Suppressing Output (The GNU Troff Manual)

### 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.