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:

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.

Comment command

#anything⟨line-break⟩

Apply comment annotation. Ignore any characters from the # character up to the next newline. Each comment can be preceded by arbitrary syntactical space, and every command can be terminated by a comment.

Simple commands

The commands in this subsection have a command code consisting of a single character, taking a fixed number of arguments. Most of them are commands for positioning and text writing. These commands are smart about whitespace. Optionally, syntactical space can be inserted before, after, and between the command letter and its arguments. All of these commands are stackable, i.e., they can be preceded by other simple commands or followed by arbitrary other commands on the same line. A separating syntactical space is necessary only when two integer arguments would clash or if the preceding argument ends with a string argument.

C id⟨white-space⟩

Typeset the glyph of the special character id. Trailing syntactical space is necessary to allow special character names of arbitrary length. The drawing position is not advanced.

c c

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

f n

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

H n

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

h n

Move the drawing position right n basic units. AT&T troff allowed negative n; GNU troff does not produce such values, but groff's output driver library handles them.

m scheme [component ...]

Select the stroke color using the components in the color space scheme. Each component is an integer between 0 and 65536. The quantity of components and their meanings vary with each scheme. This command is a groff extension.

N n

Typeset the glyph with index n in the current font. n is normally a non-negative integer. The drawing position is not advanced. The html and xhtml devices use this command with negative n to produce unbreakable space; the absolute value of n is taken and interpreted in basic units.

n b a

Indicate a break. No action is performed; the command is present to make the output more easily parsed. The integers b and a describe the vertical space amounts before and after the break, respectively. GNU troff issues this command but groff's output driver library ignores it. See v and V.

p n

Begin a new page, setting its number to n. Each page is independent, even from those using the same number. The vertical drawing position is set to 0. All positioning, writing, and drawing commands are interpreted in the context of a page, so a p command must precede them.

s n

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

t xyz...⟨white-space⟩

t xyz... dummy-arg⟨white-space⟩

Typeset word xyz; that is, set a sequence of ordinary glyphs named x, y, z, ..., terminated by a space or newline; an optional second integer argument is ignored (this allows the formatter to generate an even number of arguments). Each glyph is set at the current drawing position, and the position is then advanced horizontally by the glyph's width. A glyph's width is read from its metrics in the font description file, scaled to the current type size, and rounded to a multiple of the horizontal motion quantum. Use the C command to emplace glyphs of special characters. The t command is a groff extension and is output only for devices whose DESC file contains the tcommand directive; see groff_font(5).

u n xyz...

u xyz... dummy-arg⟨white-space⟩

Typeset word xyz with track kerning. As t, but after placing each glyph, the drawing position is further advanced horizontally by n basic units. The u command is a groff extension and is output only for devices whose DESC file contains the tcommand directive; see groff_font(5).

V n

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

v n

Move the drawing position down n basic units. AT&T troff allowed negative n; GNU troff does not produce such values, but groff's output driver library handles them.

w 

Indicate an inter-word space. No action is performed; the command is present to make the output more easily parsed. Only inter-word spaces on an output line (be they breakable or not) are thus described; those resulting from horizontal motion escape sequences are not. GNU troff issues this command but groff's output driver library ignores it. See h and H.

Graphics commands

Each graphics or drawing command in the page description language starts with the letter “D”, followed by one or two characters that specify a subcommand; this is followed by a fixed or variable number of integer arguments that are separated by a single space character. A “D”, command may not be followed by another command on the same line (apart from a comment), so each “D” command is terminated by a syntactical line break.

GNU troff output follows AT&T troff's output conventions (no space between command and subcommand, all arguments are preceded by a single space character), but groff's parser allows optional space between the command letters and makes the space before the first argument optional. As usual, each space can be any sequence of tab and space characters.

Some graphics commands can take a variable number of arguments. In this case, they are integers representing a size measured in basic units u. The h arguments stand for horizontal distances where positive means right, negative left. The v arguments stand for vertical distances where positive means down, negative up. All these distances are offsets relative to the current location.

Each graphics command directly corresponds to a troff \D escape sequence. See subsection “Drawing commands” of groff(7).

Unless indicated otherwise, each graphics command directly corresponds to a similar groff \D escape sequence; see groff(7).

Unknown D commands are assumed to be device-specific. Its arguments are parsed as strings; the whole information is then sent to the postprocessor.

In the following command reference, the syntax element ⟨line-break⟩ means a syntactical line break as defined in subsection “Separation” above.

D~ h1 v1 h2 v2 ... hn vn⟨line-break⟩

Draw B-spline from current position to offset (h1, v1), then to offset (h2, v2) if given, etc., up to (hn, vn). This command takes a variable number of argument pairs; the current position is moved to the terminal point of the drawn curve.

Da h1 v1 h2 v2⟨line-break⟩

Draw arc from current position to (h1, v1)+(h2, v2) with center at (h1, v1); then move the current position to the final point of the arc.

DC d⟨line-break⟩

DC d dummy-arg⟨line-break⟩

Draw a solid circle using the current fill color with diameter d (integer in basic units u) with leftmost point at the current position; then move the current position to the rightmost point of the circle. An optional second integer argument is ignored (this allows the formatter to generate an even number of arguments). This command is a GNU extension.

Dc d⟨line-break⟩

Draw circle line with diameter d (integer in basic units u) with leftmost point at the current position; then move the current position to the rightmost point of the circle.

DE h v⟨line-break⟩

Draw a solid ellipse in the current fill color with a horizontal diameter of h and a vertical diameter of v (both integers in basic units u) with the leftmost point at the current position; then move to the rightmost point of the ellipse. This command is a GNU extension.

De h v⟨line-break⟩

Draw an outlined ellipse with a horizontal diameter of h and a vertical diameter of v (both integers in basic units u) with the leftmost point at current position; then move to the rightmost point of the ellipse.

DF color-scheme [component ...]⟨line-break⟩

Set fill color for solid drawing objects using different color schemes; the analogous command for setting the color of text, line graphics, and the outline of graphic objects is m. The color components are specified as integer arguments between 0 and 65536. The number of color components and their meaning vary for the different color schemes. These commands are generated by GNU troff's escape sequences “\D'F ...'” \M (with no other corresponding graphics commands). No position changing. This command is a GNU extension.

Df n⟨line-break⟩

The argument n must be an integer in the range -32767 to 32767.

Dl h v⟨line-break⟩

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

Dp h1 v1 h2 v2 ... hn vn⟨line-break⟩

Draw a polygon line from current position to offset (h1, v1), from there to offset (h2, v2), etc., up to offset (hn, vn), and from there back to the starting position. For historical reasons, the position is changed by adding the sum of all arguments with odd index to the current horizontal position and the even ones to the vertical position. Although this doesn't make sense it is kept for compatibility. This command is a groff extension.

DP h1 v1 h2 v2 ... hn vn⟨line-break⟩

The same macro as the corresponding Dp command with the same arguments, but draws a solid polygon in the current fill color rather than an outlined polygon. The position is changed in the same way as with Dp. This command is a GNU extension.

Dt n⟨line-break⟩

Set the current line thickness to n (an integer in basic units u) if n>0; if n=0 select the smallest available line thickness; otherwise, the line thickness is made proportional to the type size, which is the default. For historical reasons, the horizontal position is changed by adding the argument to the current horizontal position, while the vertical position is not changed. Although this doesn't make sense, it is kept for compatibility. This command is a GNU extension.

Device control commands

Each device control command starts with the letter “x”, followed by a space character (optional or arbitrary space or tab in GNU troff) and a subcommand letter or word; each argument (if any) must be preceded by a syntactical space. All “x”, commands are terminated by a syntactical line break; no device control command can be followed by another command on the same line (except a comment).

The subcommand is basically a single letter, but to increase readability, it can be written as a word, i.e., an arbitrary sequence of characters terminated by the next tab, space, or newline character. All characters of the subcommand word but the first are simply ignored. For example, GNU troff outputs the initialization command “x i” as “x init” and the resolution command “x r” as “x res”.

In the following, the syntax element ⟨line-break⟩ means a syntactical line break as defined in subsection “Separation” above.

xF name⟨line-break⟩

(Filename control command)
Use name as the intended name for the current file in error reports. This is useful for remembering the original file name when groff uses its internal piping mechanism. The input file is not changed by this command. This command is a GNU extension.

xf n s⟨line-break⟩

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

xH n⟨line-break⟩

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

xi ⟨line-break⟩

(init control command)
Initialize device. This is the third command of the header.

xp ⟨line-break⟩

(pause control command)
Parsed but ignored. The classical documentation reads pause device, can be restarted.

xr n h v⟨line-break⟩

(resolution control command)
Resolution is n, while h is the minimal horizontal motion, and v the minimal vertical motion possible with this device; all arguments are positive integers in basic units u per inch. This is the second command of the header.

xS n⟨line-break⟩

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

xs ⟨line-break⟩

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

xt ⟨line-break⟩

(trailer control command)
Generate trailer information, if any. In groff, this is currently ignored.

xT xxx⟨line-break⟩

(Typesetter control command)
Set the name of the output driver to xxx, a sequence of non-whitespace characters terminated by whitespace. The possible names correspond to those of groff's -T option. This is the first command of the header.

xu n⟨line-break⟩

(underline control command)
Configure underlining of spaces. If n is 1, start underlining of spaces; if n is 0, stop underlining of spaces. This is needed for the cu request in nroff mode and is ignored otherwise. This command is a GNU extension.

xX anything⟨line-break⟩

(X-escape control command)
Send string anything uninterpreted to the device. If the line following this command starts with a + character this line is interpreted as a continuation line in the following sense. The + is ignored, but a newline character is sent instead to the device, the rest of the line is sent uninterpreted. The same applies to all following lines until the first character of a line is not a + character. This command is generated by the groff escape sequence \X. Line continuation is a GNU extension.

Legacy compressed encoding

AT&T troff primarily emitted glyphs by writing two digits (a motion) followed by a single character corresponding to a glyph. This syntax is less a command itself than a compressed encoding of the c and h commands.

ddc

Move right dd (exactly two decimal digits) basic units u, then print glyph with single-letter name c.

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.