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groff: Suppressing Output

 
 5.32 Suppressing Output
 =======================
 
  -- Escape sequence: \O[num]
      Suppress GNU 'troff' output of glyphs and geometric objects.  The
      sequences '\O2', '\O3', '\O4', and '\O5' are intended for internal
      use by 'grohtml'.
 
      '\O0'
           Disable the emission of glyphs and geometric objects to the
           output driver, provided that this sequence occurs at the
           outermost suppression level (see '\O3' and '\04' below).
           Horizontal motions corresponding to non-overstruck glyph
           widths still occur.
 
      '\O1'
           Enable the emission of glyphs and geometric objects to the
           output driver, provided that this sequence occurs at the
           outermost suppression level.
 
      '\O0' and '\O1' also reset the four registers 'opminx', 'opminy',
      'opmaxx', and 'opmaxy' to -1.  These four registers mark the top
      left and bottom right hand corners of a box encompassing all
      written or drawn output.
 
      '\O2'
           At the outermost suppression level, enable emission of glyphs
           and geometric objects, and write to the standard error stream
           the page number and values of the four aforementioned
           registers encompassing glyphs written since the last
           interpolation of a '\O' sequence, as well as the page offset,
           line length, image file name (if any), horizontal and vertical
           device motion quanta, and input file name.  Numeric values are
           in basic units.
 
      '\O3'
           Begin a nested suppression level.  'grohtml' uses this
           mechanism to create images of output preprocessed with 'gpic',
           'geqn', and 'gtbl'.  At startup, GNU 'troff' is at the
           outermost suppression level.  'pre-grohtml' generates these
           sequences when processing the document, using GNU 'troff' with
           the 'ps' output device, Ghostscript, and the PNM tools to
           produce images in PNG format.  They start a new page if the
           device is not 'html' or 'xhtml', to reduce the number of
           images crossing a page boundary.
 
      '\O4'
           End a nested suppression level.
 
      '\O[5PFILE]'
           At the outermost suppression level, write the name 'file' to
           the standard error stream at position P, which must be one of
           'l', 'r', 'c', or 'i', corresponding to left, right, centered,
           and inline alignments within the document, respectively.  FILE
           is a name associated with the production of the next image.
 
  -- Register: \n[.O]
      Output suppression nesting level applied by '\O3' and '\O4' escape
      sequences.
 

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