Provided by: groff_1.23.0-3build2_amd64 bug

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⟩.

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 output format referred to collectively in groff documentation as “AT&T troff”.

       “A  Typesetter-independent  TROFF”  by  Brian  W.  Kernighan, 1982, AT&T Bell Laboratories
       Computing Science Technical Report No. 97, provides additional insights  into  the  device
       and font description file formats and device-independent output format.

       groff(1)
              documents the -Z option and contains pointers to further groff documentation.

       groff(7)
              describes the groff language, including its escape sequences and system of units.

       groff_font(5)
              details the device scaling parameters of device DESC files.

       troff(1)
              generates the device-independent intermediate output documented here.

       roff(7)
              presents historical aspects and the general structure of roff systems.

       groff_diff(7)
              enumerates  differences  between the intermediate output produced by AT&T troff and
              that of groff.

       gxditview(1)
              is a viewer for intermediate output.

       Roff.js
              ⟨https://github.com/Alhadis/Roff.js/⟩ is a viewer for intermediate  output  written
              in JavaScript.

       grodvi(1),  grohtml(1), grolbp(1), grolj4(1), gropdf(1), grops(1), and grotty(1) are groff
       postprocessors.