groff_out - GNU roff intermediate output format
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.
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.
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.
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 groff(7) 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 groff_font(5).
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.
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.
#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.
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. | ||
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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 groff_font(5).
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 groff_font(5).
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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. | ||
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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. |
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~ h 1 v
1 h
2 v
2 ... h
n v
n
⟨line-break⟩
Draw B-spline from current position to offset (h 1 , v 1 ), then to offset (h 2 , v 2 ) if given, etc., up to (h n , v n ). This command takes a variable number of argument pairs; the current position is moved to the terminal point of the drawn curve.
Da h 1 v 1 h 2 v 2 ⟨line-break⟩
Draw arc from current position to (h 1 , v 1 )+(h 2 , v 2 ) with center at (h 1 , v 1 ); 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 h 1 v 1 h 2 v 2 ... h n v n ⟨line-break⟩
Draw a polygon line from current position to offset (h 1 , v 1 ), from there to offset (h 2 , v 2 ), etc., up to offset (h n , v n ), 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 h 1 v 1 h 2 v 2 ... h n v n ⟨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.
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.
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 grops(1) to get its representation as a PostScript file, or gropdf(1) 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 grotty(1) 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 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.
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 groff_diff(7).
/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: 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.
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.
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.