groff_out(5) File Formats Manual groff_out(5)
Name
groff_out - GNU roff device-independent page description language
Description
The fundamental operation of the GNU troff formatter, troff(1), 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 device-independent output
language, never to the language intended for use by document authors.
Device-independent output commands comprise several categories: glyph
output; font, color, and text size selection; motion of the drawing
position; page advancement; drawing of geometric primitives; and device
extension 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.
Background
As groff(1) is a wrapper program around GNU troff and automatically
calls an output driver, users seldom encounter this format under normal
circumstances. groff offers the option -Z to inhibit postprocessing
such that GNU troff's output is sent to the standard output stream just
as it is when running GNU troff directly.
The purpose of device-independent output is to facilitate the
development of postprocessors by providing a common programming
interface for all devices. It is a distinct, and much simpler,
language from that of the formatter, troff. The device-independent
output can be thought of as a "page description language".
In the following discussion, the term troff output describes what is
output by GNU troff, while page description denotes the language
accepted by the parser that interprets this output for the output
drivers. This parser handles whitespace more flexibly than AT&T
troff's implementation, recognizes a GNU extension to the language, and
supports a legacy compressed encoding of a subset of commands for
compatibility; otherwise, the formats are the same. (The parser for
device-independent output can be found in the groff sources at src/
libs/libdriver/input.cpp.)
When Brian Kernighan designed AT&T troff's device-independent page
description language circa 1980, he had to balance readability and
maintainability against severe constraints on file size and
transmission speed to the output device. A decade later, when James
Clark wrote groff, these constraints were no longer as tight.
Syntax
roff's page description language is a sequence of tokens: single-letter
commands or their arguments. Some commands accept a subcommand as a
first argument, followed by one or more further arguments.
AT&T device-independent troff used whitespace minimally when producing
output. GNU troff, in contrast, attempts to make its output more
human-readable. The whitespace characters--tab, space, and
newline--are always meaningful. They are never used to represent
spacing in the document; that is done with horizontal (h, H) and
vertical (v, V) positioning commands. Any sequence of space and/or tab
characters is equivalent to a single space, separating commands from
arguments and arguments from each other. Space is required only where
omitting it would cause ambiguity. A line break separates commands.
The comment character is a pound/hash sign (#), and marks the remainder
of the line as a comment. A line comprising only whitespace after
comment removal does nothing but separate input tokens.
For example, the relative horizontal motion command (h) and the command
to write one glyph (c), each take a single argument; the former a
signed integer, and the latter a printable ISO 646/"ASCII" character.
A series of such commands could validly occur without spaces on an
input line, but GNU troff follows each with a newline.
Some commands have a more complex syntax; the GNU troff extension
command for writing glyph sequences (t) accepts a variable number of
arguments. Those that draw geometric objects (D) or control the device
(x) furthermore recognize subcommand arguments. Such commands thus
must end with a newline. In GNU troff, the device extension
(sub)command "x X" uniquely supports a line continuation syntax; a
single input line contains any other.
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 "s"
(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.
Output structure
Device-independent troff output is organized into three parts: a
header, a body, and a trailer.
The task of the header is to set general device parameters. GNU troff
guarantees that its header consists of the following three lines:
x T device
x res n h v
x init
with the parameters n, h, and v set as outlined in subsection "Device
Control Commands" below. The parser for the device-independent page
description language format is able to interpret additional whitespace
and comments as well even in the header.
The body contains the document's visible content. Once an output
driver interprets "x init", it prepares to handle commands in general.
Processing terminates when a "x stop" command is encountered; the last
line of any GNU troff page description output always contains such a
command.
Semantically, the body is page-oriented. The p command starts a new
page. Positioning, writing, and drawing commands are performed within
a page, so they cannot occur before the first p command. The output
driver reckons absolute positioning (by the H and V commands) with
respect to the current page's origin at the top left corner, and all
other positioning relative to the drawing position on the page.
The trailer advances the drawing position to the bottom of the page and
informs the device that the document (or "job") has ended.
Command reference
This section describes all page description output commands, both from
AT&T troff as well as extension commands issued by GNU troff.
Comment command
#anything<line-break>
Apply comment annotation. Ignore any characters from the
# character up to the next newline. Each comment can be
preceded by arbitrary syntactical space, and 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 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
inter-word spaces on an output line (be they breakable or not)
are thus described; those resulting from 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 page description language
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.
GNU troff output follows AT&T troff's output conventions (no space
between command and subcommand, all arguments are preceded by a single
space character), but groff's 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.
Each graphics command directly corresponds to a troff \D escape
sequence. See subsection "Drawing commands" of groff(7).
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 GNU 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 GNU 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 GNU troff's escape sequences "\D'F ...'" \M (with
no other corresponding graphics commands). No position
changing. This command is a GNU 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 command is superseded
by "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 GNU 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 GNU 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 GNU extension.
Device control commands
Each device control command starts with the letter "x", followed by a
space character (optional or arbitrary space or tab in GNU troff) 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, GNU troff outputs the initialization command "x i" as "x
init" and the resolution command "x r" as "x res".
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 its internal piping mechanism. The input file
is not changed by this command. This command is a GNU
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 header.
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 header.
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 device-independent 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 header.
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 GNU 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. Line continuation is a GNU extension.
Legacy compressed encoding
AT&T troff primarily emitted glyphs by writing two digits (a motion)
followed by a single character corresponding to a glyph. This syntax
is less a command itself than a compressed encoding of the c and h
commands.
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 is impractical because
the widths of the glyphs have a greater magnitude in basic units than
two decimal digits can represent. 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.
Compatibility
The page description language of AT&T troff was first documented in "A
Typesetter-independent TROFF", by Brian Kernighan, and by 1992 the AT&T
troff manual was updated to incorporate a description of it.
groff's page description language is compatible with this specification
except in the following aspects.
o AT&T device-independent troff's quasi-device independence is not yet
implemented.
o The printing hardware of the early 1980s differed from today's.
groff's output device names also differ from those of AT&T troff For
example, the PostScript device in AT&T troff, post (implemented by
the driver command dpost), has a resolution of only 720 units per
inch, suitable for printers of decades past. groff's ps device has
a resolution of 72000 units per inch. In principle, by implementing
a rescaling mechanism, groff could come to emulate AT&T's post
device.
o While the B-spline command D~ is reliably interpreted by groff's
page description language parser, some output drivers don't
implement drawing routines for it.
o In GNU troff, the argument to the commands s and x H uses an
implicit unit of scaled points z whereas AT&T troff uses spacing
points p. This isn't an incompatibility, but a compatible
extension, for both units coincide for any device without a
sizescale directive in its DESC file, including all postprocessors
from AT&T and groff's text (nroff-mode) devices. groff devices that
use sizescale either do not exist for AT&T troff have a different
name, or seem to have a different resolution. So conflicts are very
unlikely.
o The drawing position after the commands "Dp", "DP", and "Dt", are
processed is illogical. Since old versions of GNU troff had this
wart, we've retained it for compatibility, but may change it in the
future. Wrap these drawing commands with the \Z escape sequence to
both overcome the illogical positioning and keep your input working
consistently regardless of the wart's presence in the
implementation.
The differences between groff and classical troff are documented in
groff_diff(7).
Files
/opt/local/share/groff/1.24.1/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 page
description language and that of GNU troff. It has since been expanded
and revised by Bernd Warken <groff-bernd.warken-72@web.de> and G.
Branden Robinson <g.branden.robinson@gmail.com>.
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 page
description language 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, (CSTR #97),
provides additional insights into the device and font description file
formats and device-independent page description language.
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 scaling parameters of DESC (device description)
files.
troff(1)
generates the language documented here.
roff(7)
presents historical aspects and the general structure of roff
systems.
groff_diff(7)
enumerates differences between the output of AT&T troff and that
of GNU troff.
gxditview(1)
is a viewer for device-independent troff output.
Roff.js <https://github.com/Alhadis/Roff.js/> is a viewer for device-
independent troff output written in JavaScript.
grodvi(1), grohtml(1), grolbp(1), grolj4(1), gropdf(1), grops(1), and
grotty(1) are groff postprocessors.
groff 1.24.1 2026-05-15 groff_out(5)
groff 1.24.1 - Generated Mon May 18 15:45:48 CDT 2026