File: groff.info,  Node: Suppressing Output,  Next: Colors,  Prev: Environments,  Up: GNU troff Reference

5.32 Suppressing Output
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 -- 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.