File: groff.info,  Node: Output Language Compatibility,  Prev: Intermediate Output Examples,  Up: gtroff Output

6.1.4 Output Language Compatibility
-----------------------------------

The intermediate output 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 incorprate a description of it.

   The GNU 'troff' 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.