8.1.4 Output Language Compatibility

The intermediate output language of AT&T troff was first documented in the UNIX troff manual, with later additions documented in A Typesetter-indenpendent TROFF, written by Brian Kernighan.

The gtroff intermediate output format is compatible with this specification except for the following features.

• The classical quasi device independence is not yet implemented.
• The old hardware was very different from what we use today. So the groff devices are also fundamentally different from the ones in AT&T troff. For example, the AT&T PostScript device is called post and has a resolution of only 720 units per inch, suitable for printers 20 years ago, while groff's ps device has a resolution of 72000 units per inch. Maybe, by implementing some rescaling mechanism similar to the classical quasi device independence, groff could emulate AT&T's post device.
• The B-spline command ‘D~’ is correctly handled by the intermediate output parser, but the drawing routines aren't implemented in some of the postprocessor programs.
• The argument of the commands ‘s’ and ‘x H’ has the implicit unit scaled point ‘z’ in gtroff, while AT&T troff has point (‘p’). This isn't an incompatibility but a compatible extension, for both units coincide for all devices without a sizescale parameter in the DESC file, including all postprocessors from AT&T and groff's text devices. The few groff devices with a sizescale parameter 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 position changing after the commands ‘Dp’, ‘DP’, and ‘Dt’ is illogical, but as old versions of gtroff used this feature it is kept for compatibility reasons.