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5.27 Suppressing output

— Escape: \Onum

Disable or enable output depending on the value of num:

\O0
Disable any glyphs from being emitted to the device driver, provided that the escape occurs at the outer level (see \O[3] and \O[4]). Motion is not suppressed so effectively \O[0] means pen up.
\O1
Enable output of glyphs, provided that the escape occurs at the outer level.

\O0 and \O1 also reset the four registers ‘opminx’, ‘opminy’, ‘opmaxx’, and ‘opmaxy’ to −1. See Register Index. These four registers mark the top left and bottom right hand corners of a box which encompasses all written glyphs.

For example the input text:

          
          Hello \O[0]world \O[1]this is a test.

produces the following output:

          
          Hello       this is a test.
\O2
Provided that the escape occurs at the outer level, enable output of glyphs and also write out to stderr the page number and four registers encompassing the glyphs previously written since the last call to \O.
\O3
Begin a nesting level. At start-up, gtroff is at outer level. The current level is contained within the read-only register .O. See Built-in Registers.
\O4
End a nesting level. The current level is contained within the read-only register .O. See Built-in Registers.
\O[5Pfilename]
This escape is grohtml specific. Provided that this escape occurs at the outer nesting level write the filename to stderr. The position of the image, P, must be specified and must be one of l, r, c, or i (left, right, centered, inline). filename is associated with the production of the next inline image.