" to denote
the language accepted by the parser implemented in groff's internal C++
library used by most of its output drivers.
Language concepts
During the run of troff, the roff input is cracked down to the
information on what has to be printed at what position on the intended
device. So the language of the intermediate output format can be quite
small. Its only elements are commands with or without arguments. In
this document, the term "command" always refers to the intermediate
output language, never to the roff language used for document
formatting. There are commands for positioning and text writing, for
drawing, and for device controlling.
Separation
Classical troff output had strange requirements on whitespace. The
groff output parser, however, is smart about whitespace by making it
maximally optional. The whitespace characters, i.e., the tab, space,
and newline characters, always have a syntactical meaning. They are
never printable because spacing within the output is always done by
positioning commands.
Any sequence of space or tab characters is treated as a single
syntactical space. It separates commands and arguments, but is only
required when there would occur a clashing between the command code and
the arguments without the space. Most often, this happens when
variable length command names, arguments, argument lists, or command
clusters meet. Commands and arguments with a known, fixed length need
not be separated by syntactical space.
A line break is a syntactical element, too. Every command argument can
be followed by whitespace, a comment, or a newline character. Thus a
syntactical line break is defined to consist of optional syntactical
space that is optionally followed by a comment, and a newline
character.
The normal commands, those for positioning and text, consist of a
single letter taking a fixed number of arguments. For historical
reasons, the parser allows stacking of such commands on the same line,
but fortunately, in groff intermediate output, every command with at
least one argument is followed by a line break, thus providing
excellent readability.
The other commands -- those for drawing and device controlling -- have
a more complicated structure; some recognize long command names, and
some take a variable number of arguments. So all D and x commands were
designed to request a syntactical line break after their last argument.
Only one command, `x X' has an argument that can stretch over several
lines, all other commands must have all of their arguments on the same
line as the command, i.e., the arguments may not be split by a line
break.
Lines containing only spaces and/or a comment are treated as empty and
ignored.
Argument units
Some commands accept integer arguments that represent measurements, but
the scaling units of the formatter's language are never used. Most
commands assume a scaling unit of "u" (basic units), and others use "z"
(scaled points); These are defined by the parameters specified in the
device's DESC file; see groff_font(5) and, for more on scaling units,
groff(7) and Groff: The GNU Implementation of troff, the groff Texinfo
manual. Color-related commands use dimensionless integers.
Note that single characters can have the eighth bit set, as can the
names of fonts and special characters (this is, glyphs). The names of
glyphs and fonts can be of arbitrary length. A glyph that is to be
printed will always be in the current font.
A string argument is always terminated by the next whitespace character
(space, tab, or newline); an embedded # character is regarded as part
of the argument, not as the beginning of a comment command. An integer
argument is already terminated by the next non-digit character, which
then is regarded as the first character of the next argument or
command.
Document parts
A correct intermediate output document consists of two parts, the
prologue and the body.
The task of the prologue is to set the general device parameters using
three exactly specified commands. The groff prologue is guaranteed to
consist of the following three lines (in that order):
x T device
x res n h v
x init
with the arguments set as outlined in subsection "Device Control
Commands" below. However, the parser for the intermediate output
format is able to swallow additional whitespace and comments as well.
The body is the main section for processing the document data.
Syntactically, it is a sequence of any commands different from the ones
used in the prologue. Processing is terminated as soon as the first
x stop command is encountered; the last line of any groff intermediate
output always contains such a command.
Semantically, the body is page oriented. A new page is started by a
p command. Positioning, writing, and drawing commands are always done
within the current page, so they cannot occur before the first
p command. Absolute positioning (by the H and V commands) is done
relative to the current page, all other positioning is done relative to
the current location within this page.
Command reference
This section describes all intermediate output commands, the classical
commands as well as the groff extensions.
Comment command
#anything<line-break>
A comment. Ignore any characters from the # character up to the
next newline. Each comment can be preceded by arbitrary
syntactical space; every command can be terminated by a comment.
Simple commands
The commands in this subsection have a command code consisting of a
single character, taking a fixed number of arguments. Most of them are
commands for positioning and text writing. These commands are smart
about whitespace. Optionally, syntactical space can be inserted
before, after, and between the command letter and its arguments. All
of these commands are stackable, i.e., they can be preceded by other
simple commands or followed by arbitrary other commands on the same
line. A separating syntactical space is necessary only when two
integer arguments would clash or if the preceding argument ends with a
string argument.
C id<white-space>
Typeset the glyph of the special character id. Trailing
syntactical space is necessary to allow special character names
of arbitrary length. The drawing position is not advanced.
c c Typeset the glyph of the ordinary character character c. The
drawing position is not advanced.
f n Select the font mounted at position n. n cannot be negative.
H n Horizontally move the drawing position to n basic units from the
left edge of the page. n cannot be negative.
h n Move the drawing position right n basic units. AT&T troff
allowed negative n; GNU troff does not produce such values, but
groff's output driver library handles them.
m scheme [component ...]
Select the stroke color using the components in the color space
scheme. Each component is an integer between 0 and 65536. The
quantity of components and their meanings vary with each scheme.
This command is a groff extension.
mc cyan magenta yellow
Use the CMY color scheme with components cyan, magenta,
and yellow.
md Use the default color (no components; black in most
cases).
mg gray
Use a grayscale color scheme with a component ranging
between 0 (black) and 65536 (white).
mk cyan magenta yellow black
Use the CMYK color scheme with components cyan, magenta,
yellow, and black.
mr red green blue
Use the RGB color scheme with components red, green, and
blue.
N n Typeset the glyph with index n in the current font. n is
normally a non-negative integer. The drawing position is not
advanced. The html and xhtml devices use this command with
negative n to produce unbreakable space; the absolute value of n
is taken and interpreted in basic units.
n b a Indicate a break. No action is performed; the command is
present to make the output more easily parsed. The integers b
and a describe the vertical space amounts before and after the
break, respectively. GNU troff issues this command but groff's
output driver library ignores it. See v and V.
p n Begin a new page, setting its number to n. Each page is
independent, even from those using the same number. The
vertical drawing position is set to 0. All positioning,
writing, and drawing commands are interpreted in the context of
a page, so a p command must precede them.
s n Set type size to n scaled points (unit z in GNU troff). AT&T
troff used unscaled points (p) instead; see section
"Compatibility" below.
t xyz...<white-space>
t xyz... dummy-arg<white-space>
Typeset word xyz; that is, set a sequence of ordinary glyphs
named x, y, z, ..., terminated by a space or newline; an
optional second integer argument is ignored (this allows the
formatter to generate an even number of arguments). Each glyph
is set at the current drawing position, and the position is then
advanced horizontally by the glyph's width. A glyph's width is
read from its metrics in the font description file, scaled to
the current type size, and rounded to a multiple of the
horizontal motion quantum. Use the C command to emplace glyphs
of special characters. The t command is a groff extension and
is output only for devices whose DESC file contains the tcommand
directive; see groff_font(5).
u n xyz...
u xyz... dummy-arg<white-space>
Typeset word xyz with track kerning. As t, but after placing
each glyph, the drawing position is further advanced
horizontally by n basic units. The u command is a groff
extension and is output only for devices whose DESC file
contains the tcommand directive; see groff_font(5).
V n Vertically move the drawing position to n basic units from the
top edge of the page. n cannot be negative.
v n Move the drawing position down n basic units. AT&T troff
allowed negative n; GNU troff does not produce such values, but
groff's output driver library handles them.
w Indicate an inter-word space. No action is performed; the
command is present to make the output more easily parsed. Only
adjustable, breakable inter-word spaces are thus described;
those resulting from \~ or horizontal motion escape sequences
are not. GNU troff issues this command but groff's output
driver library ignores it. See h and H.
Graphics commands
Each graphics or drawing command in the intermediate output starts with
the letter D followed by one or two characters that specify a
subcommand; this is followed by a fixed or variable number of integer
arguments that are separated by a single space character. A D command
may not be followed by another command on the same line (apart from a
comment), so each D command is terminated by a syntactical line break.
troff output follows the classical spacing rules (no space between
command and subcommand, all arguments are preceded by a single space
character), but the parser allows optional space between the command
letters and makes the space before the first argument optional. As
usual, each space can be any sequence of tab and space characters.
Some graphics commands can take a variable number of arguments. In
this case, they are integers representing a size measured in basic
units u. The h arguments stand for horizontal distances where positive
means right, negative left. The v arguments stand for vertical
distances where positive means down, negative up. All these distances
are offsets relative to the current location.
Unless indicated otherwise, each graphics command directly corresponds
to a similar groff \D escape sequence; see groff(7).
Unknown D commands are assumed to be device-specific. Its arguments
are parsed as strings; the whole information is then sent to the
postprocessor.
In the following command reference, the syntax element <line-break>
means a syntactical line break as defined in subsection "Separation"
above.
D~ h1 v1 h2 v2 ... hn vn<line-break>
Draw B-spline from current position to offset (h1, v1), then to
offset (h2, v2) if given, etc., up to (hn, vn). This command
takes a variable number of argument pairs; the current position
is moved to the terminal point of the drawn curve.
Da h1 v1 h2 v2<line-break>
Draw arc from current position to (h1, v1)+ (h2, v2) with center
at (h1, v1); then move the current position to the final point
of the arc.
DC d<line-break>
DC d dummy-arg<line-break>
Draw a solid circle using the current fill color with diameter d
(integer in basic units u) with leftmost point at the current
position; then move the current position to the rightmost point
of the circle. An optional second integer argument is ignored
(this allows the formatter to generate an even number of
arguments). This command is a groff extension.
Dc d<line-break>
Draw circle line with diameter d (integer in basic units u) with
leftmost point at the current position; then move the current
position to the rightmost point of the circle.
DE h v<line-break>
Draw a solid ellipse in the current fill color with a horizontal
diameter of h and a vertical diameter of v (both integers in
basic units u) with the leftmost point at the current position;
then move to the rightmost point of the ellipse. This command
is a groff extension.
De h v<line-break>
Draw an outlined ellipse with a horizontal diameter of h and a
vertical diameter of v (both integers in basic units u) with the
leftmost point at current position; then move to the rightmost
point of the ellipse.
DF color-scheme [component ...]<line-break>
Set fill color for solid drawing objects using different color
schemes; the analogous command for setting the color of text,
line graphics, and the outline of graphic objects is m. The
color components are specified as integer arguments between 0
and 65536. The number of color components and their meaning
vary for the different color schemes. These commands are
generated by the groff escape sequences \D'F ...' and \M (with
no other corresponding graphics commands). This command is a
groff extension.
DFc cyan magenta yellow<line-break>
Set fill color for solid drawing objects using the CMY
color scheme, having the 3 color components cyan,
magenta, and yellow.
DFd <line-break>
Set fill color for solid drawing objects to the default
fill color value (black in most cases). No component
arguments.
DFg gray<line-break>
Set fill color for solid drawing objects to the shade of
gray given by the argument, an integer between 0 (black)
and 65536 (white).
DFk cyan magenta yellow black<line-break>
Set fill color for solid drawing objects using the CMYK
color scheme, having the 4 color components cyan,
magenta, yellow, and black.
DFr red green blue<line-break>
Set fill color for solid drawing objects using the RGB
color scheme, having the 3 color components red, green,
and blue.
Df n<line-break>
The argument n must be an integer in the range -32767 to 32767.
0<=n<=1000
Set the color for filling solid drawing objects to a
shade of gray, where 0 corresponds to solid white, 1000
(the default) to solid black, and values in between to
intermediate shades of gray; this is obsoleted by command
DFg.
n<0 or n>1000
Set the filling color to the color that is currently
being used for the text and the outline, see command m.
For example, the command sequence
mg 0 0 65536
Df -1
sets all colors to blue.
This command is a groff extension.
Dl h v<line-break>
Draw line from current position to offset (h, v) (integers in
basic units u); then set current position to the end of the
drawn line.
Dp h1 v1 h2 v2 ... hn vn<line-break>
Draw a polygon line from current position to offset (h1, v1),
from there to offset (h2, v2), etc., up to offset (hn, vn), and
from there back to the starting position. For historical
reasons, the position is changed by adding the sum of all
arguments with odd index to the current horizontal position and
the even ones to the vertical position. Although this doesn't
make sense it is kept for compatibility. This command is a
groff extension.
DP h1 v1 h2 v2 ... hn vn<line-break>
The same macro as the corresponding Dp command with the same
arguments, but draws a solid polygon in the current fill color
rather than an outlined polygon. The position is changed in the
same way as with Dp. This command is a groff extension.
Dt n<line-break>
Set the current line thickness to n (an integer in basic
units u) if n>0; if n=0 select the smallest available line
thickness; otherwise, the line thickness is made proportional to
the type size, which is the default. For historical reasons,
the horizontal position is changed by adding the argument to the
current horizontal position, while the vertical position is not
changed. Although this doesn't make sense, it is kept for
compatibility. This command is a groff extension.
Device control commands
Each device control command starts with the letter x followed by a
space character (optional or arbitrary space/tab in groff) and a
subcommand letter or word; each argument (if any) must be preceded by a
syntactical space. All x commands are terminated by a syntactical line
break; no device control command can be followed by another command on
the same line (except a comment).
The subcommand is basically a single letter, but to increase
readability, it can be written as a word, i.e., an arbitrary sequence
of characters terminated by the next tab, space, or newline character.
All characters of the subcommand word but the first are simply ignored.
For example, troff outputs the initialization command x i as x init and
the resolution command x r as x res. But writings like x i_like_groff
and x roff_is_groff are accepted as well to mean the same commands.
In the following, the syntax element <line-break> means a syntactical
line break as defined in subsection "Separation" above.
xF name<line-break>
(Filename control command)
Use name as the intended name for the current file in error
reports. This is useful for remembering the original file name
when groff uses an internal piping mechanism. The input file is
not changed by this command. This command is a groff extension.
xf n s<line-break>
(font control command)
Mount font position n (a non-negative integer) with font named s
(a text word); see groff_font(5).
xH n<line-break>
(Height control command)
Set character height to n (a positive integer in scaled
points z). Classical troff used the unit points (p) instead;
see section "Compatibility" below.
xi <line-break>
(init control command)
Initialize device. This is the third command of the prologue.
xp <line-break>
(pause control command)
Parsed but ignored. The classical documentation reads pause
device, can be restarted.
xr n h v<line-break>
(resolution control command)
Resolution is n, while h is the minimal horizontal motion, and v
the minimal vertical motion possible with this device; all
arguments are positive integers in basic units u per inch. This
is the second command of the prologue.
xS n<line-break>
(Slant control command)
Set slant to n degrees (an integer in basic units u).
xs <line-break>
(stop control command)
Terminates the processing of the current file; issued as the
last command of any intermediate troff output.
xt <line-break>
(trailer control command)
Generate trailer information, if any. In groff, this is
currently ignored.
xT xxx<line-break>
(Typesetter control command)
Set the name of the output driver to xxx, a sequence of non-
whitespace characters terminated by whitespace. The possible
names correspond to those of groff's -T option. This is the
first command of the prologue.
xu n<line-break>
(underline control command)
Configure underlining of spaces. If n is 1, start underlining
of spaces; if n is 0, stop underlining of spaces. This is
needed for the cu request in nroff mode and is ignored
otherwise. This command is a groff extension.
xX anything<line-break>
(X-escape control command)
Send string anything uninterpreted to the device. If the line
following this command starts with a + character this line is
interpreted as a continuation line in the following sense. The
+ is ignored, but a newline character is sent instead to the
device, the rest of the line is sent uninterpreted. The same
applies to all following lines until the first character of a
line is not a + character. This command is generated by the
groff escape sequence \X. The line-continuing feature is a
groff extension.
Obsolete command
In classical troff output, emitting a single glyph was mostly done by a
very strange command that combined a horizontal move and the printing
of a glyph. It didn't have a command code, but is represented by a
3-character argument consisting of exactly 2 digits and a character.
ddc Move right dd (exactly two decimal digits) basic units u, then
print glyph with single-letter name c.
In groff, arbitrary syntactical space around and within this
command is allowed to be added. Only when a preceding command
on the same line ends with an argument of variable length a
separating space is obligatory. In classical troff, large
clusters of these and other commands were used, mostly without
spaces; this made such output almost unreadable.
For modern high-resolution devices, this command does not make sense
because the width of the glyphs can become much larger than two decimal
digits. In groff, it is used only for output to the X75, X75-12, X100,
and X100-12 devices. For others, the commands t and u provide greater
functionality and superior troubleshooting capacity.
Postprocessing
The roff postprocessors are programs that have the task to translate
the intermediate output into actions that are sent to a device. A
device can be some piece of hardware such as a printer, or a software
file format suitable for graphical or text processing. The groff
system provides powerful means that make the programming of such
postprocessors an easy task.
There is a library function that parses the intermediate output and
sends the information obtained to the device via methods of a class
with a common interface for each device. So a groff postprocessor must
only redefine the methods of this class. For details, see the
reference in section "Files" below.
Example
This section presents the intermediate output generated from the same
input for three different devices. The input is the sentence hell
world fed into groff on the command line.
o High-resolution device ps
shell> echo "hell world" | groff -Z -T ps
x T ps
x res 72000 1 1
x init
p1
x font 5 TR
f5
s10000
V12000
H72000
thell
wh2500
tw
H96620
torld
n12000 0
x trailer
V792000
x stop
This output can be fed into the postprocessor grops(1) to get its
representation as a PostScript file, or gropdf(1) to output directly to
PDF.
o Low-resolution device latin1
This is similar to the high-resolution device except that the
positioning is done at a minor scale. Some comments (lines starting
with #) were added for clarification; they were not generated by the
formatter.
shell> "hell world" | groff -Z -T latin1
# prologue
x T latin1
x res 240 24 40
x init
# begin a new page
p1
# font setup
x font 1 R
f1
s10
# initial positioning on the page
V40
H0
# write text 'hell'
thell
# inform about a space, and do it by a horizontal jump
wh24
# write text 'world'
tworld
# announce line break, but do nothing because ...
n40 0
# ... the end of the document has been reached
x trailer
V2640
x stop
This output can be fed into the postprocessor grotty(1) to get a
formatted text document.
o Classical style output
As a computer monitor has a very low resolution compared to modern
printers the intermediate output for the X devices can use the jump-
and-write command with its 2-digit displacements.
shell> "hell world" | groff -Z -T X100
x T X100
x res 100 1 1
x init
p1
x font 5 TR
f5
s10
V16
H100
# write text with old-style jump-and-write command
ch07e07l03lw06w11o07r05l03dh7
n16 0
x trailer
V1100
x stop
This output can be fed into the postprocessor xditview(1x) or
gxditview(1) for displaying in X.
Due to the obsolete jump-and-write command, the text clusters in the
classical output are almost unreadable.
Compatibility
The intermediate output language of the classical troff was first
documented in [CSTR #97]. The groff intermediate output format is
compatible with this specification except for the following features.
o The classical quasi device independence is not yet implemented.
o The old hardware was very different from what we use today. So the
groff devices are also fundamentally different from the ones in
classical troff. For example, the classical PostScript device was
called post and had a resolution of 720 units per inch, while groff's
ps device has a resolution of 72000 units per inch. Maybe, by
implementing some rescaling mechanism similar to the classical quasi
device independence, these could be integrated into modern groff.
o The B-spline command D~ is correctly handled by the intermediate
output parser, but the drawing routines aren't implemented in some of
the postprocessor programs.
o The argument of the commands s and x H has the implicit unit scaled
point z in groff, while classical troff had point (p). This isn't an
incompatibility, but a compatible extension, for both units coincide
for all devices without a sizescale parameter, including all
classical and the groff text devices. The few groff devices with a
sizescale parameter either did not exist, had a different name, or
seem to have had a different resolution. So conflicts with classical
devices are very unlikely.
o The position changing after the commands Dp, DP, and Dt is illogical,
but as old versions of groff used this feature it is kept for
compatibility reasons.
The differences between groff and classical troff are documented in
groff_diff(7).
Files
/opt/local/share/groff/1.23.0/font/devname/DESC
describes the output device name.
Authors
James Clark wrote an early version of this document, which described
only the differences between AT&T device-independent troff's output
format and that of GNU roff. The present version was completely
rewritten in 2001 by Bernd Warken <groff-bernd.warken-72@web.de>.
See also
Groff: The GNU Implementation of troff, by Trent A. Fisher and Werner
Lemberg, is the primary groff manual. You can browse it interactively
with "info groff".
"Troff User's Manual" by Joseph F. Ossanna, 1976 (revised by Brian W.
Kernighan, 1992), AT&T Bell Laboratories Computing Science Technical
Report No. 54, widely called simply "CSTR #54", documents the language,
device and font description file formats, and device-independent output
format referred to collectively in groff documentation as "AT&T troff".
"A Typesetter-independent TROFF" by Brian W. Kernighan, 1982, AT&T Bell
Laboratories Computing Science Technical Report No. 97, provides
additional insights into the device and font description file formats
and device-independent output format.
groff(1)
documents the -Z option and contains pointers to further groff
documentation.
groff(7)
describes the groff language, including its escape sequences and
system of units.
groff_font(5)
details the device scaling parameters of device DESC files.
troff(1)
generates the device-independent intermediate output documented
here.
roff(7)
presents historical aspects and the general structure of roff
systems.
groff_diff(7)
enumerates differences between the intermediate output produced
by AT&T troff and that of groff.
gxditview(1)
is a viewer for intermediate output.
Roff.js <https://github.com/Alhadis/Roff.js/> is a viewer for
intermediate output written in JavaScript.
grodvi(1), grohtml(1), grolbp(1), grolj4(1), gropdf(1), grops(1), and
grotty(1) are groff postprocessors.
groff 1.23.0 2 July 2023 groff_out(5)
groff 1.23.0 - Generated Sat Dec 23 10:03:20 CST 2023