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.

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.

Typeset the glyph of the ordinary
    character c. The drawing position is not advanced.

Select the font mounted at
    position n. n cannot be negative.

Horizontally move the drawing position to
    n basic units from the left edge of the page. n cannot be negative.

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.

Select the stroke color using the
    components in the color space scheme. Each component is an integer between 0 and . The quantity of components and their meanings vary with each scheme. This command is a groff extension.

    Use the CMY color scheme with components
        cyan, magenta, and yellow.

    Use the default color
        (no components; black in most cases).

    Use a grayscale color scheme with a component ranging
        between 0 (black) and (white).

    Use the CMYK color scheme with components
        cyan, magenta, yellow, and black.

    Use the RGB color scheme with components
        red, green, and blue.

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.

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.

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.

Set type size to
    n scaled points (unit z in GNU troff). AT&T troff used unscaled points (p) instead; see section “Compatibility” below.

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).

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).

Vertically move the drawing position to
    n basic units from the top edge of the page. n cannot be negative.

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.

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.

Draw B-spline from current position to offset
    then to offset if given, etc., up to This command takes a variable number of argument pairs; the current position is moved to the terminal point of the drawn curve.

Draw arc from current position to
    with center at then move the current position to the final point of the arc.

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.

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.

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.

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.

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 . 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.

    Set fill color for solid drawing objects using the CMY color scheme,
        having the 3 color components cyan, magenta, and yellow.

    Set fill color for solid drawing objects to the default fill color value
        (black in most cases). No component arguments.

    Set fill color for solid drawing objects to the shade of gray given by
        the argument, an integer between 0 (black) and (white).

    Set fill color for solid drawing objects using the CMYK color scheme,
        having the 4 color components cyan, magenta, yellow, and black.

    Set fill color for solid drawing objects using the RGB color scheme,
        having the 3 color components red, green, and blue.

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
                Df -1

        sets all colors to blue.

    This command is a GNU extension.

Draw line from current position to offset
    (integers in basic units u); then set current position to the end of the drawn line.

Draw a polygon line from current position to offset
    from there to offset etc., up to offset and from there back to the starting position. As the polygon is closed, the end of drawing is the starting point, so the position doesn't change. This command is a groff extension.

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. This command is a GNU extension.

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. 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.

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.

Mount font position
    n (a non-negative integer) with font named s (a text word); see groff_font(5).

Set character height to
    n (a positive integer in scaled points z). Classical troff used the unit points (p) instead; see section “Compatibility” below.

Initialize device.
    This is the third command of the header.

Parsed but ignored.
    The classical documentation reads pause device, can be restarted.

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.

Set slant to
    n degrees (an integer in basic units u).

Terminates the processing of the current file; issued as the last
    command of device-independent troff output.

Generate trailer information, if any.
    In groff, this is currently ignored.

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.

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.

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.

•   AT&T device-independent troff's quasi-device independence is not yet implemented.

•   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.

•   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.

•   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.

•   Temporarily, there existed some confusion on the positioning after the D commands that are groff extensions. This has been clarified by establishing the classical rule for all groff drawing commands:

    The position after a graphic object has been drawn is at its end; for circles and ellipses, the "end" is at the right side.

From this, the positionings specified for the drawing commands above follow quite naturally.

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.