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 ⟨groff-bernd.warken-72@web.de⟩.