Ubuntu Manpage: groff_out - GNU roff intermediate output format

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.

       • 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.

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.

• 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).

Files

       /usr/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.