6.2.4 Output Language Compatibility

The page description language of AT&T troff was first documented in A Typesetter-independent TROFF, by Brian Kernighan, and by 1992 the AT&T troff manual was updated to incorporate a description of it.

groff’s page description language is compatible with this specification except in the following aspects.

• AT&T device-independent troff’s quasi-device independence is not yet implemented.
• The printing hardware of the early 1980s differed from today’s. groff’s output device names also differ from those of AT&T troff. For example, the PostScript device in AT&T troff, post (implemented by the driver command dpost), has a resolution of only 720 units per inch, suitable for printers of decades past. groff’s ps device has a resolution of 72000 units per inch. In principle, by implementing a rescaling mechanism, groff could come to emulate AT&T’s post device.
• While the B-spline command ‘D~’ is reliably interpreted by groff’s page description language parser, some output drivers don’t implement drawing routines for it.
• In GNU troff, the argument to the commands ‘s’ and ‘x H’ uses an implicit unit of scaled points ‘z’ whereas AT&T troff uses spacing points ‘p’. This isn’t an incompatibility, but a compatible extension, for both units coincide for any device without a sizescale directive in its DESC file, including all postprocessors from AT&T and groff’s text (nroff-mode) devices. groff devices that use sizescale either do not exist for AT&T troff, have a different name, or seem to have a different resolution. So conflicts are very unlikely.
• The drawing position after the commands ‘Dp’, ‘DP’, and ‘Dt’ are processed is illogical. Since old versions of GNU troff had this wart, we’ve retained it for compatibility, but may change it in the future. Wrap these drawing commands with the \Z escape sequence to both overcome the illogical positioning and keep your input working consistently regardless of the wart’s presence in the implementation.