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Name | Description | Language concepts | Command reference | Postprocessing | Example | Compatibility | Files | Authors | See also | COLOPHON |
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groff_out(5) File Formats Manual groff_out(5)
groff_out - GNU roff intermediate output format
The fundamental operation of the formatter is the translation of
the 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 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 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.
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.
Empty lines, i.e., lines containing only space and/or a comment,
can occur everywhere. They are just ignored.
Argument units
Some commands take integer arguments that are assumed to
represent values in a measurement unit, but the letter for the
corresponding scaling indicator is not written with the output
command arguments; see and Groff: The GNU Implementation of
troff, the groff Texinfo manual, for more on this topic. Most
commands assume the scaling indicator “u”, the basic unit of the
device, some use “z”, the scaled point unit of the device, while
others, such as the color commands, expect plain integers. Note
that these scaling indicators are relative to the chosen device.
They are defined by the parameters specified in the device's DESC
file; see
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.
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 only necessary when two integer arguments
would clash or if the preceding argument ends with a string
argument.
C xxx⟨white-space⟩
Print a glyph (special character) named xxx. The trailing
syntactical space or line break is necessary to allow
glyph names of arbitrary length. The glyph is printed at
the current print position; its size is read from the font
description file. The print position is not changed.
c c Print glyph with single-letter name c at the current print
position; its size is read from the font description file.
The print position is not changed.
f n Set font to font number n (a non-negative integer).
H n Move right to the absolute vertical position n (a non-
negative integer in basic units u) relative to left edge
of current page.
h n Move n (a non-negative integer) basic units u horizontally
to the right. AT&T troff allowed negative n; GNU troff
does not produce such values, but groff's output driver
library handles them.
m color-scheme [component] ...
Set the color for text (glyphs), line drawing, and the
outline of graphic objects using different color schemes;
the analogous command for the filling color of graphic
objects is DF. 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 sequence \m. They are groff extensions.
mc cyan magenta yellow
Set color using the CMY color scheme, having the
3 color components cyan, magenta, and yellow.
md Set color to the default color value (black in most
cases). No component arguments.
mg gray
Set color to the shade of gray given by the
argument, an integer between 0 (black) and 65536
(white).
mk cyan magenta yellow black
Set color using the CMYK color scheme, having the
4 color components cyan, magenta, yellow, and
black.
mr red green blue
Set color using the RGB color scheme, having the
3 color components red, green, and blue.
N n Print glyph with index n (an integer, normally non-
negative) of the current font. The print position is not
changed. If -T html or -T xhtml is used, negative values
are emitted also to indicate an unbreakable space with
given width. For example, N -193 represents an
unbreakable space which has a width of 193u.
n b a Inform the device about a line break, but no positioning
is done by this command. In classical troff, the integer
arguments b and a informed about the space before and
after the current line to make the intermediate output
more human readable without performing any action. In
groff, they are just ignored, but they must be provided
for compatibility reasons.
p n Begin a new page in the outprint. The page number is set
to n. This page is completely independent of pages
formerly processed even if those have the same page
number. The vertical position on the outprint is
automatically set to 0. All positioning, writing, and
drawing is always done relative to a page, so a p command
must be issued before any of these commands.
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 a word xyz; that is, set a sequence of ordinary
glyphs named x, y, z, ..., terminated by a space character
or a line break; 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. Only ordinary characters can
be set using this command; use the C command to emplace
special characters. The t command is a groff extension
and is output only for devices whose DESC file contains
the tcommand directive; see
u n xyz...⟨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 (u). The u command
is a groff extension and is output only for devices whose
DESC file contains the tcommand directive; see
V n Move down to the absolute vertical position n (a non-
negative integer in basic units u) relative to upper edge
of current page.
v n Move n basic units u down (n is a non-negative integer).
AT&T troff allowed negative n; GNU troff does not produce
such values, but groff's output driver library handles
them.
w Describe an adjustable space. This performs no action; it
is present for documentary purposes. The spacing itself
must be performed explicitly by a move command.
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
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
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.
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.
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.
• 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 to get its
representation as a PostScript file, or to output directly to
PDF.
• 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 to get a formatted
text document.
• 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 or for displaying
in X.
Due to the obsolete jump-and-write command, the text clusters in
the classical output are almost unreadable.
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.
• The classical quasi device independence is not yet implemented.
• 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.
• 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.
• 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.
• 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
/usr/local/share/groff/1.23.0/font/devname/DESC
describes the output device name.
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⟩.
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.
documents the
-Z option and contains pointers to further groff
documentation.
describes the
groff language, including its escape sequences and system
of units.
details the device scaling parameters of device
DESC files.
generates the device-independent intermediate output documented
here.
presents historical aspects and the general structure of
roff systems.
enumerates differences between the intermediate output produced
by AT&T
troff and that of groff.
is a viewer for intermediate output.
Roff.js
⟨https://github.com/Alhadis/Roff.js/⟩ is a viewer for
intermediate output written in JavaScript.
and are groff postprocessors.
This page is part of the groff (GNU troff) project. Information
about the project can be found at
⟨http://www.gnu.org/software/groff/⟩. If you have a bug report
for this manual page, see ⟨http://www.gnu.org/software/groff/⟩.
This page was obtained from the project's upstream Git repository
⟨https://git.savannah.gnu.org/git/groff.git⟩ on 2022-12-17. (At
that time, the date of the most recent commit that was found in
the repository was 2022-12-14.) If you discover any rendering
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groff 1.23.0.rc1.3569-94746-d1i4rtDyecember 2022 groff_out(5)
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