Provided by: groff_1.22.4-3_amd64 bug

NAME

       groff_out - groff intermediate output format

DESCRIPTION

       This  manual  page  describes  the  intermediate  output  format  of  the GNU roff(7) text
       processing system groff(1).  This output is produced by a run of the GNU troff(1) program.
       It  contains  already all device-specific information, but it is not yet fed into a device
       postprocessor program.

       As the GNU roff processor groff(1) is a wrapper program around  troff  that  automatically
       calls  a  postprocessor,  this output does not show up normally.  This is why it is called
       intermediate within the groff system.  The groff program provides the option -Z to inhibit
       postprocessing, such that the produced intermediate output is sent to standard output just
       like calling troff manually.

       In this document, the term troff output describes what is output by the GNU troff program,
       while  intermediate  output  refers  to  the  language that is accepted by the parser that
       prepares this output for the postprocessors.  This parser is  smarter  on  whitespace  and
       implements obsolete elements for compatibility, otherwise both formats are the same.  Both
       formats can be viewed directly with gxditview(1).

       The main purpose of the intermediate output concept is to facilitate  the  development  of
       postprocessors  by  providing  a  common  programming interface for all devices.  It has a
       language of its own that is completely different from the groff(7)  language.   While  the
       groff  language is a high-level programming language for text processing, the intermediate
       output language is a kind of low-level assembler language by specifying all  positions  on
       the page for writing and drawing.

       The  pre-groff  roff  versions  are  denoted  as classical troff.  The intermediate output
       produced by groff is fairly readable, while classical troff output was hard to  understand
       because of strange habits that are still supported, but not used any longer by GNU troff.

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.

       Empty  lines,  i.e.,  lines  containing only space and/or a comment, can occur everywhere.
       They are just ignored.

   Argument Units
       Some commands take integer arguments that are assumed to represent values in a measurement
       unit,  but the letter for the corresponding scale indicator is not written with the output
       command arguments; see groff(7) and Groff: The GNU  Implementation  of  troff,  the  groff
       Texinfo  manual,  for more on this topic.  Most commands assume the scale indicator u, the
       basic unit of the device, some use z, the scaled point unit of the device,  while  others,
       such  as  the  color commands expect plain integers.  Note that these scale indicators are
       relative to the chosen device.  They are  defined  by  the  parameters  specified  in  the
       device's DESC file; see groff_font(5).

       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⟨end-of-line⟩
              A  comment.   Ignore  any  characters  from  the # character up to the next newline
              character.

       This command is the only possibility for commenting  in  the  intermediate  output.   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 only
       necessary when two integer arguments would clash or if the preceding argument ends with  a
       string argument.

       C xxx⟨white-space⟩
              Print  a  glyph  (special  character) named xxx.  The trailing syntactical space or
              line break is necessary to allow glyph names of arbitrary  length.   The  glyph  is
              printed at the current print position; the glyph's size is read from the font file.
              The print position is not changed.

       c c    Print glyph with single-letter name c at the current print  position;  the  glyph's
              size is read from the font file.  The print position is not changed.

       f n    Set font to font number n (a non-negative integer).

       H n    Move  right  to  the  absolute vertical position n (a non-negative integer in basic
              units u) relative to left edge of current page.

       h n    Move  n  (a  non-negative  integer)  basic  units  u  horizontally  to  the  right.
              [CSTR #54] allows negative values for n also, but groff doesn't use this.

       m color-scheme [component ...]
              Set  the  color for text (glyphs), line drawing, and the outline of graphic objects
              using different color schemes; the analogous  command  for  the  filling  color  of
              graphic  objects  is  DF.   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 sequence
              \m.  No position changing.  These commands are a groff extension.

              mc cyan magenta yellow
                     Set color using the CMY color scheme, having the 3  color  components  cyan,
                     magenta, and yellow.

              md     Set  color  to  the default color value (black in most cases).  No component
                     arguments.

              mg gray
                     Set color to the shade of gray given by the argument, an integer  between  0
                     (black) and 65536 (white).

              mk cyan magenta yellow black
                     Set  color  using the CMYK color scheme, having the 4 color components cyan,
                     magenta, yellow, and black.

              mr red green blue
                     Set color using the RGB color scheme, having the  3  color  components  red,
                     green, and blue.

       N n    Print  glyph  with index n (an integer, normally non-negative) of the current font.
              The print position is not changed.  If -T html or -T xhtml is used, negative values
              are  emitted  also to indicate an unbreakable space with given width.  For example,
              N -193 represents an unbreakable space which has a width of 193u.  This command  is
              a groff extension.

       n b a  Inform  the  device about a line break, but no positioning is done by this command.
              In classical troff, the integer arguments b and a informed about the  space  before
              and  after  the  current  line  to make the intermediate output more human readable
              without performing any action.  In groff, they are just ignored, but they  must  be
              provided for compatibility reasons.

       p n    Begin  a  new  page  in  the  outprint.  The page number is set to n.  This page is
              completely independent of pages formerly processed even if those have the same page
              number.   The  vertical  position  on  the outprint is automatically set to 0.  All
              positioning, writing, and drawing is always done relative to a page, so a p command
              must be issued before any of these commands.

       s n    Set  point  size to n scaled points (this is unit z in GNU troff).  Classical troff
              used the unit points (p) instead; see section “Compatibility” below.

       t xyz...⟨white-space⟩
       t xyz... dummy-arg⟨white-space⟩
              Print a word, i.e., a sequence of glyphs with single-letter names x,  y,  z,  etc.,
              terminated  by  a  space  character  or  a  line  break; an optional second integer
              argument is ignored (this allows the  formatter  to  generate  an  even  number  of
              arguments).  The first glyph should be printed at the current position, the current
              horizontal position should then be increased by the width of the first  glyph,  and
              so  on for each glyph.  The widths of the glyph are read from the font file, scaled
              for the current point size, and rounded to a multiple of the horizontal resolution.
              Special  characters  (glyphs  with  names  longer  than  a single letter) cannot be
              printed using this command; use the C command for those glyphs.  This command is  a
              groff  extension; it is only used for devices whose DESC file contains the tcommand
              keyword; see groff_font(5).

       u n xyz...⟨white-space⟩
              Print word with track kerning.  This is the same as the t command except that after
              printing each glyph, the current horizontal position is increased by the sum of the
              width of that glyph and n (an integer in basic units u).  This command is  a  groff
              extension;  it  is  only  used  for  devices  whose DESC file contains the tcommand
              keyword; see groff_font(5).

       V n    Move down to the absolute vertical position n  (a  non-negative  integer  in  basic
              units u) relative to upper edge of current page.

       v n    Move  n  basic  units  u  down  (n  is  a non-negative integer).  [CSTR #54] allows
              negative values for n also, but groff doesn't use this.

       w      Informs about a paddable whitespace to increase readability.   The  spacing  itself
              must be performed explicitly by a move command.

   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).   No
              position changing.  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.

              No position changing.  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 actual 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;  if  n<0  set  the  line  thickness
              proportional to the point size (this is the default before the first Dt command was
              specified).  For historical reasons, the horizontal position is changed  by  adding
              the  argument to the actual 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 actually just ignored.

       xT xxx⟨line-break⟩
              (Typesetter control command)
              Set name of device to word  xxx,  a  sequence  of  characters  ended  by  the  next
              whitespace character.  The possible device names coincide with those from the groff
              -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, this is only used
       for the devices X75, X75-12, X100, and X100-12.  For other devices, the commands t  and  u
       provide a better functionality.

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.

EXAMPLES

       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/1.22.4/font/devname/DESC
              Device description file for device name.

       src/libs/libdriver/input.cpp
              Defines  the  parser  and postprocessor for the intermediate output.  It is located
              relative to the top directory of  the  groff  source  tree.   This  parser  is  the
              definitive specification of the groff intermediate output format.

AUTHORS

       James  Clark wrote an early version of this document, which described only the differences
       between ditroff(7)'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

       A reference like groff(7) refers to a manual page; here groff in section 7 of the man page
       documentation system.  To read the example, look up section 7 in your desktop help  system
       or call from the shell prompt

              shell> man 7 groff

       For more details, see man(1).

       groff(1)
              option -Z and further readings on groff.

       groff(7)
              for details of the groff language such as numerical units and escape sequences.

       groff_font(5)
              for details on the device scaling parameters of the DESC file.

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

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

       groff_diff(7)
              The differences between the intermediate output in groff and classical troff.

       gxditview(1)
              Viewer for the intermediate output.

       grodvi(1), grohtml(1), grolbp(1), grolj4(1), grops(1), grotty(1)
              the groff postprocessor programs.

       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”.

       The classical troff output language is described in two  AT&T  Bell  Labs  CSTR  documents
       available on-line at Bell Labs CSTR site ⟨http://cm.bell-labs.com/cm/cs/cstr.html⟩.

       [CSTR #97]
              A  Typesetter-independent  TROFF  by  Brian  Kernighan  is  the  original  and most
              comprehensive  documentation  on  the  output  language;  see  CSTR  #97   ⟨http://
              cm.bell-labs.com/cm/cs/cstr/97.ps.gz⟩.

       [CSTR #54]
              The  1992  revision  of  the  Nroff/Troff  User's Manual by J. F. Ossanna and Brian
              Kernighan isn't as comprehensive as [CSTR #97] regarding the output  language;  see
              CSTR #54 ⟨http://cm.bell-labs.com/cm/cs/cstr/54.ps.gz⟩.